Steam turbines; a practical work on the development, advantages, and disadvantages of the steam turbine; the design, selection, operation, and maintenance of steam turbine and turbo-generator plants

Front matter

Class T J 7 Z t

Bonk • L; ^

Copyright N° _ia_L£.

Introduction

'"THE steam turbine is not only one of the most ancient forms * of prime-mover, but is also one of the most recent engineering developments, and perhaps the most talked-of at the present time. Remarkable advancements in its development have been made during the past ten years, and this has given a great incentive to steam engineering, while the electrical engineer has been stim¬ ulated to work out new ideas in designing generators suitable for the very high speeds of rotation. The steam turbine has forged rapidly to the fiont, and in spite of the early and serious handicaps, in the way of steam economy, has taken its place beside the best reciprocating engines of the present day. The turbine possesses many advantages over the reciprocating engine, and its field of greatest influence is likely to find in the immediate future a more serious competitor in the gas engine than in the reciprocating engine.

<51 With the relative merits or shortcomings of the steam turbine and the steam engine as prime-movers, the pages following are concerned to a certain extent, for modern engineering has not yet defined with sufficient clearness the respective fields of these agents, into either of wdiich the other agent may enter w r ith a certain degree of success. The rivalry between these two forms of prime-mover has, however, given place to the mutual recognition of the merits of both, and cordial co-operation in certain classes of work for which neither alone is fully adapted; as, for instance, the use of the low-pressure steam turbine with the reciprocating engine, in which a higher economy is effected than that obtained by the use of either alone.

<| Steam turbine developments of recent years have been in the direction of higher economies in the production of power from steam, and while the ultimate limit has not yet been reached along this line, still the steam turbines of today are highly efficient machines, measuring up to the best energy economy.

<fl The pages wdiich follow have been written for the practical man, for the man less interested in the finer points of theory than in the results accomplished and the way they are secured by the most successful builders of the steam turbine. Nevertheless, the fundamental principles have been brought out in sufficient detail for intelligent mechanical design. Between the designs of dif¬ ferent builders, little attempt has been made to draw comparisons; but the facts have been stated with the idea that the reader may form his own conclusions on points where a difference of opinion may arise.

T~T

TWO 1875-K.V.A. THREE-PHASE 11,000-VOLT LOW PRESSURE UNITS. INSTALLED FOR THE DUQUESNE LIGHT

COMPANY AT RANKIN, PENNSYLVANIA

Courtesy of Westinghouse Machine Company

Contents

History

Hero type.

Watt's patent.

Part I

Introduction. The steam turbine is one of the most recent engineering developments, and perhaps the most talked of, at the present time. During the past ten years the most marked improve¬ ments in its development have been made, and this has given a great impetus to engineering, especially steam engineering, although the very high speeds of rotation have driven the electrical engineer to work out new ideas in designing generators suitable for these higher speeds. The turbine has forged rapidly to the front, and, in spite of an early and serious handicap in the way of steam economy, has taken its place beside the best reciprocating engines of the present time. Many claim it to be superior in the matter of steam economy, but this will be discussed more fully later on. The turbine evidently possesses many advantages over the reciprocating engine, and, in its field of greatest usefulness, is likely to find in the near future a more severe competitor in the gas engine than in the reciprocating engine. For some classes of work, the steam turbine in its present state of development is entirely unadapted.

The steam turbine consists essentially of nozzles or guide pas¬ sages which direct the steam onto vanes or buckets attached to the periphery of rotating wheels, the essential elements of which are shown in Fig. 1. The simplest form of turbine is perhaps one of the type in which a jet of steam impinges upon the buckets of a wheel, in much the same manner that a stream of water impinges upon the buckets of a Pelton water-wheel; there is, in fact, a great similarity between water turbines and steam turbines. The under¬ lying principles are the same in either case, but the application of those principles is different. Steam flowing through a properly designed nozzle, with 150 pounds boiler pressure on one side, and the usual turbine vacuum on the other, will attain a velocity of about 4,000 feet per second, or about twice the muzzle velocity of a rifle ball. Water, to attain this enormous velocity, would have to flow from a head of about 234,000 feet. When this is compared with the ordinary water head of 150 feet or less, or even with such an ex¬ ceptionally high head as 3,000 feet, which is sometimes met with in water powers on the Pacific Slope, a glimpse will be had of the magnitude of the problem confronting the steam turbine engineer. To put this in other words, the steam turbine designer has to deal with a velocity equivalent to that produced by a head of water nearly

Fig. 1. Elements of DeLaval Turbine.

1,500 times as high as Niagara Falls. It will at once be seen, then, that the velocity of rotation of a simple turbine wheel to attain the best efficiency must be enormous. If this total head is to be used in one wheel, the peripheral speed must be nearly 2,000 feet per second, and at such speeds, the centrifugal force is so great that it is no easy matter to design a wheel that will not burst, even were there available some material stronger than any we now know. As it is, about 1,200 feet per second is considered the practical limit of peripheral velocity for a wheel built of the best nickel steel.

A little mathematical calculation will show that wheels of five feet in diameter will revolve 4,600 times per minute to attain a velocity of even 1,200 feet per second at the periphery. It is the problem of the steam turbine designer to reduce these speeds to more manage¬ able rates without at the same time making too great a sacrifice of efficiency.

From a thermodynamic standpoint, the turbine and reciprocating engine are not unlike, but the force of the steam acts differently in them. In both, it is the heat energy of the steam that does the work. In the one, the steam slowly expands, exerting pressure on a piston; in the other, it expands in narrow passages, pushing the particles ahead faster and faster and thus obtaining velocity which is then imparted to the vanes of a rotating wheel. In the one, the steam acts by virtue of statical pressure; in the other, by virtue of its high velocity. In either case, it is the internal heat of the steam that causes the expansion and does the work. If heat is lost in any way, by condensation, radiation, etc., the work will be proportionally less. In the turbine, a difference in pressure from inlet to outlet acts as a motive force indirectly, and then only in so far as it causes a rapid flow of steam.

Advantages. The well-known expression, work = force X space , embodies the idea that a given amount of work may be accomplished in a certain time by increasing the total force of the steam on the piston at the expense of the number of revolutions of the fly-wheel per minute, or vice versd. For example, a Corliss engine running at 180 revolutions per minute requires a mighty effort behind the piston to develop 1,000 horse-power, and this tre¬ mendous force demands a large cylinder, a heavy frame, and an immense fly-wheel. If, now, an engine were built to run at 800 revolutions per minute, much less push behind the piston would be necessary to develop the same horse-power, and, therefore, the parts could all be made smaller, and the whole weight very much reduced.

To go a step further, and consider the steam turbine, which must run at 2,000 to 3,000 revolutions per minute, it is clearly seen that this enormous speed reduces the mass of the parts even more. The heavy fly-wheel is no longer necessary, as the rotating parts are moving at a sufficiently high speed to ^acquire an immense inertia, and there is always a constant effort exerted on the vanes by the team, thereby producing an absolutely steady turning moment. Furthermore, the motive parts of the turbine revolve, which is in direct contrast to the reciprocating engine, in which the piston is moving backward and forward, and the turning moment is continu¬ ally changing from a maximum to a minimum. It is clear, therefore, that for a given horse-power, the steam turbines produce smaller machines, lighter foundations, and consequently smaller power houses. A fair idea of the relative space occupied may be gained from Fig. 2.

Again, the generator, on account of this high speed, will be smaller and less expensive. The turbine requires oil in its bearings

Fig. 2. Comparative Sizes of 5000-K. W. Corliss Engine and Generator and Curtis

Turbo-Generator of Same Power.

only; hence there is no oil to go over in the condensed steam, and the condensation may be used for boiler feed without any danger of carrying oil into the boilers. The turbine requires somewhat less attendance than the reciprocating engine, and the whole machine is compact and simple. To do its best, the turbine requires a higher vacuum than is ordinarily obtained for the reciprocating engine, and hence needs very much larger condensers, more cooling water, and additional air-pump capacity All this in a measure offsets some of its advantages, and frequently more trouble arises from the air pumps and condensers than from the turbine itself. The turbine may of course operate at the usual vacuum with a somewhat greater steam consumption and a slightly lower efficiency.

The reciprocating engine has its own advantages, and in certain classes of work will doubtless hold its own, but for all such apparatus as blowers, centrifugal pumps, generators, etc., which may be direct connected to a turbine, the reciprocating engine is rapidly becoming a thing of the past, and even for factories where belt drives are used, the steam turbine has been suggested.

History. The steam turbine is not only one of the most recent engineering developments, but is, at the same time, perhaps, one of the most ancient forms of prime mover. In a book written by Hero of Alexan¬ dria, over 100 years before the beginning of the Chris¬ tian era, a very simple form of steam turbine is described.

It consisted of a hollow sphere mounted on hollow trunions, through which the steam into the sphere. On opposite sides of the sphere were outlets consisting of pipes bent at right angles in lines tangent to the equator of the sphere, in such a manner that the reaction of steam escaping through these pipes caused the sphere to revolve on its trunions, in much the same way that the water escaping from the arms of a lawn sprinkler causes it to revolve. This turbine, which is illus¬ trated in Fig. 3, is the simplest form of the pure reaction motor.

In 1629, Branca, an Italian, invented a turbine much like a miniature water wheel, which was driven by a jet of steam fiom a nozzle directed against the buckets of the wheel. This is the simplest form of an impulse turbine, and is illustrated by Fig. 4.

In 1784, Wolfgang de Ivempelen designed a turbine of the lawn sprinkler type, similar in principle to Hero’s engine, the chief dif~ ference being the substitution of a horizontal revolving tube for the hollow sphere which Hero used. Steam, escaping from the outlets in opposite ends of the tube, caused it to revolve by reaction, just

as the escaping water causes the lawn sprinkler to revolve.

In 1784 it is said that James Watt took out a patent on a tur¬ bine, but, as we all so well know, devoted his gen¬ ius to the devel¬ opment of the reciprocating engine. At this time, both types of engine were

'Fig. 4. Branca’s Impulse Turbine. . . , ,,

in about the

same crude form, and it is possible that had Watt devoted his energies to the turbine instead of to the reciprocating engine we might not have had the ordinary form of steam engine in its present

Fig. 6. Reaction Wheel of Avery & Foster.

high state of efficiency, for the turbine possesses so many advantages. This is proved by the fact that it has at last come to the front in spite of the great commercial success of the reciprocating engine.

In 1821, Avery & I oster tcck out the first patent granted for a turbine by the United States Patent Office. This was on the Hero lines, and was really an improvement on the Wolfgang de Kempelen turbine of 17S4. This turbine appears to be the first to attain com¬ mercial success. Several were built under the Avery patent and were used to run sawmills near Syracuse, N. Y.

Steam entered a hollow shaft, Fig. 5, through a stuffing box, passed through to the hollow arms, and escaped through plain open¬ ings in opposite ends of the arms. The speed of rotation was enormous, the periphery of a 7 ft. wheel traveling at the rate of about 14 miles per minute. The wear was excessive, and this, combined with inability to get proper packing for the stuf¬ fing boxes, rather than the lack of steam economy, doubtless caused its failure, for the reciprocating en¬ gine of those days had not reached its highest state of economy. Had CD

Avery used the present Fig - 6 * Real & Pichon Com P°^ nd Turbine,

expanding nozzles instead of plain openings for his steam outlets, his steam consumption would doubtless have been less, but the speed of rotation more. Diverging nozzles were used as early as 1838 , but as they were not correctly proportioned, they were a hin¬ drance rather than a help, and the idea seems to have been given up for a time.

As early as 1827, a compound turbine was patented by Real Si Pichon, the idea being to reduce the velocity of rotation by passing the steam through successive wheels G, Fig. 6, separated by disks B B containing outlets C to permit the passage of the steam from one chamber to another. H is one of the blades, F the shaft, and M the steam exhaust. This is the principle on which the present Rateau turbine works.

The chief cause for early failure in turbine work was lack of comprehensive knowledge of the flow of steam. It was not until 1840 or thereabouts that anyone seemed to get at the real facts or appreciate the true significance of the situation. In this year Pilbrow patented a machine that was a distinct advance in the right direction, and his patent claims show that he, at least, understood some of the fundamental principles. In 1842, he attempted to re¬ duce the speed of rotation by compounding, passing steam through successive wheels revolving in opposite directions. There are, of course, grave objections to such an arrangement. He later invented a turbine with several nozzles that could be successively cut out of action as the load on the turbine varied. This, in its crude form, is the fundamental idea of the arrangement of the nozzles used in the DeLaval and Curtis turbines, but Pilbrow used a converging, instead of a diverging nozzle, and his wheel was unlike others of the impulse type.

In 1840, Wilson patented the forerunner of the Parsons type of turbine. He passed steam successively through rows of running and stationary vanes, gradually expanding it until the exhaust pressure was reached. A view of Wilson’s invention is shown in Fig. 7; a, b } and c, are vanes which are attached to and rotate with the drum D , while d, e, and / are stationary guide vanes. Steam enters at the left, passes through the turbine longitudinally, and exhausts at the right. Wilson appears to have been among the first to realize that the volume of steam increases as it expands to lower pressures, to provide for the same by increased size of passages, and, what is per¬ haps most important, to claim this in his patent.

In 1858, Hartman Bros, pat¬ ented a turbine consisting of two revolving disks c and c' fixed to a shaft D f as shown in Fig. 8.

Between them was a segment of stationary reversing blades d d.

Steam entered from a nozzle F and was exhausted at II; G is the casing. This turbine embodied the essential element of the one-stage Curtis turbine of the present day.

Perrigault & Farcot, about 1870, patented a compound tur¬ bine in which the steam, as it left the buckets, passed through suc¬ cessive passages and again and again impinged upon the face of the same wheel. This is the prin¬ ciple adopted in the Riedler- Stumpf turbine and is illustrated by Fig. 9. Steam enters this turbine through the nozzle A, passes through the wheel buckets to the other side, and discharges

Fig. 8. Hartman's Compound Impulse Turbine.

into pipe B, which brings the steam around again to the inlet side. Here it discharges through the opening 5, against another bucket of the same wheel, whence it is picked up by the opening x t in the pipe C, and so on, finally exhausting through the pipe D. Somewhat earlier, a turbine was invented that returned the steam in a similar

* manner, but to another set of blades on the same wheel. Tnis idea has also been perfected by Riedler & Stumpf.

Moorehouse patented, in 1877, an improvement on the type suggested by Real & Pichon in 1827. His chief claim was an allow¬ ance for the increased volume of expanding steam in this type of turbine.

DeLaval took out a patent on a reaction turbine of the Hero type in 1883. It differed from the Avery turbine in detail, but not in principle. This turbine was extensively used for running cream separators, and was commercially successful, but was later aban¬ doned for the present type of DeLaval motor.

Fig. 9. Compound Turbine with One Wheel.

In 1885, Parsons took out his first turbine patent on a motor along the lines previously suggested by Wilson, and is responsible for the successful development of this type of motor. His first tur¬ bine, shown in Fig. 10, took steam in the center A, and exhausted at both ends through the exhaust passage E E , thus avoiding any end-thrust on the shaft B . At the same time, he patented his famous flexible bearing, now in general use. In 18SS, he patented the present arrangement of grouping several rows of blades together increasing the drum diameters step by step to provide for proper expansion, at the same time patenting his balancing pistons, at present employed to relieve end-thrust.

The expanding nozzle had been patented in 1867 for use iD steam injectors, but it was not until 1894 that anyone patented its use in connection with a turbine. In this year, DeLaval secured this patent and used the nozzle in connection with his turbine, for the purpose of expanding the steam and getting a high velocity of jet with in¬ creased kinetic energy.

During 1894 and 1895 there were issued a large number of patents, many of which have been successfully developed. Among them were Parsons’, Rateau’s, and the first patents for the use of buckets of the Pclton type.

In 189G, Curtis patented the use of an expanding nozzle in com¬ bination with a compound wheel of the type suggested by the Hart-

Fig. 10. Early Parsons Turbine.

l man patents in 1858. Others had used both the expanding nozzle and the same type of wheel, and only two years earlier patents were taken out for a converging nozzle with a similar wheel. From a study of nozzles, it will appear that the converging nozzle could be used economically by increasing the number of stages used in the expansion, but the turbine would be larger than the Curtis, and probably less efficient.

Patents were issued in 1898 to Riedlei & Stumpf, whose turbine appears to be an improvement on the Perrigault & Farcot, patented about 1870.

In 1900, the Zoelly patents were issued. This turbine in prin¬ ciple is similar to the Rateau, but different in construction.

The steam turbine patents issued since 1900 are altogether too numerous even to mention, but from them a number of commercial machines have been developed, and are now on the market. The principal commercial turbines will be described later.

It must not be thought that this summary -is at all exhaustive, or that even all the noteworthy turbine patents have been mentioned. There are hundreds of them, and it is possible here only to men¬ tion those that are the immediate forerunners of our present com¬ mercial types. This brief summary will show that the commercial success of the turbine has been due to a more complete knowledge of the properties of steam, improved details, and the possibility of better workmanship, rather than to the development of new principles, for the distinctive fundamental ideas of all of our commercial tur¬ bines had been suggested years ago. It should be borne in mind, however, that no fundamental principle can be successfully worked out unless the minutest detail is correct, and these details may, and in the case of the turbine did, prevent the successful carrying out of the early ideas.

Fundamental Principles. The underlying principles of steam and water turbines are alike—a moving fluid impinges upon curved vanes or buckets attached to the periphery of a wheel, thus causing it to revolve. The vanes or buckets change the direction of the actua¬ ting fluid and absorb part of its energy, the fluid leaving the turbine with a comparatively low velocity., To insure reasonable economy, the fluid must impinge upon the vanes in a direction tangential to their surface at the point of impact, so as not to impart any shock and to avoid spattering. Further, the residual velocity at outlet should be as low as possible.

In either class of turbine, rotation is caused, not by the statical pressure of the actuating fluid, but by the velocity which it imparts to the rotating turbine wheels. The kinetic energy of the fluid

IFF 2

passing through the turbine is equal to —-—, where W equals the

weight of the fluid per second and V is its velocity at entrance. Evi¬ dently, the smaller IF, the larger V must be to develop the same

WV 7

power. If the fluid leaves the turbine with the velocity V a , then- ~

2 g represents the energy not absorbed by the turbine. If V a is small, this wasted energy will be likewise small.

Since the fundamental principles of the turbines are the same, it would seem at first sight as if steam and water turbines could be built on similar lines. But this is not so, because the difference in density and in elasticity of the two fluids requires different applications of those principles. In the steam turbine, not only must proper steps be taken to abstract the energy from the steam jet, but also to make that energy a maximum by providing for the proper expansion of the steam.

To make more clear the differences just mentioned, it should be remembered that water is an inelastic fluid; that is, one having a constant volume under all conditions of pressure. Therefore, in flowing through a nozzle, if the velocity at the outlet is to be greater than the velocity in the pipe, the area of the outlet must be smaller than the cross-section of the pipe. Steam, on the other hand, is an elastic fluid and expands rapidly as it flows through a nozzle. If the increase in volume were in exact ratio to the increase in velocity, then, for maximum efficiency, the nozzle would be parallel-sided. This happens when the pressure at discharge is about 60 % of the initial pressure. But when discharging into a low pressure, the volume of the steam increases more rapidly than the velocity, and hence, if the mouth of the nozzle is to be capable of discharging the same weight of steam per second as the throat (the condition for maximum efficiency), the cross-sectional area of the nozzle must con¬ stantly increase toward the outlet.

Steam will expand as it passes through the turbine and if the passages are correctly proportioned, so that this expansion can take place only in one direction, that is, in the line of flow, the steam particles will be forced forward in a nearly uniform jet; the steam, by virtue of this expansion, will attain a very high velocity and the jet will consequently have a high kinetic energy.

Water turbines use a relatively small head and a large quantity of fluid; with the steam turbines, the quantity of fluid is small, but the head is very large. To develop large powers with any form of turbine, it is necessary that a number of wheels be used. With water turbines, each wheel acts under full head, each using a relatively small quantity of water. With steam turbines, however, it is the head that must be divided into different steps; i. e., a single steam turbine can use the full quantity of fluid but, if desired to run at relatively low speeds, it can use but a portion of the total head. To develop 1,000 H. P. on a turbine shaft, with a head of 150 feet, would require approximately 4,800 pounds of fluid per second, depending somewhat upon the design of the wheel. With a head of 3,000 feet there would be required 242 pounds of fluid per second, and with a head of 234,000 feet, comparable with that of a steam turbine, the requirement would be about 3J pounds of fluid per second. It is thus clearly seen that the difficulty in developing large powers with the water turbine is that of providing

for a sufficiently large quantity of fluid through the turbines, and with the steam turbine, that of handling the great velocities re¬ sulting from the enormous head.

In all steam turbines, the steam is expanded in suitable nozzles or passages. In some the expan¬ sion is all in the nozzles; in others, partly in the nozzles and partly in the vanes or blades. In some, the total expansion from boiler to exhaust takes place in one nozzle, and the energy is absorbed by a single wheel; such a turbine is called a single-stage turbine . In others, the expansion in one set of nozzles is only partial, and after passing through one or more wheels, the steam again passes through another set of nozzles and set of wheels, and so on, until exhaust pressure is reached. This is a multi-stage turbine. There may be all the way from one to forty stages, or even more, in turbines of this type where all expansion is in the stationary vanes; and in turbines where part of the expansion is in the running vanes, there may be 100 stages or more. Some turbines use nozzles for expanding the steam, and some use stationary vanes for the purpose, these vanes being so shaped as to provide suitable passage areas to permit of steam expansion. The principle is the same whether., nozzles or blades are used, but blades are generally

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o

Fig. 11. Apparatus for Showing Force of a Jet.

used where many stages are employed and the drop in pressure is small from stage to stage.

Before taking up the actual study of steam turbines, it will be necessary to have a clear conception of a few elementary principles of mechanics. Suppose a hollow cube to be filled with some fluid (water or steam) at a given pressure, and to have an opening in one side that can readily be closed. The arrangement is such that when the outlet is opened, the internal pressure will remain the same. If the outlet is opened, the fluid will rush out, as shown in Fig. 11, and, if the jet is supposed to strike against a board free to move, the jet will exert a force upon that board tending to swing it in the direction of the jet. This force is called an impulse. At the same time there will be a tendency on the part of the cube to move in the opposite direction, and the force thus developed is called a reaction. It may be explained in this way:

Suppose each side of the cube to be one foot square, the area of the opening, one square inch, and the internal pressure, 100 pounds per square inch. There will be 144 X 100 pounds pressure on each side of the cube with the outlet closed, but when the one-inch outlet is opened, the total pressure on the side containing the outlet will be reduced by the pressure of 100 pounds on the opening itself. This will leave an unbalanced force of 100 pounds acting in the opposite direction, which is the origin cf the reactive force. This explanation is not strictly correct, but serves to give an idea of these two forces, impulse and reaction.

Hero’s turbine was a reaction turbine pure and simple, Branca’s, an impulse turbine; but what is called a reaction turbine at the present time is not a simple reaction turbine in any sense, but one running under the combined influence of reaction and impulse. Likewise, the so-called impulse turbine is not a pure impulse turbine, but acts under the combined influence of impulse and reaction. There is no pure reaction turbine now on the market. The so-called impulse turbine being rather simpler of explanation, for the present only this type will be considered in the following explanations. How