In operation all that is necessary is to watch the steam pressure at the
"Throttle" and "Inlet" gages, to see that neither this pressure nor the
steam temperature varies much; to keep the vacuum constant, as well as
pressures on the water glands and those indicated by the "Relay Oil" and
"Bearing Oil" gages; to take care that the temperatures of the oil
flowing to and from the bearings does not exceed 135 degrees Fahr. (at
which temperature the hand can comfortably grasp the copper oil-return
pipes); to see that oil flows freely at all vents on the bearings, and
that the governor parts are periodically oiled. So far as the generator
is concerned, it is only essential to follow the practice common in all
electric power plant operation, which need not be reviewed here.
_Stopping the turbine_ is practically the reverse of starting, the
successive steps being as follows: starting the auxiliary oil pump,
freeing it of water and allowing it to run slowly; removing the load
gradually; breaking the vacuum when the load is almost zero, shutting
off the condenser injection and taking care that the steam exhausts
freely into the atmosphere; shutting off the gland water when the load
and vacuum are off; pulling the automatic stop to trip the valve and
shut off steam and, as the speed of the turbine decreases, speeding up
the auxiliary oil pump to maintain pressure on the bearings; then, when
the turbine has stopped, shutting down the auxiliary oil pump, turning
off the cooling water, opening the steam chest drains and slightly
oiling the oil inlet valve-stem. During these operations the chief
particulars to be heeded are: not to shut off the steam before starting
the auxiliary oil pump nor before the vacuum is broken, and not to shut
off the gland water with vacuum on the turbine. The automatic stop
should also remain unhooked until the turbine is about to be started up
again.
General
Water used in the glands of the turbine must be free from scale-forming
impurities and should be delivered at the turbine under a steady
pressure of not less than 15 pounds. The pressure in the glands will
vary from 4 to 10 pounds. This water may be warm. In the use of water
for the cooling coils and of oil for the lubricating system, nothing
more is required than ordinary good sense dictates. An absolutely pure
mineral oil must be supplied, of a non-foaming character, and it should
be kept free through filtering from any impurities.
The above refers particularly to Allis-Chalmers turbines of the type
ordinarily used for power service. For turbines built to be run
non-condensing, the part relating to vacuum does not, of course, apply.
IV. WESTINGHOUSE-PARSONS STEAM TURBINE
While the steam turbine is simple in design and construction and does
not require constant tinkering and adjustment of valve gears or taking
up of wear in the running parts, it is like any other piece of fine
machinery in that it should receive intelligent and careful attention
from the operator by inspection of the working parts that are not at all
times in plain view. Any piece of machinery, no matter how simple and
durable, if neglected or abused will in time come to grief, and the
higher the class of the machine the more is this true.
Any engineer who is capable of running and intelligently taking care of
a reciprocating engine can run and take care of a turbine, but if he is
to be anything more than a starter and stopper, it is necessary that he
should know what is inside of the casing, what must be done and avoided
to prevent derangement, and to keep the machine in continued and
efficient operation.
In the steam turbine the steam instead of being expanded against a
piston is made to expand against and to get up velocity in itself. The
jet of steam is then made to impinge against vanes or to react against
the moving orifice from which it issues, in either of which cases its
velocity and energy are more or less completely abstracted and
appropriated by the revolving member. The Parsons turbine utilizes a
combination of these two methods.
[Illustration: FIG. 34]
Fig. 34 is a sectional view of the standard Westinghouse-Parsons
single-flow turbine. A photograph of the rotor R R R is reproduced in
Fig. 35, while in Fig. 36 a section of the blading is shown upon a
larger scale. Between the rows of the blading upon the rotor extend
similar rows of stationary blades attached to the casing or stator. The
steam entering at A (Fig. 34), fills the circular space surrounding the
rotor and passes first through a row of stationary blades, 1 (Fig. 37),
expanding from the initial pressure P to the slightly lower pressure
P{1}, and attaining by that expansion a velocity with which it is
directed upon the moving blade 2. In passing through this row of blades
it is further expanded from pressure P{1} to P{2} and helps to push the
moving blades along by the reaction of the force with which it issues
therefrom. Impinging upon the second row of stationary blades 3, the
direction of flow is diverted so as to make it impinge at a favorable
angle upon the second row of revolving blades 4, and the action is
continued until the steam is expanded to the pressure of the condenser
or of the medium into which the turbine finally exhausts. As the
expansion proceeds, the passages are made larger by increasing the
length of the blades and the diameter of the drums upon which they are
carried in order to accommodate the increasing volume.
[Illustration: FIG. 35]
[Illustration: FIG. 36]
[Illustration: FIG. 37]
It is not necessary that the blades shall run close together, and the
axial clearance, that is the space lengthwise of the turbine between the
revolving and the stationary blades, varies from 1/8 to 1/2 inch; but in
order that there may not be excessive leakage over the tops of the
blades, as shown, very much exaggerated, in Fig. 38, the radial
clearance, that is, the clearance between the tops of the moving blades
and the casing, and between the ends of the stationary blades and the
shell of the rotor, must be kept down to the lowest practical amount,
and varies, according to the size of the machine and length of blade,
from about 0.025 to 0.125 of an inch.
[Illustration: FIG. 38]
In the passage A (Fig. 34) exists the initial pressure; in the passage B
the pressure after the steam has passed the first section or diameter of
the rotor; in the passage C after it has passed the second section. The
pressure acting upon the exposed faces of the rows of vanes would crowd
the rotor to the left. They are therefore balanced by pistons or
"dummies" P P P revolving with the shaft and exposing in the annular
spaces B^1 and C^1 the same areas as those of the blade sections which
they are designed to balance. The same pressure is maintained in B^1 as
in B, and in C^1 as in C by connecting them with equalizing pipes E E.
The third equalizing pipe connects the back or right-hand side of the
largest dummy with the exhaust passage so that the same pressure exists
upon it as exists upon the exhaust end of the rotor. These dummy pistons
are shown at the near end of the rotor in Fig. 35. They are grooved so
as to form a labyrinth packing, the face of the casing against which
they run being grooved and brass strips inserted, as shown in Fig. 39.
The dummy pistons prevent leakage from A, B^1 and C^1 to the condenser,
and must, of course, run as closely as practicable to the rings in the
casing, the actual clearance being from about 0.005 to 0.015 of an inch,
again depending on the size of the machine.
[Illustration: FIG. 39]
The axial adjustment is controlled by the device shown at T in Fig. 34
and on a larger scale in Fig. 40. The thrust bearing consists of two
parts, T{1} T{2}. Each consists of a cast-iron body in which are placed
brass collars. These collars fit into grooves C, turned in the shaft as
shown. The halves of the block are brought into position by means of
screws S{1} S{2} acting on levers L{1} L{2} and mounted in the bearing
pedestal and cover. The screws are provided with graduated heads which
permit the respective halves of the thrust bearing to be set within one
one-thousandth of an inch.
[Illustration: FIG. 40]
The upper screw S{2} is set so that when the rotor exerts a light
pressure against it through the thrust block and lever the grooves in
the balance pistons are just unable to come in contact with the dummy
strips in the cylinder. The lower screw S{1} is then adjusted to permit
about 0.008 to 0.010 of an inch freedom for the collar between the
grooves of the thrust bearing.
These bearings are carefully adjusted before the machine leaves the
shop, and to prevent either accidental or unauthorized changes of their
adjustment the adjusting screw heads are locked by the method shown in
Fig. 40. The screw cannot be revolved without sliding back the latch
L{3}. To do this the pin P{4} must be withdrawn, for which purpose the
bearing cover must be removed.
In general this adjustment should not be changed except when there has
been some wear of the collars in the thrust bearing; nevertheless, it is
a wise precaution to go over the adjustment at intervals. The method of
doing this is as follows: The machine should have been in operation for
some time so as to be well and evenly heated and should be run at a
reduced speed, say 10 per cent. of the normal, during the actual
operation of making the adjustment. Adjust the upper screw which, if
tightened, would push the spindle away from the thrust bearing toward
the exhaust. Find a position for this so that when the other screw is
tightened the balance pistons can just be heard to touch, and so the
least change of position inward of the upper screw will cause the
contact to cease. To hear if the balance pistons are touching, a short
piece of hardwood should be placed against the cylinder casing near the
balance piston. If the ear is applied to the other end of the piece of
wood the contact of the balance pistons can be very easily detected. The
lower screw should then be loosened and the upper screw advanced from
five to fifteen one-thousandths, according to the machine, at which
position the latter may be considered to be set. The lower screw should
then be advanced until the under half of the thrust bearing pushes the
rotor against the other half of the thrust bearing, and from this
position it should be pushed back ten or more one-thousandths, to give
freedom for the rotor between the thrusts, and locked. A certain amount
of care should be exercised in setting the dummies, to avoid straining
the parts and thus obtain a false setting.
The object in view is to have the grooves of the balance pistons running
as close as possible to the collars in the cylinder, but without danger
of their coming in actual contact, and to allow as little freedom as
possible in the thrust bearing itself, but enough to be sure that it
will not heat. The turbine rotor itself has scarcely any end thrust, so
that all the thrust bearing has to do is to maintain the
above-prescribed adjustment.
The blades are so gaged that at all loads the rotor has a very light but
positive thrust toward the running face of the dummy strips, thus
maintaining the proper clearance at the dummies as determined by the
setting of the proper screw adjustment.
Main Bearings
The bearings which support the rotor are shown at F F in Fig. 34 and in
detail in Fig. 41. The bearing proper consists of a brass tube B with
proper oil grooves. It has a dowel arm L which fits into a corresponding
recess in the bearing cover and which prevents the bearing from turning.
On this tube are three concentric tubes, C D E, each fitting over the
other with some clearance so that the shaft is free to move slightly in
any direction. These tubes are held in place by the nut F, and this nut,
in turn, is held by the small set-screw G. The bearing with the
surrounding tubes is placed inside of the cast-iron shell A, which rests
in the bearing pedestal on the block and liner H. The packing ring M
prevents the leakage of oil past the bearing. Oil enters the chamber at
one end of the bearing at the top and passes through the oil grooves,
lubricating the journal, and then out into the reservoir under the
bearing. The oil also fills the clearance between the tubes and forms a
cushion, which dampens any tendency to vibration.
[Illustration: FIG. 41]
The bearings, being supported by the blocks or "pads" H, are
self-alining. Under these pads are liners 5, 10, 20, and 50 thousandths
in thickness. By means of these liners the rotor may be set in its
proper running position relative to the stator. This operation is quite
simple. Remove the liners from under one bearing pad and place them
under the opposite pad until a blade touch is obtained by turning the
rotor over by hand. After a touch has been obtained on the top, bottom,
and both sides, the total radial blade clearance will be known to equal
the thickness of the liners transferred. The position of the rotor is
then so adjusted that the radial blade clearance is equalized when the
turbine is at operating temperature.
On turbines running at 1800 revolutions per minute or under, a split
babbitted bearing is used, as shown in Figs. 42a and 42b. These bearings
are self-alining and have the same liner adjustment as the
concentric-sleeve bearings just described. Oil is supplied through a
hole D in the lower liner pad, and is carried to the oil groove F
through the tubes E E. The oil flows from the middle of this bearing to
both ends instead of from one end to the other, as in the other type.
[Illustration: FIG. 42A]
[Illustration: FIG. 42B]
Packing Glands
Where the shaft passes through the casing at either end it issues from a
chamber in which there exists a vacuum. It is necessary to pack the
shaft at these points, therefore, against the atmospheric pressure, and
this is done by means of a water-gland packing W W (Fig. 34). Upon the
shaft in Fig. 35, just in front of the dummy pistons, will be seen a
runner of this packing gland, which runner is shown upon a larger scale
and from a different direction in Fig. 43. To get into the casing the
air would have to enter the guard at A (Fig. 44), pass over the
projecting rings B, the function of which is to throw off any water
which may be creeping along the shaft by centrifugal force into the
surrounding space C, whence it escapes by the drip pipe D, hence over
the five rings of the labyrinth packing E and thence over the top of the
revolving blade wheel, it being apparent from Fig. 43 that there is no
way for the air to pass by without going up over the top of the blades;
but water is admitted to the centrally grooved space through the pipe
shown, and is revolved with the wheel at such velocity that the pressure
due to centrifugal force exceeds that of the atmosphere, so that it is
impossible for the air to force the water aside and leak in over the
tips of the blades, while the action of the runner in throwing the water
out would relieve the pressure at the shafts and avoid the tendency of
the water to leak outward through the labyrinth packing either into
the vacuum or the atmosphere.
[Illustration: FIG. 43]
[Illustration: FIG. 44]
The water should come to the glands under a head of about 10 feet, or a
pressure of about 5 pounds, and be connected in such a way that this
pressure may be uninterruptedly maintained. Its temperature must be
lower than the temperature due to the vacuum within the turbine, or it
will evaporate readily and find its way into the turbine in the form of
steam.
[Illustration: FIG. 45]
In any case a small amount of the steaming water will pass by the gland
collars into the turbine, so that if the condensed steam is to be
returned to the boilers the water used in the glands must be of such
character that it may be safely used for feed water. But whether the
water so used is to be returned to the boilers or not it should never
contain an excessive amount of lime or solid matter, as a certain amount
of evaporation is continually going on in the glands which will result
in the deposit of scale and require frequent taking apart for cleaning.
[Illustration: FIG. 46]
When there is an ample supply of good, clean water the glands may be
packed as in Fig. 45, the standpipe supplying the necessary head and the
supply valve being opened sufficiently to maintain a small stream at the
overflow. When water is expensive and the overflow must be avoided, a
small float may be used as in Fig. 46, the ordinary tank used by
plumbers for closets, etc., serving the purpose admirably.
When the same water that is supplied to the glands is used for the
oil-cooling coils, which will be described in detail later, the coils
may be attached to either of the above arrangements as shown in Fig. 47.
[Illustration: FIG. 47]
When the only available supply of pure water is that for the boiler
feed, and the condensed steam is pumped directly back to the boiler, as
shown in Fig. 48, the delivery from the condensed-water pumps may be
carried to an elevation 10 feet above the axis of the glands, where a
tank should be provided of sufficient capacity that the water may have
time to cool considerably before being used. In most of these cases, if
so desired, the oil-cooling water may come from the circulating pumps of
the condenser, provided there is sufficient pressure to produce
circulation, as is also shown in Fig. 48.
[Illustration: FIG. 48]
When the turbine is required to exhaust against a back pressure of one
or two pounds a slightly different arrangement of piping must be made.
The water in this case must be allowed to circulate through the glands
in order to keep the temperature below 212 degrees Fahrenheit. If this
is not done the water in the glands will absorb heat from the main
castings of the machine and will evaporate. This evaporation will make
the glands appear as though they were leaking badly. In reality it is
nothing more than the water in the glands boiling, but it is
nevertheless equally objectionable. This may be overcome by the
arrangement shown in Fig. 49, where two connections and valves are
furnished at M and N, which drain away to any suitable tank or sewer.
These valves are open just enough to keep sufficient circulation so that
there is no evaporation going on, which is evidenced by steam coming out
as though the glands were leaking. These circulating valves may be used
with any of the arrangements above described.
[Illustration: FIG. 49]
The Governor
On the right-hand end of the main shaft in Fig. 34 there will be seen a
worm gear driving the governor. This is shown on a larger scale at A
(Fig. 50). At the left of the worm gear is a bevel gear driving the
spindle D of the governor, and at the right an eccentric which gives a
vibratory motion to the lever F. The crank C upon the end of the shaft
operates the oil pump. The speed of the turbine is controlled by
admitting the steam in puffs of greater or less duration according to
the load. The lever F, having its fulcrum in the collar surrounding the
shaft, operates with each vibration of the eccentric the pilot valve.
The valve is explained in detail later.
[Illustration: FIG. 50]
This form of governor has been superseded by an improved type, but so
many have been made that it will be well to describe its construction
and adjustment. The two balls W W (Fig. 50) are mounted on the ends of
bell cranks N, which rest on knife edges. The other end of the bell
cranks carry rollers upon which rest a plate P, which serves as a
support for the governor spring S. They are also attached by links to a
yoke and sleeve E which acts as a fulcrum for the lever F. The governor
is regulated by means of the spring S resting on the plate P and
compressed by a large nut G on the upper end of the governor spindle,
which nut turns on a threaded quill J, held in place by the nut H on the
end of the governor spindle and is held tight by the lock-nut K. To
change the compression of the spring and thereby the speed of the
turbine the lock-nut must first be loosened and the hand-nut raised to
lower the speed or lowered to raise the speed as the case may be. This
operation may be accomplished while the machine is either running or at
rest.
The plate P rests upon ball bearings so that by simply bringing pressure
to bear upon the hand-wheel, which is a part of the quill J, the spring
and lock-nut may be held at rest and adjusted while the rest of the
turbine remains unaffected. Another lever is mounted upon the yoke E on
the pin shown at I, the other end of which is fastened to the piston of
a dash-pot so as to dampen the governor against vibration. Under the
yoke E will be noticed a small trigger M which is used to hold the
governor in the full-load position when the turbine is at rest.
The throwing out of the weights elevates the sleeve E, carrying with it
the collar C, which is spanned by the lever F upon the shaft H. The
later turbines are provided with an improved form of governor operating
on the same principle, but embodying several important features. First,
the spindle sleeve is integral with the governor yoke, and the whole
rotates about a vertical stationary spindle, so that two motions are
encountered--a rotary motion and an up and down motion, according to the
position taken by the governor. This spiral motion almost entirely
eliminates the effect of friction of rest, and thereby enhances the
sensitiveness of the governor. Second, the governor weights move outward
on a parallel motion opposed directly by spring thrust, thus relieving
the fulcrum entirely of spring thrust. Third, the lay shaft driving the
governor oil pump and reciprocator is located underneath the main
turbine shaft, so that the rotor may be readily removed without in the
least disturbing the governor adjustment.
The Valve-Gear
The valve-gear is shown in section in Fig. 51, the main admission being
shown at V{1} at the right, and the secondary V{2} at the left of the
steam inlet. The pilot valve F receives a constant reciprocating motion
from the eccentric upon the layshaft of the turbine through the lever F
(Fig. 50). These reciprocations run from 150 to 180 per minute. The
space beneath the piston C is in communication with the large steam
chest, where exists the initial pressure through the port A; the
admission of steam to the piston C being controlled by a needle valve
B. The pilot valve connects the port E, leading from the space beneath
the piston to an exhaust port I.
[Illustration: FIG. 51]
When the pilot valve is closed, the pressures can accumulate beneath the
piston C and raise the main admission valve from its seat. When the
pilot valve opens, the pressure beneath the piston is relieved and it is
seated by the helical spring above. If the fulcrum E (Fig. 50) of the
lever F were fixed the admission would be of an equal and fixed
duration. But if the governor raises the fulcrum E, the pilot valve F
(Fig. 51) will be lowered, changing the relations of the openings with
the working edges of the ports.
The seating of the main admission valve is cushioned by the dashpot, the
piston of which is shown in section at G (Fig. 51). The valve may be
opened by hand by means of the lever K, to see if it is perfectly free.
The secondary valve is somewhat different in its action. Steam is
admitted to both sides of its actuating piston through the needle valves
M M, and the chamber from which this steam is taken is connected with
the under side of the main admission valve, so that no steam can reach
the actuating piston of the secondary valve until it has passed through
the primary valve. When the pilot valve is closed, the pressures
equalize above and below the piston N and the valve remains upon its
seat. When the load upon the turbine exceeds its rated capacity, the
pilot valve moves upward so as to connect the space above the piston
with the exhaust L, relieving the pressure upon the upper side and
allowing the greater pressure below to force the valve open, which
admits steam to the secondary stage of the turbine.