rated capacity that stage is taking all the steam for which the blade
area will afford a passage. The port connecting the upper side of the
piston N with the exhaust may be permanently closed by means of the hand
valve Q, to be found on the side of the secondary pilot valve chest,
thus cutting the secondary valve entirely out of action. No dashpot is
necessary on this valve, the compression of the steam in the chamber W
by the fall of the piston being sufficient to avoid shock.
The timing of the secondary valve is adjusted by raising or lowering the
pilot valve by means of the adjustment provided. It should open soon
enough so that there will not be an appreciable drop in speed before the
valve comes into play. The economy of the machine will be impaired if
the valve is allowed to open too soon.
Safety Stop Governor
This device is mounted on the governor end of the turbine shaft, as
shown in Figs. 52 and 53. When the speed reaches a predetermined limit,
the plunger A, having its center of gravity slightly displaced from the
center of rotation of the shaft, is thrown radially outward and strikes
the lever B. It will easily be understood that when the plunger starts
outward, the resistance of spring C is rapidly overcome, since the
centrifugal force increases as the square of the radius, or in this case
the eccentricity of the center of gravity relative to the center of
rotation. Hence, the lever is struck a sharp blow. This releases the
trip E on the outside of the governor casing, and so opens the steam
valve F, which releases steam from beneath the actuating piston of a
quick-closing throttle valve, located in the steam line. Thus, within a
period of usually less than one second, the steam is entirely shut off
from the turbine when the speed has exceeded 7 or 8 per cent of the
normal.
[Illustration: FIG. 52]
[Illustration: FIG. 53]
The Oiling System
Mounted on the end of the bedplate is the oil pump, operated from the
main shaft of the turbine as previously stated. This may be of the
plunger type shown in Fig. 54, or upon the latest turbine, the rotary
type shown in Fig. 55. Around the bedplate are located the oil-cooling
coils, the oil strainer, the oil reservoir and the oil pipings to the
bearing.
[Illustration: FIG. 54]
The oil reservoir, cooler, and piping are all outside the machine and
easily accessible for cleaning. Usually a corrugated-steel floor plate
covers all this apparatus, so that it will not be unsightly and
accumulate dirt, particularly when the turbine is installed, so that all
this apparatus is below the floor level; i.e., when the top of the
bedplate comes flush with the floor line. In cases where the turbine is
set higher, a casing is usually built around this material so that it
can be easily removed, and forms a platform alongside the machine.
[Illustration: FIG. 55]
The oil cooler, shown in Fig. 56, is of the counter-current type, the
water entering at A and leaving at B, oil entering at C (opening not
shown) and leaving at D. The coils are of seamless drawn copper, and
attached to the cover by coupling the nut. The water manifold F is
divided into compartments by transverse ribs, each compartment
connecting the inlet of each coil with the outlet of the preceding coil,
thus placing all coils in series. These coils are removable in one piece
with the coverplate without disturbing the rest of the oil piping.
[Illustration: FIG. 56]
Blading
[Illustration: FIG. 57]
The blades are drawn from a rod consisting of a steel core coated with
copper so intimately connected with the other metal that when the bar is
drawn to the section required for the blading, the exterior coating
drawn with the rest of the bar forms a covering of uniform thickness as
shown in Fig. 57. The bar after being drawn through the correct section
is cut into suitable lengths punched as at A (Fig. 58), near the top of
the blade, and has a groove shown at B (Fig. 59), near the root, stamped
in its concave face, while the blade is being cut to length and punched.
The blades are then set into grooves cut into the rotor drum or the
concave surface of the casing, and spacing or packing pieces C (Fig. 59)
placed between them. These spacing pieces are of soft iron and of the
form which is desired that the passage between the blades shall take.
The groove made upon the inner face of the blade is sufficiently near to
the root to be covered by this spacing piece. When the groove has been
filled the soft-iron pieces are calked or spread so as to hold the
blades firmly in place. A wire of comma section, as shown at A (Fig.
59), is then strung through the punches near the outer ends of the
blades and upset or turned over as shown at the right in Fig. 58. This
upsetting is done by a tool which shears the tail of the comma at the
proper width between the blades. The bent-down portion on either side
of the blade holds it rigidly in position and the portion retained
within the width of the blade would retain the blade in its radial
position should it become loosened or broken off at the root. This comma
lashing, as it is called, takes up a small proportion only of the blade
length or projection and makes a job which is surprisingly stiff and
rigid, and yet which yields in case of serious disturbance rather than
to maintain a contact which would result in its own fusing or the
destruction of some more important member.
[Illustration: FIG. 58]
[Illustration: FIG. 59]
Starting Up the Turbine
When starting up the turbine for the first time, or after any extended
period of idleness, special care must be taken to see that everything is
in good condition and that all parts of the machine are clean and free
from injury. The oil piping should be thoroughly inspected and cleaned
out if there is any accumulation of dirt. The oil reservoirs must be
very carefully wiped out and minutely examined for the presence of any
grit. (Avoid using cotton waste for this, as a considerable quantity of
lint is almost sure to be left behind and this will clog up the oil
passages in the bearings and strainer.)
The pilot valves should be removed from the barrel and wiped off, and
the barrels themselves cleaned out by pushing a soft cloth through them
with a piece of wood. In no case should any metal be used.
If the turbine has been in a place where there was dirt or where there
has been much dust blowing around, the bearings should be removed from
the spindle and taken apart and thoroughly cleaned. With care this can
be done without removing the spindle from the cylinder, by taking off
the bearing covers and very carefully lifting the weight of the spindle
off the bearings, then sliding back the bearings. It is best to lift the
spindle by means of jacks and a rope sling, as, if a crane is used,
there is great danger of lifting the spindle too high and thereby
straining it or injuring the blades. After all the parts have been
carefully gone over and cleaned, the oil for the bearing lubrication
should be put into the reservoirs by pouring it into the governor gear
case G (Fig. 34). Enough oil should be put in so that when the governor,
gear case, and all the bearing-supply pipes are full, the supply to the
oil pump is well covered.
Special care should be taken so that no grit gets into the oil when
pouring it into the machine. Considerable trouble may be saved in this
respect by pouring the oil through cloth.
A very careful inspection of the steam piping is necessary before the
turbine is run. If possible it should be blown out by steam from the
boilers before it is finally connected to the turbine. Considerable
annoyance may result by neglecting this precaution, from particles of
scale, red lead, gasket, etc., out of the steam pipe, closing up the
passages of the guide blades.
When starting up, always begin to revolve the spindle without vacuum
being on the turbine. After the spindle is turning slowly, bring the
vacuum up. The reason for this is, that when the turbine is standing
still, the glands do not pack and air in considerable quantity will rush
through the glands and down through the exhaust pipe. This sometimes has
the effect of unequal cooling. In case the turbine is used in
conjunction with its own separate condenser, the circulating pump may be
started up, then the turbine revolved, and afterward the air pump put in
operation; then, last, put the turbine up to speed. In cases, however,
where the turbine exhausts into the same condenser with other machinery
and the condenser is therefore already in operation, the valve between
the turbine and the condenser system should be kept closed until after
the turbine is revolved, the turbine in the meantime exhausting through
the relief valve to atmosphere.
Care must always be taken to see that the turbine is properly warmed up
before being caused to revolve, but in cases where high superheat is
employed always revolve the turbine just as soon as it is moderately
hot, and before it has time to become exposed to superheat.
In the case of highly superheated steam, it is not undesirable to
provide a connection in the steam line by means of which the turbine may
be started up with saturated steam and the superheat gradually applied
after the shaft has been permitted to revolve.
For warming up, it is usual practice to set the governor on the trigger
(see Fig. 50) and open the throttle valve to allow the entrance of a
small amount of steam.
It is always well to let the turbine operate at a reduced speed for a
time, until there is assurance that the condenser and auxiliaries are in
proper working order, that the oil pump is working properly, and that
there is no sticking in the governor or the valve gear.
After the turbine is up to speed and on the governor, it is well to
count the speed by counting the strokes of the pump rod, as it is
possible that the adjustment of the governor may have become changed
while the machine has been idle. It is well at this time, while there is
no load on the turbine, to be sure that the governor controls the
machine with the throttle wide open. It might be that the main poppet
valve has sustained some injury not evident on inspection, or was
leaking badly. Should there be some such defect, steps should be taken
to regrind the valve to its seat at the first opportunity.
On the larger machines an auxiliary oil pump is always furnished. This
should be used before starting up, so as to establish the oil
circulation before the turbine is revolved. After the turbine has
reached speed, and the main oil pump is found to be working properly, it
should be possible to take this pump out of service, and start it again
only when the turbine is about to be shut down.
If possible, the load should be thrown on gradually to obviate a sudden,
heavy demand upon the boiler, with its sometimes attendant priming and
rush of water into the steam pipe, which is very apt to take place if
the load is thrown on too suddenly. A slug of water will have the effect
of slowing down the turbine to a considerable extent, causing some
annoyance. There is not likely to be the danger of the damage that is
almost sure to occur in the reciprocating engine, but at the same time
it is well to avoid this as much as possible. A slug of water is
obviously more dangerous when superheated steam is being employed, owing
to the extreme temperature changes possible.
Running
While the turbine is running, it should have a certain amount of careful
attention. This, of course, does not mean that the engineer must stand
over it every minute of the day, but he must frequently inspect such
parts as the lubricators, the oiling system, the water supply to the
glands and the oil-cooling coil, the pilot valve, etc. He must see that
the oil is up in the reservoir and showing in the gage glass provided
for that purpose, and that the oil is flowing freely through the
bearings, by opening the pet cocks in the top of the bearing covers. An
ample supply of oil should always be in the machine to keep the suction
in the tank covered.
Care must be taken that the pump does not draw too much air. This can
usually be discovered by the bubbling up of the air in the governor
case, when more oil should be added.
It is well to note from time to time the temperature of the bearings,
but no alarm need be occasioned because they feel warm to the touch; in
fact, a bearing is all right as long as the hand can be borne upon it
even momentarily. The oil coming from the bearings should be preferably
about 120 degrees Fahrenheit and never exceed 160 degrees.
It should generally be seen that the oil-cooling coil is effective in
keeping the oil cool. Sometimes the cooling water deposits mud on the
cooling surface, as well as the oil depositing a vaseline-like
substance, which interferes with the cooling effect. The bearing may
become unduly heated because of this, when the coil should be taken out
at the first opportunity and cleaned on the outside and blown out by
steam on the inside, if this latter is possible. If this does not reduce
the temperature, either the oil has been in use too long without being
filtered, or the quality of the oil is not good.
Should a bearing give trouble, the first symptom will be burning oil
which will smoke and give off dense white fumes which can be very
readily seen and smelled. However, trouble with the bearings is one of
the most unlikely things to be encountered, and, if it occurs, it is due
to some radical cause, such as the bearings being pinched by their caps,
or grit and foreign matter being allowed to get into the oil.
If a bearing gets hot, be assured that there is some very radical cause
for it which should be immediately discovered and removed. Never, under
any circumstances, imagine that you can nurse a bearing, that has
heated, into good behavior. Turbine bearings are either all right or all
wrong. There are no halfway measures.
The oil strainer should also be occasionally taken apart and thoroughly
cleaned, which operation may be performed, if necessary, while the
turbine is in operation. The screens should be cleaned by being removed
from their case and thoroughly blown out with steam. In the case of a
new machine, this may have to be done every two or three hours. In
course of time, this need only be repeated perhaps once a week. The
amount of dirt found will be an indication of the frequency with which
this cleaning is necessary.
The proper water pressure, about five pounds per square inch, must be
maintained at the glands. Any failure of this will mean that there is
some big leak in the piping, or that the water is not flowing properly.
The pilot valve must be working freely, causing but little kick on the
governor, and should be lubricated from time to time with good oil.
Should it become necessary, while operating, to shut down the condenser
and change over to non-condensing operation, particular care should be
observed that the change is not made too suddenly to non-condensing, as
all the low-pressure sections of the turbine must be raised to a much
higher temperature. While this may not cause an accident, it is well to
avoid the stresses which necessarily result from the sudden change of
temperature. The same reasons, of course, do not hold good in changing
from non-condensing to condensing.
Shutting Down
When shutting down the turbine the load may be taken off before closing
the throttle; or, as in the case of a generator operating on an
independent load, the throttle may be closed first, allowing the load to
act as a brake, bringing the turbine to rest quickly. In most cases,
however, the former method will have to be used, as the turbine
generally will have been operating in parallel with one or more other
generators. When this is the case, partially close the throttle just
before the load is to be thrown off, and if the turbine is to run
without load for some time, shut off the steam almost entirely in order
to prevent any chance of the turbine running away. There is no danger of
this unless the main valve has been damaged by the water when wet steam
has been used, or held open by some foreign substance, when, in either
case, there may be sufficient leakage to run the turbine above speed,
while running light. At the same time, danger is well guarded against by
the automatic stop valve, but it is always well to avoid a possible
danger. As soon as the throttle is shut, stop the condenser, or, in the
case where one condenser is used for two or more turbines, close the
valve between the turbine and the condenser. Also open the drains from
the steam strainer, etc. This will considerably reduce the time the
turbine requires to come to rest. Still more time may be saved by
leaving the field current on the generator.
Care should be taken, when the vacuum falls and the turbine slows down,
to see that the water is shut off from the glands for fear it may leak
out to such an extent as to let the water into the bearings and impair
the lubricating qualities of the oil.
Inspection
At regular intervals thorough inspection should be made of all parts of
the turbine. As often as it appears necessary from the temperature of
the oil, depending on the quality of the oil and the use of the turbine,
remove the oil-cooling coil and clean it both on the inside and outside
as previously directed; also clean out the chamber in which it is kept.
Put in a fresh supply of oil. This need not necessarily be new, but may
be oil that has been in use before but has been filtered. We recommend
that an oil filter be kept for this purpose. Entirely new oil need only
be put into the turbine when the old oil shows marked deterioration.
With a first-class oil this will probably be a very infrequent
necessity, as some new oil has to be put in from time to time to make up
the losses from leakage and waste.
Clean out the oil strainer, blowing steam through the wire gauze to
remove any accumulation of dirt. Every six months to a year take off the
bearing covers, remove the bearings, and take them apart and clean out
thoroughly. Even the best oil will deposit more or less solid matter
upon hot surfaces in time, which will tend to prevent the free
circulation of the oil through the bearings and effectively stop the
cushioning effect on the bearings. Take apart the main and secondary
valves and clean thoroughly, seeing that all parts are in good working
order. Clean and inspect the governor and the valve-gear, wiping out any
accumulation of oil and dirt that may appear. Be sure to clean out the
drains from the glands so that any water that may pass out of them will
run off freely and will not get into the bearings.
At the end of the first three months, and after that about once a year,
take off the cylinder cover and remove the spindle. When the turbine is
first started up, there is very apt to be considerable foreign matter
come over in the steam, such as balls of red lead or small pieces of
gasket too small to be stopped by the strainer. These get into the guide
blades in the cylinder and quite effectively stop them up. Therefore,
the blades should be gone over very carefully, and any such additional
accumulation removed. Examine the glands and equilibrium ports for any
dirt or broken parts. Particularly examine the glands for any deposit of
scale. All the scale should be chipped off the gland parts, as, besides
preventing the glands from properly packing, this accumulation will
cause mechanical contact and perhaps cause vibration of the machine due
to lack of freedom of the parts. The amount of scale found after the
first few inspections will be an indication of how frequently the
cleaning should be done. As is discussed later, any water that is
unsuitable for boiler feed should not be used in the glands.
In reassembling the spindle and cover, very great care must be taken
that no blades are damaged and that nothing gets into the blades. Nearly
all the damage that has been done to blades has resulted from
carelessness in this respect; in fact, it is impossible to be too
careful. Particular care is also to be taken in assembling all the
parts and in handling them, as slight injury may cause serious trouble.
In no case should a damaged part be put back until the injury has been
repaired.
If for any reason damaged blades cannot be repaired at the time, they
can be easily removed and the turbine run again without them until it is
convenient to put in new ones; in fact, machines have been run at full
load with only three-quarters of the total number of blades. In such an
event remove the corresponding stationary blades as well as the moving
blades, so as not to disturb the balance of the end thrust.
Conditions Conducive to Successful Operation
In the operation of the turbine and the conditions of the steam, both
live and exhaust play a very important part. It has been found by
expensive experimenting that moisture in the steam has a very decided
effect on the economy of operation; or considerably more so than in the
case of the reciprocating engine. In the latter engine, 2 per cent. of
moisture will mean very close to 2 per cent. increase in the amount of
water supplied to the engine for a given power. On the other hand, in
the turbine 2 per cent. moisture will cause an addition of more nearly 4
per cent. It is therefore readily seen that the drier the entering
steam, the better will be the appearance of the coal bill.
By judicious use of first-class separators in connection with a suitable
draining system, such as the Holly system which returns the moisture
separated from the steam, back to the boilers, a high degree of quality
may be obtained at the turbine with practically no extra expense during
operation. Frequent attention should be given the separators and traps
to insure their proper operation. The quality of the steam may be
determined from time to time by the use of a throttling calorimeter. Dry
steam, to a great extent, depends upon the good and judicious design of
steam piping.
Superheated steam is of great value where it can be produced
economically, as even a slight degree insures the benefits to be derived
from the use of dry steam. The higher superheats have been found to
increase the economy to a considerable extent.
When superheat of a high degree (100 degrees Fahrenheit or above) is
used special care must be exercised to prevent a sudden rise of the
superheat of any amount. The greatest source of trouble in this respect
is when a sudden demand is made for a large increase in the amount of
steam used by the engine, as when the turbine is started up and the
superheater has been in operation for some time before, the full load is
suddenly thrown on. It will be readily seen that with the turbine
running light and the superheater operating, there is a very small
amount of steam passing through; in fact, practically none, and this may
become very highly heated in the superheater, but loses nearly all its
superheat in passing slowly to the turbine; then, when a sudden demand
is made, this very high temperature steam is drawn into the turbine.
This may usually be guarded against where a separately fired superheater
is used, by keeping the fire low until the load comes on, or, in the
case where the superheater is part of the boiler, by either not starting
up the superheater until after load comes on, or else keeping the
superheat down by mixing saturated steam with that which has been
superheated. After the plant has been started up there is little danger
from this source, but such precautions should be taken as seem best in
the particular cases.
Taking up the exhaust end of the turbine, we have a much more striking
departure from the conditions familiar in the reciprocating engine. Due
to the limits imposed upon the volume of the cylinder of the engine, any
increase in the vacuum over 23 or 24 inches, in the case, for instance,
of a compound-condensing engine, has very little, if any, effect on the
economy of the engine. With the turbine, on the other hand, any increase
of vacuum, even up to the highest limits, increases the economy to a
very considerable extent and, moreover, the higher the vacuum the
greater will be the increase in the economy for a given addition to the
vacuum. Thus, raising the vacuum from 27 to 28 inches has a greater
effect than from 23 to 24 inches. For this reason the engineer will
readily perceive the great desirability of maintaining the vacuum at the
highest possible point consistent with the satisfactory and economical
operation of the condenser.