The steam engine indicator and its appliances. Being a comprehensive treatise for the use of constructing, erecting and operating engineers, superintendents, master mechanics, and students, describing in a clear and concise manner the practical application and use of the steam engine indicator ...

Front matter

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Dedication

To the young Engineers of America, who by thoughtful and careful study ; are seeking to make their opportunities and overcome obstacles in the care and management of the Steam Engine; this volume is respectfully inscribed by

The Author.

Preface

The preparation of this book has occupied most of the author's spare time for a number of years. Originally the matter was not intended for publication, but the manuscript has grown so large and complete, which consideration, combined with many repeated requests, has induced the writer to publish the matter in book form.

Let every Engineer make his own Indicator book as he proceeds in his study and practice, and it will prove invaluable in after years. The present work has been compiled in this way, from data continually obtained during the author's professional career, extending over a third of a century.

The introduction of algebraical for mules have been avoided. These are readily found in the many valuable mechanical Pocket- Books. The writer has endeavored to discuss the principle and use of the Indicator in as plain common sense words as the subject and the English language will admit of.

Special attention has been given to the requirements of the young progressive student in Steam Engineering. The preparation of the following chapters has been a work of pleasure to the author, and if they prove beneficial to his fellow- workmen, he will be amply repaid.

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CHAPTER.

Brief History of the Indicator, - I.

Purpose of the Indicator, - II.

Definition of Technical Terms, ----- III.

Construction of the Steam Engine Indicator, - IV.

Indicator Appliances, '----- = V.

Indicator Appliances, Continued, - VI.

Indicator Reducing Motion, - - VII.

Drum Stop and Electrical Attachment, - VIII.

Care and Use of Indicator, - IX.

To Take Diagrams, ------- X.

Indicator Diagrams, .-.._. XL

Study of Diagrams, ------- XII.

Lines and Points of the Diagrams, - XIII.

Isothermal Curve, ------- XIV.

Adiabatic Curve and Point of Cut-off, - XV.

The Foot Pound and Measurement of Diagram, - - XVI.

Expansion of Steam, ______ XVII.

Hyperbolic Logarithms- ------ XVIII.

Theory of Action of Steam Expansion in Cylinders, . . XIX

Reading the Diagrams, _--___ XX.

Different Methods of Computing Water Consumption, - XXI.

Indicator Testing Device, ------ XXII.

Planimeters, .----._ XXIII.

Comparison Diagrams from Throttling and Cut-off Engines, XXIV.

The Economy of Expansion, - XXV.

The Point of Cut-off, ------ XXVI.

Back Pressure and Compression, ----- XXVII.

Combining the Diagrams from Compound Engines, - - XXVIII.

Diagrams From Gas and Oil Engines and Ammonia Compressors, XXIX.

Making Calorimeter Tests, - XXX.

Miscellaneous Diagrams, - - - - - XXXI.

Engine Economy, .---••. _ XXXII.

Chapter I. Brief History Of The Indicator

The idea embodied in this important and instructive little instrument, was originated by the celebrated James Watts during the latter part of the last century, at a very early period in the history of the steam engine, and it has since then in its improved forms, materially contributed to the perfection and efficiency of our modern steam engines , not only by enabling the engineer to ascertain the exact values of the forces from which the power is derived, but also by pointing out the precise periods, in relation to the different parts of the stroke, at which these elements of power come into action. The original machine of Watts, consisted simply of a cylinder, about six inches long and one inch in diameter, in which there was a closely fitted piston; and was attached to the engine cylinder by means of a suitable pipe, fitted with a valve to open or close communication between them. A long open coiled spring was used of which one end was fastened to the piston, and the other to the cover of the indicator cylinder; this spring resisted the pressure of the steam, in one direction, and also the pressure of the atmosphere in the other.

12 Steam Engine htdicator

A pointer was connected to the piston, and moved directly with it and served to locate the atmospheric line, or zero ; and all motion of the pointer was above or below that point. There was no paper drum ; the pointer merely indicating on a scale, the highest and lowest pressure in the engine cylinder, measured from the atmospheric line. An improvement on this instrument was made by adding a flat slide to which a sheet of paper was secured and giving it a coincident motion, on a reduced scale, with the engine piston, by attaching a cord from it to the crosshead or some other moving part of the engine, and returning the same by means of a counterweight. The machine in this improved form, though crude in comparison and less compact in construction, was almost identical in principle of its operation, to the many different instruments now so extensively in use, and it enabled him to ascertain the exact mean effective steam pressure throughout the stroke, and also the proportion which the vacuum in the cylinder, at different parts of the stroke, bore to that in the condenser, in order to determine the dimensions of cylinder required for any given power, as also the relative proportion proper to be given to the steam and exhaust ports, of the slow speed engines of his experiments.

Having attained the objects, he left for succeeding engineers to devise, improve, and put into such a compact and portable form, as to be easily applicable to steam engines of every description ; to such an extent as we find the modern indicator of to-day.

One of the first to improve on the instrument of Watts, and nearly one-half a century later, was Wm. Macnaught of Glasgow, who constructed an instrument represented at one-quarter actual size in elevation, Fig. i, and in plan, Fig. 2.

Unlike the indicators of to-day, that have a parallel and multiplied movement of the pencil as compared with the piston ; in this instrument the movement of both are coincident,

And Its Appliances.

that is, whatever motion is imparted to the piston by the steam, the pencil moves precisely the same distance either way. As constructed in this way, the spiral spring opposing the force of the steam against the piston, is liable to disarrangement, and unevenness ; on account of the greater length of spring necessary to obtain cards of a convenient height for computation.

Its consequent general adoption has led to many very important, and decided improvements in the construction of the instrument, and very materially aided in elevating the standard of duty of steam engines, and also demonstrated the economy resulting from a liberal use of the expansive power of steam. From a glance at Figure 1 , it will be perceived that the Indicator is neither more nor less than a small single acting steam engine with the addition of a spiral spring on the opposite side of the steam piston, to resist the force of the steam.

The many treatises at the present day on the steam engine indicator, leaves little to be said in reference to the actual manipulation of the instrument in practice : and we now find almost all of the Stationary, Locomotive, Marine, and other engineers imbued with the necessity of understanding the working of the indicator in all its details, not only in the interest of their employers, but particularly for their own personal benefit, in acquiring a knowledge of the intricate working of steam, or other operating forces, of their engines, in all their various details, and which becomes necessary to enable them to reach a place in the higher ranks of engineering science.

Fig. 2.

The principal parts of an indicator, of the most complete construction, consists of an outside cylinder, or body, inside of which is secured an inner or working cylinder ; with a nicely fitting piston, of exactly one-half of a square inch in area, equal to about .7978 of an inch in diameter, working therein.

The piston is so closely fitted to the working cylinder of the indicator, that at the pressures of from two to three hundred pounds per square inch, there is but a very small amount of steam that can pass by or through it ; but all indicators are subject to a certain amount of leakage of the piston, and for which ample means are always provided for its escape ; in order to avoid any unnecessary back pressure on the side of the piston opposite the steam pressure.

Where the indicator is to be used for obtaining cards, from pressures ranging from four to six hundred pounds per square inch, it is advisable to use a reduced size of working cylinder and piston ; the area of which, shall only be one-fourth of a square inch; equal to about .5641 of an inch in diameter, thus avoiding the use of indicator springs of a very high tension. The different sizes of the working cylinders, and their pistons, are made interchangeable in the indicator, so that either can be removed at any time, and the other substituted.

One end of the piston rod is connected to the piston, by a ball connected joint, and the opposite end to some part of a pencil mechanism, for producing a so-called parallel, or straight line movement of the pencil, up and down, in reference to the paper drum.

„ The parallel motion is another beautiful and ingenious invention of Watts, as he applied it to steam engines, for the purpose of guiding the piston rod, back and forth in a straight line ; and without the intervention of any guides, or other means, for the purpose; thereby eliminating friction to a great extent ; with smooth working, and providing a mechanism that has since been used in various modified forms, and adapted, to a certain extent, to different kinds of machinery where a motion of this principle is desired.

The parallel motion, as applied to his engines, was considered by Watts, notwithstanding his other inventions, to be the one which gave him the greatest satisfaction and pride, as it performed its mission with the best practical results ; even though not absolutely mathematically correct in some respects. In most of our modern indicators this principle of mechanism, for their pencil movement has been almost universally adopted in its various and possible modified forms, and constant efforts in this direction are being made, with a view of eliminating the slight inaccuracies that are known to exist in the original parallel motion ; particularly in the constant varying ratio of the movement, that exists between the piston and the extreme end of the system or the point at which the pencil is located.

The parallel or straight line movement of the pencil is usually produced by a system of levers and links of definite lengths, and pivoted in such a manner that by any movement of the system, up or down, the end of the lever carrying the pencil is always expected to move in a straight line, and also in some exact ratio to the movement of the indicator piston.

Chapter II Purpose Of The Indicator

The Steam Engine Indicator is an instrument originally designed for recording the varying pressure of steam in an engine cylinder ; at any, and all points during the revolution of the engine, and has subsequently been applied under many, and various other circumstances, wherever the record, and measure of an irregular pressure has been desired.

The production of this record is the result of two motions, and is traced by the indicator pencil upon paper that is secured to a light cylinder, called the paper drum.

To this drum is imparted a rotary oscillating motion upon its axis, at right angles to the motion of the pencil ; such motion being derived from the cross head, or any part of the engine having a movement coincident with it. •

The motion of the pencil is produced by the varying pressures of steam acting against the indicator piston ; in opposition to the strength of a spring of known tension. Consequently, an indicator properly attached too, and communicating with the interior of the cylinder of a steam engine in operation, and the drum given a motion (on a reduced scale) corresponding to that of the engine piston, will, (on bringing the pencil in contact with the paper upon the drum, during its oscillation), trace the outline of an irregular figure upon the paper, which is usually known as, and called an Indicator Diagram,

And Its Appliances.

an example of which is represented in Fig. 3, CD. E. F. B. B. and shows the varying pressure acting against one side only of

Fig. 3. the piston, during a revolution of the engine; and such pressures can be properly located, and correctly measured.

The upper line C. D. E. F. of the diagram represents the force of the steam impelling the piston during its forward stroke ; while the back pressure line B. B. shows its retardation on the return stroke ; the average height of each being measured, (by the scale of the spring), from the atmospheric line A. A.

The difference between their average height, represents the Mean Effective Pressure, usually designated the M. E. P. of the diagram.

The atmospheric line A. A, is also drawn by the indicator but at a time when steam communication between it, and the engine cylinder is closed; and consequently subjecting both sides of the Indicator Piston to atmospheric pressure only.

To show the pressure on the other side of the engine piston, another diagram must be taken from- the opposite end of the cylinder.

In most calculations of the diagrams, it becomes desirable to draw, by hand, additional lines, from and by which, the actual lines may be compared.

First, the straight line V. V. is drawn to represent the absolute vacuum, or absence of all pressure.

Second, the line V. O. represents the clearance volume, and is drawn at right angles, (or perpendicular), to the atmospheric line.

Third, the line O. O. is drawn to show the full boiler pressure in order that it may be seen on the diagram how near that pressure has been realized.

The different lines, and events of the stroke, as shown by the diagram during a revolution of the engine, and the name by which they are designated will be noted, and explained in a later chapter.

Diagrams taken from different engines, and under varying conditions, will present themselves in an almost endless variety of forms, depending entirely upon circumstances connected with the operation.

The earlier forms of the instrument, (although the same in principle), were crudely constructed, the moving parts exceedingly heavy ; inducing vibrations, sufficient to vitiate, and distort the diagrams ; also causing more or less tardiness at the different events of the stroke, such as admission of steam, point of cut-off, and also compression ; because of the heavy parts being unable to respond so promptly to change of pressure ; thereby making them unreliable, and imperfect in their action, to an extent depending upon the weight of the pencil mechanism, rapidity of rotative speed, and also suddenness of change of pressure ; and therefore preventing their being successfully adaptable, only to engines of very low rotative speeds.

A nd Its Applia?ices. 1 9

The prevalence of many high speed engines in use at the present time, renders the use of the old type of indicators almost useless, and wholly unsuited for high speed, where any reliable results are expected, as many details which gave little trouble at low speed, seriously affect the results at the higher speeds.

The present improved construction of some of the various modern indicators, in which perfection is attained as near as can be expected, consists principally in superior designs, simplified construction, a better adaptation of the pencil mechanism, also finer adjustment, convenience of manipulation, and especially extreme lightness of the moving parts, thereby practically eliminating the effects of inertia, and momentum of these parts.

These qualities in any Indicator are absolutely indispensable in order to secure accurate and reliable results from high speed engines.

The information that may be derived from a careful and attentive application of the Indicator to engines of all descriptions, is almost incalculable to the engineer ; because many facts are accurately determined by its use, that cannot be obtained in any other way, with any great degree of satisfaction or correctness ; consequently its use has enabled the engineer to discover many unforseen, and necessarily unknown defects existing in the engine, that have formerly been veiled in mystery, and at the present time, its value is so universally recognized, and relied upon, that most manufacturers of high grade engines, make provision for its application ; and do not consider their engines complete until the valves have been correctly adjusted by the use, and assistance of the indicator, and set in such manner, as that the maximum efficiency of the engine shall have been attained. •

Chapter III. Definitions Of Technical Terms

The many terms, generally used in connection with the study of steam engineering, are all measurable ; each with reference to some established unit, and by which they are clearly defined, and their correctness recognized.

Some of these are indispensable to the engineer, a few of which are briefly described, and explained as follows :

The Unit of Work is equal in amount to the power required to lift one pound, one foot high, and is called the Foot Pound.

The Unit of Heat, or Thermal unit, is the quantity of heat necessary to raise the temperature of one pound of water, one degree, or from 39 to 40 Fah.

Also, one unit of heat is equal to 772 foot pounds or units of work.

The Sensible Heat of any body, as Air, Water, or Steam, is the heat that is sensible to our touch, and in extent is as shown, and measured by the thermometer.

Latent Heat of steam is the quantity of heat, (expressed in heat units), required to vaporize water, that has previously been heated to a temperature equal to the resulting steam, of vaporization.

Specific Heat is the quantity of heat measured in thermal units, necessary to raise one unit of weight of the substance, through one degree of temperature.

Saturated, or Dry Steam, is steam confined under pressure in contact with, the water from which it is formed, and contains just sufficient heat to maintain the water in a state of steam, and will vary in pressure, and density corresponding to varying temperatures.

When saturated steam suffers any loss of heat, a condensation of some of the steam also takes place.

Superheated Steam is steam containing an excess of heat.

If to saturated steam more heat be added, its temperature will increase, and the steam is said to be superheated, because its temperature will be greater than that corresponding to saturated steam of the same pressure. This excess may be parted with, without condensation.

Horse Power (H. P.) is the standard used for measuring the power of a steam engine, and is equal to lifting 33,000 pounds one foot high in one minute of time, or 550 pounds in one second, or any equivalent thereof in opposition to the force of gravity.

Indicated Horse Power, (I. H. P.) is the horse power of an engine as computed from the Indicator Diagram.

If the mean area of the piston in square inches be multiplied by the mean effective pressure in pounds per square inch exerted against it, and also by its speed in feet per minute ; this product on being divided by 33,000, will be the indicated horse power of the engine.

Net Horse Power is the indicated horse power of an engine less the horse power which is consumed in overcoming its own friction.

Boiler Pressure is the pressure above atmosphere, or the pressure as shown by a correct steam gauge.

Initial Pressure is the pressure in the cylinder acting against the piston, at or near the commencement of the stroke of the engine.

Absolute Pressure is the pressure of the steam calculated from absolute vacuum.

It is the pressure as shown by a steam gauge, with the pressure or weight of the atmosphere added thereto.

Terminal Pressure is the pressure above the line of absolute vacuum that would exist in an engine cylinder, provided the release of the steam had not taken place until the end of the stroke had been reached.

Usually the release happens earlier, and in such case its position may be located by continuing the expansion curve by hand from any point of release, to the end of the diagram.

The terminal pressure must always be measured from the vacuum line ; consequently it is the absolute terminal pressure.

Mean Effective Pressure (M. E. P.) is the average of all the varying pressures at different parts of the stroke, exerted against the engine piston to impel it forward ; less all the pressure that acts on the opposite side of the piston to retard its progress.

Back Pressure Line, in a non-condensing engine, represents the loss that occurs, from the retardation of the escaping steam ; due to a'tmospheric or other pressure

It is indicated on the diagram by its height above the atmospheric line, and is expressed in pounds per square inch.

On the diagram of a condensing engine it is indicated by its height above a line drawn by hand to represent the absolute or perfect vacuum.

Total Back Pressure is represented on the diagram by its height above the line of Absolute Vacuum.

Initial Expansion is the fall of pressure along the steam line; which often happens in an engine cylinder, between initial pressure, and the point at cut-off.

Compression is a result caused by the action of the piston in compressing into the clearance space, all steam remaining in the cylinder after the exhaust valve closes.

Clearance is all of the space or waste room between the piston at the end of its stroke, and the face of the valve.

Its volume or amonnt is usually expressed in its per centage of the piston displacement.

Piston Displacement is the distance passed over by the piston in traversing a single stroke.

Its volume is equal to the area of the piston in square inches, multiplied by the length of stroke, in inches, the product is the volume of displacement in cubic inches.

Wire Drawing is a term sometimes applied to the action of steam, and arises in consequence of very restricted steam passages, also dilatory valve motion, thereby reducing more or less, the pressure in the engine cylinders, and usually considered to be a loss in the matter of economy.

Valve Lap is the excess of length of the valve at each end ; (when at the middle of its stroke) over the extreme outer edge of the steam ports ; and is designed to serve as a cut-off valve within certain limits.

Valve Lead is the amount of opening of the steam port, (which is regulated by the valve) for the admission of steam to the cylinder, just as the piston arrives at the corresponding extreme end of the stroke ; the entering steam thereby serving as a cushion for the reciprocating parts of the engine.

Water or Steam Consumption is the amount of steam accounted for by the indicator, per horse power per hour ; and is a measure of the economy of the engine.

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Chapter IV. Construction Of The Indicator

The principal difference in the construction of the various indicators on the market to-day lies in the pencil movement mechanism. The main objects sought in the design of this

mechanism are as follows : ist, the nearest possible approach to straight line vertical movement of pencil parallel to the axis of the steam cylinder. 2nd, constant ratio of movement, pencil to piston. 3rd, lightness of moving parts, thereby reducing the momentum of these parts to a minimum. 4th, accessibility of the parts and convenience in handling same to take diagrams.

For our intent and purpose, it is not necessary to describe more than one of the various instruments in the market to-day, and we have in preference to all others, selected the weil known improved new style Tabor Indicator, as shown in Figs. 4, 5 and 6 as being the instrument more nearly fulfilling all the con-

Fig. 5.

And Its Appliances.

ditions and requirements named ; and which are absolutely necessary for the production of the most accurate results in Indicator practice, including lightness of moving parts ; thereby reducing momentum ; and also for ease and convenience to the operator in handling the instrument while taking diagrams. In order that the particular advantages of this instrument may be thoroughly understood, it is necessary that a

brief description of its construction, and arrangement of parts, should be given here.

The principal and most important peculiarity of this instrument from all others, lies in the method employed to communicate a straight line motion to the pencil, and at the same time of producing an exactly equal ratio of movement between the piston of the Indicator and the pencil ; both of which are perfectly accomplished by means of a steel plate, through which there is a slot (as shown, full size, in Fig. 7), of such contour, or shape, as to exactly counteract the tendency to a radial movement of the pencil bar ; this slotted plate is attached in an upright position on the swivel plate of the instrument, and upon which the whole pencil mechanism is self-contained. The swivel plate is in turn secured to the cylinder cover of the instrument ; said cover serving as a center, upon which the entire pencil movement can be revolved in either direction, until it comes in contact with the paper drum. In order to make this slot in the plate available for the purpose, a small stud is secured on the pencil bar, upon which there is mounted a hardened steel roller, fitted so as to travel in the slot from one extreme to the other ; and in doing this,

Fig. 6.

the pencil is caused to move in a straight line up and down the paper drum, in an exact ratio, throughout any and all parts of the movement, of exactly five times the distance moved by the indicator piston.

The radial slot in the plate is an irregular curve and deviates slightly from a true circle ; the irregularities existing in the curve, just compensating for the error that would occur in case a radial link were substituted, and corresponding in length to the average radius of the slot ; where one end of said link is pivoted to a stationary point and the other to some part of the pencil mechanism ; a method now employed in a number of the well known indicators at the present time, and the results are, that the lines made by the pencil, which are supposed to be straight, are not so, nor are the ratios of piston and pencil equal.

A radial link, the end of which always moves in a true circle, is therefore not a successful substitute for accuracy, as compared with the irregular curve in the slot plate of the Tabor Indicator, as said slot which guides and controls every movement of the pencil being so formed as to give the most accurate results, in . Fig. 7. every position the pencil bar may assume.

There is another plate of exactly the same outline, as the slot plate (but without slot) and secured coincident with it ; the roll stud in the pencil bar projecting far enough through it to make contact with said plate, and serves to receive any pressure that may occur when the pencil is brought in contact with the paper drum in the act of taking diagrams ; and it also prevents the torsional strain that would otherwise come on the center and back links, thereby reducing the friction from this cause to a minimum.

And Its Appliances.

These plates are provided with a projecting part on their lower ends, which are drilled and tapped to provide for a screw passing through, to regulate the pressure of the pencil upon the paper drum, said screw coming in contact with a standard secured to the body of the instrument. A small upright projection on the swivel plate, serves as a fulcrum for the lower end of the back link, the upper end being connected to one end of the pencil bar.

The back and center links are made of a fine grade of steel as also the pencil bar, which is hardened and carefully drawn to a spring temper, highly polished and all given a blue finish. The piston rod is of steel and made hollow in order to reduce weight, its upper end is connected to the center link, and the lower is made solid and terminates in a ball ; this ball is provided with a universal cap and socket and to which, in turn, the indicator piston is attached by means of a thumb nut.

The piston rod is formed of three pieces ; the body, shank and ball ; and upon the shank there is formed a collar as shown at A, Fig. 8, full size, a little larger in diameter than the body, and serves as a safety stop for the pencil movement, by coming in contact with the under side of the cylinder cover just before the pencil bar reaches its extreme height, thereby preventing any further movement of the piston rod, which would be Yig. 8. likely to result in injury to the pencil movement in case of breakage of a spring when in use, or by a mistake of using too light a spring for the steam pressure.

The piston is made very light, of a hard bronze metal, truly turned, grooved on its periphery, to act as a water packing, and lapped perfectly round and straight on its face to an exact size.