Importance of Oiling System and Water Service

FIG. 60FIG. 60

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 thisAis the cylinder or casing,Bthe spindle or rotor, andCthe blades. The balancing pistons,D,E, andF, the pressure upon which counterbalances the axial thrust upon the three-bladed stages, are grooved, the brass dummy ringsG Gin the cylinder being alined within a few thousandths of an inch of the groovedwalls, 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 ringsCare consequently within a few thousandths of an inch of the wallsdof the spindle dummy groovesD. 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.

FIG. 61FIG. 61

It has sometimes been found that the groove walls, under the combined action of superheated steam and friction, in cases where actual running contact hasoccurred, 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 conductduring a prolonged run. In Fig.62, showing the spindle,Bis the thrust (made in halves), the ringsOof which fit into the grooved thrust-ringsCin the spindle. Two lugsDare 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 leversE, the latter being fulcrumed atFin the thrust-bearing cover. The screwsG, working in bushes, also fit into the thrust-bearing cover, and are capable of pushing against the ends of the leversEand thus adjusting the separate halves of the block in opposite directions.

FIG. 62FIG. 62

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 screwG. This screw is then locked in position and the top half ofthe 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 screwsGup 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 thoroughexamination 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.

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:

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.63is 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;Bis a cast-iron partition dividing the oil chamberCfrom the oil-cooling chamberD. 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 especiallyof 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.

FIG. 63FIG. 63

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 increasedin 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.

FIG. 64FIG. 64

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 considerablelength usually reaches a maximum early, afterward falling to a temperature about which the fluctuations are only slight during the remainder of the run. Fig.64illustrates 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.63again reveals at once a weakness in that design, namely, the unnecessarily close proximity in which the oil and water tanks are placed.

FIG. 65FIG. 65

A design of thermometer cup suitable for oil thermometers is given in Fig.65in whichAis an end view of the turbine bedplate,Bis a turbine bearing andCandDare 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 whereAis the steel tee piece, into which is screwed the brass thermometer cupB. 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.

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, and68are 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 certainwant of alinement between the two spindles without in any way affecting the smooth running of the whole unit.

FIG. 66FIG. 66

Fig. 67Fig.67

In Fig.66Ais the turbine spindle end andBthe 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 couplingC, 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 spindlesAandBwill 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 accommodateitself to a little want of alinement is the inherent cause.

Looking at the coupling illustrated in Fig.67, itwill be seen that something here is much better adapted to dealing with troubles of alinement. The turbine and generator spindlesAandB, respectively, are coned at the ends, and upon these tapered portions are shrunk circular headsCandDhaving 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 sleeveE. The nutsFandGprevent any lateral movement of the coupling headsCandD. 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.

FIG. 68FIG. 68

The type illustrated in Fig.68is 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,AandB, have toothed headsCandDshrunk upon them,the heads being secured by the nutsEandF. The teeth in this case are cut in the enlarged ends as shown. A sleeveG, 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.

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 pipeTand enters the condenser at the top as shown, where it at once comes into contact with the water tubes inW. These tubes fill an annular area, the central un-tubed portion below the baffle capBforming the vapor chamber. The condensed steam falls upon the bottom tube-platePand is carried away by the pipeSleading to the water pumpH. The Y pipeEterminating above the level of the water in the condenserenters the dry-air pump section pipeA. Cold circulating water enters the condenser at the bottom, through the pipeI, and entering the water chamberXproceeds upward through the tubes into the top-water chamberY, and from there out of the condenser through the exit pipe. It will be observed that the vapor extracted through the platePpasses on its journey out of the condenser through the cooling chamberDsurrounded by the cold circulating water. This, of course, is a very advantageous feature. AtRis the condenser relief, atUthe relief valve for the water chambers.

FIG. 69FIG. 69

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.

The condenser is first thoroughly dried out, particular care being given to the outside of the tubes and the bottom tube-plateP. 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 aleak 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-platePand 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 chamberWand 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.

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 closingcommunication 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.

[4]Contributed toPowerby Thomas Franklin.

Thereare one or two points of importance in the conduct of a test on a turbine and these will be briefly touched upon. Fig.70illustrates 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 pipeAto the pump, and is then forced along the pipeB(leading under ordinary circumstances to the hot-well), through the main water valveCdirectly to the measuring tanks. To enter these the water has to pass through the valvesDandE, while the valvesFandGare for quickly emptying the tanks when necessary, being of a larger bore than the inlet valves. The inlet pipesHIare 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 thehot-well. LeversKandLfulcrumed atJandJare connected to the valve spindles by auxiliary levers. The valve arrangement is such that by pulling down the leverKthe inlet valveDis opened and the inlet valveEis closed. Again, by pulling down the leverLthe outlet valveFis closed, while the outlet valveGis also simultaneously closed.

FIG. 70FIG. 70

During a consumption test the valves are operated in the following manner: The leverKis pulled down, which opens the inlet valve to the first tank and closes that to the second. The bottom leverL, however, is lifted, which for the time being opens the outlet valveF, and incidentally opens the valveG; the latter valve can; however, for the moment be neglected. When the turbine is started, and the condensed steambegins 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, leverLis quickly pulled down and the valvesFandGclosed. The first tank now gradually fills, and after a definite period, say fifteen minutes, the leverKis 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, wouldnecessitate 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 itwould 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.

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 surroundingconditions. The very first considerations, when undertaking to carry out a consumption test, should be devoted to obtaining the steadiest possiblelead. 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 erectingthe 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 governorspring 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.

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:

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