Part 6

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