If for any reason damaged blades cannot be repaired at the time, they can be easily removed and the turbine run again without them until it is convenient to put in new ones; in fact, machines have been run at full load with only three-quarters of the total number of blades. In such an event remove the corresponding stationary blades as well as the moving blades, so as not to disturb the balance of the end thrust.

Conditions Conducive to Successful Operation

In the operation of the turbine and the conditions of the steam, both live and exhaust play a very important part. It has been found by expensive experimenting that moisture in the steam has a very decided effect on the economy of operation; or considerably more so than in the case of the reciprocating engine. In the latter engine, 2 per cent. of moisture will mean very close to 2 per cent. increase in the amount of water supplied to the engine for a given power. On the other hand, in the turbine 2 per cent. moisture will cause an addition of more nearly 4 per cent. It is therefore readily seen that the drier the entering steam, the better will be the appearance of the coal bill.

By judicious use of first-class separators in connection with a suitable draining system, such as the Holly system which returns the moisture separated from the steam, back to the boilers, a high degree of quality may be obtained at the turbine with practically no extra expense during operation. Frequent attention should be given the separators and traps to insure their proper operation. The quality of the steam may be determined from time to time by the use of a throttling calorimeter. Dry steam, to a great extent, depends upon the good and judicious design of steam piping.

Superheated steam is of great value where it can be produced economically, as even a slight degree insures the benefits to be derived from the use of dry steam. The higher superheats have been found to increase the economy to a considerable extent.

When superheat of a high degree (100 degrees Fahrenheit or above) is used special care must be exercised to prevent a sudden rise of the superheat of any amount. The greatest source of trouble in this respect is when a sudden demand is made for a large increase in the amount of steam used by the engine, as when the turbine is started up and the superheater has been in operation for some time before, the full load is suddenly thrown on. It will be readily seen that with the turbine running light and the superheater operating, there is a very small amount of steam passing through; in fact, practically none, and this may become very highly heated in the superheater, but loses nearly all its superheat in passing slowly to the turbine; then, when a sudden demand is made, this very high temperature steam is drawn into the turbine. This may usually be guarded against where a separately fired superheater is used, by keeping the fire low until the load comes on, or, in the case where the superheater is part of the boiler, by either not starting up the superheater until after load comes on, or else keeping the superheat down by mixing saturated steam with that which has been superheated. After the plant has been started up there is little danger from this source, but such precautions should be taken as seem best in the particular cases.

Taking up the exhaust end of the turbine, we have a much more striking departure from the conditions familiar in the reciprocating engine. Due to the limits imposed upon the volume of the cylinder of the engine, any increase in the vacuum over 23 or 24 inches, in the case, for instance, of a compound-condensing engine, has very little, if any, effect on the economy of the engine. With the turbine, on the other hand, any increase of vacuum, even up to the highest limits, increases the economy to a very considerable extent and, moreover, the higher the vacuum the greater will be the increase in the economy for a given addition to the vacuum. Thus, raising the vacuum from 27 to 28 inches has a greater effect than from 23 to 24 inches. For this reason the engineer will readily perceive the great desirability of maintaining the vacuum at the highest possible point consistent with the satisfactory and economical operation of the condenser.

The exhaust pipe should always be carried downward to the condenser when possible, to keep the water from backing up from the condenser into the turbine. If the condenser must be located above the turbine, then the pipe should be carried first downward and then upward in the U form, in the manner of the familiar "entrainer," which will be found effectively to prevent water getting back when the turbine is operating.

Condensers

As has been previously pointed out, the successful and satisfactory operation of the turbine depends very largely on the condenser. With the reciprocating engine, if the condenser will give 25 inches vacuum, it is considered fairly good, and it is allowed to run along by itself until the vacuum drops to somewhere below 20 inches, when it is completely gone over, and in many cases practically rebuilt and the vacuum brought back to the original 25 inches. It has been seen that this sort of practice will never do in the case of the turbine condenser and, unless the vacuum can be regularly maintained at 27 or 28 inches, the condenser is not doing as well as it ought to do, or it is not of the proper type, unless perhaps the temperature and the quantity of cooling water available render a higher vacuum unattainable.

On account of the great purity of the condensed steam from the turbine and its peculiar availability for boiler feed (there being no oil of any kind mixed with it to injure the boilers), the surface condenser is very desirable in connection with the turbine. It further recommends itself by reason of the high vacuum obtainable.

Where a condenser system capable of the highest vacuum is installed, the need of utilizing it to its utmost capacity can hardly be emphasized too strongly. A high vacuum will, of course, mean special care and attention, and continual vigilance for air leaks in the exhaust piping, which will, however, be fully paid for by the great increase in economy.

It must not be inferred that a high vacuum is essential to successful operation of this type of turbine, for excellent performance both in the matter of steam consumption and operation is obtained with inferior vacuum. The choice of a condenser, however, is a matter of special engineering, and is hardly within the province of this article.

Oils

There are several oils on the market that are suitable for the purpose of the turbine oiling system, but great care must be exercised in their selection. In the first place, the oil must be pure mineral, unadulterated with either animal or vegetable oils, and must have been washed free from acid. Certain brands of oil require the use of sulphuric acid in their manufacture and are very apt to contain varying degrees of free acid in the finished product. A sample from one lot may have almost no acid, while that from another lot may contain a dangerous amount.

Mineral oils that have been adulterated, when heated up, will partially decompose, forming acid. These oils may be very good lubricants when first put into use, but after awhile they lose all their good qualities and become very harmful to the machine by eating the journals in which they are used. These oils must be very carefully avoided in the turbine, as the cheapness of their first cost will in no way pay for the damage they may do. A very good and simple way to test for such adulterations is to take up a quantity of the oil in a test tube with a solution of borax and water. If there is any animal or vegetable adulterant present it will appear as a white milk-like emulsion which will separate out when allowed to stand. The pure mineral oil will appear at the top as a clear liquid and the excess of the borax solution at the bottom, the emulsion being in between. A number of oils also contains a considerable amount of paraffin which is deposited in the oil-cooling coil, preventing the oil from being cooled properly, and in the pipes and bearings, choking the oil passages and preventing the proper circulation of the oil and cushioning effect in the bearing tubes. This is not entirely a prohibitive drawback, the chief objection being that it necessitates quite frequently cleaning the cooling coil, and the oil piping and bearings.

Some high-class mineral oils of high viscosity are inclined to emulsify with water, which emulsion appears as a jelly-like substance. It might be added that high-grade oils having a high viscosity might not be the most suitable for turbine use.

Since the consumption of oil in a turbine is so very small, being practically due only to leakage or spilling, the price paid for it should therefore be of secondary importance, the prime consideration being its suitability for the purpose.

In some cases a central gravity system will be employed, instead of the oil system furnished with the turbine, which, of course, will be a special consideration.

For large installations a central gravity oiling system has much to recommend it, but as it performs such an important function in the power plant, and its failure would be the cause of so much damage, every detail in connection with it should be most carefully thought out, and designed with a view that under no combination of circumstances would it be possible for the system to become inoperative. One of the great advantages of such a system is that it can be designed to contain very large quantities of oil in the settling tanks; thus the oil will have quite a long rest between the times of its being used in the turbine, which seems to be very helpful in extending the life of the oil. Where the oil can have a long rest for settling, an inferior grade of oil may be used, providing, however, that it is absolutely free of acid.



V. PROPER METHOD OF TESTING A STEAM TURBINE[3]

[3] Contributed to Power by Thomas Franklin.

The condensing arrangements of a turbine are perhaps mainly instrumental in determining the method of test. The condensed steam alone, issuing from a turbine having, for example, a barometric or jet condenser, cannot be directly measured or weighed, unless by meter, and these at present are not sufficiently accurate to warrant their use for test purposes, if anything more than approximate results are desired. The steam consumed can, in such a case, only be arrived at by measuring the amount of condensing water (which ultimately mingles with the condensed steam), and subtracting this quantity from the condenser's total outflow. Consequently, in the case of turbines equipped with barometric or jet condensers, it is often thought sufficient to rely upon the measurement taken of the boiler feed, and the boiler's initial and final contents. Turbines equipped with surface-condensing plants offer better facilities for accurate steam-consumption calculations than those plants in which the condensed exhaust steam and the circulating water come into actual contact, it being necessary with this type simply to pump the condensed steam into a weighing or measuring tank.

In the case of a single-flow turbine of the Parsons type, the covers should be taken off and every row of blades carefully examined for deposits, mechanical irregularities, deflection from the true radial and vertical positions, etc. The blade clearances also should be gaged all around the circumference, to insure this clearance being an average working minimum. On no account should a test be proceeded with when any doubt exists as to the clearance dimensions.



The dummy rings of a turbine, namely, those rings which prevent excessive leakage past the balancing pistons at the high-pressure end, should have especial attention before a test. A diagrammatic sketch of a turbine cylinder and spindle is shown in Fig. 60, for the benefit of those unfamiliar with the subject. In this A is the cylinder or casing, B the spindle or rotor, and C the blades. The balancing pistons, D, E, and F, the pressure upon which counterbalances the axial thrust upon the three-bladed stages, are grooved, the brass dummy rings G G in the cylinder being alined within a few thousandths of an inch of the grooved walls, as indicated. After these rings have been turned (the turning being done after the rings have been calked in the cylinder), it is necessary to insure that each ring is perfectly bedded to its respective grooved wall so that when running the several small clearances between the groove walls and rings are equal. A capital method of thus bedding the dummy rings is to grind them down with a flour of emery or carborundum, while the turbine spindle is slowly revolving under steam. Under these conditions the operation is performed under a high temperature, and any slight permanent warp the rings may take is thus accounted for. The turbine thrust-block, which maintains the spindle in correct position relatively to the spindle, may also be ground with advantage in a similar manner.

The dummy rings are shown on a large scale in Fig. 61, and their preliminary inspection may be made in the following manner:

The spindle has been set and the dummy rings C are consequently within a few thousandths of an inch of the walls d of the spindle dummy grooves D. The clearances allowed can be gaged by a feeler placed between a ring and the groove wall. Before a test the spindle should be turned slowly around, the feelers being kept in position. By this means any mechanical flaws or irregularities in the groove walls may be detected.



It has sometimes been found that the groove walls, under the combined action of superheated steam and friction, in cases where actual running contact has occurred, have worn very considerably, the wear taking the form of a rapid crumbling away. It is possible, however, that such deterioration may be due solely to the quality of the steel from which the spindle is forged. Good low-percentage carbon-annealed steel ought to withstand considerable friction; at all events the wear under any conditions should be uniform. If the surfaces of both rings and grooves be found in bad condition, they should be re-ground, if not sufficiently worn to warrant skimming up with a tool.

As the question of dummy leakage is of very considerable importance during a test, it may not be inadvisable to describe the manner of setting the spindle and cylinder relatively to one another to insure minimum leakage, and the methods of noting their conduct during a prolonged run. In Fig. 62, showing the spindle, B is the thrust (made in halves), the rings O of which fit into the grooved thrust-rings C in the spindle. Two lugs D are cast on each half of the thrust-block. The inside faces of these lugs are machined, and in them fit the ball ends of the levers E, the latter being fulcrumed at F in the thrust-bearing cover. The screws G, working in bushes, also fit into the thrust-bearing cover, and are capable of pushing against the ends of the levers E and thus adjusting the separate halves of the block in opposite directions.



The top half of the turbine cylinder having been lifted off, the spindle is set relatively to the bottom half by means of the lower thrust-block screw G. This screw is then locked in position and the top half of the cover then lowered into place. With this method great care must necessarily be exercised when lowering the top cover; otherwise the brass dummy rings may be damaged.

A safer method is to set the dummy rings in the center of the grooves of the spindle, and then to lower the cover, with less possibility of contact. There being usually plenty of side clearance between the blades of a turbine, it may be deemed quite safe to lock the thrust-block in its position, by screwing the screws G up lightly, and then to turn on steam and begin running slowly.

Next, the spindle may be very carefully and gradually worked in the required direction, namely, in that direction which will tend to bring the dummy rings and groove walls into contact, until actual but very light contact takes place. The slightest noise made by the rubbing parts inside the turbine can be detected by placing one end of a metal rod onto the casing in vicinity of the dummy pistons, and letting the other end press hard against the ear. Contact between the dummy rings and spindle being thus demonstrated, the spindle must be moved back by the screws, but only by the slightest amount possible. The merest fraction of a turn is enough to break the contact, which is all that is required. In performing this operation it is important, during the axial movements of the spindle, to adjust the halves of the thrust-block so that there can exist no possible play which would leave the spindle free to move axially and probably vibrate badly.

After ascertaining the condition of the dummy rings, attention might next be turned to the thrust-block, which must not on any account be tightened up too much. It is sufficient to say that the actual requirements are such as will enable a very thin film of oil to circulate between each wall of the spindle thrust-grooves and the brass thrust-blocks ring. In other words, there should be no actual pressure, irrespective of that exerted by the spindle when running, upon the thrust-block rings, due to the separate halves having been nipped too tightly. The results upon a test of considerable friction between the spindle and thrust-rings are obvious.

The considerations outlined regarding balancing pistons and dummy rings can be dispensed with in connection with impulse turbines of the De Laval and Rateau types, and also with double-flow turbines of a type which does not possess any dummies. The same general considerations respecting blade conditions and thrust-blocks are applicable, especially to the latter type. With pure so-called impulse turbines, where the blade clearances are comparatively large, the preliminary blade inspection should be devoted to the mechanical condition of the blade edges and passages. As the steam velocities of these types are usually higher, the importance of minimizing the skin friction and eliminating the possibility of eddies is great.

Although steam leakage through the valves of a turbine may not materially affect its steam consumption, unless it be the leakage through the overload valve during a run on normal full load, a thorough examination of all valves is advocated for many reasons. In a turbine the main steam-inlet valve is usually operated automatically from the governor; and whether it be of the pulsating type, admitting the steam in blasts, or of the non-pulsating throttling type, it is equally essential to obtain the least possible friction between all moving and stationary parts. Similar remarks apply to the main governor, and any sensitive transmitting mechanism connecting it with any of the turbine valves. If a safety or "runaway" governor is possessed by the machine to be tested, this should invariably be tried under the requisite conditions before proceeding farther. The object of this governor being automatically to shut off all steam from the turbine, should the latter through any cause rise above the normal speed, it is often set to operate at about 12 to 15 per cent. above the normal. Thus, a turbine revolving at about 3000 revolutions per minute would be closed down at, say, 3500, which would be within the limit of "safe" speed.

Importance of Oiling System and Water Service

The oil question, being important, should be solved in the early stages previously, if possible, to any official or unofficial consumption tests. Whether the oil be supplied to the turbine bearings by a self-contained system having the oil stored in the turbine bedplate or by gravity from a separate oil source, does not affect the question in its present aspect. The necessary points to investigate are four in number, and may be headed as follows:

(a) Examination of pipes and partitions for oil leakage.

(b) Determination of volume of oil flowing through each bearing per unit of time.

(c) Examination for signs of water in oil.

(d) Determination of temperature rise between inlet and outlet of oil bearings.

The turbine supplied with oil by the gravity or any other separate system holds an advantage over the ordinary self-contained machine, inasmuch as the oil pipes conveying oil into and from the bearings can be easily approached and, if necessary, repaired. On the other hand, the machine possessing its own oil tank, cooling chamber and pump is somewhat at a disadvantage in this respect, as a part of the system is necessarily hidden from view, and, further, it is not easily accessible. The leakage taking place in any system, if there be any, must, however, be detected and stopped.

Fig. 63 is given to illustrate a danger peculiar to the self-contained oil system, in which the oil and oil-cooling chambers are situated adjacently in the turbine bedplate. One end of the bedplate only is shown; B is a cast-iron partition dividing the oil chamber C from the oil-cooling chamber D. Castings of this kind have sometimes a tendency to sponginess and the trouble consequent upon this weakness would take the form of leakage between the two chambers. Of course this is only a special case, and the conditions named are hardly likely to exist in every similarly designed plant. The capacity of oil, and especially of hot oil, to percolate through the most minute pores is well known. Consequently, in advocating extreme caution when dealing with oil leakage, no apology is needed.



It may be stated without fear of contradiction that the oil in a self-contained system, namely, a system in which the oil, stored in a reservoir near or underneath the turbine, passes only through that one turbine's bearings, and immediately back to the storage compartment, deteriorates more rapidly than when circulating around an "entire" system, such as the gravity or other analogous system. In the latter, the oil tanks are usually placed a considerable distance from the turbine or turbines, with the oil-cooling arrangements in fairly close proximity. The total length of the oil circuit is thus considerably increased, incidentally increasing the relative cooling capacity of the whole plant, and thereby reducing the loss of oil by vaporization.

The amount of oil passing through the bearings can be ascertained accurately by measurement. With a system such as the gravity it is only necessary to run the turbine up to speed, turn on the oil, and then, over a period, calculate the volume of oil used by measuring the fall of level in the storage tank and multiplying by its known cross-sectional area. In those cases where the return oil, after passing through the bearings, is delivered back into the same tank from which it is extracted, it is of course necessary, during the period of test, to divert this return into a separate temporary receptacle. Where the system possesses two tanks, one delivery and one return (a superior arrangement), this additional work is unnecessary. The same method can be applied to individual turbines pumping their own oil from a tank in the bedplate; the return oil, as previously described, being temporarily prevented from running back to the supply.

The causes of excessive oil consumption by bearings are many. There is an economical mean velocity at which the oil must flow along the revolving spindle; also an economical mean pressure, the latter diminishing from the center of the bearing toward the ends. The aim of the economist must therefore be in the direction of adjusting these quantities correctly in relation to a minimum supply of oil per bearing; and the principal factors capable of variation to attain certain requirements are the several bearing clearances measured as annular orifices, and the bearing diameters.

It is not always an easy matter to detect the presence of water in an oil system, and this difficulty is increased in large circuits, as the water, when the oil is not flowing, generally filters to the lowest members and pipes of the system, where it cannot usually be seen. A considerable quantity of water in any system, however, indicates its presence by small globular deposits on bearings and spindles, and in the worst cases the water can clearly be seen in a small sample tapped from the oil mains. There is only one effective method of ridding the oil of this water, and this is by allowing the whole mass of oil in the system to remain quiescent for a few days, after which the water, which falls to the lowest parts, can be drained off. A simple method of clearing out the system is to pump all the oil the whole circuit contains through the filters, and thence to a tank from which all water can be taken off. One of the ordinary supply tanks used in the gravity system will serve this purpose, should a temporary tank not be at hand. If necessary, the headers and auxiliary pipes of the system can be cleaned out before circulating the oil again, but as this is rather a large undertaking, it need only be resorted to in serious cases.



It is seldom possible to discover the correct and permanent temperature rise of the circulating oil in a turbine within the limited time usually alloted for a test. After a continuous run of one hundred hours it is possible that the temperature at the bearing outlets may be lower than it was after the machine had run for, say, only twenty hours. As a matter of fact an oil-temperature curve plotted from periodical readings taken over a continuous run of considerable length usually reaches a maximum early, afterward falling to a temperature about which the fluctuations are only slight during the remainder of the run. Fig. 64 illustrates an oil-temperature curve plotted from readings taken over a period of twenty-four hours. In this case the oil system was of the gravity description, the capacity of the turbine being about 6000 kilowatts. The bearings were of the ordinary white-metal spherical type. Over extended runs of hundreds and even thousands of hours, the above deductions may be scarcely applicable. Running without break for so long, a small turbine circulating its own lubricant would possibly require a renewal of the oil before the run was completed, in the main owing to excessive temperature rise and consequent deterioration of the quality of the oil. Under these conditions the probabilities are that several temperature fluctuations might occur before the final maximum, and more or less constant, temperature was reached. In this connection, however, the results obtained are to a very large extent determined by the general mechanical design and construction of the oiling system and turbine. A reference to Fig. 63 again reveals at once a weakness in that design, namely, the unnecessarily close proximity in which the oil and water tanks are placed.



A design of thermometer cup suitable for oil thermometers is given in Fig. 65 in which A is an end view of the turbine bedplate, B is a turbine bearing and C and D are the inlet and outlet pipes, respectively. The thermometer fittings, which are placed as near the bearing as is practicable, are made in the form of an angular tee fitting, the oil pipes being screwed into its ends. The construction of the oil cup and tee piece is shown in the detail at the left where A is the steel tee piece, into which is screwed the brass thermometer cup B. The hollow bottom portion of this cup is less than 1/16 of an inch in thickness. The top portion of the bored hole is enlarged as shown, and into this, around the thermometer, is placed a non-conducting material. The cup itself is generally filled with a thin oil of good conductance.

Allied to the oil system of a turbine plant is the water service, of comparatively little importance in connection with single self-contained units of small capacity, where the entire service simply consists of a few coils and pipes, but of the first consideration in large installations having numerous separate units supplied by oil and water from an exterior source. The largest turbine units are often supplied with water for cooling the bearings and other parts liable to attain high temperature. Although the water used for cooling the bearings indirectly supplements the action taking place in the separate oil coolers, it is of necessity a separate auxiliary service in itself, and the complexity of the system is thus added to. A carefully constructed water service, however, is hardly likely to give trouble of a mechanical nature. The more serious deficiencies usually arise from conditions inherent to the design, and as such must be approached.

Special Turbine Features to be Inquired into

Before leaving the prime mover itself, and proceeding to the auxiliary plant inspection, it may be well to instance a few special features relating to the general conduct of a turbine, which it is the duty of a tester to inquire into. There are certain specified qualifications which a machine must hold when running under its commercial conditions, among these being lack of vibration of both turbine and machinery driven, be it generator or fan, the satisfactory running of auxiliary turbine parts directly driven from the turbine spindle, minimum friction between the driving mediums, such as worm-wheels, pumps, fans, etc., slight irregularities of construction, often resulting in heated parts and excessive friction and wear, and must therefore be detected and righted before the final test. Furthermore, those features of design—and they are not infrequent in many machines of recent development—which, in practice, do not fulfil theoretical expectations, must be re-designed upon lines of practical consistency. The experienced tester's opinion is often at this point invaluable. To illustrate the foregoing, Figs. 66, 67, and 68 are given, representing, respectively, three distinct phases in the evolution of a turbine part, namely, the coupling. Briefly, an ordinary coupling connecting a driving and a driven shaft becomes obstinate when the two separate spindles which it connects are not truly alined. The desire of turbine manufacturers has consequently been to design a flexible coupling, capable of accommodating a certain want of alinement between the two spindles without in any way affecting the smooth running of the whole unit.



In Fig. 66 A is the turbine spindle end and B the generator spindle end, which it is required to drive. It will be seen from the cross-sectional end view that both spindle ends are squared, the coupling C, with a square hole running through it, fitting accurately over both spindle ends as shown. Obviously the fit between the coupling and spindle in this case must be close, otherwise considerable wear would take place; and equally obvious is the fact than any want of alinement between the two spindles A and B will be accompanied by a severe strain upon the coupling, and incidentally by many other troubles of operation of which this inability of the coupling to accommodate itself to a little want of alinement is the inherent cause.

Looking at the coupling illustrated in Fig. 67, it will be seen that something here is much better adapted to dealing with troubles of alinement. The turbine and generator spindles A and B, respectively, are coned at the ends, and upon these tapered portions are shrunk circular heads C and D having teeth upon their outer circumferences. Made in halves, and fitting over the heads, is a sleeve-piece, with teeth cut into its inner bored face. The teeth of the heads and sleeve are proportioned correctly to withstand, without strain, the greatest pressure liable to be thrown upon them. There is practically no play between the teeth, but there exists a small annular clearance between the periphery of the heads and the inside bore of the sleeve, which allows a slight lack of alinement to exist between the two spindles, without any strain whatever being felt by the coupling sleeve E. The nuts F and G prevent any lateral movement of the coupling heads C and D. For all practical requirements this type of coupling is satisfactory, as the clearances allowed between sliding sleeve and coupling heads can always be made sufficient to accommodate a considerable want of alinement, far beyond anything which is likely to occur in actual practice. Perhaps the only feature against it is its lack of simplicity of construction and corresponding costliness.



The type illustrated in Fig. 68 is a distinct advance upon either of the two previous examples, because, theoretically at least, it is capable of successfully accommodating almost any amount of spindle movement. The turbine and generator spindle ends, A and B, have toothed heads C and D shrunk upon them, the heads being secured by the nuts E and F. The teeth in this case are cut in the enlarged ends as shown. A sleeve G, made in halves, fits over the heads, and the teeth cut in each half engage with those of their respective heads. All the teeth and teeth faces are cut radially, and a little side play is allowed.

The Condenser

To some extent, as previously remarked, the condenser and condensing arrangements are instrumental in determining the lines upon which a test ought to be carried out. In general, the local features of a plant restrict the tester more or less in the application of his general methods. A thorough inspection, including some preliminary tests if necessary, is as essential to the good conduct of the condensing plant as to the turbine above it. It may be interesting to outline the usual course this inspection takes, and to draw attention to a few of the special features of different plants. For this purpose a type of vertical condenser is depicted in Fig. 69. Its general principle will be gathered from the following description:

Exhaust steam from the turbine flows down the pipe T and enters the condenser at the top as shown, where it at once comes into contact with the water tubes in W. These tubes fill an annular area, the central un-tubed portion below the baffle cap B forming the vapor chamber. The condensed steam falls upon the bottom tube-plate P and is carried away by the pipe S leading to the water pump H. The Y pipe E terminating above the level of the water in the condenser enters the dry-air pump section pipe A. Cold circulating water enters the condenser at the bottom, through the pipe I, and entering the water chamber X proceeds upward through the tubes into the top-water chamber Y, and from there out of the condenser through the exit pipe. It will be observed that the vapor extracted through the plate P passes on its journey out of the condenser through the cooling chamber D surrounded by the cold circulating water. This, of course, is a very advantageous feature. At R is the condenser relief, at U the relief valve for the water chambers.



A new condenser, especially if it embody new and untried features, generally requires a little time and patience ere the best results can be obtained from it. Perhaps the quickest and most satisfactory method of getting at the weak points of this portion of a plant is to test the various elements individually before applying a strict load test. Thus, in dealing with a condenser similar to that illustrated in Fig. 69, the careful tester would probably make, in addition to a thorough mechanical examination, three or four individual vacuum and water tests. A brief description of these will be given. The water test, the purpose of which is to discover any leakage from the tubes, tube-plates, water pipes, etc., into portions of the steam or air chambers, should be made first.

Water Tests of Condenser

The condenser is first thoroughly dried out, particular care being given to the outside of the tubes and the bottom tube-plate P. Water is then circulated through the tubes and chambers for an hour or two, after which the pumps are stopped, all water is allowed to drain out and a careful examination is made inside. Any water leaking from the tubes above the bottom baffle-plate will ultimately be deposited upon that plate. It is essential to stop this leakage if there be any, otherwise the condensed steam measured during the consumption test will be increased to the extent of the leakage. A slight leakage in a large condenser will obviously not affect the results to any serious extent. The safest course to adopt when a leak is discovered and it is found inopportune to effect immediate repair is to measure the actual volume of leakage over a specified period, and the quantity then being known it can be subtracted from the volume of the condensed steam at the end of the consumption test.

It is equally essential that no leakage shall occur between the bottom tube-plate P and the tube ends. The soundness of the tube joints, and the joint at the periphery of the tube-plate can be tested by well covering the plate with water, the water chamber W and cooling chamber having been previously emptied, and observing the under side of the plate. It must be admitted that the practice of measuring the extent of a water leak over a period, and afterward with this knowledge adjusting the obtained quantities, is not always satisfactory. On no account should any test be made with considerable water leakage inside the condenser. The above method, however, is perhaps the most reliable to be followed, if during its conduct the conditions of temperature in the condenser are made as near to the normal test temperature as possible. There are many condensers using salt water in their tubes, and in these cases it would seem natural to turn to some analytical method of detecting the amount of saline and foreign matter leaking into the condensed steam. Unless, however, only approximate results are required, such methods are not advocated. There are many reasons why they cannot be relied upon for accurate results, among these being the variation in the percentage of saline matter in the sea-water, the varying temperature of the condenser tubes through which the water flows, and the uncertainty of such analysis, especially where the percentage leakage of pure saline matter is comparatively small.

The Vacuum Test

Having convinced himself of the satisfactory conduct of the condenser under the foregoing simple preparatory water tests, the tester may safely pass to considerations of vacuum. There exists a good old-fashioned method of discovering the points of leakage in a vacuum chamber, namely, that of applying the flame of a candle to all seams and other vulnerable spots, which in the location of big leaks is extremely valuable. Assuming that the turbine joints and glands have been found capable of preventing any inleak of air, with only a small absolute pressure of steam or air inside it, and, further, an extremely important condition, with the turbine casing at high and low temperatures, separately, a vacuum test can be conducted on the condenser alone.

This test consists of three operations. In the first place a high vacuum is obtained by means of the air pump, upon the attainment of which communication with everything else is closed, and results noted. The second operation consists in repeating the above with the water circulating through the condenser tubes, the results in this case also being carefully tabulated. Before conducting the third test, the condensers must be thoroughly warmed throughout, by running the turbine for a short time if necessary, and after closing communication with everything, allowing the vacuum to slowly fall.

A careful consideration and comparison of the foregoing tests will reveal the capabilities of the condenser in the aspect in which it is being considered, and will suggest where necessary the desirable steps to be taken.



VI. TESTING A STEAM TURBINE[4]

[4] Contributed to Power by Thomas Franklin.

Special Auxiliary Plant for Consumption Test

There are one or two points of importance in the conduct of a test on a turbine and these will be briefly touched upon. Fig. 70 illustrates the general arrangement of the special auxiliary plant necessary for carrying through a consumption test, when the turbine exhaust passes through a surface condenser. The condensed steam, after leaving the condenser, passes along the pipe A to the pump, and is then forced along the pipe B (leading under ordinary circumstances to the hot-well), through the main water valve C directly to the measuring tanks. To enter these the water has to pass through the valves D and E, while the valves F and G are for quickly emptying the tanks when necessary, being of a larger bore than the inlet valves. The inlet pipes H I are placed directly above the outlet valves, and thus, when required, before any measurements are taken, the water can flow directly through the outlet valves, the pipes terminating only a short distance above them, away to an auxiliary tank or directly to the hot-well. Levers K and L fulcrumed at J and J are connected to the valve spindles by auxiliary levers. The valve arrangement is such that by pulling down the lever K the inlet valve D is opened and the inlet valve E is closed. Again, by pulling down the lever L the outlet valve F is closed, while the outlet valve G is also simultaneously closed.



During a consumption test the valves are operated in the following manner: The lever K is pulled down, which opens the inlet valve to the first tank and closes that to the second. The bottom lever L, however, is lifted, which for the time being opens the outlet valve F, and incidentally opens the valve G; the latter valve can; however, for the moment be neglected. When the turbine is started, and the condensed steam begins to accumulate in the condenser, the water is pumped along the pipes and, both the inlet and outlet valves on the first tank being open, passes through, without any being deposited in the tank, to the drain. This may be continued until all conditions are right for a consumption test and, the time being carefully noted, lever L is quickly pulled down and the valves F and G closed. The first tank now gradually fills, and after a definite period, say fifteen minutes, the lever K is pushed up, thus diverting the flow into the second tank. While the latter is filling, the water in the first tank is measured, and the tank emptied by a large sluice valve, not shown.

The operation of alternately filling, measuring, and emptying the two measuring tanks is thus carried on until the predetermined time of duration of test has expired, when the total water as measured in the tanks, and representing the amount of steam condensed during that time, is easily found by adding together the quantities given at each individual measurement.

All that are necessary to insure successful results from a plant similar to this are care and accuracy in its operation and construction. Undoubtedly in most cases it is preferable to weigh the condensed steam instead of measuring the volume passed, and from that to calculate the weight. If dependence is being placed upon the volumetric method, it is advisable to lengthen the duration of the test considerably, and if possible to measure the feed-water evaporated at the same time. Such a course, however, would necessitate little change, and none of a radical nature, from the arrangement described. Where, however, the measuring method is adopted, the all-important feature, requiring on the tester's part careful personal investigation, is the graduation of the tanks. It facilitates this operation very considerably when the receptacles are graduated upon a weight scale. That is to say, whether or not a vertical scale showing the actual hight of water be placed inside the tank, it is advisable to have a separate scale indicating at once to the attendant the actual contents, by weight, of the tank at any time. It is the tester's duty to himself to check the graduation of this latter scale by weighing the water with which he performs the operation of checking.

Apart from the foregoing, there is little to be said about the measuring apparatus. As has been stated, accuracy of result depends in this connection, as in all others, upon careful supervision and sound and accurate construction, and this the tester can only positively insure by exhaustive inspection in the one case and careful deliberation in the conduct of the other.

It will be readily understood that the procedure—and this implies some limitations—of a test is to an extent controlled by the conditions, or particular environment of the moment. This is strictly true, and as a consequence it is often impossible, in a maker's works, for example, to obtain every condition, coinciding with those specified, which are to be had on the site of final operation only. For this reason it would appear best to reserve the final and crucial test of a machine, which test usually in the operating sense restricts a prime mover in certain directions with regard to its auxiliary plant, etc., until the machine has been finally erected on its site. Obviously, unless a machine had become more or less standardized, a preliminary consumption test would be necessary, but once this primary qualification respecting consumption had been satisfactorily settled, there appears to be no reason why exhaustive tests in other directions should not all be carried out upon the site, where the conditions for them are so much more favorable.

When the steam consumption of a steam turbine is so much higher than the guaranteed quantity, it usually takes little less than a reconstruction to put things right. The minor qualifications of a machine, however, which can be examined into and tested with greater ease, and usually at considerably less expense, upon the site, and consequently under specified conditions, may be advantageously left over until that site is reached, where it is obvious that any shortcomings and general deficiency in performance will be more quickly detected and diagnosed.

Test Loads from the Tester's View-point

Before proceeding to describe the points of actual interest in the consumption test, a few considerations respecting test loads will be dealt with from the tester's point of view. Here again we often find ourselves restricted, to an extent, by the surrounding conditions. The very first considerations, when undertaking to carry out a consumption test, should be devoted to obtaining the steadiest possible lead [Transcriber: load?]. It may be, and is in many cases, that circumstances are such as to allow a steady electrical load to be obtained at almost any time. On the other hand an electrical load of any description is sometimes not procurable at all, without the installation of a special plant for the purpose. In such cases a mechanical friction load, as, for example, that obtained by the water brake, is sometimes available, or can easily be procured. Whereas, however, this type of load may be satisfactory for small machines, it is usually quite impossible for use with large units, of, say, 5000 kilowatts and upward. It is seldom, however, that turbines are made in large sizes for directly driving anything but electrical plants, although there is every possibility of direct mechanical driving between large steam turbines and plants of various descriptions, shortly coming into vogue, so that usually there exist some facilities for obtaining an electrical load at both the maker's works and upon the site of operation.

One consideration of importance is worth inquiring into, and this has relation to the largest turbo-generators supplied for power-station and like purposes. Obviously, the testing of, say, a 7000-kilowatt alternator by any standard electrical-testing method must entail considerable expense, if such a test is to be carried out in the maker's works. Nor would this expense be materially decreased by transferring the operations to the power-station, and there erecting the necessary electrical plant for obtaining a water load, or any other installation of sufficient capacity to carry the required load according to the rated full capacity of the machine.

Assuming, then, that there exist no permanent facilities at either end, namely the maker's works and the power station, for adequately procuring a steady electrical-testing load of sufficient capacity, there still remains, in this instance, an alternative source of power which is usually sufficiently elastic to serve all purposes, and this is of course the total variable load procurable from the station bus-bars. It is conceivable that one out of a number of machines running in parallel might carry a perfectly steady load, the latter being a fraction of a total varying quantity, leaving the remaining machines to receive and deal with all fluctuations which might occur. Even in the event of there being only two machines, it is possible to maintain the load on one of them comparatively steady, though the percentage variation in load on either side of the normal would in the latter case be greater than in the previous one. This is accomplished by governor regulation after the machines have been paralleled. For example, assuming three turbo-alternators of similar make and capacity to be running in parallel, each machine carrying exactly one-third of the total distributed load, it is fair to regard the governor condition, allowing for slight mechanical disparities of construction, of all three machines as being similar; and even in the case of three machines of different capacity and construction, the governor conditions when the machines are paralleled are more or less relatively and permanently fixed in relation to one another. In other words, while the variation in load on each machine is the same, the relative variation in the governor condition must be constant.

By a previously mentioned system of governor regulation, however, it is possible, considering again for a moment the case of three machines in parallel, by decreasing the sensitiveness of one governor only, to accommodate nearly all the total variation in load by means of the two remaining machines, the unresponsiveness of the one governor to change in speed maintaining the load on that machine fairly constant. By this method, at any rate, the variation in load on any one machine can be minimized down to, say, 3 per cent, either side of the normal full load.

There is another and more positive method by which a perfectly steady load can be maintained upon one machine of several running in parallel. This may be carried out as follows: Suppose, in a station having a total capacity of 20,000 kilowatts, there are three machines, two of 6000 kilowatts each, and one of 8000 kilowatts, and it is desired to carry out a steady full-load test upon one of the 6000 kilowatts units. Assuming that the test is to be of six hours' duration, and that the conditions of load fluctuations upon the station are well known, the first step to take is to select a period for the test during which the total load upon all machines is not likely to fall below, say, 8000 kilowatts. The tension upon the governor spring of the turbine to be tested must then be adjusted so that the machine on each peak load is taxed to its utmost normal capacity; and even when the station load falls to its minimum, the load from the particular machine shall not be released sufficiently to allow it to fall below 6000 kilowatts. Under these conditions, then, it may be assumed that although the load on the test machine will vary, it cannot fall below 6000 kilowatts. Therefore, all that remains to be done to insure a perfectly steady load equal to the normal full load of the machine, or 6000 kilowatts, is to fix the main throttle or governing valve in such a position that the steam passing through at constant pressure is just capable of sustaining full speed under the load required. When this method is adopted, it is desirable to fix a simple hight-adjusting and locking mechanism to the governing-valve spindle. The load as read on the indicating wattmeter can then be very accurately varied until correct, and farther varied, if necessary, should any change occur in the general conditions which might either directly or indirectly bring about a change of load.

Preparing the Turbine for Testing

All preliminary labors connected with a test being satisfactorily disposed of, it only remains to place the turbines under the required conditions, and to then proceed with the test. For the benefit of those inexperienced in the operation of large turbines, we will assume that such a machine is about to be started for the purpose outlined.

It is always advisable to make a strict practice of getting all the auxiliary plant under way before starting up the turbine. In handling a turbine plant the several operations might be carried through in the following order:

(1) Circulating oil through all bearings and oil chambers.[5]

(2) Starting of condenser circulating-water pumps, and continuous circulation of circulating water through the tubes of condenser.

(3) Starting of pump delivering condensed steam from the condenser hot-well to weighing tanks.

(4) Starting of air pump, vacuum being raised as high as possible within condenser.

(5) Sealing of turbine glands, whether of liquid or steam type, no adjustment of the quantity of sealing fluid being necessary, however, at this point.

(6) Adjustment of valves on and leading to the water-weighing tanks.

(7) Opening of main exhaust valve or valves between turbine and condenser.

(8) Starting up of turbine and slowly running to speed.

(9) Application of load, and adjustment of gland-sealing steam.

[5] In a self-contained system, where the oil pump is usually driven from the turbine spindle, this would of course be impossible. In the gravity and allied systems, however, it should always be the first operation performed. The tests for oil consumption, described previously, having been carried out, it is assumed that suitable means have been adopted to restrict the total oil flow through the bearings to a minimum quantity.

The running to speed of large turbo-alternators requires considerable care, and should always be done slowly; that is to say the rate of acceleration should be slow. It is well known that the vibration of a heavy unit is accompanied by a synchronous or non-synchronous vibration of the foundation upon which it rests. The nearest approach to perfect synchronism between unit and foundation is obtained by a gradual rise in speed. A machine run up to speed too quickly might, after passing the critical speed, settle down with little visible vibration, but at a later time, even hours after, suddenly begin vibrating violently from no apparent cause. The chances of this occurring are minimized by slow and careful running to speed.

Whether the machine being tested is one of a number running in parallel, or a single unit running on a steady water load, the latter should in all cases be thrown on gradually until full load is reached. A preliminary run of two or three hours—whenever possible—should then be made, during which ample opportunity is afforded for regulating the conditions in accordance with test requirements. The tester will do well during the last hour of this trial run to station his recorders at their several posts and, for a short time at least, to have a complete set of readings taken at the correct test intervals. This more particularly applies to the electrical water, superheat and vacuum readings. In the case of a turbo-alternator the steadiness obtainable in the electrical load may determine the frequency of readings taken, both electrical and otherwise. On a perfectly steady water-tank load, for example, it may be sufficiently adequate to read all wattmeters, voltmeters, and ammeters from standard instruments at from one- to two-minute intervals. Readings at half-minute intervals, however, should be taken with a varying load, even when the variation is only slight.

The water-measurement readings may of course be taken at any suitable intervals, the time being to an extent determined by the size of the measuring tanks or the capacity of the weighing machine or machines. When designing the measuring apparatus, the object should be to minimize, within economical and practical range, the total number of weighings or measurements necessary. Consequently, no strict time of interval between individual weighings or measurements can be given in this case. It may be said, however, that it is not desirable to take these at anything less than five-minute intervals. Under ordinary circumstances a three- to five-minute interval is sufficient in the case of all steam-pressure, vacuum—including mercurial columns and barometer—superheat and temperature readings.

Gland and Hot-Well Regulation

There are two highly important features requiring more or less constant attention throughout a test, namely the gland and hot-well regulation. For the present purpose we may assume that the glands are supplied with either steam or water for sealing them. All steam supplied to the turbine obviously goes to swell the hot-well contents, and to thus increase the total steam consumption. The ordinary steam gland is in reality a pressure gland. At both ends of the turbine casing is an annular chamber, surrounding the turbine spindle at the point where it projects through the casing. A number of brass rings on either side of this chamber encircle the spindle, with only a very fine running clearance between the latter and themselves. Steam enters the gland chamber at a slight pressure, and, when a vacuum exists inside the turbine casing, tends to flow inward. The pressure, however, inside the gland is increased until it exceeds that of the atmosphere outside, and by maintaining it at this pressure it is obvious that no air can possibly enter the turbine through the glands, to destroy the vacuum. The above principle must be borne in mind during a test upon a turbine having steam-fed glands. Perhaps the best course to follow—in view of the economy of gland steam consumption necessary—is as follows:

During the preliminary non-test run, full steam is turned into both glands while the vacuum is being raised, and maintained until full load has been on the turbine for some little time. The vacuum will by this time have probably reached its maximum, and perhaps fallen to a point slightly lower, at which hight it may be expected to remain, other conditions also remaining constant. The gland steam must now be gradually turned off until the amount of steam vapor issuing from the glands is almost imperceptible. This should not lower the vacuum in the slightest degree. By gradual degrees the gland steam can be still farther cut down, until no steam vapor at all can be discerned issuing from the gland boxes. This reduction should be continued until a point is reached at which the vacuum is affected, when it must be stopped and the amount of steam flowing to the gland again increased very slightly, just enough to bring the vacuum again to its original hight. The steam now passing into the glands is the minimum required under the conditions, and should be maintained as nearly constant as possible throughout the test. Practically all steam entering the glands is drawn into the turbine, and thence to the condenser, and under the circumstances it may be assumed the increase in steam consumption arising from this source is also a minimum.

There is one mechanical feature which has an important bearing upon the foregoing question, and which it is one of the tester's duties to investigate. This is illustrated in Fig. 71, which shows a turbine spindle projecting through the casing. The gland box is let into the casing as shown. Brass rings A calked into the gland box encircle the shaft on either side of the annular steam space S. As the clearance between the turbine spindle and the rings A is in a measure instrumental in determining the amount of steam required to maintain a required pressure inside the chamber, it is obvious that this clearance should be minimum. An unnecessarily large clearance means a proportionally large increase in gland steam consumption and vice versa.



When the turbine glands are sealed with water, all water leakage which takes place into the turbine, and ultimately to the condenser hot-well, must be measured and subtracted from the hot-well contents at the end of a test.

The foregoing remarks would not apply to those cases in which the gland supply is drawn from and returned to the hot-well, or a pipe leading from the hot-well. Then no correction would be necessary, as all water used for gland purposes might be assumed as being taken from the measuring tanks and returned again in time for same or next weighing or measurement.

General Considerations

There are a few principal elementary points which it is necessary always to keep in mind during the conduct of a test. Among these are the effects of variation in vacuum, superheat, initial steam pressure, and, as already indicated, in load. There exist many rules for determining the corrections necessitated by this variation. For example, it is often assumed that 9 degrees Fahrenheit, excess or otherwise, above or below that specified, represents an increase or reduction in efficiency of about 1 per cent. It is probable that the percentage increase or decrease in steam consumption, in the case of superheat, can be more reliably calculated than in other cases, as, for example, vacuum; but the increase cannot be said to be due solely to the variation in superheat. In other words, the individuality of the particular turbine being tested always contributes something, however small this something may be, to the results obtained.

These remarks are particularly applicable where vacuum is concerned. Here again rules exist, one of these being that every additional inch of vacuum increases the economy of the turbine by something slightly under half a pound of steam per kilowatt-hour. But a moment's consideration convinces one of the utter unreliability of such rules for general application. It is, for instance, well known that many machines, when under test, have demonstrated that the total increase in the water rate is very far from constant. A machine tested, for example, gave approximately the following results, the object of the test being to discover the total increase in the water rate per inch decrease in vacuum:

From 27 inches to 26 inches, 4.5 per cent.

From 26.2 inches to 24.5 inches, 2.5 per cent.

This illustrates to what an extent the ratio of increase can vary, and it must be borne in mind that it is very probable that the variation is different in different types and sizes of machines.

There can exist, therefore, no empirical rules of a reliable nature upon which the tester can base his deductions. The only way calculated to give satisfaction is to conduct a series of preliminary tests upon the turbine undergoing observation, and from these to deduce all information of the nature required, which can be permanently recorded in a set of curves for reference during the final official tests.

In conclusion, it must be admitted that many published tests outlining the performances of certain makes of turbine are unreliable. To determine honestly the capabilities of any machine in the direction of steam economy is an operation requiring time, and unbiased and accurate supervision. By means of such assets as "floating quantities," short tests during exceptionally favorable conditions, and disregard of the vital necessity of running a test under the proper specified conditions, it is comparatively easy to obtain results apparently highly satisfactory, but which under other conditions might be just the reverse. These considerations are, however, unworthy of the tester proper.



VII. AUXILIARIES FOR STEAM TURBINES[6]

[6] Contributed to Power by Thomas Franklin.

The Jet Condenser

The jet condenser illustrated in Fig. 72 is singularly well adapted for the turbine installation. As the type has not been so widely adopted as the more common forms of jet condenser and the surface types, it may prove of interest to describe briefly its general construction and a few of its special features in relation to tests.



Referring to the figure, C is the main condenser body. Exhaust steam enters at the left-hand side through the pipe E, condensing water issuing through the pipe D at the opposite side. Passing through the short conical pipe P, the condensing water enters the cylindrical chamber W and falls directly upon the spraying cone S. The hight of this spraying cone is determined by the tension upon the spring T, below the piston R, the latter being connected to the cone by a spindle L. An increase of the water pressure inside the chamber W will thus compress the spring, and the spraying cone being consequently lowered increases the aperture between it and the sloping lower wall of the chamber W, allowing a greater volume of water to be sprayed. The piston R incidentally prevents water entering the top vapor chamber V. From the foregoing it can be seen that this condenser is of the contra-flow type, the entering steam coming immediately into contact with the sprayed water. The perforated diaphragm plate F allows the vapor to rise into the chamber V, from which it is drawn through the pipe A to the air pump. A relief valve U prevents an excessive accumulation of pressure in the vapor chamber, this valve being obviously of delicate construction, capable of opening upon a very slight increase of the internal pressure over that of the atmosphere. Condensed steam and circulating water are together carried down the pipe B to the well Z, from which a portion may be carried off as feed water, and the remainder cooled and passed through the condenser again. Under any circumstances, whether the air pump is working or not, a certain percentage of the vapor in the condenser is always carried down the pipe B, and this action alone creates a partial vacuum, thus rendering the work of the air pump easier. As a matter of fact, a fairly high vacuum can be maintained with the air pump closed down, and only the indirect pumping action of the falling water operating to rarify the contents of the condenser body. It is customary to place the condenser forty or more feet above the circulating-water pump, the latter usually being a few feet below the turbine.

Features Demanding Attention

When operating a condenser of this type, the most important features requiring preliminary inspection and regulation while running are:

(a) Circulating-water regulation.

(b) Freedom of all mechanical parts of spraying mechanism.

(c) Relief-valve regulation.

(d) Water-cooling arrangements.

The tester will, however, devote his attention to a practical survey of the condenser and its auxiliaries, before running operations commence.

A preliminary vacuum test ought to be conducted upon the condenser body, and the exhaust piping between the condenser and turbine. To accomplish this the circulating-water pipe D can be filled with water to the condenser level. The relief valve should also be water-sealed. Any existing leakage can thus be located and stopped.

Having made the condenser as tight as possible within practical limits, vacuum might be again raised and, with the same parts sealed, allowed to fall slowly for, say, ten minutes. A similar test over an equal period may then be conducted with the relief valve not water-sealed. A comparison of the times taken for an equal fall of vacuum in inches, under the different conditions, during the above two tests, will reveal the extent of the leakage taking place through the relief valve. It seems superfluous to add that the fall of vacuum in both the foregoing tests must not be accelerated in any way, but must be a result simply of the slight inevitable leakage which is to be found in every system.

On a comparatively steady load, and with consequently only small fluctuation in the volume of steam to be condensed, the conditions are most favorable for regulating the amount of circulating water necessary. Naturally, an excess of water above the required minimum will not affect the pressure conditions inside the condenser. It does, however, increase the quantity of water to be handled from the hot-well, and incidentally lowers the temperature there, which, whether the feed-water pass through economizers or otherwise, is not advisable from an economical standpoint. Thus there is an economical minimum of circulating water to be aimed at, and, as previously stated, it can best be arrived at by running the turbine under normal load and adjusting the flow of the circulating water by regulating the main valve and the tension upon the spring T. Under abnormal conditions, the breakdown of an air pump, or the sudden springing of a bad leak, for instance, the amount of circulating water can be increased by a farther opening of the main valve if necessary, and a relaxation of the spring tension by hand; or, the spring tension might be automatically changed immediately upon the vacuum falling.

The absolute freedom of all moving parts of the spraying mechanism should be one of the tester's first assurances. To facilitate this, it is customary to construct the parts, with the exception of the springs, of brass or some other non-corrosive metal. The spraying cone must be thoroughly clean in every channel, to insure a well-distributed stream of water. Nor is it less important that careful attention be given to the setting and operation of the relief valve, as will be seen later. The obvious object of such a valve is to prevent the internal condenser pressure ever being maintained much higher than the atmospheric pressure. A number of carefully designed rubber flap valves, or one large one, have been found to act successfully for this purpose, although a balanced valve of more substantial construction would appear to be more desirable.

Importance of Relief Valves

The question of relief valves in turbine installations is an important one, and it seems desirable at this point to draw attention to another necessary relief valve and its function, namely the turbine atmospheric valve. As generally understood, this is placed between the turbine and condenser, and, should the pressure in the latter, owing to any cause, rise above that of the atmosphere, it opens automatically and allows the exhaust steam to flow through it into the atmosphere, or into another condenser.

A general diagrammatic arrangement of a steam turbine, condenser, and exhaust piping is shown in Fig. 73. Connected to the exhaust pipe B, near to the condenser, is the automatic atmospheric valve D, from which leads the exhaust piping E to the atmosphere. The turbine relief valve is shown at F, and the condenser relief valve at G. The main exhaust valve between turbine and condenser is seen at H. We have here three separate relief valves: one, F, to prevent excessive pressure in the turbine: the second, D, an atmospheric valve opening a path to the air, and, in addition to preventing excessive pressure accumulating, also helping to keep the temperature of the condenser body and tubes low; the third, the condenser relief valve G, which in itself ought to be capable of exhausting all steam from the turbine, should occasion demand it.



Assuming a plant of this description to be operating favorably, the conditions would of necessity be as follows: The valves F, D, and G, all closed; the valve H open. Suppose that, owing to sudden loss of circulating water, the vacuum fell to zero. The condenser would at once fill with steam, a slight pressure would be set up, and whichever of the three valves happened to be set to blow off at the lowest pressure would do so. Now it is desirable that the first valve to open under such circumstances should be the atmospheric valve D. This being so, the condenser would remain full of steam at atmospheric pressure until the attendant had had time to close the main hand-or motor-operated exhaust valve H, which he would naturally do before attempting to regain the circulation of the condensing water. Again, assume the installation to be running under the initial conditions, with the atmospheric valve D and all remaining valves except H closed.

Suppose the vacuum again fell to zero from a similar cause, and, further, suppose the atmospheric valve D failed to operate automatically. The only valves now capable of passing the exhaust steam are the turbine and condenser relief valves F and G. Inasmuch as the pressures at exhaust in the turbine proper, on varying load, vary over a considerably greater range than the small fairly constant absolute pressures inside the condenser, it is obviously necessary to allow for this factor in the respective setting of these two relief valves. In other words, the obvious deduction is to set the turbine relief valve to blow off at a higher pressure than the condenser relief valve, even when considering the question with respect to condensing conditions only. In this second hypothetical case, then, with a closed and disabled atmospheric valve, the exhaust must take place through the condenser, until the turbine can be shut down, or the circulating water regained without the former course being found necessary.

There is one other remote case which may be assumed, namely, the simultaneous refusal of both atmospheric and condenser relief valves to open, upon the vacuum inside the condenser being entirely lost. The exhaust would then be blown through the turbine relief valve F, until the plant could be closed down.

Although the conditions just cited are highly improbable in actual practice, it can at once be seen that to insure the safety of the condenser, absolutely, the turbine relief valve must be set to open at a comparatively low pressure, say 40 pounds by gage, or thereabouts. To set it much lower than this would create a possibility of its leaking when the turbine was making a non-condensing run, and when the pressure at the turbine exhaust end is often above that of the atmosphere. From every point of view, therefore, it is advisable to make a minute examination of all relief valves in a system, and before a test to insure that these valves are all set to open at their correct relative pressures.

It must be admitted that the practice of placing a large relief valve upon a condenser in addition to the atmospheric exhausting valve is by no means common. The latter valve, where surface condensing is adopted, is often thought sufficient, working in conjunction with a quickly operated main exhaust valve. Similarly, with a barometric condenser as that illustrated in Fig. 72, the atmospheric exhaust valve D (seen in Fig. 73) is sometimes dispensed with. This course is, however, objectionable, for upon a loss of vacuum in the turbine, all exhaust steam must pass through the condenser body, or the entire plant be closed down until the vacuum is regained. The simple construction of the barometric condenser, however, is in such an event much to its advantage, and the passage of the hot steam right through it is not likely to seriously warp or strain any of its parts, as might probably happen in the case of a surface condenser.

The question of the advisability of thus adding to a plant can only be fairly decided when all conditions, operating and otherwise, are fully known. For example, if we assume a large turbine to be operating on a greatly varying load, and exhausting into a condenser, as that in Fig. 72, and, further, having an adequate stand-by to back it up, one's obvious recommendation would be to equip the installation with both a condenser relief valve and an atmospheric valve, in addition, of course, to the main exhaust valve, which is always placed between the atmospheric valve and condenser. There are still other considerations, such as water supply, condition of circulating water, style of pump, etc., which must all necessarily have an obvious bearing upon the settlement of this question; so that generalization is somewhat out of place, the final design in all cases depending solely upon general principles and local conditions.

Other Necessary Features of a Test

In connection with the condenser, of any type, and its auxiliaries, there remain a few necessary examinations and operations to be conducted, if it is desired to obtain the very best results during the test. It will be sufficient to just outline them, the method of procedure being well known, and the requirement of any strict routine being unnecessary. These include:

(1) A thorough examination of the air-pump, and, if possible, an equally careful examination of diagrams taken from it when running on full load. Also careful examination of the piping, and of any other connections between the air pump and condenser, or other auxiliaries. It will be well in this examination to note the general "lay" of the air pipes, length, hight to which they rise above condenser and air pump, facilities for drainage, etc., as this information may prove valuable in determining the course necessary to rectify deficiencies which may later be found to exist.

(2) In a surface condenser, inspection of the pumps delivering condensed steam to the measuring tanks or hot-well; inspection of piping between the condenser and the pump, and also between the pump and measuring tanks. If these pumps are of the centrifugal type it is essential to insure, for the purposes of a steam-consumption test, as much regularity of delivery as possible.

(3) In the case of a consumption test upon a turbine exhausting into a barometric condenser, and where the steam consumed is being measured by the evaporation in the boiler over the test period, time must be devoted to the feed-pipes between the feed-water measuring meter or tank and the boilers. Under conditions similar to those operating in a plant such as that shown in Fig. 72, the necessary boiler feed might be drawn from the hot-well, the remainder of the hot-well contents probably being pumped through water coolers, or towers, for circulating through the condenser. With the very best system, it is possible for a slight quantity of oil to leak into the exhaust steam, and thence to the hot-well. In its passage, say along wooden conduits, to the measuring tank or meter, this water would probably pass through a number of filters. The efficiency of these must be thoroughly insured. It is unusual, in those cases where a simple turbine steam-consumption test is being carried out, and not an efficiency test of a complete plant, to pass the measured feed-water through economizers. Should the latter course, owing to special conditions, become necessary, a careful examination of all economizer pipes would be necessary.

(4) The very careful examination of all thermometer pockets, steam- and temperature-gage holes, etc., as to cleanliness, non-accumulation of scale, etc.

Special Auxiliaries Necessary

Having outlined the points of interest and importance in connection with the more permanent features of a plant, we arrive at the preparation and fitting of those special auxiliaries necessary to carry on the test.



It is customary, when carrying out a first test, upon both prime mover and auxiliaries, to place every important stage in the expansion in communication with a gage, so that the various pressures may be recorded and later compared with the figures of actual requirement. To do this, in the case of the turbine, it is necessary to bore holes in the cover leading to the various expansion chambers, and into each of these holes to screw a short length of steam pipe, having preferably a loop in its length, to the other end of which the gage is attached. Fig. 74 illustrates, diagrammatically, a complete turbine installation, and shows the various points along the course taken by the steam at which it is desirable to place pressure gages. The figure does not show the high-pressure steam pipe, nor any of the turbine valves. With regard to these, it will be desirable to place a steam gage in the pipe, immediately before the main stop-valve, and another immediately after it. Any fall of pressure between the two sides of the valve can thus be detected. To illustrate this clearly, Fig. 75 is given, showing the valves of a turbine, and the position of the gages connected to them. The two gages E and F on either side of the main stop-valve A are also shown. The steam after passing through the valve, which, in the case of small turbines, is hand-operated, goes in turn through the automatic stop-valve B, the function of which is to automatically shut steam off should the turbine attain a predetermined speed above the normal, the steam strainer C, and finally through the governing valve D into the turbine. As shown, gages G and H are also fitted on either side of the strainer, and these, in conjunction with gages E and F, will enable any fall in pressure between the first two valves and the governing valve to be found. Up to the governing-valve inlet no throttling of the steam ought to take place under normal conditions, i.e., with all valves open, and consequently any fall in pressure between the steam inlet and this point must be the result of internal wire-drawing. By placing the gages as shown, the extent to which this wire-drawing affects the pressures obtainable can be discovered.



On varying and even on normal and steady full load, the steam is more or less reduced in pressure after passing through the governing valve D; a gage I must consequently be placed between the valve, preferably on the valve itself, and the turbine. Returning to Fig. 74, the gages shown are A, B, C, D, and E, connected to the first, second, third, fourth, and fifth expansions; also F in the turbine and exhaust space, where there are no blades, G in the exhaust pipe immediately before the main exhaust valve E (see Fig. 73), and H connected to the condenser. On condensing full load it is probable that A, B, and C will all register pressures above the atmosphere, while gages D, E, F, and G will register pressures below the atmosphere, being for this purpose vacuum gages. On the other hand, with a varying load, and consequently varying initial pressures, one or two of the gages may register pressure at one moment and vacuum at another. It will therefore be necessary to place at these points compound gages capable of registering both pressure and vacuum. With the pressures in the various stages constantly varying, however, a gage is not by any means the most reliable instrument for recording such variations. The constant swinging of the finger not only renders accurate reading at any particular moment both difficult and, to an extent, unreliable, but, in addition, the accompanying sudden changes of condition, both of temperature and pressure, occurring inside the gage tube, in a comparatively short time permanently warp this part, and thus altogether destroy the accuracy of the gage. It is well known that even with the best steel-tube gages, registering comparatively steady pressures, this warping of the tube inevitably takes place. The quicker deterioration of such gage tubes, when the gage is registering quickly changing pressures, can therefore readily be conceived, and for this reason alone it is desirable to have all gages, whatever the conditions under which they work, carefully tested and adjusted at short intervals. If it is desired to obtain reliable registration of the several pressures in the different expansions of a turbine running on a varying load, it would therefore seem advisable to obtain these by some type of external spring gage (an ordinary indicator has been found to serve well for this purpose) which the sudden internal variations in pressure and temperature cannot deleteriously affect.

In view of the great importance he must attach to his gage readings, the tester would do well to test and calibrate and adjust where necessary all the gages he intends using during a test. This he can do with a standard gage-testing outfit. By this means only can he have full confidence in the accuracy of his results.

In like manner it is his duty personally to supervise the connecting and arrangement of the gages, and the preliminary testing for leakage which can be carried out simultaneously with the vacuum test made upon the turbine casing.

Where Thermometers are Required

Equally important with the foregoing is the necessity of calibrating and testing of all thermometers used during a test. Where possible it is advisable to place new thermometers which have been previously tested at all points of high temperature. Briefly running them over, the points at which it is necessary to place thermometers in the entire system of the steam and condensing plant are as follows:

(1) A thermometer in the steam pipe on the boiler, where the pipe leaves the superheater.

(2) In the steam pipe immediately in front of the main stop-valve, near point E in Fig. 75.

(3) In the main governing valve body (see I, Fig. 75) on the inlet side.

(4) In the main governing valve body on the turbine side, which will register temperatures of steam after it has passed through the valve.

(5) In the steam-turbine high-pressure chamber, giving the temperature of the steam before it has passed through any blades.

(6) In the exhaust chamber, giving the temperature of steam on leaving the last row of blades.

(7) In the exhaust pipe near the condenser.

(8) In the condenser body.

(9) In the circulating-water inlet pipe close to the condenser.

(10) In the circulating-water outlet pipe close to the condenser.

(11) In the air-pump suction pipe close to the condenser.

(12) In the air-pump suction pipe close to the air pump.

It is not advisable to place at those vital points, the readings at which directly or indirectly affect the consumption, two thermometers, say one ordinary chemical thermometer and one thermometer of the gage type, thus eliminating the possibility of any doubt which might exist were only one thermometer placed there.

There is no apparent reason why one should attempt to take a series of temperature readings during a consumption test on varying load. The temperatures registered under a steady load test can be obtained with great reliability, but on a varying load, with constantly changing temperatures at all points, this is impossible. This is, of course, owing to the natural sluggishness of the temperature-recording instruments, of whatever class they belong to, in responding to changes of condition. As a matter of fact, the possibility of obtaining correctly the entire conditions in a system running under greatly varying loads is very doubtful indeed, and consequently great reliance cannot be placed upon figures obtained under such conditions.

A few simple calculations will reveal to the tester his special requirements in the direction of measuring tanks, piping, etc., for his steam consumption test. Thus, assuming the turbine to be tested to be of 3000 kilowatt capacity normal load, with a guaranteed steam consumption of, say, 14.5 pounds per kilowatt-hour, he calculates the total water rate per hour, which in this case would be 43,500 pounds, and designs his weighing or measuring tanks to cope with that amount, allowing, of course, a marginal tank volume for overload requirements.



VIII. TROUBLES WITH STEAM TURBINE AUXILIARIES[7]

[7] Contributed to Power by Walter B. Gump.

The case about to be described concerns a steam plant in which there were seven cross-compound condensing Corliss engines, and two Curtis steam turbines. The latter were each of 1500-kilowatt capacity, and were connected to surface condensers, dry-vacuum pumps, centrifugal, hot-well and circulating pumps, respectively. In the illustration (Fig. 76), the original lay-out of piping is shown in full lines. Being originally a reciprocating plant it was difficult to make the allotted space for the turbines suitable for their proper installation. The trouble which followed was a perfectly natural result of the failure to meet the requirements of a turbine plant, and the description herein given is but one example of a great many where the executive head of a concern insists upon controlling the situation without regard to engineering advice or common sense.