XXXVI. BOILERS FOR STATIONARY STEAM ENGINES

The boiler for a steam engine requires the most careful usage and inspection, in the first case because a good boiler may be destroyed very rapidly by careless usage, and in the second case because the durability of a boiler depends to a great extent upon matters that are beyond ordinary control, and that in many cases do not make themselves known except in their results, which can only be discovered by careful and intelligent inspection. All that the working engineer is called upon to do is, to use the boiler properly, keep it clean, and examine it at such intervals as the nature of the conditions under which it is used may render necessary.

The periods at which a boiler should be cleaned and inspected depend upon the quality of the water, whether the feed water is purified or not, and to a certain extent upon the design of the boiler; hence these periods are variable under different circumstances.

The horse power of a boiler is estimated in various ways, and there is no uniform practice in this respect. Some makers estimate a boiler to have a horse power for every fifteen square feet of heating surface it possesses, while others allow but 12 square feet.

The heating surface of a boiler of any kind is the surface that is exposed to the action of the fire on one side, and has water on the other; hence the surface of the steam space is not reckoned as heating surface, even though it may be exposed to the action of the heat. The effectiveness of the heating surface of a boiler obviously, however, depends upon the efficiency of the fire, and this depends upon the amount of draught, hence the estimation of horse power from the amount of its heating surface, while affording to a certain extent a standard of measurement or comparison while the boiler is not in use, has no definite value when the boiler is erected and at work.

Thus whatever amount of steam a boiler may produce under a poor or moderate draught, it will obviously produce more under an increased draught; hence the efficiency of the same boiler depends to a certain extent upon the draught, or in other words upon the quantity of fuel that can be consumed upon its fire bars.

The amount of water required in steam boilers varies from 16 lbs. to 40 lbs., per horse power per hour, and it has been proposed to compute the horse power of boilers from the water evaporation, taking as a standard 30 lbs. of feed water at a temperature of 70 degrees, evaporated into steam at a temperature of 212 degrees, at which temperature the steam is assumed to equal the pressure of the atmosphere.

[49]"The strength of the shell of a cylindrical boiler to resist a pressure within it, is inversely proportional to its diameter and directly, to the thickness of the plate of which it is formed.

[49] From "Steam Boilers."

"For instance, take three cylindrical boilers each made of 1/2 inch plate, the first one 2 feet 6 inches in diameter; the second twice that, or 5 feet in diameter; and the third twice that again, or 10 feet in diameter; and if the 2 foot 6 inch boiler is fit for a safe working pressure of 180 lbs. per square inch, then the 5 foot boiler will be fit for exactly one-half that amount, or 90 lbs. per square inch; and the ten foot boiler will be fit for half the working pressure of the five foot boiler, hence we have:

-----------------+------------+-------------------------- Diameter of | Thickness |Relative working pressure. boiler shell. | of plate. | -----------------+------------+-------------------------- 2 feet 6 inches.| 1/2 inch. | 180 lbs. per square inch. 5 " | " " | 90 " " " " 10 " | " " | 45 " " " " -----------------+------------+--------------------------

"The reverse applies to the thickness of the plate. For instance, if we take two cylindrical boiler shells, each 5 feet in diameter, the first one made of plate 1/2 inch thick, and the second twice that, or 1 inch thick, and if the first is equal to a safe working pressure of 90 lbs. per square inch, then the second is equal to a safe working pressure of twice as much, or 180 lbs. per square inch, providing, of course, that the riveted seams are of equal strength in each case, and that both boilers are allowed the same margin for safety; hence we have:

----------------+------------+-------------------------- Diameter of | Thickness | Safe working pressure. boiler. | of shell. | ----------------+------------+-------------------------- 5 feet. | 1/2 inch. | 90 lbs. per square inch. 5 " | 1 " | 180 " " " " ----------------+------------+--------------------------

"These principles (namely, that the strength of a boiler is, all other things or elements being equal, inversely proportional to its diameter, and directly proportional to its thickness) afford us a groundwork upon which we may lay down rules for determining by calculation the strength of the solid part[50] of any boiler shell, and the bases of these calculations are as follows:

[50] In the case of the riveted joints or seams other considerations come in, as will be shown hereafter.

"If the shell plate of a cylindrical boiler is 1/2 inch thick, there is one inch section of metal to be broken before the boiler can be divided into two pieces, that is to say there is 1/2 inch on each side of the shell, as shown in Fig. 3237, and the two together will make 1 inch. If we take a ring an inch broad, as, say, at A in Fig. 3238, we shall obviously have a section of 1 square inch of metal to break before the ring can be broken into two pieces.

"The next consideration is, what is the average strength of a plate of boiler iron? Now suppose we have a strip of boiler iron 2 inches wide and 1/2 inch thick, or, what is the same thing, a bar of boiler iron 1 inch square, and that we lay it horizontally and pull its ends apart until it breaks, how many lbs. will it bear before breaking? Now for our present purpose we may assume this to be 47,040 lbs., and if this number of lbs. be divided by the diameter of the boiler in inches, it will give the bursting pressure in lbs. for any square inch in the ring, or any other square inch in the cylindrical shell of the boiler.

"The reason for dividing by the diameter of the boiler is as follows:

"Of course the steam pressure presses equally on all parts of the interior surface of the shell, and may be taken as radiating from the centre of the boiler, as in Fig. 3239, which represents an end view of a strip an inch wide, of one half of a boiler. Now leaving the riveted seam out of the question, and supposing the shell to be truly cylindrical, and the metal to be of equal quality throughout, it will take just as much pressure to burst the shell apart in one direction as it will in another, hence we may suppose that the boiler is to be burst in the direction of arrow a, and it is the section of metal at b b that is resisting rupture in that direction.

"Now suppose we divide the surface against which the steam presses into six divisions, by lines radiating from the centre C, and to find the amount of area acting on each division to burst the shell in the direction of arrow a, we drop perpendicular lines, as line e, from the lines of division to the line b b, and the length of the line divided off (by the perpendicular) on the diameter represents the effectiveness of the area of that division to burst the boiler in the direction of arrow a; thus for that part of the boiler surface situate in the first division, or from b to line e, the area acting to burst the boiler in the direction of a is represented by the length of the line k, while the general direction of the pressure on this part of the shell is represented by arrow m.

"Similarly, for that part of the shell situate between vertical line e and vertical line f, the general direction of the steam pressure is denoted by the arrow l, while the proportion of this part that is acting to sever the boiler in the direction of a is represented by the distance n, or from the line e to line f measured on the line b b.

"By carrying out this process we shall perceive that, although the pressure acts upon the whole circumference, yet its effectiveness in bursting the boiler in any one direction is equal to the boiler diameter. Thus in Fig. 3240, the pressure acting in the direction of the arrows a (and to burst the boiler apart at b b) is represented by the diametral line b b, while the pressure actually exerted upon the whole boiler shell is represented by the circumference of the boiler.

"To proceed, then, it will now be clear that the ultimate strength of the boiler material, multiplied by twice the thickness of the boiler shell plate in inches or decimal parts of an inch, and this sum divided by the internal diameter of the boiler, in inches, gives the pressure (in lbs. per square inch) at which the boiler shell will burst."

We have here only considered the strength of the solid plate of the shell, and may now consider the strength of the riveted joints, because, as the boiler cannot be any stronger as a whole than its weakest part is, and as the riveted joints are the weakest parts of a cylindrical boiler,[51] therefore the strength of the riveted joint determines the strength of the boiler.

[51] It may be here noted that the riveted joint of a flat plate is stronger than the flat surface of the plate, because at the joint the plate is doubled, or one plate overlaps the other.

[52]"The strains to which a riveted joint is subjected are as follows: That acting to shear the rivet across its diameter is called the shearing strain. But the same strain acts to tear the plate apart; hence, when spoken of with reference to the action on the plate, it is called the tearing strain.

[52] From "Steam Boilers."

"The same strain also acts to crush and rupture the plate between the rivet hole and the edge of the plate, and in this connection it is called the crushing strain.

"Thus, Fig. 3241 represents a single riveted lap joint, in which the joint at rivets A, B, and C is intact, the metal outside of D has crushed, the rivets E, F have sheared, and the plate has torn at H, leaving a piece J on the rivets K L.

"It is obvious that, since it is the same strain that has caused these different kinds of rupture, the joint has, at each location, simply given way where it was the weakest.

"If a riveted joint was to give way by tearing only, the indication would be that the proportion of strength was greatest in the rivets, which might occur from the plate being of inferior metal to the rivets, or from the rivets being too closely spaced. If the rivets were to shear and the plate remain intact, it would indicate insufficient strength in the rivets, which might occur from faulty material in the rivets, from smallness of rivet diameter, or from the rivets being too widely spaced.

"The object then, in designing a riveted joint is to have its resistance to tearing and shearing proportionately equal, whatever form of joint be employed."

The English Board of Trade recommends that the rivet section should always be in excess of the plate section, whereas, in ordinary American practice, for stationary engine boilers, the plate and rivet percentages are made equal.

The forms of riveted joints employed in boiler work are as follows:

Fig. 3242 represents a single riveted lap joint. Fig. 3243 represents a double riveted lap joint, chain riveted; and Fig. 3244, a double riveted lap joint, with the rivets arranged zigzag.

Fig. 3245 represents a single and Fig. 3246 a double riveted butt joint, so called because the ends of the boiler plate abut together. The plates on each side of joint are called butt straps.

The advantages of the butt joint are, first, that the boiler shell is kept more truly cylindrical, and the joint is not liable to bend as it does in the lap joints, in the attempt of the boiler (when under pressure) to assume the form of a true circle, and second that the rivets are placed in double shear. That is to say, if in a lap joint the rivet was to shear between the plates, the joint would come apart, whereas, in a butt joint, the rivet must shear on each side of the plate, and therefore in two places.

Fig. 3247 represents a form of joint much used in locomotive practice in the United States. It is a lap joint, with a covering plate on the inside of the joint; rivets E and F are in single and rivets D in double shear.

[53]"When we have to deal with comparatively thin boiler plates, there is no difficulty in obtaining a sufficiently high percentage of strength in the joints, by using the ordinary double riveted joint, but when we have to deal with thick plates, as in the case of large marine boilers, as 1 inch or upwards, a more costly form of joint must be employed, in order to obtain the required percentage of strength at the joint; hence the ordinary double riveted joint is replaced by various other forms as follows:

[53] From "Steam Boilers."

"First, a triple zigzag riveted lap joint, such as shown in Fig. 3248, or a chain riveted joint as in Fig. 3249, in both of which the third row of rivets enables the rivet pitch to be increased, thus increasing the plate percentage, while the third row of rivets also increases the rivet percentage.

"Second, by employing butt joints with butt straps, either double or treble riveted.

"A double riveted butt joint with double straps is shown in Fig. 3250, and a treble with double straps in Figs. 3251 and 3252.

"Third. By various arrangements of the rivets in conjunction with butt joints and double straps, with which it is not necessary, at this point, to deal.

"One of the great advantages obtained by the use of the double strap is that of bringing the rivet into double shear (or in other words, the rivet must shear on each side of the plate, or in two places, instead of between the plates only, before the joint can give way by shearing), and thus obtaining an increased calculated strength of 1-3/4 times the ordinary or single shear, the rule being to find the rivet strength in the ordinary way (as before explained), and then multiply the result by 1.75.

"The Board of Trade rules for spacing the rivets of these joints are as follows:

"Dimension E is the distance from the edge of the plate to the centre of the rivet hole. Dimension V is the distance between the rows of rivets, dimension p is the pitch of the rivets, which is always measured from centre to centre of the rivets, and dimension pd is the diagonal pitch of the rivets.

"The rule for finding dimension E, whether the plates and rivets are either of steel or iron, is as follows:

"Multiply the diameter of the rivet by 3 and divide by 2, the formula being as follows:

3 × d ------- = E. 2

"To find the distance V between the rows of rivets in chain riveted joints. This distance must not be less than twice the rivet diameter, and a more desirable rule is four times the rivet diameter plus 1 divided by 2, thus:

4_d_ + 1 -------- = V. 2

"To find the distance between the rows of zigzag riveted joints:

_____________________________ \/(11_p_ + 4_d_) × (p + 4_d_) ------------------------------- = V, 10

that is, multiply 11 times the pitch plus 4 times the rivet diameter, by the pitch plus 4 times the rivet diameter, then extract the square root and divide by 10.

"To find diagonal pitch pd, multiply the pitch p by 6, then add 4 and divide by 10, thus:

6_p_ + 4 -------- = pd." 10

Fig. 3253 represents a form of high percentage joint, used upon marine boilers of 10 to 14 feet diameter, and carrying from 100 to 190 lbs. pressure of steam. The rivets are what are termed unevenly pitched, or, that is to say, on each side of the joint, there are three rows of rivets, of which the inner and outer rows are wider pitched than the middle row.

[54]"The advantage gained by this spacing is that the shear of the outer row of rivets is added to the plate section at the narrow pitch, that is to say, if the plate section broke through the line of rivet holes at the narrow pitch, it has yet to shear the outer row of rivets before the plate can separate."

[54] From "Steam Boilers."

Fig. 3254 represents a second example of joint with rivets unevenly pitched, this form finding much favor in recent practice. The four inner rows of rivets are spaced at narrow pitch and the two outer rows are wide pitched.

[55]"The strength percentage of this joint is calculated from three points of view, as follows:

[55] From "Steam Boilers."

"First. The plate section at the wide pitched rivets.

"Second. The rivet section in one pitch.

"Third. The plate section at the narrow pitch plus half the double shear of the outer or wide pitched rivet."

The steam pressures generally employed in the boilers of stationary engines range from about 60 to 100 lbs. per square inch, and as a result of these comparatively low pressures less perfect forms of construction are employed than would be permissible if higher pressures were used.

The strength of the shell plate of boilers of small diameter is always largely in excess of the requirements, and as a result the strength of the joints may bear a very low percentage to that of the solid plate, and yet give a sufficient factor of safety for the working pressure.

Take, for example, a boiler shell of 36 inches internal diameter with a shell plate 1/4 inch thick, and allowing the strength of the material to be 48,000 lbs. per inch of section, and with a factor of safety of 4, the working pressure will be 166 lbs. per square inch, thus:

Strength Plate thickness of the material. × 2.

48000 × (.25 × 2) --------------------------------- = 666-2/3 lbs. = bursting pressure. 36

Diameter of boiler.

By dividing this 666 by the factor of safety 4 we get 166-2/3 lbs. as the working pressure of the shell plate independent of the riveted joint. Usually, however, such a boiler would not be used for a pressure above about 60 lbs. per inch, and this leaves a wide margin for the reduction of strength caused by the riveted joints.

Suppose, for example, that a single riveted lap joint is used, and the strength of this joint is but 50 per cent. of that of the solid plate, and we have as follows:

Strength of % strength Twice material. of riveted the plate joint. thickness.

48000 × .50 × (.25 × 2) ---------------------------------- = 83-1/3 lbs. = W.P. 36 × 4

Internal diam. Factor of of boiler. safety.

Here then we find that the working pressure of the solid plate is double that of the riveted joint, and that the working pressure of the boiler is 83 lbs. per square inch, notwithstanding that the strength of the riveted joints is but 50 per cent. of that of the solid plate. Such a boiler would not, however, be used for a pressure of over 60 lbs. per square inch.

If the above-named boiler was double riveted so as to bring the percentage of joint strength up to say 70 per cent, of that of the solid plate, its working pressure would be 116 lbs. per square inch, thus:

Strength of % strength Twice material of riveted the plate joint. thickness.

48000 × .70 × (.25 × 2) ---------------------------------- = 116-2/3 lbs. = W.P. 36 × 4

Internal diam. Factor of of boiler. safety.

But in practice such a boiler would not be used for pressures above about 75 lbs. per square inch, hence the shell plate thickness is still largely in excess of the requirements, and it may be remarked that plates less than 1/4 inch thick are not used on account of the difficulty of caulking them and keeping them steam tight.

On account therefore of the excessive strength of the shell plates in boilers of small diameter, butt straps are rarely used in stationary boilers, while punching the rivet holes and other inferior modes of construction are employed. We may now consider the circumferential seams of the boilers for stationary engines, such boilers sometimes being of great length in proportion to the diameter.

In proportion as the length of a boiler (in proportion to its diameter) is increased, the construction of the circumferential or transverse seams, as they are sometimes called, becomes of more importance.

The strength of the circumferential seams is so much greater than that of the longitudinal seams that it is often taken for granted that they are sufficiently strong if made with a lap joint and single riveted, but that such is not always the case will be shown presently.

In Fig. 3255 is represented a boiler composed of three strakes (i. e., three rings or sections), and it is clear that as the thickness of the shell is doubled at the circumferential seams where the ends of the middle strake pass within the end strakes, therefore the strength of the lapped joint of the shell to resist rupture in a transverse direction, as denoted by the arrows A, B, is actually increased by reason of the lap of the riveted joint. But suppose this boiler to be supported at the ends only, and the weight of the shell and of the water within it will be in a direction to cause the middle of the boiler to sag down, and therefore places a shearing strain on the rivets of the circumferential seams.

Moreover, the temperature of the outside of the boiler cannot be made or maintained uniform, because the fire passing beneath the bottom of the boiler first will keep it hotter, causing it to expand more, and this expansion acts to shear the rivets of the circumferential seams. In proportion as the heat of the fire varies in intensity, the amount of the expansion will vary, and the consequence is that the circumferential seams may get leaky or the joint may work, especially in boilers that are long in proportion to their diameters. It is clear, therefore, that for the very best construction at least a double riveted circumferential joint should be employed.

Leaving these considerations out of the question, however, we may find the amount of stress on the circumferential seams by multiplying the area of the end of the boiler by the working pressure, and dividing by the cross-sectional area of all the rivets in one circumferential seam.

Suppose, for example, that the diameter of the boiler is 36 inches, the working pressure 60 lbs. per square inch, and that there are in each circumferential seam 50 rivets, each 3/4 inch in diameter, and we proceed as follows:

The area of a circle 36 inches in diameter = 1017.87 square inches.

The area of a rivet 3/4 inches in diameter = .4417 square inch.

Then

Area of Working boiler end. pressure.

1017.87 × 60 ------------------ = 2765 lbs. per cross-sectional square inch of rivet. 50 × .4417

Number Area of of rivets. each rivet.

By multiplying the area of the boiler end by the working pressure, we get the total steam pressure acting to shear the rivets, and by multiplying the number of rivets by the area of one rivet, we get the total area resisting the steam pressure, and then by dividing the one quantity into the other, we get the shearing stress per square inch of rivet section.

In the case of longitudinal seams, we have as follows, the pitch being say 2-1/8 and the rivets 3/4.

Diameter Steam Pitch. of boiler pressure. in inches.

36 × 60 × 2.125 ------------------------------ = 5196 lbs. per square inch of rivet 2 × .4417 area.

Rivets in Area of one pitch. rivet.

It is seen, therefore, that the stress placed by the steam pressure on the transverse seam is about one-half of that it places on the longitudinal seam. But, as before remarked, the transverse seam is subject to racking strains, from which the longitudinal seams are exempt; thus, for example, the expansion of the boiler diameter, whether uniform or not, does not strain the longitudinal seam, whereas it may severely strain the transverse seam.

The English Board of Trade rules, in assigning values to the various constructions and qualities of workmanship, assign a certain value, in the form of an addition to the factor of safety, which takes into account the difference in the stress upon the transverse and longitudinal seams, the quantities in each case having been determined both from experiment and from experience. A comparison of the different values may be made as follows:

The rules take a boiler shell made of the best material, with all the rivet holes drilled after the strakes are rolled into shape and put together, with all the seams (both longitudinal and transverse) fitted with double butt straps each at least five-eighths of the thickness of the shell plates they cover, and with all the seams at least double riveted, with rivets having an allowance of not more than 75 per cent. over the single shear, and provided that the boilers have been open to the inspection of their surveyors during the whole period of construction, and say that such a boiler shell shall be allowed a factor of safety (divisor of seam strength) of 5.

But for every departure from this, which they deem the best mode of construction, a penalty in the shape of an addition to the factor of safety is made. These additions to the factors of safety with reference to the longitudinal as compared to the transverse seams, are given in the following table:

An addition of .25 is also made to the factor of safety, when the strakes are not entirely under or over. In Fig. 3256 for example, strake b is within or under strake a at one end and strake c at the other end, hence b is entirely under; strake c is over b and d, and therefore entirely over; while strake d is under c, and over e, and therefore not entirely under nor entirely over.

When the rivet holes are punched they do not match properly, and unless the holes are punched somewhat smaller than the required size and reamed out afterwards, some rivets receive more stress than others, and may consequently shear in detail. It is customary, however, to punch the holes for ordinary stationary boilers, and it is with seams having punched holes therefore that we have at present to deal.

In the United States the rivet diameter and plate percentages are, in the boilers of stationary engines, usually made equal, and the reasons advanced both for and against this are as follows:

First, in favor of a greater plate percentage than rivet section, it is advanced that the plate gets thinner by wear, whereas the rivet does not, hence the wear reduces the plate section; that the plate is weakened by the punching process, and requires a greater percentage to make up its strength as compared to the rivet; that the rivets are usually of better material than the plates.

In favor of a greater rivet section than plate section, it is advanced that the shearing strength of iron is but about four-fifths of the tensile strength, and that with equal plate and rivet sections the rivet is therefore the weakest; that with punched holes the rivets may be sheared in detail, and that the rivets may be sheared gradually by the working of the joint from varying expansion and contraction.

From these premises the assumption is drawn that the weakening of the plate from being punched and from corrosion about offsets the excess of the tensile over the shearing strength, and that it is best therefore to employ such a pitch that the area of the rivet and of the metal left between the rivet holes shall be equal.

In order to do this the diameter of the rivet must be determined, and the following are the proportions given by the various authorities named:

TABLE OF THE DIAMETERS OF RIVETS FOR VARIOUS THICKNESSES OF PLATES WITH SINGLE RIVETED LAP JOINT.

---------+------------------------------------------------------- | DIAMETER OF RIVETS. +-------+---------+----------+----------+-------+------- Thickness|Lloyds'|Liverpool| English |Fairbairn.| Unwin.|Wilson. of Plate.|Rules. | Rules. |Dockyards.| | | ---------+-------+---------+----------+----------+-------+------- in. | in. | in. | in. | in. | in. | in. 5/16 | 5/8 | 5/8 | 1/2 | 5/8 | 11/16 | 5/8 3/8 | 5/8 | 5/8 | 5/8 | 3/4 | 3/4 | 11/16 7/16 | 5/8 | 3/4 | 3/4 | 21/32 | 13/16 | 3/4 1/2 | 3/4 | 13/16 | 3/4 | 3/4 | 7/8 | 3/4 ---------+-------+---------+----------+----------+-------+------- 9/16 | 3/4 | 13/16 | 7/8 | 27/32 | 7/8 | 7/8 5/8 | 3/4 | 7/8 | 7/8 | 15/16 | 15/16 | 7/8 11/16 | 7/8 | 7/8 | 7/8 | 1-1/32 |1 | 7/8 3/4 | 7/8 | 15/16 | 1 | 1-1/8 |1-1/16 | 1 ---------+-------+---------+----------+----------+-------+------- 13/16 | 7/8 | 1 | 1 | 1-7/32 |1-3/32 | 1 7/8 |1 | 1-1/8 | 1-1/8 | ... |1-1/8 | 1 15/16 |1 | 1-3/16 | 1-1/8 | ... |1-3/16 | 1-1/8 1 |1 | 1-1/4 | 1-1/8 | ... |1-1/4 | 1-1/8 ---------+-------+---------+----------+----------+-------+-------

From the above it is seen that with thin plates the diameter of rivet employed is about twice the thickness of the plate, whereas as the thickness of plate increases the proportion of rivet diameter decreases, and the reasons for this are, first, that with rivets twice the thickness of thick plates and pitched so as to equalize the rivet and plate sections the pitch would be too great to permit of the seams being caulked steam tight.

The diameter of the rivet having been determined, the rivet area and area of plate left between the rivet holes may be made equal by determining the pitch by the following rule:

Rule.--To the area of the rivet divided by the plate thickness add the diameter of the rivet, and the sum so obtained is the pitch. The correctness of this rule may be shown as follows:

Suppose the rivet diameter to be 7/8 inch = decimal equivalent .875, and its area will be .6013 square inch. Suppose the thickness of the plate to be 9/16 = decimal equivalent .5625, then by the rule:

Rivet area. Plate thickness = .5625 ) .6013 ( 1.0689 5625 ---- 38800 33750 ----- 50500 45000 ----- 55000 50625

To this 1.0689 we are to add the rivet diameter, thus:

1.0689 .8750 = rivet diameter. ------ 1.9439 = pitch of the rivets.

We have thus found the required pitch to be 1.9439 inches, and as the joint is single riveted there are two half rivets or one whole one to one pitch, and if we subtract the diameter of the rivet from the pitch we shall get the width of the metal or plate left between the rivets, thus:

1.9439 = pitch of rivets. .8750 = diameter of rivet. ------ 1.0689 = distance in inches between the rivets.

If now we multiply this distance between the rivets by the thickness of the plate, we shall get the area of the plate that is left between the rivet holes, thus:

1.0689 = width of plate between rivets. .5625 = thickness of plate. ------ 53445 21378 64134 53445 --------- Area of plate = .60125625 between rivets

Here then we find the area of plate left between the rivet holes to be 6.01 square inches, and as the area of the rivet is 6.01 square inches, the two are shown to be equal.

We may now place the various rivet diameters and the pitches that will make the rivet area and plate area in a single riveted joint equal in a table as follows:

TABLE OF RIVET DIAMETERS AND PITCHES FOR SINGLE RIVETED LAP JOINTS.

-------------------+------------------+-------- Thickness of Plate.|Diameter of Rivet.| Pitch. -------------------+------------------+-------- 1/4 | 1/2 | 1-1/4 5/16 | 5/8 | 1-5/8 3/8 | 11/16 | 1-11/16 7/16 | 3/4 | 1-3/4 1/2 | 3/4 | 1-5/8 9/16 | 7/8 | 2 5/8 | 7/8 | 1-7/8 11/16 | 7/8 | 1-3/4 3/4 | 1 | .. 13/16 | 1 | 2 7/8 | 1 | 1-1/8 15/16 | 1-1/8 | 2-1/8 1 | 1-1/8 | 2-1/8 1-1/16 | 1-1/8 | 2-1/8 1-1/8 | 1-3/16 | 2-1/4 1-1/4 | 1-3/16 | 2-1/8 -------------------+------------------+--------

The rivets in double riveted lap joints, and in butt strap joints having a single cover, are spaced alike, because in both cases there are two rivets in one pitch, and the rivets are in single shear.

As there are two rivets in one pitch (instead of only one as in a single riveted joint), therefore the percentage of rivet section is doubled, and the plate section must therefore be doubled if the plate and rivet sections are to be made equal, and the rule for finding the required pitch is as follows:

Rule.--To the amount of rivet area in one pitch, divided by the thickness of the plate, add the diameter of the rivet.

Example.--Let the plate thickness be as in the last example 9/16, decimal equivalent = .5625, and the rivet diameter be 7/8 inch = decimal equivalent .875, the area of one rivet being .6013 square inch, and the pitch is calculated as follows:

.6013 = area of one rivet. 2 = the rivets in one pitch. ------ Plate thickness = .5625 ) 1.2026 ( 2.1377 1.1250 ------ 7760 2.137 5625 .875 = rivet diameter. ---- ----- 21350 3.012 = pitch. 16875 ----- 43750 39375 ----- 43750 39375 ----- 4375

We find, therefore, that the pitch is 3.012, or 3 inches (which is near enough for practical purposes), and we may now make it clear that this is correct.

In Fig. 3257 the joint is shown drawn one-half full size, and the length a of plate left between the rivet holes measures (as nearly as it is necessary to measure it) 2-5/32 inches, or 2.156, and if we multiply this by the thickness of the plate = .5625 inch, we get 1.2 square inches as the area of the plate left between the rivet holes.

Now there are two rivets in a pitch (as one-half of B, one-half of C, and the whole of F), and as the area of each rivet is .6, therefore the area of the two will be 1.2, and the plate section and rivet section are shown to be equal.

The area at a is obviously the same as that at A, because the pitches of both rows of rivets are equal, this being an ordinary zigzag riveted joint.

We may now consider the diagonal pitch of the rivets, using the rule below.

The pitch × 6, + 4 times the rivet diameter ------------------------------------------- = the diagonal divided by 10 pitch _p__{D}.

In this example the pitch has been found to be 3 inches, hence we have

.875 = diameter of rivet. 4 = constant. ----- 3.500

3 = pitch of the rivets. 6 = constant. -- 18 3.5 = rivet diameter multiplied by 4. ---- 10 ) 21.5 (2.15 = the diagonal pitch. 20 -- 15 10 -- 50

The diagonal pitch, that is, the distance _p__{D}, Fig. 3257, is therefore found to be 2.15, or 2-1/8 inch full.

The amount of metal left between the rivets, measured on the diagonal pitch, is twice the dimension H multiplied by the thickness of the plate, and as this (with the diagonal pitch determined as above) always exceeds the pitch A or a, therefore if the plate fails, it will be along the line a, and not through the diagonal pitch.

We may now consider the total amount that the plates overlap in a double riveted lap joint zigzag riveted, this amount being twice the distance E, added to the distance V between the rows of rivets.

The distance E, Fig. 3257, is usually made one and a half times the diameter of the rivet, this being found to give sufficient strength to prevent the edge of the plate from tearing out and to prevent the rivet from shearing the plate out to the edge, rupture not being found to occur in either of these directions.

The rule for finding the distance V, when the diagonal pitch has been determined by the rules already explained, is as follows:

Rule.--To the pitch multiplied by 11, add 4 times the rivet diameter, then multiply by the pitch, plus 4 times the rivet diameter. Then extract the square root and divide by 10.

Placed in formula, the rule appears as follows, d representing the rivet diameter, and p the pitch.

__________________________ \/(11_p_ + 4_d_)(p + 4_d_) ---------------------------- = distance V between the rows of rivets. 10

As this rule involves the extraction of the square root of the sum of quantities above the line, and as in determining the diagonal pitch, we have already determined the distance V, it is unnecessary to our purpose to carry out this latter calculation, as it is easier to find the diagonal pitch, and then, after drawing the joint, the distance between the rows of rivets can be measured if it is required, as it might be in finding the length of plate required to roll into a strake for a boiler of a given diameter and having a double riveted lap joint.

We may now consider chain riveted joints in comparison with zigzag riveted joints, which is especially necessary, because it has been assumed by some that the second row of rivets in a chain riveted joint added nothing to the strength of the joint.

Fig. 3258 represents a chain riveted joint, having the same thickness of plate, rivet diameter and pitch as the zigzag riveted joint in Fig. 3257, and it will be seen that the plate sections at a and at a are the same in the two figures, and as there are four half rivets, which are equal to two rivets, in one pitch, therefore the strength of the two joints is equal.

Each joint can be as efficiently caulked as the other, as the rivet spacing is the same and the edge of the plate is the same distance from the rivets in both cases.

The pitch of the rivets is obtained by the same rule as for zigzag riveted joints, and all we have now to consider is the distance apart of the two rows of rivets or distance V in the Fig. 3258, and for this there are two rules, the first being that it shall not be less than twice the diameter of the rivet, which would leave a dimension at H in the figure equal to the diameter of the rivet. The second rule is that a better proportion than the above is to multiply the diameter of the rivet by 3. This makes the dimension at H equal to twice the rivet diameter.

When the joints have double buttstraps, the rivets may be spaced as wide as the necessity for tight caulking will admit, because, on account of the rivets being in double shear, the rivet percentage exceeds the plate percentage.

The allowance for the rivets being in double shear is 75 per cent., or in other words, a rivet in double shear is allowed 1.75 times the area of the same size rivet in single shear.

STATIONARY ENGINE BOILERS.

The simplest form of horizontal boiler is the plain cylinder boiler, an example of which is given in Fig. 3259, and which is largely used in iron works and coal mines.

Boilers of this class are easily cleaned, because the whole interior can be readily got at to clean.

As the bottom of this boiler gets thinned from wear, the boiler is turned upside down, thus prolonging its life.

Fig. 3260 represents an internally fired flue boiler, known as the Cornish or Lancashire boiler. The furnace is at one end of the flues, the fire passing through them to the chimney. There is here obviously more heating surface than in the plain cylinder boiler, but somewhat less facility for cleaning.

The Galloway boiler is of this class, but has vertical water tubes placed at intervals in the flues. These water tubes are wider at the top than at the bottom. They serve to break up the body of heat that passes through the flues, and increase the heating surface while extracting more of the heat and promoting the circulation of the water in the boiler.

A water tube is one in which the water is inside and the fire outside, as distinguished from a fire tube, in which the fire passes through the tube and the water is outside. A water tube is stronger than a fire tube, because the former is subject to bursting pressure and the latter to collapsing pressure.

Vertical boilers are internally fired, and in the ordinary forms have no return tubes or flues, examples of those used for small stationary engines being given as follows.

Fig. 3261 represents an ordinary form with vertical tubes. The upper ends of the tubes here pass through the steam space--a condition that under the moderate pressures and firing that this class of boiler is subjected to is of less importance than it is in boilers having higher chimneys and therefore a more rapid draught, and using higher pressures of steam. Furthermore, the small diameters and lengths or heights in which these boilers are made give them ample strength with shells and tubes of less thickness, while the condition of tube ends with steam on one side and fire on the other is permissible without the injurious effects that ensue under rapid combustion and high pressures.

The crown sheet of the fire boxes or furnaces of this class of boiler is very effective heating surface, first, because of the great depth (and therefore weight) of water resting upon it insuring constant contact between the water and the plate, while there is no danger of the crown sheet burning from shortness of water.

A similar boiler, but with the upper ends of the tubes below the water level, is shown in Fig. 3262.

From the small diameters of these boilers, the flat surfaces are not stayed except to the extent that the holding power of the tubes serves that end.

A return flue vertical boiler is shown in Figs. 3263 and 3264. The whole of the surfaces having contact with the fire also have contact with the water, and the height of the crown sheet removes it from the intense heat of the fire. It is stayed to the top of the boiler. The fire box or combustion chamber being taper increases the effectiveness of its sides as heating surface, since the heat in its vertical passage impinges against it.

The products of combustion pass from the top of the combustion chamber through short horizontal flues, which enter an annular space surrounding the lower section of the boiler, and from this space vertical flues pass to a corresponding space at the bottom of the boiler.

The passage of the steam generated at the sides of the combustion chamber is facilitated by the taper of the chamber, which gives increased room for the steam as it gathers in ascending.

Vertical boilers for high pressures, as from 60 to 120 lbs. per inch, are represented in the figures from 3265 to 3269.

In boilers of this class, a majority contain water tubes, which, when properly arranged, promote rapid evaporation and circulation.

A boiler with Field tubes is shown in Fig. 3265. It consists of an outer shell and a cylindrical fire box, from the crown sheet of which a number of Field tubes are suspended in the fire box or combustion chamber.

Fig. 3266 is a sectional view of a Field tube, the construction being as follows:

The outer tube, which is expanded into the tube plate, is enclosed at its lower end, and has at its upper end in the water space of the boiler a perforated mouth piece, from which is suspended an inner tube that extends nearly to the bottom of the outer tube.

As the outer tube is bathed in the fire, steam is generated very rapidly, and a thorough and rapid circulation is kept up, the water passing down the inner and up the outer tubes, as denoted by the arrows.

The outer tube is spread out at the upper end to a slight cone, so that it cannot be forced out of the tube sheet by the pressure, and as it hangs free, there is no liability for it to loosen or get leaky from expansion and contraction.

From the great amount of heating surface obtained with these tubes, the fire box may be kept at a minimum diameter for the duty, while still leaving a wide space for the water leg, which facilitates the circulation.

The damper, which is suspended in the uptake, spreads the fire sideways.

Fig. 3267 represents the arrangement of Field tubes in a boiler.

A boiler of this form may for a given capacity be made lighter and smaller than in any other of the ordinary forms, while the rapid circulation acts to keep the tubes clean.

The inner tubes may be thin, because they are under pressure both inside and out, while the outer tubes may be thin, because they are under a bursting strain, whereas a fire tube is under collapsing pressure.

A design of high rate boilers, in which the uptake does not come into contact with the water, and water tubes are employed, is shown in Fig. 3268. In the fire box is an inclined tube which promotes the circulation, and is very effective heating surface, and in the combustion chamber are a number of vertical water tubes.

Two manholes give access for cleaning purposes.

The efficiency of the heating surface in this class of boiler is increased from the fact that, as the heat does not pass direct through the boiler, it impinges against the surface. In Fig. 3269, for example, the exit from the spherical fire box is on one side of the boiler, and the uptake on the other, the heat passing from the fire box into a combustion chamber, and thence through the horizontal fire tubes to the uptake.

The crown sheet is here stayed by gusset stays, but if made spherical, as in Fig. 3270, the stays may be omitted.

Figs. 3271, 3272, and 3273 illustrate a 60-inch horizontal return tubular boiler constructed by the Hartford Steam Boiler Inspection and Insurance Company. This class of boiler has found much favor in the United States. It is an externally fired, return tube boiler, the fire passing beneath the boiler and returning through the tubes to the front end of the boiler, whence it passes through the drum to the chimney.

The boiler is supported on the brackets B, B´, the front one, B, resting on an iron plate imbedded in the brickwork, and the back ones on rollers which rest on the plates P´ imbedded in the brickwork. This allows the boiler to expand and contract endways under variations of temperature without racking the brickwork.

A, A, etc., are for holding the brickwork together. The blow-off pipe C is for emptying or blowing down the boiler. The feed-pipe F enters the front end of the boiler, passes along it, and then crosses over. A pipe H from the steam space of the boiler supplies steam to the steam gauge G, and to the upper end of the gauge glass, which is on the casting K. The lower end of the gauge glass receives water from a pipe which passes into the water space of the boiler; at J are the three gauge cocks for testing the height of the water in the boiler.

The manhole affords ingress into the boiler for inspecting and for scaling or cleaning it, the nozzles being for a safety valve. At E is a hand-hole for washing out and cleaning the boiler. P is a damper in the fire door for admitting air above the fire bars, and R is a damper for regulating the draught.

In the brick walls that support the boiler there are air spaces to prevent the conduction of the heat through and prevent cracking of the brickwork. The tubes are arranged in vertical and horizontal rows and are equally spaced throughout.

Fig. 3274 represents the front end, and Fig. 3275 a longitudinal sectional view of the front end of a boiler of this class. In this case, however, the pipes for the water gauge pass direct into the boiler.

In some practice the tubes are arranged as in Fig. 3276, being wider pitched or spaced in the middle of the boiler to increase the circulation of the water in the boiler.

Another arrangement is shown in Fig. 3277, the tubes being staggered or arranged zigzag. This permits of the employment of a greater number of tubes, but does not afford such free circulation of the water.

Fig. 3278 represents an arrangement where the tubes are in rows both vertically and horizontally.

Fig. 3279 represents a boiler by the Erie Iron Works, the details of the setting being as follows:

Fig. 3280 is an end view of the setting with the brickwork in section.

Fig. 3281 side view of the boiler and setting.

Fig. 3282 a front end view of the boiler, and Fig. 3283 a ground plan of the brickwork. When the front plate of the boiler setting extends above the middle of the boiler, as in Fig. 3279, it is said to have a "full arch front." Whereas when this plate or casting extends to the middle only of the boiler, it is said to have a half arch front.

Figs. 3284, 3285, 3286, and 3287 show the setting for a half arch front boiler, the dimensions of the settings of both these boilers being given in the following tables:

MEASUREMENTS FOR SETTING TUBULAR STATIONARY BOILERS WITH FULL ARCH FRONTS.

REFERENCE LETTERS ON DIAGRAMS.

------+---+----+----+----+----+----+------+----+----+----+----+---- No. | A | B | C | D | E | F | G | H | I | J | K | L ------+---+----+----+----+----+----+------+----+----+----+----+---- |Ft.|Ins.|Ins.|Ins.|Ins.|Ins.| Ins. |Ins.|Ins.|Ins.|Ins.|Ins. 1 | 7| 32 | 12 | 20 | 16 | 45 |44 | 7 | 32 | 64 | 85 | 26 2 | 7| 34 | 12 | 20 | 16 | 48 |47 | 8 | 34 | 66 | 90 | 26 3 | 8| 36 | 12 | 20 | 16 | 48 |47 | 8 | 36 | 68 | 92 | 26 3-1/2| 10| 36 | 12 | 20 | 16 | 48 |47 | 8 | 36 | 68 | 92 | 26 4 | 8| 42 | 14 | 20 | 16 | 48 |47 | 8 | 42 | 74 | 98 | 27 5 | 10| 42 | 14 | 20 | 16 | 48 |47 | 8 | 42 | 74 | 98 | 27 6 | 10| 44 | 14 | 24 | 16 | 48 |47 | 10 | 44 | 76 |100 | 27 7 | 12| 44 | 14 | 24 | 16 | 48 |46-1/2| 10 | 44 | 76 |100 | 27 7-1/2| 14| 44 | 14 | 24 | 16 | 47 |45-1/2| 10 | 44 | 76 | 99 | 26 8 | 12| 48 | 16 | 24 | 16 | 47 |45-1/2| 10 | 48 | 88 |103 | 26 9 | 14| 48 | 16 | 24 | 16 | 47 |45-1/2| 10 | 48 | 88 |103 | 26 10 | 12| 54 | 16 | 24 | 20 | 50 |48-1/2| 10 | 54 | 94 |112 | 26 10-1/2| 15| 54 | 16 | 24 | 20 | 50 |48-1/2| 10 | 54 | 94 |112 | 26 11 | 12| 60 | 18 | 24 | 20 | 50 |48-1/2| 12 | 60 |108 |118 | 26 12 | 14| 60 | 18 | 24 | 20 | 50 |48-1/2| 12 | 60 |108 |118 | 26 13 | 16| 60 | 18 | 26 | 20 | 50 |48 | 12 | 60 |108 |118 | 26 14 | 15| 66 | 18 | 28 | 20 | 50 |48-1/2| 12 | 66 |114 |124 | 26 15 | 16| 66 | 18 | 28 | 20 | 50 |48 | 12 | 66 |114 |124 | 26 16 | 16| 72 | 20 | 30 | 20 | 50 |48 | 12 | 72 |120 |130 | 26 ------+---+----+----+----+----+----+------+----+----+----+----+----

------+----+-------+----+----+----+----+----+----+----+------+------ | | | | | | | | | | | NO. | | | | | | | | | | |COMMON | | | | | | | | | | |BRICK | | | | | | | | | |NO. OF|ABOVE No. | M | N | O | P | Q | R | S | T | U | FIRE |FLOOR | | | | | | | | | |BRICK.|LEVEL. ------+----+-------+----+----+----+----+----+----+----+------+------ |Ins.|Ft.Ins.|Ins.|Ins.|Ins.|Ins.|Ins.|Ins.|Ins.| | 1 | 19 | 11-6 | 20 | 40 | 12 | 16 | 36 | 34 | 4 | 600 | 6800 2 | 22 | 11-6 | 20 | 40 | 12 | 16 | 36 | 34 | 4 | 600 | 7500 3 | 22 | 12-6 | 24 | 40 | 12 | 16 | 36 | 34 | 4 | 650 | 7700 3-1/2| 22 | 14-6 | 28 | 46 | 12 | 16 | 42 | 42 | 4 | 720 | 8500 4 | 21 | 12-8 | 24 | 40 | 12 | 16 | 36 | 34 | 4 | 730 | 8500 5 | 21 | 14-8 | 28 | 46 | 12 | 16 | 42 | 42 | 4 | 770 | 9600 6 | 21 | 15-0 | 28 | 46 | 12 | 16 | 42 | 42 | 4 | 880 | 10500 7 | 21 | 17-0 | 32 | 52 | 12 | 16 | 48 | 49 | 4 | 940 | 10800 7-1/2| 21 | 19-0 | 36 | 58 | 12 | 16 | 54 | 84 | 4 | 1120 | 11500 8 | 21 | 17-2 | 32 | 52 | 12 | 20 | 48 | 49 | 4 | 1120 | 13600 9 | 21 | 19-2 | 36 | 58 | 12 | 20 | 54 | 84 | 4 | 1140 | 15700 10 | 24 | 17-6 | 32 | 52 | 12 | 20 | 48 | 49 | 4 | 1160 | 16200 10-1/2| 24 | 20-8 | 36 | 56 | 16 | 20 | 54 | 90 | 4 | 1270 | 17500 11 | 24 | 17-10 | 32 | 50 | 16 | 24 | 48 | 49 | 4 | 1400 | 20500 12 | 24 | 19-10 | 36 | 56 | 16 | 24 | 54 | 84 | 4 | 1500 | 23000 13 | 24 | 22-0 | 40 | 56 | 16 | 24 | 54 | 96 | 4 | 1540 | 25300 14 | 24 | 21-2 | 36 | 56 | 16 | 24 | 54 | 90 | 4 | 1590 | 26000 15 | 24 | 22-2 | 40 | 56 | 16 | 24 | 54 | 96 | 4 | 1620 | 27000 16 | 24 | 22-6 | 40 | 56 | 16 | 24 | 54 | 96 | 4 | 1750 | 30000 ------+----+-------+----+----+----+----+----+----+----+------+------

NOTE.--In setting "Standard" boilers, the side walls should be so built that the longitudinal seams of the shell will be protected from the fire.

MEASUREMENTS FOR SETTING TUBULAR STATIONARY BOILERS WITH HALF ARCH FRONTS.

REFERENCE LETTERS ON DIAGRAMS.

------+---+----+----+----+----+------+------+----+----+----+------- No. | A | B | C | D | E | F | G | H | I | J | K ------+---+----+----+----+----+------+------+----+----+----+------- |Ft.|Ins.|Ins.|Ins.|Ins.|Ins. | Ins. |Ins.|Ins.|Ins.| Ins. 1 | 7 | 32 | 14 | 20 | 16 |46 |45 | 7 | 32 | 64 | 73 2 | 7 | 34 | 14 | 20 | 16 |46 |45 | 8 | 34 | 66 | 75 3 | 8 | 36 | 14 | 20 | 16 |46 |45 | 8 | 36 | 68 | 77 3-1/2|10 | 36 | 14 | 20 | 16 |46 |45 | 8 | 36 | 68 | 77 4 | 8 | 42 | 18 | 20 | 16 |46 |45 | 8 | 42 | 74 | 83 5 |10 | 42 | 18 | 20 | 16 |46 |45 | 8 | 42 | 74 | 83 6 |10 | 44 | 18 | 24 | 16 |46 |45 | 10 | 44 | 76 | 85 7 |12 | 44 | 18 | 24 | 16 |46 |44-1/2| 10 | 44 | 76 | 85 7-1/2|14 | 44 | 18 | 24 | 16 |46 |44-1/2| 10 | 44 | 76 | 85 8 |12 | 48 | 19 | 24 | 16 |50 |48-1/2| 10 | 48 | 88 | 93 9 |14 | 48 | 19 | 24 | 16 |50 |48-1/2| 10 | 48 | 88 | 93 10 |12 | 54 | 19 | 24 | 20 |50 |48-1/2| 10 | 54 | 94 | 99 10-1/2|15 | 54 | 19 | 24 | 20 |50 |48-1/2| 10 | 54 | 94 | 99 11 |12 | 60 | 21 | 24 | 20 |46-3/4|45-1/2| 12 | 60 |108 |101-3/4 12 |14 | 60 | 21 | 24 | 20 |46-3/4|45 | 12 | 60 |108 |101-3/4 13 |16 | 60 | 21 | 26 | 20 |46-3/4|45 | 12 | 60 |108 |101-3/4 14 |15 | 66 | 24 | 28 | 20 |47 |45-1/2| 12 | 66 |114 |108 15 |16 | 66 | 24 | 28 | 20 |47 |45-1/2| 12 | 66 |114 |108 16 |16 | 72 | 24 | 30 | 20 |48 |46-1/2| 12 | 72 |120 |115 ------+---+----+----+----+----+------+------+----+----+----+-------

------+------+------+--------+----+------+----+----+----+----+---- No. | L | M | N | O | P | Q | R | S | T | U ------+------+------+--------+----+------+----+----+----+----+---- | Ins. | Ins. |Ft. Ins.|Ins.| Ins. |Ins.|Ins.|Ins.|Ins.|Ins. 1 |26 |20 | 10-3 | 20 |33 | 12 | 16 | 36 | 34 | 4 2 |26 |20 | 10-3 | 20 |33 | 12 | 16 | 36 | 34 | 4 3 |26 |20 | 11-3 | 24 |33 | 12 | 16 | 36 | 34 | 4 3-1/2|26 |20 | 13-3 | 28 |39 | 12 | 16 | 42 | 42 | 4 4 |27 |19 | 11-3 | 24 |32-1/2| 12 | 16 | 36 | 34 | 4 5 |27 |19 | 13-3 | 28 |38-1/2| 12 | 16 | 42 | 42 | 4 6 |27 |19 | 13-7 | 28 |38-1/2| 12 | 16 | 42 | 42 | 4 7 |27 |19 | 15-7 | 32 |44-1/2| 12 | 16 | 48 | 49 | 4 7-1/2|27 |19 | 17-7 | 36 |50-1/2| 12 | 16 | 54 | 84 | 4 8 |26 |24 | 15-7 | 32 |48 | 12 | 20 | 48 | 49 | 4 9 |26 |24 | 17-7 | 36 |54 | 12 | 20 | 54 | 84 | 4 10 |26 |24 | 15-11 | 32 |48-1/2| 12 | 20 | 48 | 49 | 4 10-1/2|26 |24 | 19-1 | 36 |52-1/2| 16 | 20 | 54 | 90 | 4 11 |26 |20-3/4| 16-1 | 32 |47 | 16 | 24 | 48 | 49 | 4 12 |26 |20-3/4| 18-1 | 36 |53 | 16 | 24 | 54 | 84 | 4 13 |26 |20-3/4| 20-3 | 40 |53 | 16 | 24 | 54 | 96 | 4 14 |26 |21 | 19-5 | 36 |52-1/2| 16 | 24 | 54 | 90 | 4 15 |26 |21 | 20-5 | 40 |52-1/2| 16 | 24 | 54 | 96 | 4 16 |28-1/4|19-3/4| 20-7 | 40 |52-1/2| 16 | 24 | 54 | 96 | 4 ------+------+------+--------+----+------+----+----+----+----+----

------+------+------+----+----+----+------+------ | | | | | | | NO. | | | | | | |COMMON | | | | | | |BRICK | | | | | |NO. OF|ABOVE No. | V | W | X | Y | Z | FIRE |FLOOR | | | | | |BRICK.|LEVEL. ------+------+------+----+----+----+------+------ | Ins. | Ins. |Ins.|Ins.|Ins.| | 1 |36 | 9 | 24 | 12 | 7 | 600 | 6150 2 |36 | 9 | 28 | 12 | 7 | 600 | 6200 3 |36 | 9 | 28 | 12 | 15 | 650 | 6700 3-1/2|36 | 9 | 28 | 12 | 25 | 720 | 7050 4 |32-3/4|12-1/4| 32 | 16 | 11 | 730 | 7700 5 |32-3/4|12-1/4| 32 | 16 | 25 | 770 | 8700 6 |32-3/4|12-1/4| 36 | 16 | 25 | 880 | 8800 7 |32-1/4|12-1/4| 36 | 20 | 35 | 940 | 9300 7-1/2|32-1/4|12-1/4| 36 | 24 | 45 | 1120 | 9500 8 |36-1/4|12-1/4| 36 | 20 | 35 | 1120 | 11100 9 |36-1/4|12-1/4| 36 | 24 | 45 | 1140 | 12900 10 |34 |14-1/2| 42 | 20 | 35 | 1160 | 13200 10-1/2|34 |14-1/2| 42 | 24 | 57 | 1270 | 14200 11 |31 |14-1/2| 48 | 20 | 37 | 1400 | 16700 12 |30-1/2|14-1/2| 48 | 24 | 45 | 1500 | 19200 13 |30-1/2|14-1/2| 48 | 24 | 65 | 1540 | 21500 14 |31 |14-1/2| 54 | 24 | 57 | 1590 | 22100 15 |31 |14-1/2| 54 | 24 | 65 | 1620 | 23100 16 |27-1/2|19 | 54 | 24 | 65 | 1750 | 26000 ------+------+------+----+----+----+------+------

NOTE.--In setting "Standard" boilers, the side walls should be so built that the longitudinal seams of the shell will be protected from the fire.

THE EVAPORATIVE EFFICIENCIES OF BOILERS.

[56]"Many tests have been undertaken to ascertain the evaporative power of different classes of boilers in actual work; but few of these are of any value, owing to the unreliable means usually employed to measure the quantity of water evaporated. The easiest method, and consequently the one most frequently adopted, is to measure the quantity by the difference of its height in the water-gauge glass at the beginning and end of the trial, and also at intermediate stages. This method is very rude and uncertain, since there can be little doubt that in many boilers at work the surface of the water is not level, but is usually higher over the furnace, or where the greatest ebullition occurs. The difference in height at any moment will greatly depend upon the intensity of the ebullition, which is ever varying during the intervals between firing. With mechanical firing the difference of height is probably reduced to a minimum.

[56] From "A Treatise on Steam Boilers," by Robert Wilson.

"The meters employed for measuring the water are sometimes not trustworthy. The only sure method of ascertaining the quantity of water evaporated is by actual measurement with a cistern or vessel whose cubic contents are accurately known. The quantity of water in the boiler before and after the trial should be measured at the same temperature, which should not exceed 212° to insure accuracy. But even when the amount of water introduced and the quantity passed off from the boiler are accurately ascertained, there yet remains a doubt as to how much has been actually evaporated, and how much may have passed off in priming, unless the trial has been conducted with the boiler open to the atmosphere, which appears to be the only condition under which accuracy can be insured, unless a suitable apparatus can be provided for accurately measuring the weight and temperature of all the steam and water given off, when the boiler is working above atmospheric pressure.

"There are very few boilers that do not prime more or less, and the quantity of water passed off in this manner is sometimes very considerable, and has led to the impossible results of 16 and 17 lbs. of water evaporated per lb. of ordinary coal in locomotive and water-tube boilers being seriously recorded. Externally fired boilers, that have given the moderate result of 5 lbs. of water per lb. of coal at atmospheric pressure, have shown the unexpected result of 10 and 12 lbs. of water evaporated at 40 lbs. pressure. In fact, unless the amount of water passed over with the steam by priming or foaming, when working under pressure, can be accurately ascertained, the evaporative results are not to be relied upon, however careful in other respects the trial may have been conducted. It is customary to give the quantity of water evaporated from and at a temperature of 212°, or the boiling point at atmospheric pressure, to which the results of evaporation are reduced."

The quantity corresponding to any temperature of feed water and working pressure can readily be found with the aid of the annexed table, taken from The Encyclopædia Britannica, wherein are presented the relations of the properties of steam, as now accepted by the best authorities.

TABLE GIVING THE PRESSURE, TEMPERATURE, AND VOLUME OF STEAM.

-----------+-----------+-----------+----------+---------+------------- Total pres-| | | | |Relative vol- sure per |Gauge pres-| Sensible |Total heat|Weight of| ume of steam square inch| sure or |temperature|in degrees|one cubic|compared with measured | pressure | in |from zero |foot of | the water from a | above | Fahrenheit| of | steam. |from which it vacuum. |atmosphere.| degrees. | Fahren- | lbs. | was lbs. | lbs. | | heit. | | evaporated. -----------+-----------+-----------+----------+---------+------------- 1 | --- | 102.1 | 1144.5 | .0030 | 20582 2 | --- | 126.3 | 1151.7 | .0058 | 10721 3 | --- | 141.6 | 1156.6 | .0085 | 7322 4 | --- | 153.1 | 1160.1 | .0112 | 5583 5 | --- | 162.3 | 1162.9 | .0138 | 4527 6 | --- | 170.2 | 1165.3 | .0163 | 3813 7 | --- | 176.9 | 1167.3 | .0189 | 3298 8 | --- | 182.9 | 1169.2 | .0214 | 2909 9 | --- | 188.3 | 1170.8 | .0239 | 2604 10 | --- | 193.3 | 1172.3 | .0264 | 2358 11 | --- | 197.8 | 1173.7 | .0289 | 2157 12 | --- | 202.0 | 1175.0 | .0314 | 1986 13 | --- | 205.9 | 1176.2 | .0338 | 1842 14 | --- | 209.6 | 1177.3 | .0362 | 1720 14.7 | 0 | 212.0 | 1178.1 | .0380 | 1642 15 | .3 | 213.1 | 1178.4 | .0387 | 1610 16 | 1.3 | 216.3 | 1179.4 | .0411 | 1515 17 | 2.3 | 219.6 | 1180.3 | .0435 | 1431 18 | 3.3 | 222.4 | 1181.2 | .0459 | 1357 19 | 4.3 | 225.3 | 1182.1 | .0483 | 1290 20 | 5.3 | 228.0 | 1182.9 | .0507 | 1229 21 | 6.3 | 230.6 | 1183.7 | .0531 | 1174 22 | 7.3 | 233.1 | 1184.5 | .0555 | 1123 23 | 8.3 | 235.3 | 1185.2 | .0580 | 1075 24 | 9.3 | 237.8 | 1185.9 | .0601 | 1036 25 | 10.3 | 240.1 | 1186.6 | .0625 | 996 26 | 11.3 | 242.3 | 1187.3 | .0650 | 958 27 | 12.3 | 244.4 | 1187.8 | .0673 | 926 28 | 13.3 | 246.4 | 1188.4 | .0696 | 895 29 | 14.3 | 248.4 | 1189.1 | .0719 | 866 30 | 15.3 | 250.4 | 1189.8 | .0743 | 838 31 | 16.3 | 252.2 | 1190.4 | .0766 | 813 32 | 17.3 | 254.1 | 1190.9 | .0779 | 789 33 | 18.3 | 255.9 | 1191.5 | .0812 | 767 34 | 19.3 | 257.6 | 1192.0 | .0835 | 746 35 | 20.3 | 259.3 | 1192.5 | .0858 | 726 36 | 21.3 | 260.9 | 1193.0 | .0881 | 707 37 | 22.3 | 262.6 | 1193.5 | .0905 | 688 38 | 23.3 | 264.2 | 1194.0 | .0929 | 671 39 | 24.3 | 265.8 | 1194.5 | .0952 | 655 40 | 25.3 | 267.3 | 1194.9 | .0974 | 640 41 | 26.3 | 268.7 | 1195.4 | .0996 | 625 42 | 27.3 | 270.2 | 1195.8 | .1020 | 611 43 | 28.3 | 271.6 | 1196.2 | .1042 | 598 44 | 29.3 | 273.0 | 1196.6 | .1065 | 595 45 | 30.3 | 274.4 | 1197.1 | .1089 | 572 46 | 31.3 | 275.8 | 1197.5 | .1111 | 561 47 | 32.3 | 277.1 | 1197.9 | .1133 | 550 48 | 33.3 | 278.4 | 1198.3 | .1156 | 539 49 | 34.3 | 279.7 | 1198.7 | .1179 | 529 50 | 35.3 | 281.0 | 1199.1 | .1202 | 518 51 | 36.3 | 282.3 | 1199.5 | .1224 | 509 52 | 37.3 | 283.5 | 1199.9 | .1246 | 500 53 | 38.3 | 284.7 | 1200.3 | .1269 | 491 54 | 39.3 | 285.9 | 1200.6 | .1291 | 482 55 | 40.3 | 287.1 | 1201.0 | .1314 | 474 56 | 41.3 | 288.2 | 1201.3 | .1336 | 466 57 | 42.3 | 289.3 | 1201.7 | .1364 | 458 58 | 43.3 | 290.4 | 1202.0 | .1380 | 451 59 | 44.3 | 291.6 | 1202.4 | .1403 | 444 60 | 45.3 | 292.7 | 1202.7 | .1425 | 437 61 | 46.3 | 293.8 | 1203.1 | .1447 | 403 62 | 47.3 | 294.8 | 1203.4 | .1469 | 424 63 | 48.3 | 295.9 | 1203.7 | .1493 | 417 64 | 49.3 | 296.9 | 1204.0 | .1516 | 411 65 | 50.3 | 298.0 | 1204.3 | .1538 | 405 66 | 51.3 | 299.0 | 1204.6 | .1560 | 399 67 | 52.3 | 300.0 | 1204.9 | .1583 | 393 68 | 53.3 | 300.9 | 1205.2 | .1605 | 388 69 | 54.3 | 301.9 | 1205.5 | .1627 | 383 70 | 55.3 | 302.9 | 1205.8 | .1648 | 378 71 | 56.3 | 303.9 | 1206.1 | .1670 | 373 72 | 57.3 | 304.8 | 1206.3 | .1692 | 368 73 | 58.3 | 305.7 | 1206.6 | .1714 | 363 74 | 59.3 | 306.6 | 1206.9 | .1736 | 359 75 | 60.3 | 307.5 | 1207.2 | .1759 | 353 76 | 61.3 | 308.4 | 1207.4 | .1782 | 349 77 | 62.3 | 309.3 | 1207.7 | .1804 | 345 78 | 63.3 | 310.2 | 1208.0 | .1826 | 341 79 | 64.3 | 311.1 | 1208.3 | .1848 | 337 80 | 65.3 | 312.0 | 1208.5 | .1869 | 333 81 | 66.3 | 312.8 | 1208.8 | .1891 | 329 82 | 67.3 | 313.6 | 1209.1 | .1913 | 325 83 | 68.3 | 314.5 | 1209.4 | .1935 | 321 84 | 69.3 | 315.3 | 1209.6 | .1957 | 318 85 | 70.3 | 316.1 | 1209.9 | .1980 | 314 86 | 71.3 | 316.9 | 1210.1 | .2002 | 311 87 | 72.3 | 317.8 | 1210.4 | .2024 | 308 88 | 73.3 | 318.6 | 1210.6 | .2044 | 305 89 | 74.3 | 319.4 | 1210.9 | .2067 | 301 90 | 75.3 | 320.2 | 1211.1 | .2089 | 298 91 | 76.3 | 321.0 | 1211.3 | .2111 | 295 92 | 77.3 | 321.7 | 1211.5 | .2133 | 292 93 | 78.3 | 322.5 | 1211.8 | .2155 | 289 94 | 79.3 | 323.3 | 1212.0 | .2176 | 286 95 | 80.3 | 324.1 | 1212.3 | .2198 | 283 96 | 81.3 | 324.8 | 1212.5 | .2219 | 281 97 | 82.3 | 325.6 | 1212.8 | .2241 | 278 98 | 83.3 | 326.3 | 1213.0 | .2263 | 275 99 | 84.3 | 327.1 | 1213.2 | .2285 | 272 100 | 85.3 | 327.9 | 1213.4 | .2307 | 270 101 | 86.3 | 328.5 | 1213.6 | .2329 | 267 102 | 87.3 | 329.1 | 1213.8 | .2351 | 265 103 | 88.3 | 329.9 | 1214.0 | .2373 | 262 104 | 89.3 | 330.6 | 1214.2 | .2393 | 260 105 | 90.3 | 331.3 | 1214.4 | .2414 | 257 106 | 91.3 | 331.9 | 1214.6 | .2435 | 255 107 | 92.3 | 332.6 | 1214.8 | .2456 | 253 108 | 93.3 | 333.3 | 1215.0 | .2477 | 251 109 | 94.3 | 334.0 | 1215.3 | .2499 | 249 110 | 95.3 | 334.6 | 1215.5 | .2521 | 247 111 | 96.3 | 335.3 | 1215.7 | .2543 | 245 112 | 97.3 | 336.0 | 1215.9 | .2564 | 243 113 | 98.3 | 336.7 | 1216.1 | .2586 | 241 114 | 99.3 | 337.4 | 1216.3 | .2607 | 239 115 | 100.3 | 338.0 | 1216.5 | .2628 | 237 116 | 101.3 | 338.6 | 1216.7 | .2649 | 235 117 | 102.3 | 339.3 | 1216.9 | .2674 | 233 118 | 103.3 | 339.9 | 1217.1 | .2696 | 231 119 | 104.3 | 340.5 | 1217.3 | .2738 | 229 120 | 105.3 | 341.1 | 1217.4 | .2759 | 227 121 | 106.3 | 341.8 | 1217.6 | .2780 | 225 122 | 107.3 | 342.4 | 1217.8 | .2801 | 224 123 | 108.3 | 343.0 | 1218.0 | .2822 | 222 124 | 109.3 | 343.6 | 1218.2 | .2845 | 221 125 | 110.3 | 344.2 | 1218.4 | .2867 | 219 126 | 111.3 | 344.8 | 1218.6 | .2889 | 217 127 | 112.3 | 345.4 | 1218.8 | .2911 | 215 128 | 113.3 | 346.0 | 1218.9 | .2933 | 214 129 | 114.3 | 346.6 | 1219.1 | .2955 | 212 130 | 115.3 | 347.2 | 1219.3 | .2977 | 211 131 | 116.3 | 347.8 | 1219.5 | .2999 | 209 132 | 117.3 | 348.3 | 1219.6 | .3020 | 208 133 | 118.3 | 348.9 | 1219.8 | .3040 | 206 134 | 119.3 | 349.5 | 1220.0 | .3060 | 205 135 | 120.3 | 350.1 | 1220.2 | .3080 | 203 136 | 121.3 | 350.6 | 1220.3 | .3101 | 202 137 | 122.3 | 351.2 | 1220.5 | .3121 | 200 138 | 123.3 | 351.8 | 1220.7 | .3142 | 199 139 | 124.3 | 352.4 | 1220.9 | .3162 | 198 140 | 125.3 | 352.9 | 1221.0 | .3184 | 197 141 | 126.3 | 353.5 | 1221.2 | .3206 | 195 142 | 127.3 | 354.0 | 1221.4 | .3228 | 194 143 | 128.3 | 354.5 | 1221.6 | .3250 | 193 144 | 129.3 | 355.0 | 1221.7 | .3273 | 192 145 | 130.3 | 355.6 | 1221.9 | .3294 | 190 146 | 131.3 | 356.1 | 1222.0 | .3315 | 189 147 | 132.3 | 356.7 | 1222.2 | .3336 | 188 148 | 133.3 | 357.2 | 1222.3 | .3357 | 187 149 | 134.3 | 357.8 | 1222.5 | .3377 | 186 150 | 135.3 | 358.3 | 1222.7 | .3397 | 184 155 | 140.3 | 361.0 | 1223.5 | .3500 | 179 160 | 145.3 | 363.4 | 1224.2 | .3607 | 174 165 | 150.3 | 366.0 | 1224.9 | .3714 | 169 170 | 155.3 | 368.2 | 1225.7 | .3821 | 164 175 | 160.3 | 370.8 | 1226.4 | .3928 | 159 180 | 165.3 | 372.9 | 1227.1 | .4035 | 155 185 | 170.3 | 375.3 | 1227.8 | .4142 | 151 190 | 175.3 | 377.5 | 1228.5 | .4250 | 148 195 | 180.3 | 379.7 | 1229.2 | .4357 | 144 200 | 185.3 | 381.7 | 1229.8 | .4464 | 141 210 | 195.3 | 386.0 | 1231.1 | .4668 | 135 220 | 205.3 | 389.9 | 1232.3 | .4872 | 129 230 | 215.3 | 393.8 | 1233.5 | .5072 | 123 240 | 225.3 | 397.5 | 1234.6 | .5270 | 119 250 | 235.3 | 401.1 | 1235.7 | .5471 | 114 260 | 245.3 | 404.5 | 1236.8 | .5670 | 110 270 | 255.3 | 407.9 | 1237.8 | .5871 | 106 280 | 265.3 | 411.2 | 1238.8 | .6070 | 102 290 | 275.3 | 414.4 | 1239.8 | .6268 | 99 300 | 285.3 | 417.5 | 1240.7 | .6469 | 96 -----------+-----------+-----------+----------+---------+-------------

Here we see that at 212° the total quantity of heat in the steam is 1178.1°, which gives a difference of 966.1°. This heat, usually termed latent, is absorbed in performing the work of expanding the particles of water from the liquid to the gaseous state. Now, suppose the water is evaporated at 60 lbs. pressure, the steam will have a temperature of 307°, and a total heat of 1207°. If the feed has been introduced at 60°, it is evident that 1147° of heat have been imparted. As the amount evaporated is inversely proportional to the quantity of heat required, we have 1147 ÷ 966 = 1.2. Multiplying by this factor, the quantity evaporated at 60 lbs. pressure from 60°, we obtain the amount that would be evaporated at 212° by the same quantity of fuel.

By the same table can be ascertained the comparatively small increase of heat required to evaporate water at higher pressures. Suppose we take water evaporated at 45 lbs. pressure from a feed temperature of 60°, then each lb. of water will require 1202.7-60 = 1142.7 for its conversion into steam. If we take the pressure at 100 lbs., we shall have 1216.9-60 = 1156.9° as the quantity required. The difference between these two total quantities is only 14.2°, and is so small as to be scarcely worth considering. Leaving out of account the loss due to the slight reduction of the conducting power of the material, the increased amount of heat required for the higher pressure will be only 1/36 of the total heat required at 60 lbs. With an evaporation of 7 lbs. of water from 1 lb. of coal, it will be obtained by using 1/563 more fuel, or about 1 lb. in about 556 lbs., a quantity not appreciable to the ordinary modes of weighing coal. The economy is then manifest of using steam of high pressures when at the same time advantage is taken of the facilities it offers for working the steam more expansively to the engine cylinders.

The saving that may be effected by heating the feed water may be shown as follows:

If we take the normal temperature of the feed water at 60°, the temperature of the heated water at 212°, and the boiler pressure at 20 lbs., the total heat imparted to the steam in one case is

1192.5° - 60° = 1132.5° and in the other case 1192.5° - 212° = 980.5° 152° the difference being 152°, or a saving of ------- 1132.5°

which is 13.4 per cent. If the pressure be taken at 120 lbs., instead of 20 lbs., the saving will be 13.1 per cent, showing a slight diminution in the economy effected by heating the feed water when a high boiler pressure is employed.

THE CARE AND MANAGEMENT OF STATIONARY ENGINE BOILERS.

The first thing to do in taking charge of a stationary engine boiler is to know from personal inspection that the safety fittings and the boiler-feeding apparatus are in good order.

The safety valve is the first thing to inspect, as it is liable to stick in its seat, especially in cases in which it is set at a greater pressure than is got up in the boiler, because in that case it is not lifted from the seat and in time sticks fast there.

In such cases it is proper to lift the valve at least once a day while steam is on. For this purpose a cord may be attached to the lever, passing over a pulley directly above the lever, and thence to some convenient place near the boiler, but where it is not liable to get caught and pulled accidentally.

Before lighting the fire, see that there is sufficient water in the boiler. If there is a gauge glass on the boiler, it should show three-quarters full, or three-quarters of a glass, as it is called.

The gauge glass may show a false water level, and to be sure that such is not the case, open the top gauge cock and the cock at the bottom of the gauge glass, letting the water run through the gauge glass. Open and close the cock below the gauge glass two or three times to see that the water comes to the same level each time.

If the steam pressure has been allowed to fall in the boiler without any of the cocks being opened, there will be a partial vacuum in the boiler, and air must be let in before the true water level will be shown either by the gauge glass or by the gauge cocks.

Opening the upper gauge cock will let in the air, and it should not be closed again until enough steam has been got up in the boiler to expel the air again, or in other words, until steam begins to issue from it.

The grate bars and ash pit should be cleaned of clinker, ashes, etc., and it should be seen that the tubes are clear of ashes, etc., before the fire is laid; if the grate is a shaking one, the lever should be applied to see that the grate will shake properly.

TO LIGHT THE FIRE--In the case of anthracite or hard coal, as it is sometimes termed, first cover the bars with a thin layer of coal and then put in pieces of lighted greasy waste (if it is at hand) distributed about the furnace, taking especial care to light the fire at the fire-door end and in the corners, because the fire will spread from the front end towards the back easier than it will from the back end towards the front.

The fire should light from the bottom and not from the top, hence the thinnest pieces of the wood should be put in first.

If there is any soft coal at hand, a small quantity of it will accelerate lighting the fire, as it burns easier and quicker than hard coal.

Before putting on the coal the wood should be well lighted, the bottom portion of it having ceased flaming.

This causes the lighted wood to spread over the bars and the fire to light evenly.

Charge the coal lightly, first covering the places that have burned up the most.

FIRING.--The fire door should be kept open as little as possible, as it admits cold air that is detrimental to the combustion, as well as to the draught, hence firing should be done quickly.

A good fireman will maintain as even a temperature as possible in the fire box by charging the coal lightly and quickly.

Some firemen will, after the fire is at its proper depth all over the grates, charge the fire in the front end, that is, at the fire-door end, and push it back as it burns up, to keep up the thickness of the fire at the back.

The thickness of the fire depends upon the size and kind of coal.

With small coal a fire from 4 to 6 inches deep will answer, while, if the lumps are five or six inches in diameter, a fire from a foot to 15 inches deep may be maintained, as is done in some locomotives.

The object is to have the fire thick enough to prevent it from burning through in spots or letting cold draughts of air pass through it.

The sides of the furnace require particular attention, not only because cold air is more likely to get through there, but also in boilers having fire boxes the cool sides of the box keep the temperature of the fuel down, hence a thicker fire is necessary around the sides than in the middle of the furnace or fire box.

Three things are to be considered in cleaning a fire--first, that the boiler pressure will fall during, and for a short time after, the cleaning; second, that the depth of fire will be diminished by the cleaning; and third, that the temperature of the fire will fall during the cleaning.

SHAKING GRATE BARS.

When a furnace has shaking grate bars, the cleaning of the fire is greatly facilitated, and with bars that shake singly (and good coal) the fire is often not disturbed during the day, except to shake the bars occasionally, passing the poker through it and using the hoe to keep it evenly spread.

If the grate shakes in sections, more cleaning will be required to break up the clinker, while, if the bars do not shake, the cleaning assumes greater importance.

Before cleaning, therefore, see that there is sufficient water in the boiler, that it need not be fed while cleaning, nor just after cleaning the fire.

Prepare for cleaning by having a thick fire on the grate, so that after cleaning it will burn up quickly, and let the cleaning be done as quickly as possible.

The tools used for cleaning the fire are the slice bar, Fig. 3288, which is pushed along the top of the fire bars to loosen up the fire, and let the ashes fall through the bars.

The hoe, Fig. 3289, which is used to push the fire to the back of the furnace and to pull it forward. The poker, Fig. 3290, which dislodges any clinker that may be between the bars, and lets the ashes fall through.

The clinker hook or devil's claw, Fig. 3291, which is used to haul clinker out of the fire, and the rake, Fig. 3292, which is used to spread the fire evenly over the bars after it is cleaned.

In cleaning a fire, first use the slice bar to loosen up the fire and let the ashes fall through, and also dislodge clinkers from the surface of the bars. Then push the fire to the back of the furnace. Next use the poker to clean out clinker from between the exposed part of the bars. Then with the hoe pull a part of the fire forward and pull out the clinker that may be in this part, doing so with the hoe as far as possible, as that will save time, but if it should be necessary, use the clinker hook.

Then pull forward a second portion of the fire, and spread it on the bars, removing the clinker as before. When all the fire has thus been cleaned, use the rake to spread it evenly over the bars, and put on a light charge of coal, covering the brightest parts of the fire first, and taking care that no part of the fire bars is left uncovered.

The cleaning should be done quickly.

DRAUGHT.--The draught should be decreased while the fire is being cleaned, but the damper should never be entirely closed, as this might cause an explosion in the fire box and tubes.

During a temporary interruption, as in the case of the engine stopping, partly close all the dampers, as it is wasteful to make steam and blow it off through the safety valve.

COMBUSTION.--A blue flame is evidence of incomplete combustion, but there may be a blue flame and imperfect combustion at the back end of the furnace, and a white flame and perfect combustion at the other end.

This is likely to occur with heavy firing near the fire door, and a thin fire at the tube sheet end of the fire box. In this case the unconsumed gases produced near the fire door (as evidenced by the blue flame) are consumed in passing over the bright fire at the tube plate end of the furnace.

AT NIGHT.--Always leave plenty of water in the boiler when leaving it for the night, not only to allow for any leak, but also because it gives a fair start in the morning and more time to remedy any defect in the feed pump if it arise.

By plenty of water, very nearly a full gauge is meant, or if there is no gauge glass to the boiler, let the water stand above the second or middle cock.

The usual method of leaving the fire for the night is to bank it. There is an element of danger, however, in banking a fire, unless it is done to suit the circumstances, because steam may generate very rapidly, and perhaps more rapidly than the safety valve can carry it off.

A safe method is to clean the fire, leaving the clinker and ashes covering the front half of the grate and the fire piled up on the back half.

The damper and ash pit door should be closed tight, the fire door open, and the fire well covered with fresh coal, choosing small rather than large coal.

If this method is found not to keep up the fire sufficiently, the same plan may be employed, except that the ashes and clinker may be removed, and if this still leaves too cold a boiler, and too poor a fire in the morning, the fire may be left spread over the grate, but heavily covered with fresh coal, the draught being stopped as much as possible by closing the dampers and opening the furnace door.

To further insure safety, the weight on the safety valve lever should be pushed towards the valve, so as to cause the safety valve to blow off at a less pressure than during the day.

IN THE MORNING.--In starting up a banked fire in the morning, first close the fire door and open the damper, so as to give the fire all the draught possible, and let it burn up a little; then, if it has been piled up at the back of the furnace, clean out the ashes by passing the T bar beneath the fire, and spread it over the grate, letting it burn up a little before making up a fire.

BOILER-FEED.--The fireman should endeavor, if possible, to so regulate the boiler feed that it is kept going as nearly continuously as possible while maintaining a uniform quantity of water in the boiler, and this, with uniform firing, will give the greatest economy.

When pumps are used to feed with, the amount of the lift of the valves can be regulated by a screw, so as to vary the amount of water the pump will deliver, and in this case it is comparatively easy to set them so that the pump may be kept going without putting too much water in the boiler.

When injectors are used, however, the feed will be intermittent, and a uniform quantity of water in the boiler is best obtained by feeding at short intervals, stopping the feed when the fire door is opened much, as when cleaning the fire.

If the feed water is dirty, the gauge glass should be kept clean by first shutting off the upper cock and opening the lower one, so as to let the water blow through the lower cock, and then shutting off the lower cock from the boiler, and opening the upper one, which will let the steam blow all the water out of the glass. This should be done two or three times a day, so as to keep the holes in the boiler and those in the cocks from closing up with fur or scale.

If the water falls in the glass, or if the gauge cocks show the water to be falling, notwithstanding that the feed pump has been started, it is evident that the pump is not working.

This may occur from a stuck valve, a leak in the suction pipe, from the feed water being too hot, or from the pump failing to start in action from leaky or choked valves.

A stuck valve may generally be relieved by striking a few blows on the outside of the pump with a hammer and a block of wood, or if this does not answer, with the hammer only. Check valves are the ones most likely to stick.

If a pump fails to work by reason of the feed water being too hot, the remedy is to open the pet cock to let the steam out of the pump, but if this does not succeed, cold water may be poured on the outside of the pump, which will start it, after which, in most cases, the pump will keep going and the pet cock may be closed.

If the suction pipe has a joint, a leak there will impair the action of the pump, and, if the leak is great enough, will stop it; the remedy is to make the joint tight.

Plunger pumps sometimes fail to act because the plunger has worn so small in diameter that there is sufficient air between the plunger and the pump barrel to expand and compress without lifting the valve; the remedy is obviously a new plunger of as large diameter as the pump gland will admit of, boring the gland out to admit the new plunger.

All the impurities in the water are left in the boiler when the water has evaporated, and it is obvious these impurities must be blown off or they will form scale on the internal surface of the boiler and the external surface of the tubes or flues.

This scale obstructs the passage of the heat from the iron to the water, and if let get thick enough will cause the iron to rapidly burn out.

To prevent the formation of scale, two principal methods are employed, one being to purify the feed water, and the other to occasionally blow the impurities out of the boiler.

Feed-water heaters generally serve also as purifiers, and their effectiveness is increased in proportion as the water can pass quietly through them, and has a large area on which the impurities can settle. Horizontal heaters have the advantage that they have a large settling area, and a less distance for the impurities to fall through. The water-gauge glass and the lower gauge cock are usually set so as to have a margin of about three inches of water above the tubes or crown sheet of the fire box, hence if it is known that the water is but just below the bottom of the gauge glass or gauge cock, there is no positive danger, although it is improper to let it get so low.

If the water is out of sight, and it is not known exactly how low it is, then it is dangerously low, and every minute is of vital importance.

Should the water get dangerously low in the boiler, the most dangerous thing to do is to lift the safety valve or pump in cold water, especially if it is not known how much water there is in the boiler.

As quickly as possible cover the fire with ashes, coal, earth, sand, or anything that is at hand that will smother the fire, then close the draught to the fire, leaving the fire door and the chimney damper open.

Leave all the steam outlets just as they are, and also the feed.

PRIMING.--Priming, which is also called "foaming," is that the steam carries up water into the steam space. This may arise from several causes, but it is well known that what will stop priming in some cases will cause it in others.

The known causes of priming are--first, too little room for the steam in the boiler, and it follows that a high water level may cause priming; second, it may be caused by a difference of temperature between the water and the steam in the boiler. Suppose, for example, that the pressure of the steam and water in the boiler is 160 lbs. by gauge, and its sensible temperature will be 370 degrees. Suppose then that enough steam is permitted to escape from the boiler to reduce the steam pressure to 140 lbs., and its temperature will be reduced to 361 degrees. But the water will remain at 270 degrees, and the result will be that it will pass into steam so rapidly that it will carry up the water and hold it in suspension among the steam. The water will pass with the steam into the engine cylinder, and the boiler will be said to "prime," "foam," or "work water." The same thing may happen if the water is heated very rapidly.

Priming is wasteful because it rapidly empties the boiler of its water, and dangerous because it may cause the piston to knock out the cylinder head or cover.

When the safety valve blows off, priming may be induced, especially if the engine is at work, because in this case the boiler is being forced, or, in other words, is making steam more rapidly than it is designed to do, and the passage of so large a body of steam through the water is apt to lift it.

Muddy water will sometimes cause foaming or priming, as will also insufficient circulation of the water in the boiler or sometimes the presence of grease or oil.

Priming may be detected from the discharge of water with the steam when the gauge cock is opened, the steam looking white and fluttering as it escapes, and also by violent motion of the water in the gauge glass, or by a thump or pound at the ends of the piston stroke.

To stop priming, the steam from the boiler should be decreased by slackening the speed of the engine, or if necessary, by stopping it. The true water level can then be seen, and if there is too much water in the boiler some of it may be blown off, while if the quantity of water in the boiler will permit it, the feed may be put on.

If the boiler has a surface blow-off cock, or a mechanical boiler cleaner, it is best to blow off from that, as it carries off the scum at the same time as relieving the boiler.

To prevent priming, a steady and uniform rate of boiler feed, the use of pure water, a clean boiler, and steady firing are the best means, turning on the steam slowly so as not to violently disturb the water in the boiler.

The engine as well as the boiler requires attention when the boiler primes. Thus the cylinder cocks should be opened to let out the water from the cylinder and prevent breakage of the cylinder cover.

SCALE IN BOILERS.--The steam leaves behind it all the impurities that the water contained, and these impurities deposit in form of mud and scale, which must be got rid of because it causes a loss of fuel, and if allowed to get thick enough will cause the boiler to burn.

The use of boiler compounds or scale preventatives may be resorted to with advantage, providing they are of a nature to suit the water, but mechanical cleaning must also be resorted to at periods determined by the nature of water.

Boilers are cleaned in two ways--first, by blowing off the impurities before they have formed into scale; and second, by removing at certain intervals whatever scale has formed.

Blowing down may be done in two ways--first, from the surface of the water by means of mechanical cleaners; and second, by blowing out from the bottom of the boiler.

The first draws off the impurities as they are thrown to the surface, the second draws them off after they have become more condensed and sink to the bottom.

How often a boiler should be blown down depends upon the kind of water fed to the boiler; where purifiers are used, less blowing down is obviously needed.

It is best to blow off from the bottom of the boiler when no steam is being used, as during dinner time, letting the water blow down about a quarter of the glass, or from the upper to the middle gauge cock.

As no steam is being used, the feed can then be put on to restore the quantity of water without reducing the temperature of the boiler so much. The feed should be gradual and the fire regulated to keep the steam pressure even.

How often a boiler should be washed out and cleaned depends upon the quality of the water it uses, and varies from about once a week to once a month, according to whether bad and unpurified water or purified water is used.

The first thing to do is to draw the fire, leaving the chimney damper open and closing all the other dampers so that as little cold air as possible can get into the boiler, while the heat can pass away up the chimney.

Let the steam and water all remain in the boiler until there is a gauge pressure of about 5 lbs. in the boiler.

Then open the blow-off cock and let out the water. If the water is blown off under a high pressure, then after the waste is all out the iron is hot enough to dry up the scale, making it hard and very difficult to remove.

After all the water is blown off, take out all the mud plugs and the man-hole and hand-hole covers, and wash out the boiler under as much water pressure as can be had, directing the hose so to reach all parts of the boiler and tubes, and continuing the washing until the water leaves the boiler clean.

Then with a wooden hoe on a piece of gas-pipe of small diameter for a handle, and small enough to pass through the hand-hole, draw all the loose scale to the hand-hole and remove it, letting the water run slowly, so as to carry the small pieces of scale towards the hand-hole as fast as the hoe disturbs it.

Then get inside the boiler, and a few blows with a light ball-pened hammer will loosen the scale, and a steel scraper will remove more, which must be washed down and drawn out with a hoe.

After the cleaning and scaling are complete, the engineer, with lamp in hand, should carefully examine the interior of the boiler and of the fire box, paying especial attention to the stays to see that they are not broken.

The hammer test should also be applied. It consists of sounding the boiler by light blows given by a light ball-pened hand hammer, the sound indicating defective places.

Spotted a typo or a problem in this chapter? Tell us

XXXVII. THE STEAM ENGINE

The high pressure steam engine, in whatever form it exists, consists of a frame or bed plate carrying two distinct mechanisms, first, the driving or power-transmitting mechanism, and second, the valve gear or valve motion, and to these are added such other mechanisms as the nature of the duty the engine is to perform may require.

The most prominent of these additional mechanisms is a governor for regulating the speed at which the engine is to run; nearly all steam engines require a governor in some form or other, while for electric lighting and some other purposes it constitutes the main feature in the design of the engine.

In a locomotive the air brake and the sand box are elements not found in other engines.

In a jet condensing engine, the condenser and injection water, or condensing water mechanism, is a part of the engine.

In a surface condensing engine, the air pumps and circulating pumps are a part of the engine.

In marine engines there are mechanisms for turning the engine around when no steam is up; for moving the reversing gear quickly, and for varying the point of cut off, and therefore the amount of expansion, and various other and minor mechanisms.

Referring now to the simplest form of high pressure stationary steam engine, such as represented in Figs. 3293, 3294, and 3295, its valve gear or valve motion consists of the eccentric and its strap, the eccentric rod, the valve rod guide A, the valve rod or valve spindle, and the valve v, these parts controlling the admission of steam to one side of the piston, and the exhaust from the other.

The piston, piston rod, cross head, connecting rod, crank, crank shaft, main shaft or driving shaft, and the fly wheel constitute the driving or power-transmitting mechanism.

The steam side of the piston is that against which the steam is pressing, as side S in Fig. 3295. The exhaust side, E, of the piston is that on which the steam is passing out or exhausting.

The governor for a common D valve engine regulates the engine speed by varying the opening in the bore of the pipe through which the steam passes from the boiler to the steam chest, leaving a wider opening in proportion as the engine runs slower, and reducing the opening when the engine runs faster. Assuming the engine to be running at its slowest, or its load to be so great that a full supply of steam is required in order to keep the engine up to its proper speed, and the governor will be open at its widest, so that all the further action the governor can have is to reduce the steam pipe opening, and thus cause the pressure in the steam chest to be less than that in the steam pipe.

This action is called wire-drawing the steam, and the governor is called a throttling governor.

An engine bed or bed plate is a frame that is seated or bedded to its foundation along its whole length.

An engine frame is seated to its foundations at two or more places, but not continuously throughout its length.

THE CYLINDER.

Cylinders are secured to the engine frames in three principal ways, as follows: by bolting them down to the bed plate; by bolting them to one end of the bed plate, so that they may expand and contract without springing the bed plate; and in vertical engines, by bolting them to the top of the frames.

The bores of cylinders require to be parallel, so that the piston rings may fit to the bore without requiring to expand and contract in diameter at different parts of the stroke.

Cylinders are designated for size by the diameter of the cylinder bore and the length of the stroke; thus, a 10 × 12 cylinder has a piston of ten inches diameter and 12 inches stroke.

The wear of a cylinder bore is (if the engine is kept in proper line and the piston rings, or packing rings as they are sometimes termed, fit to the bore with an equal pressure throughout the stroke) greatest near the middle of the length and least at the ends of the stroke. But when the piston rings are set out by the steam pressure, and the point of cut off occurs early in the stroke, the wear may be greatest at the ends of the cylinder bore, because of the pressure of the steam diminishing during the expansion.

The counterbore of a cylinder is a short length at each end of the cylinder, that is made of larger diameter than the rest of the bore, so that the piston head may travel completely over the working bore, and thus prevent the formation of a shoulder at each end of the cylinder. Such a shoulder forms when there is a part of the bore over which the piston does not pass. The length of the counterbore should exceed the amount of the taper on the connecting rod key, so that as the connecting rod length alters from the wear, the piston shall not strike the cylinder head.

The clearance of a cylinder is the amount of space that exists between the face of the piston when it is at the end of its stroke and that of the valve when it covers the port, the piston being at the end of the stroke, and as this space exists at each end of the cylinder, the total clearance for a revolution is twice the above amount.

The clearance at the crank end of the cylinder is reduced by the piston rod passing through it.

The amount of clearance may be measured by the following method, which has been given by Professor John E. Sweet:

See that the piston and valves are made tight, and the valves disconnected; arrange to fill the clearance spaces with water through the indicator holes, or holes drilled for the purpose. Turn the engine on the dead centre; make marks on the cross-head and guide that correspond; weigh a pail of water, and from it fill all the clearance space. Weigh the remaining water, so as to determine how much is used. Then weigh out exactly the same amount of water, turn the engine off the centre, pour in the second charge of water, and turn back until the water comes to the same point that it did in the first case. Make another mark on the cross-head, and the distance between these marks is exactly what you really wish to know; that is, it is just what piston travel equals the clearance. This gives the proportion that the clearance space bears to the space in the cylinder occupied by the steam at the end of the piston stroke. Thus, if it takes one pound of water to fill this space, and to admit the one pound of water the piston must be moved one inch, then the clearance bears the same relation to the capacity of the engine as one inch bears to the stroke of the piston. Thus, under these circumstances, in an engine of ten-inch stroke, it would be said the engine had ten per cent. clearance.

When a cylinder is to be rebored, the boring bar should be set true or central to the circumference of the counterbore, so that the bore of the cylinder may be brought to its original position with reference to the bore of the stuffing box.

Cylinders require lubricating, both to avoid friction and wear of the cylinder bore, as well as of the valve and valve seat. The amount of lubrication required depends upon the degree of tightness of the piston rings, upon the speed of the piston, upon the amount of pressure of the valve to its seat, and upon the method of operating the side valve.

Cylinders with releasing valve gears require freely lubricating, because the closure of the valve depends upon the dash pot, and undue friction retards the closing motion.

The less the movement of the valve at the moment of its release, the easier it is to move it, because the friction is less, and less lubrication is required.

Cylinders are lubricated by automatic oilers placed on the steam pipe of the engine, the oil being distributed over the surfaces by the steam.

Cylinder oilers sometimes have a pump to force the oil in, and in others the steam in the oiler condenses, and the water thus formed floats the oil over the top of a tube, or up to an orifice through which the oil gradually feeds as the condensation proceeds.

In other oil feeders, the feed is regulated by increasing or diminishing the opening through which the steam passes from the cup to the steam pipe.

Sight oil feeders are those in which there is a glass tube or body, in which the passage of the oil can be seen as it drops.

Cylinder cocks are employed at each end of the cylinder to let out the water that condenses from the steam when admitted to a cold or partly cooled cylinder. The two cocks are usually connected together by a rod, so that both may operate together.

Cylinder relief valves are valves at each end of the cylinder to relieve the cylinder from the charges of water that sometimes enter from the boiler with the live steam.

Steam ports give a quicker admission in proportion as their length is increased, and this reduces the amount of valve travel, and are sometimes given a length equal to the diameter of the cylinder bore.

The bottoms of the steam ports are sometimes so placed as to be below the level of the cylinder bore, so as to drain off the water of condensation of the steam.

Rule to find the required area of steam port.

Multiply the area in square inches of the piston, by the number opposite to the given piston speed in the following table:

Speed of piston in Number by which to feet per minute. multiply the piston area. 100 0.02 200 0.04 300 0.06 400 0.07 500 0.09 600 0.1 700 0.12 800 0.14 900 0.15 1,000 0.17

The cylinder exhaust port must be open when the valve is at the end of its travel, to an amount equal to the width of the steam port, but what this width will be in any given case depends upon the width of the bridges, the amount of the steam lap and the travel of the valve, as will be explained with reference to the slide valve.

Jacketed cylinders are those in which there is a space around the cylinder that is filled with live steam.

The object of jacketing is to prevent the loss of heat from the steam within the cylinder by radiation. The steam in the jacket should be received direct from the boiler, and should not be drawn from the jacket into the steam chest because the jacket reduces its temperature and condenses it.

The water of condensation of a steam jacket should not be allowed to accumulate in any part of the jacket, but should drain off and pass back to the boiler. To render the jacket as effective as possible, it should extend from end to end of the cylinder, the exhaust steam pipe leading directly away, so as to have as little communication with both the cylinder and the jacket as possible.

The jacket should have open communication with the boiler at all times, so as to have the pressure in the jacket at the same pressure as that in the steam chest, while the cylinder being kept hot, it will be unnecessary to blow steam through in order to warm the cylinder when starting the engine. The steam should enter the jacket at the highest point, so as to prevent the accumulation of air in the jacket. Or, if the steam is admitted at some other point, it should be so arranged as to permit its thorough circulation in the jacket. When a jacket is used, the metal of the cylinder body should be as thin as possible, because the transmission of heat through the metal is, both in time and quantity, inversely as the distance or thickness passed through.

The steam in the jacket should be as dry as possible, so that all wet steam admitted during the live steam period may be evaporated by the heat received from the steam in the jacket. The outside of the jacket should be thoroughly protected from cooling by being lagged or clothed with felt or some other material that is a non-conductor of heat.

From experiments made by Mr. Charles A. Smith, of St. Louis, it was found that the amount of variation of temperature that occurred during the stroke in a locomotive cylinder was inversely proportional to the speed of engine revolution, which shows the advantages of jacketing cylinders and of lagging them, as well as the advantage of a high rotative speed.

A lagged cylinder is one clothed, which is sometimes done with wood or metal strips, leaving an air space around the cylinder, while in others this space is filled with felt or some non-conducting material.

Experiments made by Charles E. Emery gave the following general results: The thickness of the pipes and of the non-conducting materials was kept constant.

Hair felt was the best non-conducting material of all those tested, and the value of a thickness of two inches of hair felt was taken as unity and the maximum.

The value of two inches of mineral wool as a non-conductor was 0.832 of hair felt; two inches of mineral wool and tar was 0.715. Two inches of sawdust, 0.68; two inches of a cheaper grade of mineral wool, 0.676; charcoal, 0.632; two inches of pine wood, across the grain, 0.553; two inches of loam, 0.55. This was from the Jersey flats, and almost all vegetable fibre not yet become compact. Slaked lime from the gas works, expressed decimally, with hair felt as unity, 0.48; coal ashes, 0.345; coke, only 0.277, the same as used for fuel; two inches of air space, only 0.136, which dashes a great many people's hopes, and is as interesting as any part of the data; two inches of asbestos, 0.363; two inches of Western coke, about the same as the other coke; two inches of gas house charcoal, 0.47.

These are very interesting, particularly so this matter of an air space. It has been supposed that an air space around a pipe is as good as anything we can have. The fact is, convection or circulation takes place; the air is cooled on one side of the space, descends, and rises on the other, and it is necessary to break up the air space, and that undoubtedly accounts for the efficiency of these different materials. It is the air probably that is the non-conductor; but it should be kept quiescent instead of being allowed to circulate. The air space itself is of very little value until the circulation is prevented.

THE PISTON.

In calculating the power of an engine it is the piston speed that is taken into account, and not the length of the stroke, the latter being used merely in order to obtain the piston speed.

Long strokes are usually employed upon engines running at moderate piston speeds, as from 300 to 500 feet per minute, and short strokes for piston speeds from 400 to 800 feet per minute.

The Porter Allen engine has been run noiselessly at 1,100 feet per minute.

In determining the stroke of an engine the nature of the valve-operating mechanism is taken into account.

In releasing mechanisms, or those in which connection between the eccentric rod and valve spindle is broken in order to permit the valve to close quickly, too high a speed of revolution may cause the tripping mechanism to fail to act, hence a high piston speed is obtained by means of employing a comparatively long stroke.

In positive valve gears, or those in which the valve is controlled throughout the whole of its movement by the eccentric, the valve mechanism may operate quicker without danger of missing, hence the piston speed may be greater.

When the stroke equals the diameter of the cylinder bore, the cylinder presents the least amount of exposed surface in proportion to its cubical contents.

To obtain the same amount of expansion in a short as in a long stroke engine, the steam must be expanded through an equal proportion of the stroke; thus, if the steam is cut off at half stroke in both cases, the amount of this expansion will be equal.

Pistons are made an easy fit to the cylinder bore, a steam-tight fit between the two being obtained by means of the piston rings.

Solid pistons are provided with snap piston rings.

A snap piston ring is one that is larger in diameter than the cylinder bore, and is closed in to get it into the cylinder, while it depends on its own spring outwards for its fit to the cylinder bore, having no supplementary rings or springs to force it out.

Piston rings that are expanded by supplementary springs should be tapering in thickness, the thickest part being opposite to the split, and the thinnest at the split. This causes the ring to conform itself to the cylinder bore, and makes it sit more evenly around its whole circumference. These rings are made larger in diameter than the cylinder bore, in proportion of about 1/8 inch per foot of diameter, the split being closed when the ring is sprung into place in the cylinder. But if made of brass, the split must be left open enough to allow for the expansion, or otherwise the ring expanding more than the cylinder will seize and cut single.

The split of a piston ring should be placed on the bottom of the piston (in a horizontal engine), so that the piston head, in resting on the cylinder bore, will cover up the opening of the ring.

When two or more rings are employed, the splits may be placed on the lower half of the cylinder, so as to cover up their splits as much as possible.

The follower of a piston is a plate or cover that is employed to hold the piston rings in place, and the piston rings should be so fitted that the follower should be bolted firmly up, or otherwise the bolts may come loose and work out, and getting between the piston and the cylinder cover, may cause the piston to knock the cylinder cover out.

Piston followers are necessary when the rings are set out by springs or other parts adjustable within the piston head. Snap piston rings, however, permit the use of a solid piston, dispensing with the need for a follower.

The effectiveness of a piston ring may be tested, when the construction of the engine will permit it, by disconnecting the valve for the head end, setting it so that it covers the port, and then taking off the cylinder cover at the head end and admitting steam through the crank end steam port, when any leak in the piston rings will be seen by the escape of the steam.

THE PISTON ROD.

Piston rods should be of slightly diminishing diameter at the ends, so that the wear shall not leave a shoulder at each end of the rod.

In determining the diameter of the piston rod, allowance is made for turning it occasionally in the lathe to restore its parallelism, the wear reducing its diameter more in the middle than at the ends. The diameter of a piston rod is found in practice to range between one-sixth and one-tenth the diameter of the cylinder bore.

Steel piston rods wear better than those of wrought iron, being free from scaly seams which are apt to cut the packing and cause the rod to wear in grooves.

The best method of securing a piston rod to a piston head and to the cross head is by a taper seat and a key, so that no nut is needed, and the cylinder cover need not have a recess to receive the nut when the piston is at the end of the stroke, and the amount of clearance is correspondingly reduced.

Piston head key ways are sometimes given so little clearance that the key completely fills the keyway when driven fully home. This prevents the edges of the keys from bulging into the clearance space in the keyway, which action is apt to cause the key to loosen in time. The key should have a safety pin at its small end.

When piston rods are threaded into the cross head, or into the piston, the threads are made an easy fit, and taper seats or split hubs secured by clamping screws are relied upon to keep the rod true to the cross head or piston, it being found that the screw alone cannot be relied upon for this purpose.

PISTON ROD PACKING.

Piston rod packing, of fibrous or similar material, should be cut in rings that will not quite fully envelop the piston rod, and the first ring should be placed with its split upwards. After two or three rings have been inserted, each having its split at a different part of the bore, so as to "break joints," the gland should be screwed up enough so as to carry the packing home to the back of the stuffing box. This process should be continued until the stuffing box is filled for about two-thirds of its depth, when the gland may be screwed home.

The gland should be screwed up quite evenly, so that the packing in the stuffing box shall be compressed equally all around the rod, and will not cause the gland to bind on the rod or in the stuffing box bore.

The wrench should be applied first to one nut, giving it a turn or two, and then to the other, and after the gland is firmly home the nuts should be eased back about two turns.

When a gland requires packing, it is proper to take out all the old packing that has become hard and set.

A leak in piston rod packing may sometimes be remedied by taking out three or four rings of the packing and reversing it.

If the packing is tightened up while the engine is running, it should be done very gently and evenly, as a very little screwing up may stop the leak, while excessive screwing produces undue friction.

Piston rods are in some of the most advanced practice packed with metallic packing, or packing composed of soft metal. In some forms of metallic packing the construction is such that the gland and packing do not attempt to restrain the line of motion of the piston rod, this duty being left to the guide blocks and guide bars, where it properly belongs.

THE CROSS HEAD.

In engines having Corliss frames, the cross head is provided with shoes and adjusting screws, to take up the wear.

When guide bars are shaped thus __|¯¯ the cross head is provided with gibs (usually of brass composition) to take up the wear.

In either case care must be taken to make the adjustment correct, and thus keep the piston rod in line. The shoes or gibs should not bear hard upon the guides, but be an easy sliding fit without lost motion.

Cross head pins should be kept eased away on the two parts of their circumference which are within the connecting rod brasses or boxes and near the joint faces of the same. This is necessary because the wear is greatest on the crowns of the boxes, and the pins are apt to wear oval. In some engines, the surface of the pin is cut away, but if it is not, and the pin can be revolved in the cross head, it is a good plan to give it half a turn occasionally, which will keep it round.

THE GUIDE BARS.

The guide bars of an engine require to be set exactly in line with the axis of the cylinder bore, so that they may guide the piston to travel in a straight line. They should be an easy sliding fit to the cross-head guide.

The top bar is more difficult to lubricate than the bottom one, especially when it receives the most pressure, as is the case when the top of the fly-wheel runs towards the cylinder.

Cast iron guide bars wear better than either brass, iron, or steel ones, so long as they are properly lubricated. The face of each guide bar should be cut away, so that the ends of the cross head guides will travel past it. This will prevent a shoulder forming at the ends of the bar as the face wears away. Such shoulders are apt to cause a knock as the connecting rods are lined up, because in the lining the connecting rod is restored to its original length, and the path of the cross-head guides along the bars may be altered.

THE CONNECTING ROD.

There are two principal kinds of connecting rods, the "strap ended" and the "solid ended." The solid ended wear the best, but are more difficult to get on and off the engine.

Connecting rod straps are secured to the stub ends (as the ends of the rod are called), either by bolts or by one or two gibs, and the brasses are set up by a taper key or wedge.

The taper for connecting rod keys is about an inch per foot.

The angularity of a connecting rod is a term that applies to its path of motion, which is (during all parts of the stroke except on the dead centre) at an angle to the line of engine centres. The effect of this angularity is to cause the piston motion to be accelerated at one part of the stroke and retarded at another, thus causing the point of cut-off to occur at different points of the two strokes.

The direction of the variation is to cause the point of cut-off to occur later on the stroke when the piston is moving from the head end of the cylinder towards the crank.

The amount of variation caused in the two points of cut off by the connecting rod depends upon the proportion that exists between the length of the crank and that of the connecting rod, and is less in proportion as the length of the connecting rod is greater than that of the crank.

An ordinary length of connecting rod is six times the length of the crank, or six cranks, as it is commonly termed.

Fig. 3296 represents a cylinder, piston and rod, cross head, connecting rod, and crank.

The piston b is shown in the middle of the cylinder, the cross head at E, and the crank pin at B, instead of being at G´, as it would but for the connecting rod, or if the connecting rod was infinitely long.

Now take a pair of compasses and set it from b to E, and then try it from a to D, and from c to F, and it will be seen that the three cross head positions D, E, and F correspond correctly to the three piston positions a, b, c. Then take a pair of compasses and set them to the length of the connecting rod (from E to B) and try them from D to A, from B to E, and from C to F, and it will be seen that crank pin positions A, B, and C correspond to cross head positions D, E and F, and therefore that the crank is not at half stroke when the piston is in the middle of the cylinder. Take these same compasses, and resting one point at (G´) mark the arc H, and that is where the cross head would be when the crank was at (G´). Now then we see that the connecting rod causes the piston to move slower while running from a to b than it does while running from b to c.

THE D SLIDE VALVE.

The various events which are governed by the D slide valve of a steam engine are as follows:

The live steam period is that during which the steam is admitted from the steam chest into the cylinder and the steam admitted during this period is termed live steam.

The point of cut off is that at which the valve closes the steam port, and the admission of steam into the cylinder is stopped, hence the point of cut off is at the end of the live steam period.

The period of expansion is that during which the steam is allowed to expand in the cylinder, and therefore begins at the point of cut off, and ends at the point of release.

The point of release is that at which the valve opens the port and permits the steam to escape.

The point of compression is that at which the exhaust port is closed, which occurs before the piston has reached the end of its stroke; the steam that has not passed out of the cylinder is therefore compressed, the compression continuing until the valve opens for the lead.

The lead of the valve is the amount the port is open to the live steam when the crank is on the dead centre.

The point of admission is that at which the port opens for the live steam to enter, and it follows that the lead and compression both act as a cushion, arresting the motion of the piston when it reaches the end of the stroke.

Cushioning begins, however, at the time the exhaust port is closed enough to arrest the escape of the steam, while compression begins when the valve has closed the exhaust port.

The construction of a common slide valve is shown in Fig. 3297, in which the valve is shown in its mid-position. P P are the cylinder steam ports (as the openings through which the steam passes from the steam chest to the cylinder are termed), and at X is the cylinder exhaust port, through which the steam escapes from the cylinder. Z is the valve exhaust port or exhaust cavity.

The lip of a valve is the width of its flange face, or the distance L, which is measured from the steam edge A to the exhaust cavity Z. At the other end of the valve, H is the lip extending from the steam edge B to the exhaust cavity.

Steam lap is the distance the steam ends (or the steam edges as they are called) A, B overlap the steam ports, this distance being shown on the ends of the valve at a C. If the valve had no steam lap, its steam edges would just cover the ports, as denoted by the dimension W.

Exhaust lap is the amount the exhaust cavity Z overlaps the bridges q q´, as at p, r.

Unequal steam lap is given to cause the point of cut off to occur at equal points in the piston stroke; thus in the figure there is more steam lap at the head end than at the crank end of the valve. But unequal lap could also be given in order to greatly vary the points of cut off for the two piston strokes, if such was desired.

Unequal exhaust lap may be given to equalize the point of release, or to equalize the points of compression.

The head end of the valve (or of the cylinder) is that which is furthest from the crank shaft, the other end, or that nearest to the crank shaft, being termed the crank end.

THE ACTION OF A COMMON SLIDE VALVE.

The action of a common slide valve may be traced as follows:

Suppose the port a to be at the head end of the cylinder and open to the amount of the lead with the crank on the corresponding dead centre, and if the valve travel be made equal to twice the lap and the lead, the various positions of the valve will be as marked in Figs. from 3298 to 3302; the event corresponding to each valve position being stated in the figures.

DOUBLE PORTED VALVES.

The term port applies strictly to the area of opening of the steam passage where it emerges upon the valve seat. The term steam passage includes the full length of the opening from the cylinder bore to the face upon which the valve is seated.

A double ported steam port is one in which there are two openings or steam ports, leading into one steam passage.

A double ported valve is one in which there are two ports at each end of the valve. These two ports in some cases admit steam to a single cylinder port, and in others to two steam ports, terminating in one steam passage.

A griddle valve is one that has two or more ports at each end upon a seat that has two or more ports for each steam passage.

Double ported valves are employed in some cases to increase the admission of live steam to the cylinder, and in others to increase the exhaust openings also. The effectiveness of a double ported valve is mainly valuable at the beginning of the stroke, and is especially valuable in cases when the travel of the valve is diminished to hasten the point of cut off, because in such cases the outer edges of the valve do not open the steam port to its full width, and a single port is apt to wire draw the steam. By the employment of more than one port, or several ports, a sufficient admission may be obtained with less valve travel.

The Allen double ported valve is one in which the second port increases the port opening for the admission only, as shown in Fig. 3303, in which the valve is moving in the direction of the arrow; the port K will receive steam through the opening at g, and from a port passing through the valve, the steam entering it as shown by the arrow. The second port forms part of the lap of the valve, and enables the travel to be short enough to be cut off at early points in the stroke, without employing so much steam lap as to widely distort the points of cut off, this latter being a defect of the D valve.

Webb's patent slide valve is circular, and is so arranged as to be free to revolve in the hoop of the valve rod, the effect being that the valve moves around, or to and fro in the hoop, without any special mechanism to produce such movement, and the result is, that the valve and port facings wear smooth and even without any tendency to become grooved.

BALANCED VALVES.

A balanced valve is one in which means are employed to relieve the back of the valve of the steam pressure, and thus prevent its being forced to its seat with unnecessary pressure.

In some of the most successful balanced valves this is accomplished by providing a cover plate, which may be set up to exclude the steam from the back of the valve which works (a sliding fit) between the valve face and the face of the cover plate. Such a method of balancing is sufficiently effective for all practical purposes, if the following conditions are observed: The valve rod must be accurately guided so as to avoid side strains; the valve must fit accurately to its seat and to the cover plate, and the adjustment so made that the valve slides freely at first, being steam tight, and yet allowing room for lubrication to enter. When the travel of a valve, balanced by a cover plate, is varied to alter the point of cut off, the construction must be such that the ends of the valve at the shortest stroke pass over the ends of the seat and cover plate faces, or otherwise the middle of the seat and cover plate faces will wear hollow.

The Buckeye, Porter-Allen, and Straight-Line Engines are examples of practically balanced valves. The first of these has a balancing device that follows up the wear; the second has an adjustment whereby the cover plate may be set up to take up the wear; and in the third the wear is reduced to a minimum, by accurately fitting and guiding the parts.

The construction of the valve in the Straight-Line Engine is shown in Fig. 3304, in which B represents the cylinder bore; the valve v rests on a parallel strip n, and on its top rests the parallel strip m, the pressure relieving plate P is set up firmly against the pieces m n, whose thicknesses are such as to leave the valve a working fit between the faces of R R and of P.

Instead of the valve sliding on a flat face, it may work upon a shaft or spindle as a centre, its face moving in an arc of a circle, and its action will be the same as a flat valve having the same proportions. Fig. 3304_a_ represents a valve V of this construction, whose shaft is at S, A being an arm fast on S, and driven by the eccentric rod R. To find the necessary amount of travel for such a valve, we draw lines, as f, g, from the inner edges of the steam ports, through the centre of the shaft S, and also draw an arc through the centre of the eye of arm A, and where lines f g cut the arc, as at d and e, are the extremes of motion of A.

PISTON VALVES.

A piston valve acts the same as a flat or plain (D) valve, having the same amount of lap lead and travel. In Fig. 3305 we have a cylinder with a flat valve on one side and a piston valve on the other, the head end ports being about to take steam, and it is seen that the eccentrics occupy the same positions for the two valves. The steam ports are, for the piston valve, annular grooves provided in the bore in which the valve fits. The piston valve is balanced because it receives its steam pressure on the ends, but it will not follow up its wear as the flat valve does, hence it is liable to leak.

SEPARATE CUT OFF VALVES.

Meyer's cut off valve is constructed as shown in Fig. 3306, M being the main valve, and v v the two cut off valves, whose sole duty is to cut off the steam at an earlier point than the main valve would do. If the engine is to have a fixed point of cut off, or, in other words, if the cut off is always to occur at some one particular point in the stroke, the valves may be set to do so, and equalize the points of cut off.

Variable points of cut off with the Meyer's valve may be obtained by shifting the position of the eccentric that operates the cut off valve, but it is usually done by means of moving the valve by a right and left hand screw, such as shown in Fig. 3306. The cut off eccentric is set ahead of the main eccentric, so that the cut off valve will close the ports before the main valve would do so; thus, in the figure the cut off valve is shown to have effected the cut off for port a by the time the main valve has fully opened port a, and is reversing its motion. If the engine requires to reverse its motion, the cut off eccentric is set exactly opposite to the crank, but otherwise, it may be set 8 or 10 degrees either ahead of or behind the crank, but if set too little ahead of the crank, the port may reopen after the cut off has been effected.

Gonzenback's cut off valve is constructed as in Fig. 3307, the steam chest having two compartments. A, A are the cylinder steam ports, C the main valve, and E the cut off valve, whose ports (as G) are made wider than the ports F.

Reducing the travel delays the point of cut off in the Gonzenback valve, whereas in the common slide valve it gives an earlier cut off.

THE ECCENTRIC.

When a single eccentric is used, it is simply termed the eccentric. If a cut off valve (or two cut off valves) are used upon the engine, then the eccentric that works the main valve is called the main eccentric, while that which works the cut off valve or valves is called the cut off eccentric. The main valve is that which works on the cylinder face; the cut off valve is that which effects the cut off.

A shifting eccentric is one that is moved across the shaft so as to alter its amount of throw, and, therefore, the amount of valve travel, the effect being to vary the point of cut off.

In engines where a constant amount of lead is given, or in other words, when the eccentric position is intended to be fixed, the eccentric should be secured to the crank shaft by a feather or key sunk into the crank shaft so as to prevent the eccentric from moving, while enabling it to be taken off and replaced without requiring any operations to adjust its position with relation to the crank.

The feather should fit tight on the sides, as well as on the top and bottom, and may have a slight taper on the sides, which will make it easier to fit the featherway or keyway to the feather, and easier to put the eccentric on or take it off.

By this means the eccentric cannot shift, and may be replaced after being taken off without having to set the whole valve motion over again.

When the amount of valve lead or of compression is varied to suit the speed at which the engine is to run, or to aid the counterbalancing of the engine, a feather cannot be used because it will not permit the eccentric to be moved to effect the adjustment.

Set screws possess disadvantages, inasmuch as that the point of the set screw may leave an indentation, which, if the eccentric is moved a trifle, may cause the set screw point to fall back into the old indentation, thus rendering it difficult to make a small adjustment of eccentric position.

An eccentric is the exact equivalent of a crank having the same amount of throw, as may be seen from Fig. 3308, in which the outer dotted circle represents the path of the crank and the inner one the path of the centre of the eccentric. A small crank is marked in, having the same throw as the eccentric has, and the motion given by this small crank is precisely the same as that given by the eccentric whose outer circumference is denoted by the full circle.

Considering the motion of both the crank and the eccentric, therefore, we may treat them precisely the same as two levers, placed a certain distance apart, revolving upon the same centre (the centre of the crank shaft), and represented by their throw-lines.

In Fig. 3309, let the full circle E E represent an eccentric upon a shaft whose centre is at C, and let the centre of the eccentric be at e. The path of revolution of the eccentric centre will be that of the dotted circle whose diameter is B, D. As the eccentric is in mid-position (e being equidistant from B and D), the valve will be in mid-position as denoted by the full lines at the bottom of the figure. Now suppose the eccentric to be revolved on the centre C, until its centre moves from e to V, its circumference being denoted by the dotted circle A A, and if we draw from V a vertical line cutting the line B, D at f, then from C to f will be the distance the eccentric would move the valve, which would then be in the position denoted by the dotted lines at the bottom of the figure. It becomes clear then that if we suppose the eccentric to have moved from mid-position to any other position, we may find how much it will have moved the valve by first drawing a circle representing the path of the centre of the eccentric, next drawing a line (as B D) through its centre, and then drawing a vertical line as (C e) through its mid-position and also a vertical line from the eccentric centre in its new position, the distance between these two vertical lines (as distance C f in the figure) being the amount the eccentric will have moved the valve.

It may have been noticed that the diameter of the eccentric does not affect the case, the distance B D, or the diameter of the circle described by the centre of the eccentric, being that which determines the amount of valve motion in all cases. This being the case, we may use the circle representing the path of the eccentric centre for tracing out the valve movement without drawing the full eccentric, and the diameter of that circle will of course equal the full travel of the valve.

The position of an eccentric upon a shaft is often given in degrees of angle, which is very convenient in some cases. If a valve has no lap or lead, the eccentric will stand at a right angle or angle of 90 degrees when the crank is on the dead centre.

The division of a circle into degrees may be explained as follows:

Suppose we take a circle of any diameter whatever and divide its circumference into 360 equal divisions, then each of these divisions will be one degree. The number 360 has been taken as the standard, and this being the case, there are 360 degrees in a circle, in a quarter of a circle there will therefore be 90 degrees, because 90 is one quarter of 360. By means of dividing a circle in degrees therefore we have a means of measuring or defining any required portion of it.

In Fig. 3310 the degrees of a circle are applied for defining the relative positions of a crank and an eccentric. As the zero position of the crank is on a dead centre, it is so placed in the figure, while as the zero position of the eccentric (which is for a valve having no steam lap) is at 90 degrees from the crank, therefore the dotted circle representing the path of the eccentric centre has its O or zero point at 90 degrees from the crank. Now suppose the eccentric centre stood at v and the eccentric throw line at c v, and it will stand at 30 degrees from O, hence the angular advance of the eccentric is in this case 30 degrees, or in other words, it is 30 degrees in advance of its zero position, or the position it would occupy when the crank is on the dead centre and the valve has no lap and no lead.

If we measure the distance apart of the crank and the eccentric in degrees, we find it is 120 degrees, hence place the crank where we may, we can find the corresponding eccentric position because it is 120 degrees ahead of the crank. The sign for degrees is a small ° placed at the right hand of the figures and slightly above them; thus, thirty degrees would be written 30°.

FINDING THE WORKING RESULTS GIVEN BY A D SLIDE VALVE.

Although not strictly within the line of duty of an engineer or engine driver, he is nevertheless sometimes called upon to find out how a valve of given proportions will dispose of the steam, or what proportions to give to a valve to accomplish certain results.

This is easy enough when either the travel of the valve or the amount of the lap and the width of the port are given, but if the width of the port alone is given, and all the other elements are to be found, it becomes a more difficult problem.

An engineer, however, is rarely called upon to solve the question from this stand-point, which properly belongs to the draughtsman or engine designer.

If the amount of valve travel is given, however, all the other elements may readily be found by the following construction:

Suppose that in Fig. 3311 a D valve is to be designed to cut off the steam when the piston has travelled from position B´ to R´, or at three-quarters of its stroke. Then to find the position the crank pin will be in when the cut off occurs, we draw a circle, B D, representing the path of the crank on the same scale that the length of the piston stroke is represented. The straight line from B to D will, therefore, represent the piston stroke without drawing the piston or cylinder at all (this being done in the figure to make the explanation clear). When the crank is on its dead centre, B, the piston, will be at B´, and the valve in the position shown (supposing it to have no lead). As soon as the crank and valves begin to move, the steam will enter steam port a, and to find where the crank will be when the piston is at three-quarters stroke, and is, therefore, in position R´, we mark a point at R three-quarters of the distance from B to D. Then, taking no account of the length of the connecting rod, we draw a vertical line Y from R to the circle, and this line gives at H the position the crank will be in when the piston is at R. We have so far, therefore, that while the piston travels from B´ to R´, the crank will travel from B to H. Now, it will be found that if we set a pair of compasses from B to F, which is half-way from B to H, and then rest the compasses at D, and mark an arc V, then a line from V to the centre of the crank will give us the proper position of the eccentric. As the centre of the crank pin and also the centre of the eccentric both travel in a circle, we may, therefore, take a circle having a diameter equal to twice the throw of the eccentric, (or, what is the same thing, equal to the full travel of the valve), and let it represent the paths of both the eccentric centre and the crank pin centre, the latter being drawn to a scale that is found by dividing the length of the piston stroke by the travel of the valve; thus, if the travel is 3 inches and the stroke 30 inches, the diameter of a 3 inch circle will represent the valve travel full size, and the piston stroke one-tenth full size, because 30 ÷ 3 = 10. It has been shown on page 376 that the length of the connecting rod affects the motion of the piston by distorting it, and it is necessary to take this into account in constructing the actual diagram, which may be done as follows:

The valve travel and point of cut off being given, to find the required amount of lap, there being no lead, draw a circle equal in diameter to the travel of the valve, and draw the line of centres B D, Fig. 3312; mark on the line of centres a point R, representing the position the piston is to be in at the time the cut off is to take place.

Set a pair of compasses to represent the length of the connecting rod on the same scale as the circle B D represents the path of the crank; thus, if the connecting rod is three times the length of the stroke, the compasses would be set to three times the diameter of the circle B D.

A straight line from B to D and passing through the centre C of the crank will represent the line of centres of the engine, which must be prolonged to the right sufficiently to rest the compasses on it and draw the arc Y, which will give at H the position of the crank when the piston is at R, and the cut off is to occur.

We have thus found that the amount of circular path the crank will move through from the dead centre to the point of cut off is from B to H, and as the eccentric is fast upon the same shaft, it will, in the same time, of course, move through the same part of a circle.

One half of its motion will be to open and one half to close the port, so that we may by means of the arcs at F get the point F, which is midway between B and H, and with the compasses set from B to F, mark from D the two arcs V and V´ whose distance apart will obviously be the same as from B to H.

Then from V to V´ draw the line P, and from this line to the centre C of the crank shaft is the amount of steam lap necessary for the valve, while from this line (P) to D is the width of the steam port.

The proof of the diagram is as follows:

When the crank is on the dead centre, the centre of the eccentric is at V, its throw line being represented by the line from V to C, and the valve is about to open the port as shown in the figure.

While the eccentric is moving from V to D, the valve will move in the direction of the arrow and will fully open the port, while the crank pin will move from B to F.

Then, while the crank moves from F to H, the eccentric will have moved the valve to the position it occupies in the figure, having closed the port and effected the cut off.

We have here found the amount of lap and the position of the eccentric necessary for a given point of cut off when the latter is given in terms of the piston stroke. If, however, the point of cut off had been given in terms of the crank pin position, we might find the required amount of lap at once, by simply drawing a line from the centre B, the point to H where the crank pin is to be when the cut off occurs.

From this line we could then draw the dotted circle G, and just meeting the line P, which would give the eccentric position.

To find the piston position, the arc Y would require to be drawn by the same means as before.

If the valve is to have lead, the diagram may be constructed as in Fig. 3313, in which the circle has a diameter equal to the travel of the valve and the cut off is to occur when the piston is at R and the crank at H.

When the valve is at the end of its travel and has fully opened the port, the eccentric will be at D, hence from D we mark an arc G distant from D to an amount equal to the width of the steam port, drop the vertical m from G, and at its lower end V´ is the position of the eccentric centre at the point of cut off. Then draw a line P, distant from m equal to the lead, which will give at V the position of the eccentric when the crank is on the dead centre, and the valve is open to the amount of the lead. The lap is obviously the distance from the centre C of the crank shaft to the arc G.

We have here found all the points necessary except the point at which the valve will open the port for the lead, and this we may find by setting a pair of compasses to the radius B H (or to radius V V´, as both these radii are equal), and from V as a centre, mark at A an arc, which will give the crank pin position at the time the port first opens for the lead, or in other words it will give the position. The proof of the construction is, that if we set the compasses to the distance between the crank pin position on the dead centre and the point of cut off (or from B to H), we may apply the compasses to the points V, V´, which represent the eccentric position when the port is opened to the amount of the lead, and when the cut off occurs.

If the point of cut off only is to be found, we mark from C, Fig. 3314, an arc G representing the amount of valve lap and arc S representing the lead. A vertical P gives the eccentric position V when the crank is on the dead centre at B, and a vertical m from G gives at V´ the eccentric position at the point of cut off. Then with the compasses set to the points V V´, we may mark from B an arc, locating at H the position of the crank at the point of cut off, and from this with compasses set to represent the length of the connecting rod on the same scale as the circle represents the path of the crank, we may, from a point on the line of centres, mark an arc Y giving at R the piston position at the point of cut off.

When, therefore, the lap is given, we mark it from the center C of the crank shaft, and find the other elements from it, whereas, when the lap is to be found, we mark the width of the port from the end D of the valve travel, and find the other elements from that.

A proof of all the constructions is given in Fig. 3314, in which the letters of reference correspond to those in the previous figures, and the positions of the parts are marked in degrees of angle.

To find the piston position at the point of cut off, measured in inches, of the piston stroke it must be borne in mind that as the circle B D represents the full travel of the valve, the diagram gives all the positions of the eccentric and valve full size, but that as it represents the crank path on a reduced scale, therefore we must multiply the measurement on the diagram by that scale.

Suppose, for example, that the piston stroke is 10 inches, and the valve travel 2-1/2 inches, and the circle being 2-1/2 inches in diameter, is, when considered with relation to the eccentric motion, full size, but when considered with relation to the piston or crank motion, it is only 1/4 the size, hence to find the piston position at the time of cut off, we must multiply the distance from B to R by 4.

LINK MOTION FOR STATIONARY ENGINES.

The ordinary mechanism employed to enable a stationary engine to be reversed or run in either direction is the Stephenson link motion. Other forms of link motion have been devised, but the Stephenson form has become almost universal.

Fig. 3315 represents this link motion or reversing gear with the parts in position for the full gear of the forward motion, and Fig. 3316 represents it in full gear for the backward motion.

The meaning of the term full gear is that the parts are in the position in which the steam follows the piston throughout the longest or greatest part of the stroke. When in full gear the link motion operates the valve almost precisely the same as if the eccentric rod was attached direct to the valve spindle and no link motion was used.

Besides enabling the engine to run in both directions, however, the link motion provides a means of reducing the amount of valve travel and thus causes the live steam to be cut off earlier in the piston stroke, thus using the steam more expansively. This is done by moving the reversing lever more upright, the earliest point of cut off being obtained when it is upright and the latch is in the notch marked O on the sector in Fig. 3315. If with the engine standing still we move the link motion from full gear forward to full gear backward and watch the valve, we shall find that the valve lead increases as the reversing lever approaches the upright position, or mid gear as it is termed, and that after passing that point it gradually diminishes again, the valve being so set that the lead is the same for full gear forward as it is for full gear backward.

The reversing lever is used to move the link into the required position and to hold it there (the end of the latch fitting into the notches in the sector being the detaining or locking device); as the link is suspended by its saddle pin S and the link hanger, therefore its motion is to swing or partly rotate on the pin S, and at the same time ending in the arc of a circle whose centre of motion is in the pin at the upper end of the link hanger which is pivoted to the lower arm of the lifting shaft (which is sometimes termed the tumbling shaft). It will clearly be seen that with the position the parts occupy in Fig. 3315, and the crank motion being in the direction of the arrow, the forward eccentric will move the top of the link to the right and therefore the valve will move to the right, while the backward eccentric will move the bottom end of the link to the left.

In full gear, however, the bottom eccentric rod has but a very slight effect indeed on the motion of the valve because both the link hanger and the link block will permit the link to swing on centre of the link block pin as a pivot. If now we turn to Fig. 3316 for the full gear backward, we shall see that these conditions are reversed and the backward eccentric becomes the effective one, being in line with the valve spindle. By shifting the link from one gear to the other, therefore, we have merely changed the direction in which the link will move the valve, and, therefore, the direction in which the engine would run.

In Fig. 3315 for the full gear the parts are shown in position, with the piston at the crank end of the cylinder, and the crank pin on the dead centre, and the eccentrics must be set as shown in the cut, the eccentric rods being open and not crossed. When, however, the crank is on the other dead centre and the piston at the head end of the cylinder, the rods will cross each other, and it is necessary to remember that the rods should be open when the piston is at the crank end of the cylinder. If, however, the running gear contains a rock shaft, or rocker (as is the case in American locomotives), then these conditions are reversed, and the eccentric rods will cross when the piston is at the crank end of the cylinder.

In setting the slide valve of an engine having a link motion, there are two distinct operations. First, to put the crank on the respective dead centres, which will be fully described on page 394 and need not be repeated; and second, to set the eccentrics in their proper positions on the shaft, and correct, if necessary, the lengths of the eccentric rods. The crank being on the dead centre, with the piston at crank end of the cylinder, the eccentric should be moved around on the shaft by hand until there is the desired amount of lead at the crank end port, and temporarily fastened there, a set screw usually being provided (in the eccentric) for this purpose. The lead is best measured with a wedge, W, Fig. 3315. The crank is then put on its other dead centre, and the lead for the head end port is measured. If the lead is to be made equal for the two ports (as is usually the case in horizontal engines) and it is found to come so, the valve setting for the forward gear is complete. If the lead is not equal, the forward eccentric rod or else the valve spindle must be altered so as to make the lead equal. In some engines adjusting screws are provided for the purpose of regulating the length of either the eccentric rod or else of the slide spindle; it does not matter which is altered. The link motion is then put in full gear for the backward motion, and, with the crank on the dead centre (it does not matter which dead centre), the eccentric is moved by hand upon the crank shaft until there is the required amount of valve lead. The eccentric is then fastened on the shaft and the crank put on the other dead centre, and the lead tried for the other port, and made equal by lengthening or shortening the backward eccentric rod. It is to be noted that altering the length of the eccentric rod or of the valve spindle makes it necessary to reset the eccentric, as it affects the amount of lead at both ports; hence, if any alteration of rod length is made, the whole process here described must be repeated after each alteration of rod length.

FLY BALL OR THROTTLING GOVERNORS.

An isochronal governor is one in which the two opposing forces are equal throughout the whole range of governor action, or, in other words, equal, let the vertical height of the plane in which the balls revolve or swing be what it may.

A dancing governor is one that acts spasmodically. Such an action may occur from undue friction in the parts of the governor or of its throttle valve.

The friction offers a greater resistance to starting the parts in motion than it does to keep them in motion after being started; hence, the parts are apt to remain at rest too long, and to move too far after being put in motion.

Rule to find the number of revolutions a governor should make. Divide the constant number 375.36 by twice the square root of the height of the cone in inches. The quotient is the proper number of revolutions per minute.

Example.--A governor with arms 30-1/2 inches long, measuring from the centre of suspension to the centre of the ball, revolves, in the mean position of the arms, at an angle of about thirty degrees with a vertical spindle forming a cone of about 26-1/2 inches high. At what number of revolutions per minute should this governor be driven? Here the height of the cone being 26.5 inches, the square root of which is 5.14 and twice the square root 10.28, we divide 375.36 by 10.28, which give us 36.5 as the proper number of revolutions per minute at which the governor should be driven.

The construction of the Pickering governor is as follows:

In Fig. 3317 it is shown in its simplest form, and in Fig. 3318 with the driving pulley and speeder (or engine speed regulating device) attached. This speeder consists of a spiral spring whose tension may be adjusted to more or less resist the rise of the governor balls, and thus enable the engine to run at a greater speed for a given amount of rise of the governor balls, hence by increasing the tension the engine speed is increased.

THE SPRING ADJUSTMENT.

The adjustment of the spring tension is made by a worm actuating a worm wheel on a rod passing through the spring, and to which one end of the spring is attached, the other acting on an arm that projects into a slot in the governor spindle. It is obvious that the speeder can be adjusted while the engine is running.

In Fig. 3319 the governor is shown with the speeder and Sawyer's valve, the latter enabling the governor valve to be opened or closed without affecting the rise and fall of the governor balls, which is done by operating the arm shown on the right, whose ends are provided with loops, so that a cord may be attached, enabling the engineer to operate the governor from a distance.

The safety stop or stop motion is shown on the right, Fig. 3320.

It acts to close the governor valve and stop the engine in case the belt that drives the governor should get off the pulley or break. This stop motion consists of a pulley suspended by a rod, and riding on the belt which supports its weight. Should the governor belt break, this pulley falls and severs the connection between the valve and the governor, closing the valve, and holding it closed. Fig. 3321 shows the governor in section to expose the construction of the valve. The valve V is what is termed a poppet or poppet valve, which is balanced, because the steam entering at I, and taking the course denoted by the arrows, acts equally on both ends of the valve and does not press it in either direction, while as the steam surrounds the valve it is not pressed sideways.

At B is a gland or stuffing box to keep the spindle or rod steam-tight. At A is the slot for receiving the arm from the speeder and from the stop motion.

P is obviously the driving pulley, imparting motion to the bevel wheels G, which drive the outer spindle S, the inner spindle s´ being connected to A. The balls are upon ribbon springs D, which are secured at their lower ends to a link fast to the spindle S.

The centrifugal force generated by the balls causes them to move outwards, their upper ends pulling down the cap to which they are secured, and this cap operates the valve.

Governors of this class are sometimes termed fly-ball governors.

STARTING A PLAIN SLIDE-VALVE ENGINE.

The method to be pursued before starting a plain slide-valve engine depends upon what the engineer knows about the condition of the engine.

If he knows the engine is in proper running order, all that is necessary is to first attend to the oil cups and start them feeding.

Then, if it is necessary, move the crank into the required position to start it easily; open the waste water cocks to relieve the cylinder of the water that will be condensed from the steam when it enters a cool cylinder, and turn on the steam; giving the throttle valve enough opening to start the engine slowly.

The best position for the crank pin to be in to enable its starting easily is midway between the horizontal and vertical position (or, in other words, at an angle of 45° to the line of centres) and inclining toward the cylinder, so that when the engine moves the piston will travel toward the crank shaft.

There are two reasons why this is the best position for starting. The first applies to all engines because there is a greater piston area for the steam to act on when the piston is moving toward the crank than there is when it is moving away from it. This occurs because the piston rod excludes the steam from a part of the face of the piston. The second applies to all plain slide-valve engines whose slide valves have equal laps and both steam ports of equal widths, because the live steam follows further on the stroke when the piston is moving toward the crank than it does when it is moving away from it, and it follows that more piston power is developed, and the engine is less likely to stop when passing the dead centre.

When first taking charge of an engine, it is proper, before starting it, to ascertain that it is in fair working order.

A complete examination of an engine should include a test of the fit of the piston to the cylinder bore, of the cross head to the guide bars, of the connecting rod brasses to the crank pin and cross head journals, and of the crank shaft to its bearings. It would also include a testing of the alignment of the crank shaft and of the guide bars, as well as the set of the valves and the adjustment of the governor.

The least examination permissible with a due regard to safety would be to move the engine throughout at least one full revolution by hand, and to see that the connecting rod brasses and the main bearings do not fit too tight to their respective journals, and to then start the engine slowly by giving it only enough steam to move it, keeping the hand on the throttle valve so as to be able to shut off steam instantly should it become necessary.

A thorough examination should be made in the following order:

First, slightly loosen the nuts on the crank shaft bearings and also the connecting rod keys.

Then move the fly wheel around until the crank points straight to the cylinder, which will bring the piston up to the outer end of the cylinder bore.

Take off the cylinder cover and also the follower from the piston head, and see that the piston rings are set out to fit the cylinder bore but not to bind it tight. Then bolt the follower up firmly in place again.

Take off the connecting rod and move the piston until it touches the cylinder cover at the other or crank end of the cylinder, and then draw a line across the side face of the cross head guide and on the guide itself.

Put on the cylinder cover and push the piston back until it abuts against it, and then make another line on the cross head guide and the guide bar, and these two lines will show the extreme positions to which the piston can be moved when the connecting rod is disconnected.

Next put on the connecting rod, carefully adjust the keys or wedges, so that the bores of the brasses fit easily to the crank pin and cross head pin, seeing that the oil holes are clear, and that oil will feed properly to the journals.

In making this adjustment it is a good plan, if there is any end play of the brasses on the crank pin, to set up the key or wedge until the rod can just be moved by hand on the pin, by first pulling the rod to one end of the pin, and then pushing it to the other.

In putting on the rod, it will be necessary to move the piston a trifle towards the crank.

In making the adjustment of the crank pin fit to the rod brasses, it is a good plan to drive the key home until the brasses are known to bind the crank pin, and then mark a line across the side face of the key and fair with the top face of the connecting rod strap, to then slacken back the key enough to ease back the brasses to a proper fit, and then mark another line on the key.

The first line will form a guide as to how much to slacken back the brasses to adjust the fit, and the second one will form a guide as to how much the key is moved when making a second adjustment, if one should be found necessary after the engine has been running.

Similarly in adjusting the main bearing boxes to the crank shaft, either the nuts, or what are called leads, may be taken to adjust the fit. Leads are necessary when the joint faces of the brasses do not meet, but are left open so that the wear can be taken up while the engine is running.

It is better, however, to let the brasses abut together, so that it may be known that the fit is correct when the nut is screwed firmly home.

The method of taking a lead is as follows: The top brass is loosened, and between the joint faces of the brasses or boxes on each side of the shaft a piece of lead wire is inserted. For a shaft of, say, four inches in diameter, the lead wire will be about 7/16 inch in diameter, or for a 10 inch shaft the wire should be 1/8 inch in diameter, and should be as long as the brass. The nuts are then screwed firmly home, and the wire will be squeezed between the brasses and thus flattened on two opposite sides, the thickness showing how far the joint faces of the brasses are apart when the bore grips the journal.

A liner, fit strip, distance piece, or shim (all these names meaning the same thing) is a strip of metal placed between the joint faces of the brasses to hold them the proper distance apart to make a working fit of the journal and brasses, when the latter are firmly bolted up.

The fit of the top brass therefore depends upon the fit strip being of the proper thickness from end to end.

Now the lead wire is the gauge for the thickness of the fit strip, the latter being made a trifle thicker than the flattened sides of the lead.

If the lead is thicker one end than the other, or if one lead is thicker than the other, the fit strips must be made so, and the leads must be marked so that it may be known which way they were placed between the brasses so that the proper fit strip may be on the proper side of the brass, and the proper end towards the crank.

Another method that is adopted in the case of large brasses is to screw down the nuts until the brasses bind the journal, and then make a mark on the nut and on the bolt thread. The nut is then slackened back as much as the judgment dictates, and a note made of how much this is, the marks forming a guide.

As the wear takes place, and the nuts screw farther down, a new mark is made on the nut, so that it may always be known how much to screw up or unscrew the nut, to make a light adjustment.

To avoid heating, it is a good plan to press some tallow into the bottom or in one corner of the oil cup, and then pour in the oil used for ordinary lubrication. So long as the bearing remains cool, the oil will feed and the tallow remain.

If the bearing heats, the tallow will melt, and, having a heavier body, will give a more suitable lubrication.

To find if the connecting rod is of the right length to give, as it should do, an equal amount of clearance (or space between the piston and the cylinder cover) at each end of the stroke, move the fly wheel a trifle in either direction, and then move it back until the crank is on the dead centre, and draw a line across the cross head guide and guide bar, and the distance between this line and that drawn when the connecting rod was disconnected, shows the amount of clearance at that end of the cylinder. Then move the crank pin over to its other dead centre, and mark a line across the cross head guide and the guide bar, and the distance between this line and that drawn before the connecting rod was put on will show the clearance at this end of the cylinder.

If the clearance is not equal for the two ends, it should be made so by putting liners behind the connecting rod brasses so as to lengthen or shorten the connecting rod (according as the case may require), and equalize the clearance, while at the same time bringing the connecting rod keys up to their proper heights.

To test the set of the valve, the steam-chest cover must be taken off, the crank placed alternately on each dead centre, and the lead measured for each port.

An unequal or an equal degree of valve lead may be given by suitably altering the length of the eccentric rod, but when the lead is equal for the two ports, its amount must be regulated by moving the position of the eccentric upon the crank shaft.

SQUARING A VALVE.--A method not uncommonly pursued in setting a valve is to what is called square it before trying it.

This squaring process consists in so adjusting the length of the eccentric rod that the valve travels an equal distance over or past the steam edge of each steam port; but since the valve does not, when set to give equal lead, travel equally past each port, therefore the work done in squaring a valve is all thrown away, and may result in altering the eccentric rod from its proper length to an improper one, necessitating that it be altered back again in order to set the lead right.

The proper method is to adjust both the length of the rod and the position of the eccentric, by testing the lead at once, lengthening the eccentric rod to increase the lead at the crank end, or vice versa.

Each alteration of eccentric position may render necessary an alteration of rod length, or vice versa, each alteration of rod length may render it necessary to alter the eccentric position, hence the lead should be tried at both ends of the cylinder after each alteration of either rod length or eccentric position.

In vertical engines the weight of the crank shaft causes it to wear the bottom brass or part of the bearing box the most, thus lowering its position, while the eccentric straps and pins wear most in the same direction; hence the wear increases the lead at the head end of the cylinder when the latter is above the crank, and at the crank end when the crank is above the cylinder.

When the cylinder is above the crank, the weight of the piston, cross head and connecting rod is counterbalanced at the end of the downward piston stroke by giving the crank end port more lead; but when the cylinder is below the crank, it is the head end port that must be given increased lead to prevent a pound or knock, or to allow for the wear downwards of the parts.

After an engine is started, the pet cocks should (if they are not automatic) be closed as soon as dry steam issues, and if this cannot be seen, it may be assumed to occur after the engine has made about 20 revolutions.

The parts that will then require particular attention are the crank pin, main bearings, cross head guides and the pump, if there is one. The former must be kept properly lubricated, so that they may not get hot and the cylinder lubricator (which is usually placed on the steam pipe) must be set to self feed properly.

If the crank shaft bearings should begin to heat, loosen the cap bolts and lubricate more freely, or, if it is at hand, some melted tallow may be applied with the oil, as a heavier lubricant may stop the heating.

The crank pin requires the most attention and is the most difficult to keep cool and to examine, because of its circular path rendering it difficult to feel it. This may be done, however, in two ways, first by standing at the end of the engine bed and gradually extending the hand, until the end of the rod meets it as it passes, and, second, by placing the hand on the connecting rod as near to the end of the guide bar as possible where its motion is diminished and moving the hand towards the crank pin, by which means the end of the crank pin may be approached gradually.

If the end of the rod is hot, the engine speed should be reduced or the engine should be stopped so that the connecting rod key or wedge may be eased back and the oil feed made more copious. Then, after the engine has been stopped for the night, the brasses should be taken out and any rough surface, either on the brasses or on the pin, smoothed down with a file.

Hot crank pins may occur from several causes, but by far the most common ones are from improper oiling, or from the engine being out of line.

A heavier oil will often stop, or at least modify, the heating, but its cause should always be discovered and remedied.

Engines that are used out of doors or are exposed to temperatures below the freezing point must be left so that steam leaks may not condense in any of the parts or pipes and burst them.

Leaky throttle valves may, for example, cause water to accumulate in the steam chest and freeze, perhaps bursting the steam-chest cover.

To prevent this let the engine stand with the crank just past the dead centre, so that the steam port will be open, and open the waste water cocks on the cylinder, and also on the steam chest if there is any.

If the cylinder is jacketed all the drain cocks for the jacket should also be opened.

A leaky check valve may cause the steam to condense in the pump and freeze it up solid or burst it or the pipes. To avoid this, open the pump pet cock.

Open all the drain cocks on the heater and water pipes.

If the water is left in the boiler all night it is liable to freeze.

To prevent this leave a well banked fire.

In extreme weather remember that on exposed engines the oil, if of such quality as sperm or lard oil, may freeze and prevent feeding until the bearings get hot and melt the oil.

To prevent this use a lighter oil, as, for example, a mineral oil. Or, in case of freezing, melt the oil in the cups with a piece of wire made red hot while getting up steam in the morning.

A good plan to prevent oil from freezing and yet have a good quality of oil is to mix two parts of lard oil with one part of kerosene.

Portable engines should stand as nearly level as possible, so that the water will stand level above the tubes and crown sheet of the fire box.

When feed water is drawn from a natural supply, as from a stream, the strainer at the end of the suction pipe should be clear of the bottom of the stream, where it is liable to be choked.

When the exhaust steam is used to feed the boiler, do not open the valve that lets the exhaust steam into the feed-water tank until a little while after the engine has started, because the oil fed to the cylinder will otherwise pass into the feed tank and may cause priming.

In engines having plunger pumps for feeding the boiler it is essential to keep the plunger properly packed, as a leak there impairs or stops the pump from acting.

A gauge glass may be cleaned when the engine is cold by shutting off the cocks leading from the boiler and filling the glass with benzine, allowing it to stand two hours; the benzine must be let out at the bottom of the glass tube, and not allowed to enter the boiler.

In starting a new engine be careful to let the bearings be slightly loose.

At first give only enough steam to just keep the engine going, and keep the hand on the throttle valve ready to shut off steam instantly if occasion should require.

PUMPS.

Pumps are divided into the following classes:

Lift pumps, in which the water flows freely away from the pump, which performs lifting duty only.

Force pumps, which deliver the water under pressure.

Plunger pumps, in which a "plunger," or "ram," as it is sometimes termed, is used.

Piston pumps have a piston instead of a plunger.

A double acting pump is one in which water enters into and is delivered from the pump at each stroke of its piston or plunger, or, in other words, one in which, while water is being drawn in at one end of the pump, it is also being forced out at the other.

A single acting pump is one in which the water enters the pump barrel during one piston or plunger stroke, and is expelled from the pump during the next stroke, hence the action of the suction and of the delivery is intermittent, although the pump is in continuous action.

For very heavy pressures plunger pumps are generally used, the plunger being termed a ram.

The advantage of the plunger or ram is that it gives a positive displacement, whereas in a piston pump a leaky piston permits the water from the suction side to pass through the leak in the piston, to the delivery side.

Piston pumps possess the advantage that there is less difference between the contents of the pump and the displacement than is the case in plunger pumps.

The displacement of a piston pump is found by multiplying the area of the pump bore by the length of the piston stroke.

The displacement of a plunger pump is less than the above, by reason of there being a certain amount of clearance or space between the circumference of the plunger and that of the cylinder bore.

It is desirable to keep the clearance space in all pumps as small as the conditions will allow, especially if the pump is liable to lose its water.

Losing the water means the falling of the suction water back into the source of supply, which may occur when the engine has to stop temporarily, and there is a leak in the suction valves.

Rotary pumps are those in which the piston revolves, an example of the most successful form of rotary pump being shown in Fig. 3322, which is that used by the Silsby fire engine.

The advantage possessed by a rotary pump is that it keeps the water passing through the suction in a continuous and uniform stream, as it has no valves.

It may therefore be run at a high velocity or attached direct to the engine shaft.

If a rotary pump leaks, the efficiency is not impaired so much as in a piston or plunger pump, all that is necessary being to run the pump at a high speed.

The principles of action of a pump may be understood from Fig. 3323, which represents a single acting plunger pump shown in section, and with the suction pipe in a tank of water, the pump being empty.

The surface of the water in the tank has the pressure of the atmosphere resting upon it, and as the pump is filled with air, the surface of the water within the pipe is also under atmospheric pressure.

Now suppose the plunger to move to the right, and as no more air can get into the pump, that already within it will expand, and will therefore become lighter, hence there will be less pressure on the surface of the water within the suction pipe than there is on the outside of it, and as a result the water will rise up the pipe, not because the plunger draws it, but because the air outside the pipe presses it up within the pipe.

The water inside the pipe will rise above that outside in proportion to the amount to which it is relieved of the pressure of the air, so that if the first outward stroke of the plunger reduces the pressure within the pump from 15 lbs. to 14 lbs. per square inch (15 lbs. per square inch being assumed to be its normal pressure), the water will be forced up the suction pipe to a distance of about 2-1/4 feet, because a column of water an inch square and 2-1/4 feet high is equal to 1 lb. in weight. In Fig. 3324 the pump plunger is shown to have moved enough to have permitted the water to rise above the suction valve, and it will continue to rise and enter the pump barrel as long as the plunger moves to the right.

When the plunger stops, the suction valve will fall back to its seat and enclose the water in the pump; but as soon as the plunger moves back to the left hand and enters the barrel pump further, the delivery valve will rise, and the plunger will expel from the pump a body of air or water equal in volume to the cubical contents of the plunger, or rather of that part of it that is within the barrel, and displaces water.

If the plunger was at the end of its first stroke to the right and the pump half filled with air, then this air will be expelled from the pump before any water is; whereas if the pump was filled with water, the latter only will be delivered.

Now suppose the first plunger stroke reduces the air pressure from 15 to 14 lbs., and that the second drawing stroke of the plunger reduces the air pressure in the pipe to 13 pounds per inch, the water will rise up it another 2-1/4 feet, and so on until such time as the rise of a column of water within the pipe is sufficient to be equal in weight to the pressure of the air upon the surface of the water without; hence it is only necessary to determine the height of a column of water that will weigh 15 lbs. per square inch of area at the base of the column to ascertain how far a suction pump will cause water to rise, and this is found by calculation or measurement to be a column nearly 34 feet high.

It is clear then, that however high the pump may be above the level of the water, the water cannot rise more than 34 feet up the suction pipe, even though all the air be excluded from it and a perfect vacuum formed, because the propelling force, that is, the atmospheric pressure, can only raise a column of water equal in weight to itself, and it is found in practice to be an unusually good pump that will lift water thirty feet.

Fig. 3325 shows the plunger making a delivery stroke, the suction valve being closed, and the delivery valve open where it will remain until the plunger stops.

To regulate the quantity of water the pump will deliver in cases where it is necessary to restrict its capacity, as in the case of maintaining a constant boiler feed without pumping too much water in the boiler, the height to which the suction valves can lift must be restricted, so as to limit the amount of water that can enter the pump at each drawing stroke.

The delivery valve should lift no more than necessary to give a free discharge without causing the valve to seat with a blow; but if the pump has a positive motion, the delivery valve must open wide enough to let the water out, or pressure enough may be got up in the pump to break it.

A check valve is merely a second delivery valve placed close to the boiler and serving to enable the pump to be taken apart if occasion should arise, without letting the water out of the boiler.

The lift and fall of both valves act to impair the capacity of the pump. Thus, while the suction valve is falling to its seat, the water already in the pump passes back into the suction pipe, and similarly, while the delivery valve is closing, the delivery water passes back.

A foot valve is virtually a second suction valve placed at the bottom or foot of the suction pipe.

The capacity of a pump is from 70 to 85 per cent. of the displacement of the plunger or piston, and varies with the speed at which the plunger or piston runs.

If a pump runs too fast, the water has not sufficient time to follow the piston or plunger, especially if the suction pipe has bends in it, as these bends increase the friction of the water against the bore of the pipe.

The speed of the piston or plunger should not exceed such as will require the water to pass through the suction pipe at a speed not greater than 500 feet per minute, and better results will be obtained at 350 feet per minute.

An air chamber placed above the suction pipe of any pump causes a better supply of water to the pump by holding a body of water close to it, and by making the supply of water up the suction pipe more uniform and continuous. Air chambers should be made as long in the neck as convenient, so that the water in passing through the pump barrel to the delivery pipe could not be forced up into the chamber, as, if such be the case, the air in the chamber is soon absorbed by the water.

Belt pumps are more economical than independent steam pumps, because the power they utilize is more nearly the equivalent of the power it takes to drive them, whereas in steam pumps there is a certain amount of steam, and therefore of power, expended in tripping the valves and in filling the clearance spaces in the cylinder. Furthermore, the main engine uses the steam expansively, whereas the steam pump does not.

Spotted a typo or a problem in this chapter? Tell us

XXXVI. BOILERS FOR STATIONARY STEAM ENGINES…

Cite this passage

MLA

APA

Chicago

XXXVI. BOILERS FOR STATIONARY STEAM ENGINES

XXXVII. THE STEAM ENGINE