Part 9

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