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.
[Illustration: FIG. 66]
[Illustration: FIG. 67]
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.
[Illustration: FIG. 68]
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.
[Illustration: FIG. 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.
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.
[Illustration: FIG. 70]
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.