fairly decided when all conditions, operating and otherwise, are fully
known. For example, if we assume a large turbine to be operating on a
greatly varying load, and exhausting into a condenser, as that in Fig.
72, and, further, having an adequate stand-by to back it up, one's
obvious recommendation would be to equip the installation with both a
condenser relief valve and an atmospheric valve, in addition, of course,
to the main exhaust valve, which is always placed between the
atmospheric valve and condenser. There are still other considerations,
such as water supply, condition of circulating water, style of pump,
etc., which must all necessarily have an obvious bearing upon the
settlement of this question; so that generalization is somewhat out of
place, the final design in all cases depending solely upon general
principles and local conditions.
Other Necessary Features of a Test
In connection with the condenser, of any type, and its auxiliaries,
there remain a few necessary examinations and operations to be
conducted, if it is desired to obtain the very best results during the
test. It will be sufficient to just outline them, the method of
procedure being well known, and the requirement of any strict routine
being unnecessary. These include:
(1) A thorough examination of the air-pump, and, if possible, an equally
careful examination of diagrams taken from it when running on full
load. Also careful examination of the piping, and of any other
connections between the air pump and condenser, or other auxiliaries. It
will be well in this examination to note the general "lay" of the air
pipes, length, hight to which they rise above condenser and air pump,
facilities for drainage, etc., as this information may prove valuable in
determining the course necessary to rectify deficiencies which may later
be found to exist.
(2) In a surface condenser, inspection of the pumps delivering condensed
steam to the measuring tanks or hot-well; inspection of piping between
the condenser and the pump, and also between the pump and measuring
tanks. If these pumps are of the centrifugal type it is essential to
insure, for the purposes of a steam-consumption test, as much regularity
of delivery as possible.
(3) In the case of a consumption test upon a turbine exhausting into a
barometric condenser, and where the steam consumed is being measured by
the evaporation in the boiler over the test period, time must be devoted
to the feed-pipes between the feed-water measuring meter or tank and the
boilers. Under conditions similar to those operating in a plant such as
that shown in Fig. 72, the necessary boiler feed might be drawn from the
hot-well, the remainder of the hot-well contents probably being pumped
through water coolers, or towers, for circulating through the condenser.
With the very best system, it is possible for a slight quantity of oil
to leak into the exhaust steam, and thence to the hot-well. In its
passage, say along wooden conduits, to the measuring tank or meter, this
water would probably pass through a number of filters. The efficiency of
these must be thoroughly insured. It is unusual, in those cases where a
simple turbine steam-consumption test is being carried out, and not an
efficiency test of a complete plant, to pass the measured feed-water
through economizers. Should the latter course, owing to special
conditions, become necessary, a careful examination of all economizer
pipes would be necessary.
(4) The very careful examination of all thermometer pockets, steam- and
temperature-gage holes, etc., as to cleanliness, non-accumulation of
scale, etc.
Special Auxiliaries Necessary
Having outlined the points of interest and importance in connection with
the more permanent features of a plant, we arrive at the preparation and
fitting of those special auxiliaries necessary to carry on the test.
[Illustration: FIG. 74]
It is customary, when carrying out a first test, upon both prime mover
and auxiliaries, to place every important stage in the expansion in
communication with a gage, so that the various pressures may be recorded
and later compared with the figures of actual requirement. To do this,
in the case of the turbine, it is necessary to bore holes in the cover
leading to the various expansion chambers, and into each of these holes
to screw a short length of steam pipe, having preferably a loop in its
length, to the other end of which the gage is attached. Fig. 74
illustrates, diagrammatically, a complete turbine installation, and
shows the various points along the course taken by the steam at which it
is desirable to place pressure gages. The figure does not show the
high-pressure steam pipe, nor any of the turbine valves. With regard to
these, it will be desirable to place a steam gage in the pipe,
immediately before the main stop-valve, and another immediately after
it. Any fall of pressure between the two sides of the valve can thus be
detected. To illustrate this clearly, Fig. 75 is given, showing the
valves of a turbine, and the position of the gages connected to them.
The two gages E and F on either side of the main stop-valve A are also
shown. The steam after passing through the valve, which, in the case of
small turbines, is hand-operated, goes in turn through the automatic
stop-valve B, the function of which is to automatically shut steam off
should the turbine attain a predetermined speed above the normal, the
steam strainer C, and finally through the governing valve D into the
turbine. As shown, gages G and H are also fitted on either side of the
strainer, and these, in conjunction with gages E and F, will enable any
fall in pressure between the first two valves and the governing valve to
be found. Up to the governing-valve inlet no throttling of the steam
ought to take place under normal conditions, i.e., with all valves open,
and consequently any fall in pressure between the steam inlet and this
point must be the result of internal wire-drawing. By placing the gages
as shown, the extent to which this wire-drawing affects the pressures
obtainable can be discovered.
[Illustration: FIG. 75]
On varying and even on normal and steady full load, the steam is more or
less reduced in pressure after passing through the governing valve D; a
gage I must consequently be placed between the valve, preferably on the
valve itself, and the turbine. Returning to Fig. 74, the gages shown are
A, B, C, D, and E, connected to the first, second, third, fourth, and
fifth expansions; also F in the turbine and exhaust space, where there
are no blades, G in the exhaust pipe immediately before the main exhaust
valve E (see Fig. 73), and H connected to the condenser. On condensing
full load it is probable that A, B, and C will all register pressures
above the atmosphere, while gages D, E, F, and G will register pressures
below the atmosphere, being for this purpose vacuum gages. On the other
hand, with a varying load, and consequently varying initial pressures,
one or two of the gages may register pressure at one moment and vacuum
at another. It will therefore be necessary to place at these points
compound gages capable of registering both pressure and vacuum. With the
pressures in the various stages constantly varying, however, a gage is
not by any means the most reliable instrument for recording such
variations. The constant swinging of the finger not only renders
accurate reading at any particular moment both difficult and, to an
extent, unreliable, but, in addition, the accompanying sudden changes of
condition, both of temperature and pressure, occurring inside the gage
tube, in a comparatively short time permanently warp this part, and thus
altogether destroy the accuracy of the gage. It is well known that even
with the best steel-tube gages, registering comparatively steady
pressures, this warping of the tube inevitably takes place. The quicker
deterioration of such gage tubes, when the gage is registering quickly
changing pressures, can therefore readily be conceived, and for this
reason alone it is desirable to have all gages, whatever the conditions
under which they work, carefully tested and adjusted at short intervals.
If it is desired to obtain reliable registration of the several
pressures in the different expansions of a turbine running on a varying
load, it would therefore seem advisable to obtain these by some type of
external spring gage (an ordinary indicator has been found to serve well
for this purpose) which the sudden internal variations in pressure and
temperature cannot deleteriously affect.
In view of the great importance he must attach to his gage readings, the
tester would do well to test and calibrate and adjust where necessary
all the gages he intends using during a test. This he can do with a
standard gage-testing outfit. By this means only can he have full
confidence in the accuracy of his results.
In like manner it is his duty personally to supervise the connecting and
arrangement of the gages, and the preliminary testing for leakage which
can be carried out simultaneously with the vacuum test made upon the
turbine casing.
Where Thermometers are Required
Equally important with the foregoing is the necessity of calibrating and
testing of all thermometers used during a test. Where possible it is
advisable to place new thermometers which have been previously tested
at all points of high temperature. Briefly running them over, the points
at which it is necessary to place thermometers in the entire system of
the steam and condensing plant are as follows:
(1) A thermometer in the steam pipe on the boiler, where the pipe leaves
the superheater.
(2) In the steam pipe immediately in front of the main stop-valve, near
point E in Fig. 75.
(3) In the main governing valve body (see I, Fig. 75) on the inlet side.
(4) In the main governing valve body on the turbine side, which will
register temperatures of steam after it has passed through the valve.
(5) In the steam-turbine high-pressure chamber, giving the temperature
of the steam before it has passed through any blades.
(6) In the exhaust chamber, giving the temperature of steam on leaving
the last row of blades.
(7) In the exhaust pipe near the condenser.
(8) In the condenser body.
(9) In the circulating-water inlet pipe close to the condenser.
(10) In the circulating-water outlet pipe close to the condenser.
(11) In the air-pump suction pipe close to the condenser.
(12) In the air-pump suction pipe close to the air pump.
It is not advisable to place at those vital points, the readings at
which directly or indirectly affect the consumption, two thermometers,
say one ordinary chemical thermometer and one thermometer of the gage
type, thus eliminating the possibility of any doubt which might exist
were only one thermometer placed there.
There is no apparent reason why one should attempt to take a series of
temperature readings during a consumption test on varying load. The
temperatures registered under a steady load test can be obtained with
great reliability, but on a varying load, with constantly changing
temperatures at all points, this is impossible. This is, of course,
owing to the natural sluggishness of the temperature-recording
instruments, of whatever class they belong to, in responding to changes
of condition. As a matter of fact, the possibility of obtaining
correctly the entire conditions in a system running under greatly
varying loads is very doubtful indeed, and consequently great reliance
cannot be placed upon figures obtained under such conditions.
A few simple calculations will reveal to the tester his special
requirements in the direction of measuring tanks, piping, etc., for his
steam consumption test. Thus, assuming the turbine to be tested to be of
3000 kilowatt capacity normal load, with a guaranteed steam consumption
of, say, 14.5 pounds per kilowatt-hour, he calculates the total water
rate per hour, which in this case would be 43,500 pounds, and designs
his weighing or measuring tanks to cope with that amount, allowing, of
course, a marginal tank volume for overload requirements.
VIII. TROUBLES WITH STEAM TURBINE AUXILIARIES[7]
[7] Contributed to _Power_ by Walter B. Gump.
The case about to be described concerns a steam plant in which there
were seven cross-compound condensing Corliss engines, and two Curtis
steam turbines. The latter were each of 1500-kilowatt capacity, and were
connected to surface condensers, dry-vacuum pumps, centrifugal, hot-well
and circulating pumps, respectively. In the illustration (Fig. 76), the
original lay-out of piping is shown in full lines. Being originally a
reciprocating plant it was difficult to make the allotted space for the
turbines suitable for their proper installation. The trouble which
followed was a perfectly natural result of the failure to meet the
requirements of a turbine plant, and the description herein given is but
one example of a great many where the executive head of a concern
insists upon controlling the situation without regard to engineering
advice or common sense.
[Illustration: FIG. 76. TURBINE AUXILIARIES AND PIPING]
Circulating Pump Fails to Meet Guarantee
Observing the plan view, it will be seen that the condensers for both
turbines receive their supply of cooling water from the same supply
pipe; that is, the pipes, both suction and discharge, leading to No. 1
condenser are simply branches from No. 2, which was installed first
without consideration for a second unit. When No. 1 was installed there
was a row of columns from the basement floor to the main floor extending
in a plane which came directly in front of the condenser. The column P
shown in the plan was so located as to prevent a direct connection
between the centrifugal circulating pump and the condenser inlet. The
centrifugal pump was direct-connected to a vertical high-speed engine,
and the coupling is shown at E in the elevation.
Every possible plan was contemplated to accommodate the engine and pump
without removing any of the columns, and the arrangement shown was
finally adopted, leaving the column P in its former place by employing
an S-connection from the pump to the condenser. It should be stated that
the pump was purchased under a guarantee to deliver 6000 gallons per
minute under a head of 50 feet, with an impeller velocity of 285
revolutions per minute. The vertical engine to which the pump was
connected proved to be utterly unfit for running at a speed beyond 225
to 230 revolutions per minute, and in addition the S-bend would
obviously reduce the capacity, even at the proper speed of the impeller.
Besides these factors there was another feature even more serious. It
was found that when No. 2 unit was operating No. 1 could not get as
great a quantity of circulating water as when No. 2 was shut down. This
was because No. 2 was drawing most of the water, and No. 1 received only
that which No. 2 could not pull from the suction pipe A. This will be
clear from the fact that the suction and discharge pipes for No. 1 were
only 16 inches, while those of No. 2 were 20 inches and 16 inches,
respectively. The condenser for No. 2 had 1000 square feet less cooling
surface than No. 1, which had 6000 square feet and was supplied with
cooling water by means of two centrifugal pumps of smaller capacity
than for No. 1 and arranged in parallel. These were each driven by an
electric motor, and were termed "The Siamese Twins," due to the way in
which they were connected.
The load factor of the plant ranged from 0.22 to 0.30, the load being
almost entirely lighting, so that for the winter season the load factor
reached the latter figure. The day load was, therefore, light and not
sufficient to give one turbine more than from one-fourth to one-third
its rated capacity. Under these conditions No. 1 unit was able to
operate much more satisfactorily than when fully loaded, because of the
fact that the cooling water was more effective. This was, of course, all
used by No. 1 unit when No. 2 was not operating. At best, however, it
was found that the vacuum could not be made to exceed 24 inches, and
during the peak, with the two turbines running, the vacuum would often
drop to 12 inches. A vacuum of 16 inches or 18 inches on the peak was
considered good.
An Investigation
Severe criticism "rained" heavily upon the engineer in charge, and
complaints were made in reference to the high oil consumption. An
investigation on the company's part followed, and the firm which
furnished the centrifugal pump and engine was next in order to receive
complaints. Repeated efforts were made to increase the speed of the
vertical engine to 285 revolutions per minute, but such a speed proved
detrimental to the engine, and a lower speed of about 225 revolutions
per minute had to be adopted.
A thorough test on the pump to ascertain its delivery at various speeds
was the next move, and a notched weir, such as is shown in the
elevation, was employed. The test was made on No. 2 cooling tower, not
shown in the sketch, and showed that barely 3000 gallons per minute were
being delivered to the cooling tower. While the firm furnishing the pump
was willing to concede that the pump might not be doing all it should,
attention was called to the fact that there might be some other
conditions in connection with the system which were responsible for the
losses. Notable among these was the hydraulic friction, and when this
feature of the case was presented, the company did not seem at all
anxious to investigate the matter further; obviously on account of
facing a possible necessity for new piping or other apparatus which
might cost something.
Approximately 34 feet was the static head of water to be pumped over No.
2 cooling tower. Pressure gages were connected to the suction,
discharge, and condenser inlet, as shown at G, G' and G'' respectively.
When No. 1 unit was operating alone the gage G showed practically zero,
indicating no vacuum in the suction pipe. Observing the same gage when
No. 2 unit was running, a vacuum as high as 2 pounds was indicated,
showing that No. 2 was drawing more than its share of cooling water from
the main A and hence the circulating pump for No. 1 was fighting for all
it received. Gage G' indicated a pressure of 21 pounds, while G''
indicated 18.5 pounds, showing a difference of 2.5 pounds pressure lost
in the S-bend. This is equivalent to a loss of head of nearly 6 feet,
0.43 pound per foot head being the constant employed. The total head
against which the pump worked was therefore
G' + G = 21 + 2,
or
23
---- = 53
0.43
feet approximately. Since the static head was 34 feet, the head lost in
friction was evidently
53-34 = 19
feet, or
1900
---- = 36
53
per cent., approximately.
Supply of Cooling Water Limited
In addition to this the supply of cooling water was limited, the vacuum
being extremely low at just the time when efficient operation should be
had. The natural result occurred, which was this: As the load on the
turbine increased, the amount of steam issuing into the condenser
increased, beating [Transcriber: heating?] the circulating water to a
temperature which the cooling tower (not in the best condition) was
unable to decrease to any great extent. The vacuum gradually dropped
off, which indicated that the condenser was being filled with vapor, and
in a short time the small centrifugal tail-pump lost its prime,
becoming "vapor bound," and the vacuum further decreased. The steam
which had condensed would not go into the tail-pump because of the
tendency of the dry-pump to maintain a vacuum. When a certain point was
reached the dry-vacuum pump started to draw water in its cylinder, and
the unit had to be shut down immediately.
Vapor-bound Pumps
As the circulating water gradually rose in temperature the circulating
pump also became "vapor bound," so that the unit would be tied up for
the rest of the night, as this pump could not be made to draw hot water.
The reason for this condition may be explained in the following way.
When the circulating pump was operating and there was a suction of 2
pounds indicated at G, the water was not flowing to the pump of its own
accord, but was being pulled through by force. This water would flow
through the pump until a point was reached when the water became hot
enough to be converted into vapor, this occurring at a point where the
pressure was sufficiently reduced to cause the water to boil. Naturally
this point was in the suction pipe and vapor was thus maintained behind
the pump as long as it was operating. In this case the pump was merely
maintaining a partial vacuum, but not drawing water. After the vacuum
was once lost, by reason of the facts given, it could not be regained,
as the circulating water, piping and condenser required a considerable
period of time in which to cool.
Before any radical changes were made it was decided that a man should
crawl in the suction pipe A, and remove such sand, dirt, or any other
obstacles as were believed to cause the friction. After this had been
done and considerable sand had been removed, tests were resumed with
practically the same results as before. The investigation was continued
and the dry-vacuum pumps were overhauled, as they had been damaged by
water in the cylinders, and furthermore needed re-boring. In short, the
auxiliaries were restored to the best condition that could be brought
about by the individual improvement of each piece of apparatus. As this
was not the seat of the trouble, however, the remedy failed to effect a
"cure." It was demonstrated that the steam consumption of the turbines
was greatly increased due to priming of the boilers, as well as
condensation in the turbine casing; hence, the ills above mentioned were
aggravated.
Changes in Piping
After a great deal of argument from the chief engineer, and the firm
which furnished the pump, both making a strong plea for a change in the
piping, the company accepted the inevitable, and the dotted portion
shows the present layout. The elbow M was removed, and a tee put in its
place to which the piping D was connected. The circulating pump was
removed to the position shown, and a direct connection substituted for
the S-bend. The discharge pipe C was carried from No. 1 unit separately,
as shown in the elevation, and terminated at No. 1 cooling tower
instead of No. 2, which shortened the distance about 60 feet, the total
length of pipe (one way) from No. 1 unit being originally 250 feet. In
this way the condensing equipment was made practically separate for each
turbine, as it should have been in the first place.
With the new piping a vacuum of 24 inches on the peak could be reached.
While this is far from an efficient value, yet it is better than the
former figure. The failure to reach a vacuum of 28 inches or better is
due primarily to a lack of cooling water, but an improvement in this
regard could be made by reconstructing the cooling towers, which at
present do not offer the proper amount of cooling surface. The screens
used were heavy galvanized wire of about 3/16-inch mesh, which became
coated in a short time, and must be thoroughly cleaned to permit the
water to drop through them. The supply of cooling water was taken from a
30-inch pipe line several miles long and fed from a spring. The amount
of water varied considerably and was at times quite insufficient for the
load on the plant. Instead of meeting this condition with the best
apparatus possible, a chain of difficulties were added to it, with the
results given.
INDEX
Acceleration, rate of, 147
Adjustment, axial, 65
making, 66
Air-pump, examining, 163
Allis-Chalmers Co. steam turbine, 41
Auxiliaries, 2, 154
special, 165
Auxiliary plant for consumption test, 137
spring on governor dome, 28
Axial adjustment, 65
Baffler, 36
functions, 39
Bearings, main, 69
Blades, construction details, 44
inspecting, 104
Blading, Allis-Chalmers turbine, 48
Westinghouse-Parsons turbine, 59, 92
Blueprints, studying, 11
Buckets, moving, 14
stationary, 14
Bushings, 36
Carbon packing, 19
ring, 20
Central gravity oiling system, 111
Circulating pump fails to meet guarantee, 172
Clearance, 15, 150
adjusting, 18
between moving and stationary buckets, 4
gages, 17
measuring, 18
radial, 63
Comma lashing, 95
Condensers, 108, 131
jet, 154
Conditions for successful operation, 105
Cooling water supply limited, 177
Coupling, 127
Cover-plate, 4
-plate, lowering, 9
Curtis turbine, 11
turbine in practice, 1
setting valves, 31, 32
De Laval turbines, 118
Draining system, 105
Dummy leakage, 115
pistons, 63, 65
rings, 43, 113, 114
Equalizing pipes, 64
Exhaust end of turbine, 107
pipe, 107
Expanding nozzles, 14
Feed-pipes, 164
Flow, rate, 38
Foundation drawings, 2
rings, 44, 46
Fourth-stage wheel, 14
Franklin, Thomas, 112, 137, 154
Gages, calibrating and adjusting, 169
clearance, 17
for test work, 165
Generator, 53
Glands, examination for scale, 104
packing, 71, 77
regulation, 148