Engineering practice and theory, for steam engineers

Front matter

T3 en

Chap, Copyright N<».

Shelf ^\A) I \

Dedication

To those owners of steam plants, Master Mechanics of factories, Chief Engineers in charge of steam machinery, and Mechanical Engineers engaged in designing and erecting plants, who have ever been ready and willing to offer substantial encouragement to me while employed in work that is for the purpose of shortening the distance between the office and the engine room, improving the condition of the steam engineer, and affording greater security to those who live and work in the vicinity of steam boilers, this book is respectfully dedicated by

The Author.

New Haven, Conn.

CHAPTER i

The steam engineer holds a responsible position. He must know more than just enough to start and stop his engine. What constitutes a ( 'practical engineer. ' ' The "theoretical engineer, ' ' and whyhe is so called. A combination of the two is desirable. Different kinds of steam and why they are so called. How water circulates in a boiler. The unit of measurement. Pounds pressure and what the term means. Molecules of water Heat a form of motion.

Chapter 2

Safe pressures for our boilers and how to calculate them. Strength of joints. Pitch of rivets and thickness of plates. Why braces are used. Pitch of braces. Hollow stay bolts and their uses. Water legs. Fusible plugs and where they should be located. Troublesome plugs and a heroic remedy. A comparison.

Chapter 3

Safety valves and their location. Rules. Distance from fulcrum to weight. What is the fulcrum. Why this rule is given. Proving the rule. Determining the weight to be put on the lever. Calculating the pressure at which a valve will lift.

CONTENTS.

Length of lever. Determining the proper area of safety valve. Lever and pop valves. Caring for safety valves.

Chapter 4

Heating surface of boilers and how to calculate it. Water tube boilers. What is the horse power of a boiler. Heating surface necessary. Latent, sensible and total heat. Proving the latent heat theory. How to condudl a test. Equivalent weight of wood and coal.

Chapter 5

Calculating the duty of a boiler. Conducting a calorimeter test, and determining the water in steam. Water evaporated per pound of coal, and per pound of combustible. Equivalent evaporation. A problem in proportion. Another rule to produce the same result. Percentage of moisture in steam. Its effect on the process. Accounting for the heat absorbed.

Chapter 6

Two standards. A comparison. Heat units required to evaporate one pound of water. Either standard may be used. Actual horse power of boilers. Actual and assumed conditions. Forcing boilers. Changing firemen often. Refusing to carry overpressure. A live engineer better than a dead hero. Penalty for carrying excessive pressure.

CONTENTS.

Chapter 7

Beginning of the day's work. Warming the cylinder. The air pump should be started. Operating the valve gear by hand. Starting a compound condensing engine. Water in the cylinder. The steam is admitted slowly. Calculating the power of engines. Single and double acting engines.

Chapter 8

The indicator a recording steam gage. The pressure and the time important factors. A defective diagram. The loop and what it shows. The crosshead and the indicator drum. The mean effective pressure. Dividing the diagram. Any accurate rule or scale may be used

Chapter 9

Too slow a process for this age. Planimeters and their use. Power of a double engine. The compound engine and how to calculate its power. All of the steam not used. Objedl in building compound engines. Expansion in a single cylinder. Two kinds of constants. Horse power constant of the low pressure cylinder of a compound engine.

Chapter 10

Estimating the power of a compound engine by using one cylinder. Proportion of the two pressures. Adding them together. An illustration. Volumes

CONTEXTS.

of the two cylinders. Ratio of expansion for each cylinder. Adding the clearance. Another rule for the combined ratio of expansion. Ratio directly from the diagrams.

CHAPTER ii.

Receivers. Great variation in sizes. Ratio of cylinders. One to seven. Converting a simple into a compound engine. When it will pay. Tandem and cross compound engines. Advantages and disadvantages. The steeple compound engine. Engineers have a choice of styles.

Chapter 12

Horse power of a triple expansion engine. Care must be exercised when taking diagrams. The load will change. One indicator may answer every purpose. Calculating the combined ratio of expansion from the diagram. Making the'calculation without using the intermediate cylinder. Four cylinder triple expansion engines.

Chapter 13

Quadruple expansion engines. Condensing and non-condensing. Total expansion rate. Several forms of condensers. Advantages and disadvantages. The hot well. Air and circulating pumps. Using condensing water several times over. Cooling towers. Great economy of water. Intelligent supervision needed.

CONTENTS.

Chapter 14

Weight of water. Temperature at maximum density. A pint and a pound. Foot pounds. Duty of a pumping engine. 100 pounds of coal and 1000 pounds of steam. Various heights of reservoirs. Pressure and height of a column of water. Weight and fri<5tion. Indicating the water cylinder. Improvements in pumping engines.

Chapter 15

When it is not necessary to weigh the coal. Time required to use iooo pounds of steam. Steam and water. Pounds of coal burned and pounds of water evaporated. The Corliss pumping engine. The George F. Blake engine. The Allis triple expansion pumping engine. The high duty attachment on the Worthington pumping engine. Philosophy of its operation. Result when the water main bursts. The engine at New Haven > Conn.

Chapter 16

The pressure that a pump will work against. Boiler feeders. Pressure required to run a pump. Diameter of water piston required. Diameter of steam piston. Four rules and their application. Piston, plunger and power pumps. Capacity of pumps. Size of pump required to raise a given quantity of water. Diameter of water pipe. Speed

CONTEXTS.

of water in pipes. Air-tight covering for suction pipes.

Chapter 17

Lifting and nonlifting injectors. Double tube, single tube, fixed and automatic injectors. Peculiarities of the different kinds. Effect of varying steam pressure. Internal parts must bear a certain relation to each other. Wear of tubes and accumulation of scale. Cleaning an injector. It must be properly connected. Cause of failure to force hot water. A high and a low lift. Taking water under pressure. Operating against a higher pressure than is carried on the boiler. Theory of the injector. Loss of heat. Calculating the velocity of steam. Discharge of steam through an orifice. Weight of steam and of water delivered to the boiler. Foot valves.

Chapter 18

Object sought in covering a steam pipe. Amount of steam condensed in a pipe. Square feet of surface and degrees difference of temperature. Amount of coal wasted. Porous coverings. Sensible and latent heat. Heat lost. Equivalent in horse power. Saving effected by using pipe covering.

Chapter 19

Heating with exhaust steam. Removing cylinder

CONTENTS.

oil and grease. Live steam heating. Varying amounts needed. Reducing valves. Back pressure on the piston and pressure in the system. Loss of heat in doing work. Power used. Steam used per minute. Heat units disappear in doing work. The indirect system of heating with forced blast.

Chapter 20

Reducing pressure by the engine and by the reducing valve. Temperature of steam after passing the reducing valve. Explanation of the phenomenon. Specific heat of steam. Condensation in the radiators. Open tanks. Return traps. Theory of their operation. Receivers and pumps. Heating the cold water. Steam users should appreciate a good engineer.

A NATURAL CONSEQUENCE.

When people visit the engine and boiler rooms of a mill, factory or electric station, they form opinions of the ability and general qualifications of the engineer in charge, and these opinions are based on the general appearance of the rooms and the machinery in them, therefore every engineer should care for his plant to the best of his ability, regardless of the compensation received, for this is the best way to secure a better situation.

When an engine revolves in the direction indicated by the arrow, it runs "over." This is theproper way for an engine to run.

i

:3

When an engine revolves in the direction indicated by the arrow, it runs "under." This plan should be avoided whenever it is possible to do so.

CHAPTER i. DUTIES OF THE STEAM ENGINEER

When we consider the great variety of conditions under which the steam engineer of the present time is called upon to pursue his labors, the different kinds of machines that he is expected to understand, and the weight of responsibility that rests upon him, the necessity of a thorough knowledge of the principles which govern the action of all his machines and appliances is at once recognized.

It is not enough for him to know how to start and stop his engine day after day, but he must understand its construction, and also have a complete knowledge of his boilers and every adjundl of his plant, not only that he may properly care for this important machinery, but that it may be operated in the most economical way, and when repairs become necessary they may be made in the best and least expensive manner.

To do all that is required along these lines calls foi a cool head, steady nerves, good judgment and that self-confidence which only comes through a knowledge of every detail of the work in hand.

l6 ENGINEERING PRACTICE

This constitutes what is very properly called a c 'practical engineer. n When a man thoroughly understands why it is that certain causes produce certain effects throughout the plant, it is proper to call him a "theoretical engineer," and a combination of the two makes a very desirable man to have in charge of a steam plant.

We frequently find ourselves unable to readily reply to questions, the answers to which we naturally think are familiar to us, but find it otherwise at times.

Every engineer knows what steam is, but not all can define it when the matter is referred to them. Steam is an elastic fluid resulting from the combination of heat with water. When steam is in contact with the- water from which it was generated, but without water held mechanically in suspension, its temperature corresponds to its pressure, and it is known as saturated or dry steam.

If it holds water in suspension it is called wet steam, but if subjected to more heat after being separated from the water that it was generated from, its temperature is then increased, without producing a corresponding increase of pressure, and it is then called superheated steam. In other words, superheated steam has a higher temperature than its pressure calls for, and in this condition it is nearly a perfect gas, so that it is sometimes called gaseous steam.

When we build a fire under a horizontal boiler the water directly above the fire is heated first, and as a natural consequence rises to the highest point possible. The space vacated by this water is at once filled by water rushing forward from the rear part of the boiler, and the space vacated by this water is in turn filled by the heated water that first arose to the surface. This process will be continued as long as heat is applied to the water.

In order that we may make necessary calculations it is advisable for us to have some standard unit for the measurement of heat, and this is known as the British Thermal Unit, and is often designated by its initial letters B. T. U.

It means the amount of heat required to raise the temperature of one pound of water at 39 Fah. one degree, or from 39 to 40 . It is assumed that the temperature of the water is 39 because at that point it has attained its maximum density. The Universal Heat Unit would be a more appropriate name.

After a great many heat units have been added to the water in the boiler, steam is generated, and as more heat is applied the pressure increases, the amount being indicated by the pointer of the steam gage. Here another problem is presented, for we may be asked to explain what it means when the pointer indicates 50 pounds pressure. We should i8 ENGINEERING PRACTICE

reply that it means 50 pounds to the square inch on the surface of the boiler, and the pounds are so much weight just as when tea and sugar are weighed. A column of water 1 inch square and 27.7 inches high at a temperature of 62 weighs one pound, and 50 pounds would be 50 x 27.7 = 1,385 inches, or 115 feet high. If we had suitable appliances for holding this in place, and allowing the steam pressure to act on the bottom of it, we could measure the pressure by observing the height, but this would be higher than some of the water gages on vertical boilers, and they are too high for convenience.

If a steam gage was connected at the base of this column it would indicate 50 pounds, and if the size of the column was changed the pointer would still indicate 50 pounds as long as the height remained the same, for the simple reason that the height indicates the pressure per square inch.

A molecule of water is the smallest particle of water that can exist, and when water is at 39 Fah. it is at its maximum density, because these molecules lie as closely together as possible.

If heat is applied until 212 at sea level is reached they continue to separate, and if more heat is then applied they are forced much farther apart, and the water is turned into steam. The pressure is light at first, but as more water is evaporated in a closed vessel the pressure increases, the molecules are

AND THEORY. I( j

forced closer together, and the steam becomes more dense. Heat is a form of motion, hence the greater the temperature the more rapid will be the motion of the molecules.

A TYPE OF BOILER THAT WILL NOT BECOME OBSO- LETE FOR MANY YEARS TO COME.

Chapter 2

SAFE WORKING PRESSURE. STAY BOLTS AND BRACES,

FUSIBLE PLUGS.

We do not wish to raise more pressure on our boiler than it will safely stand, hence must understand something of the rules that are given us for the purpose of determining the safe working pressure of any boiler. The following is a very good one :

Multiply one-fifth of the tensile strength of the iron or steel by the strength of the joint and by the thickness of the plate. Divide by the radius, or one-half the diameter of the boiler. The quotient will be the safe working pressure.

Suppose that the tensile strength is 50,000 pounds, the joint has .70 of the strength of the solid plate which is .5 inch thick and the diameter is 72 inches. Then (50,000 -r- 5) X .70 X .5 -f- (72 -*- 2) = 97 pounds safe working pressure.

The strength of the riveted joint may be found as follows :

From the pitch of the rivets subtract the diameter of rivet, and divide the remainder by the pitch of the rivets. Multiply the quotient by 100 and the produdl will be the strength of the plate at the joint. Multiply the area of one rivet by the number of rows of rivets, and by the shearing strength of rivets, which may be taken at 38,000 pounds. Divide the product by the pitch of rivets multiplied by the thickness of plate, and by the tensile strength of plate. Multiply the quotient by 100 and the product will be the strength of the rivets. In each case the result will be the per cent of the strength of the solid plate, and the lowest must be used in the calculation.

For illustration, suppose that the pitch is 2 inches, the rivets .75 inch in diameter, the area of which is .44 square inch, and the plate is .5 inch thick. The joint has two rows of rivets.

2 — .75 Then X 100 = 63 per cent for the strength

2 of the plate.

X 100 = 67 per cent for strength

2X.5X 50,000 of rivets.

In this case we should call the strength of the joint 63 per cent of the solid plate, as the lowest must be taken. To compute the area of rivet, square its diameter and multiply by .7854 the same as for the area of any circle.

Braces are put into a boiler to strengthen the flat surfaces, as they are weak parts. They must be located near enough to each other to prevent the plate between them from springing. The following is one of the U. S. Government rules for determining the proper distance from center to center of braces or stay bolts :

Multiply the area of cross section of brace or stay bolt by 6000. Divide by the steam pressure and extract the square root of the quotient. Applying this to the case of a head f inch thick, using braces i£ inch in diameter, where we wish to carry 100 pounds pressure gives the following, the area of the brace being 1 square inch :

1 X 6000 -5- 100 = 60 the square root of whicfr is 7.75 therefore the braces should be 7.75 inches from center to center. The only real difference between a brace and a stay bolt is that the former is longer than the latter.

Stay bolts are sometimes made hollow so that when corrosion has weakened them sufficiently to allow the water to enter the hollow part the appearance of it w r ill warn the engineer of the dangerous condition of them. When hollow stay bolts are put in above the grates of an internally fired boiler they admit air above the fire and promote combustion. As complete combustion prevents smoke they answer a double purpose.

The water leg of a boiler refers to that part of a locomotive, vertical or marine boiler in which an outer and an inner shell are parallel, or nearly so, making it necessary to conned! them with stay bolts. Some tubular boilers are made with water fronts extending down below the shells, forming water legs below the furnaces.

A fusible plug is a hollow isesrplug, which should be filled with pure tin, and in the case of a tubular boiler it had ought to be located in the rear head about 3 inches above the upper row of tubes, so that if the water level falls until the plug is uncovered the filling of tin will be melted out, and the escaping steam will give sufficient notice of the condition of affairs. In other forms of boilers it is located in some convenient place just above the lowest water level that can be permitted, so that it will melt out before the boiler is burned for lack of water.

The hollow part of the iron plug should be larger on the side that is exposed to the boiler pressure, so that there will be no danger of its being blown out before it melts. Care should be taken to keep the internal part of it free from scale and sediment, as otherwise it may not melt out as soon as the water level falls below it.

I have heard of an engineer who was troubled with fusible plugs, as he found it necessary to renew them at short intervals. Desiring to make a permanent job of it he drove in tapering iron plugs instead of putting in the tin filling. He succeeded in preventing further failure of the plugs, but his action was on a level with that of the man who removes the low water alarms from his boilers, or fixes the whistles on them so that they cannot sound an alarm, even if there is great need of one. Or it might be compared to the work of the man who fastened a piece of timber between his safety valve lever and the floor timbers above it, in order to prevent the annoying noise caused by the steam blowing into the room.

ADVICE BASED ON EXPERIENCE.

If you are an engineer in charge of a steam plant, and have decided to leave your present situation, give your employer due notice that you are going to leave, and recommend a suitable man to take the place. Dishonorable acts on his part in the past does not excuse you from this duty, as your reputation for honorable dealing is at stake.

AND THEORY.

STEAM

A VERTICAL TUBULAR BOILER WITH BRICK SETTING.

This type of boiler has no water leg to corrode, or stay bolts to break. The tubes are so arranged that a large steam space is secured, and it supplies dry steam.

Chapter 3. Safety Valve Rules

Every boiler should have a safety valve located on a separate outlet from the shell, and there should be no stop valve between it and the boiler. Where an engineer takes charge of a new plant he may wish to know whether the weights are properly adjusted on the safety valves or not, and to determine this the following rule applies :

Multiply the area of valve by the pressure to be carried, and by the distance from the valve to the fulcrum. Call this answer No. 1. Multiply weight of valve and stem by their distance from the fulcrum, and call this No. 2. Multiply one-half of length of lever by its weight, and call this No. 3. Add No. 2 and No. 3 together and subtracl the sum from No. 1. Divide the remainder by the weight of ball, and the quotient will be the distance from the fulcrum to the ball.

The fulcrum in this case is the bolt which passes through the end of lever, fastening it to the bonnet of the safety valve. I give this rule for two reas-

AND THEORY. 2J

ons. First, because it was found in a standard book of reference supposed to be corredl, and second, because I have proved it and found that it gives very good results.

It is not such a hard matter to prove some of these rules as might be supposed, for in this case all that was required was to improve the chance when one of my boilers was cool and empty to make some measurements and prove some weights. Taking the ball from the lever and weighing it I found that it balanced the scales at 76 pounds. The valve and its stem weighed 4 pounds, and the lever which was 34 inches long weighed 5.5 pounds. As the valve was 4 inches in diameter its area was 12.56 square inches, and the distance from stem to fulcrum is 2 inches. The pressure to be carried is 65 pounds. Applying the foregoing rule we have 12.56 X 65 X 2 = 1,632.8 for answer No. 1. 4X2=8 for answer No. 2 and (34 -f- 2) X 5.5 = 93.5 for answ r er No. 3. Then8 + 93.5 = 101.5 and 1,632.8 — 101.5 = 1,531.3 Dividing this by 76 shows that the ball should be set at 20.15 inches from the fulcrum.

Putting the valve together I placed the weight as near to this distance from the fulcrum as could be determined in the case of such a rough casting as we usually find a safety valve ball to be, got up steam and noted the effe<?t. When the steam gage pointer indicated 65 pounds the valve commenced to blow off steam.

In using the rule for determining the weight to be put on a safety valve lever the same process is employed as above described, except that in the latter part of it we divide by the distance from the fulcrum to the ball, and the quotient is the w T eight of the ball to be put on the lever. In this case it is I o3 I -3 -*■ 20.15 = 76 pounds.

The rule for determining the pressure at which a safety valve will open is as follows :

Multiply weight of ball by its distance from the fulcrum, and call it answer No. 1. Multiply weight of valve and stem bv their distance from the fulcrum, and call it answer No. 2. Multiply one-half of the weight of lever by its length, and call it answer Xo. 3. Add these three answers together and divide the sum by the area of the valve, multiplied by its distance from the fulcrum, and the quotient will be the pressure in pounds.

Applying this rule to the valve already mentioned gives the following result: 76 X 20.15 = 1,531.4 for No. 1. 4 X 2 = 8 for No. 2. (34 -£- 2)

When designing a lever for a safety valve of this boiler we take the maximum pressure that the boiler can safely withstand, and proceed as directed in the first rule in this chapter. In this case as we do not know what the length of the lever is, we will assume a length for it, and give it a trial. Taking the pressure at 100 pounds, and the extreme length of lever at 34 inches, we proceed as follows : 12.56 X 100 X 2 = 2,512 for No. i. 4 X 2 — 8 for No. 2. (34-^-2) X 5-5 = 93-5 for No. 3. 8 + 93.5= IOI -5 and 2,512 — 101.5 = 2,410.5. Dividing this by 76 shows that the ball should be 31.7 inches from the fulcrum for this pressure. As the lever must project beyond this, 34 inches will be right for it. It will be noted that the addition of 2 or 3 inches affects the result but very little.

It is a good plan to base the calculations for determining the necessary area for a safety valve on the grate surface, assuming a good natural draft. A lever valve should have one square inch area for each two square feet of surface. If a grate is 5 feet square, then 5X5-^-2 = 12.5 square inches. Referring to a table of areas of circles we find that this corresponds to a 4 inch circle, therefore the valve should be 4 inches in diameter.

If a pop valve is used, one square inch for each 3 square feet will answer, because this kind of valve is more efficient than a lever valve, and 5X5-^-3 = 8.3 square inches, which calls for a 3^ inch valve, or the next size larger, if this cannot be procured.

Safety valves should be kept free from dust and dirt, they should be tried every day to demonstrate that they will lift at the required pressure, and when found leaking they should be ground in without delay.

3°

This pop safety valve has no lever to be overloaded, and its nickel seat prevents it from sticking, thus giving it a clear title to the name, "safety valve. n The short lever shown is for the purpose of testing the valve by hand.

AN AXIOM.

When an engineer is notified that after a certain date his services will no longer be required, he should run his engine to the specified time, and leave everything in good order for his successor.

' mww %

Chapter 4

HEATING SURFACE OF BOILERS. CONDUCTING TESTS.

The heating surface of a steam boiler refers tc those portions of the shell and tubes which are exposed to the action of fire on one side, and covered with water on the other. If we take a horizontal tubular boiler 66 inches in diameter and 15 feet long, with 96 three inch tubes, we may calculate the heating surface in it as follows, assuming that one-half of the shell is exposed to the fire :

The circumference of a circle is found by multiplying its diameter by 3.1416 and 66 X 3.1416 = 207.34 inches, or 17.28 feet. Multiplying by the length we have 17.28 X 15 = 259 square feet, one-half of which is 129.5 square feet in the shell. There are 96 tubes each 15 feet long, making 1,440 feet in length. It requires 1.373 ^ eet ^ n length of 3 inch tubing to make one square foot of heating surface, therefore 1,440 -f- 1.373 = 1,049 square feet of heating surface in the tubes. There is but a small amount in the heads, and they may be omitted, for it cannot be called very effective heating surface. Then 129.5 + I >°49 = 1,178.5 square feet of heating surface in the boiler.

When calculating the heating* surface of other boilers, if the tubes are 3? inches in diameter, divide the total length by 1.17 and if they are 4 inches divide by 1.02 to obtain the square feet in them.

In the case of water tube boilers ascertain the length of all the tubes in inches, multiply by the circumference in inches, divide by 144 and the quotient will be the number of square feet of heating surface.

Or multiply the length of one tube in feet by the number of tubes in the boiler, and in the case of 4 inch tubes, which are very commonly used, divide by -955 The quotient will be the number of square feet of heating surface.

Where tubes of other sizes are in use their surface may be determined by the following table, taking the inside surface for fire tube, and the outside for water tube boilers :

EXTERNAL DIAMETER. INSIDE SURFACE. OUTSIDE SURFACE.

I

4.46

3.82

1.25

3-45

3.06

2.86

2-55

2-45

2.18

2.ir

1. 91

2.25

1.85

2-5

1.67

1.27

3-5

1. 17

1.09

1.02

•95

The horse power of a boiler is its power to evaporate a given amount of water per hour under stated conditions, which are explained in chapter 6.

No inflexible rule for the amount of heating surface required per horse power can be given, but the following will be found convenient in making estimates where the amount of water evaporated is not known :

Tubular boilers,

15 square feet

Locomotive "

1 5

» rr

Vertical rr rr

Flue rr //

Cylinder

Water tube ''

n rr

The latent heat of steam is the heat that is not indicated by the thermometer, and the sensible heat of steam is that which is indicated by the thermometer. When reference is made to the total heat of steam we mean the latent and sensible heat added together.