[Illustration: FIG. 181.]
_Water Ends._ There are properly speaking four kinds of water ends to
steam and power pumps:
1, A solid plunger, with a stuffing box used for heavy pressings in
hydraulic apparatus, or as shown in Fig. 182, for larger plungers.
2, A piston packed with fibrous material within the cylinder. See Fig.
181. The letter P in Fig. 182 and the following cuts indicates the
plunger.
3, A plunger packed with a metal ring around the outside, as
illustrated in Fig. 183.
4, Two plungers, Fig. 184, connected outside of the cylinder with a
stuffing box in two cylinder heads, through which the plungers work.
These are more fully explained and illustrated as they occur in many
examples as they are referred to in the oncoming chapters of this work.
The construction of the water ends of single cylinder and duplex pumps
is practically the same; any slight differences which may be found are
confined to minor details, which in no way affect the general design or
operation of the pump.
_The steam or power ends_ of numerous and varied makes of pumps
are also as shown in the following pages of this work; all pumps
actuated by power—steam, electric, etc.—are possessed of these two
distinguishing features—1, a steam or power end, and 2, the water end.
NOTE.—This statement has exception in the cases of large pumping
engines having a fly wheel or supplemental cylinders attached to
an accumulator, in which case the steam is worked expansively.
[Illustration: FIG. 182.]
[Illustration: FIG. 183.]
_The steam end of the ordinary single steam pump, and also of the
duplex pump, differs from the steam cylinder of the steam engine in
that the former has four ports to each cylinder, i. e., two steam ports
and two cushioning ports as shown hereafter in figures._
Under the division of the work allotted to the “Steam Pump” will be
found all necessary further notice of the steam ends of Pumps.
_Pump Valves._ The valve apparatus is perhaps the most important part
of any form of pump and its design has a material bearing upon its
efficiency.
[Illustration: FIG. 184.]
[Illustration: FIG. 185.]
The valves shown in Fig. 181 are carried by two plates or decks, the
suction valves being attached to the lower plate and the delivery
valves to the upper one. The upper deck, and sometimes both decks,
are removable. The valves are secured to the plates by means of bolts
or long machine screws, which, in turn, are screwed into the bridge
across the board in the plate, as shown in Figs. 185 and 186 or capped
as in Fig. 187. The valves in all pumps except the large sizes, which
may properly be classed with pumping engines, are of _the flat rubber
disc type_, with a hole in the center to enable the valve to rise
easily on the bolt, the latter serving as a guide. _A conical spring_
is employed to hold the valve firmly to its seat, the spring being held
in position by the head of the bolt, or cap, as shown.
Certain improvements in pump-valves have been made which tend to
increase the durability and to prevent the liability of sticking, which
is not an uncommon occurrence after the valves have become badly worn.
The improved forms of pump valves are shown in Figs. 186 and 187.
[Illustration: FIG. 186.]
[Illustration: FIG. 187.]
When these valves leak through wear the disc may be reversed, using
the upper side of the disc next to the valve seat. This can be done
with ordinary valves also, provided the spring has not injured the
upper surface of the disc. Valve seats are generally pressed into the
plates, although instances may be found where they are screwed. When
pressed in they may be withdrawn by substituting a bolt having longer
screw threads than the regular bolt, and provided with a nut, as shown
in Fig. 188. The bolt is slipped through a yoke and screwed into the
bridge. By turning the nut the seat can generally be started without
difficulty.
[Illustration: FIG. 188.]
Fig. 189 represents the customary _gland and stuffing-box_ in which
the gland is adjusted by the nuts C and D upon two studs. After the
adjustment has been properly made lock-nuts are tightened which leaves
the gland free yet preserves the alignment.
It has been proven by practice—after long and costly experiments—that
a number of small valves instead of one large one are far the most
durable; _durability_ being the question. Corliss, Leavitt, Holly and
other leading pump builders had occasion to find the truth of this
statement early in their careers. The “slamming” of large valves under
moderate speeds proved itself an almost insurmountable difficulty until
the principle of keeping the valve area as low as possible within
reasonable limits had been fully demonstrated.
[Illustration: FIG. 189.]
To illustrate the advantage of having a number of comparatively small
valves instead of one large one, suppose a pump to be fitted with four
3-1/2-inch delivery valves at each end, the valves covering ports 2-1/2
inches in diameter. The area of each port is 4·9 square inches. In
order to provide an equal area between the valve and the seat the valve
must rise a distance equal to one-fourth the diameter of the port.
The combined area of the four ports is 19·6 square inches, which
corresponds to the area of a circular opening 5 inches in diameter,
one-fourth of which is 1-1/4 inches. It will be understood that the
smaller valves can seat much more quickly and with less jar than the
larger one, hence a larger number of small valves is not only better
because of the great reduction in slippage, but they are also more
economical, being subjected to less wear and tear.
_The lift of valves_ for moderate or low speed pumps is seen in Fig.
190 and those for higher speeds in Fig. 191. These engravings clearly
show the relative position of the suction and discharge valves during
the movements of the piston.
[Illustration: FIG. 190.]
[Illustration: FIG. 191.]
_Pump slip_ or slippage is a term used to denote the difference between
the calculated and the actual discharge of a pump, and is generally
expressed as a percentage of the calculated discharge. Thus, when the
slippage is given as 15 per cent. it indicates that the loss due to
slip amounts to 15 per cent. of the calculated discharge. Slippage
is due to two causes, the time required for the suction and discharge
valves to seat.
When pumps run very fast the piston speed is so high that the water
cannot enter the pump fast enough to completely fill the cylinder and
consequently a partial cylinder full of water is delivered at each
stroke. _High speeds also increase slippage_, due to the _seating of
the valves_. Fig. 191 represents a sectional view of the water end of a
pump, showing the position of the valves during a quick reversal in the
direction of the arrows, which illustrates the position of the valves
corresponding to high speed. The valves in a pump, like almost every
other detail in the operation of machinery, do not act instantaneously,
but require time to reach the seats.
When pumps run at high speed the piston will move a considerable
distance, while the valves are descending to their seats, and water
flows back into the pump cylinder until the valves are tightly closed.
The valves will remain in the raised position shown in Fig. 191 until
the piston stops at the end of the stroke, and under high speed the
piston will reach the position on the return stroke indicated by the
dotted line _L_ by the time the valves are closed. The cylinder will be
filled up to this point with water from the delivery chamber so that no
vacuum can be formed until after the piston reaches this position. The
volume of water that can be drawn into the cylinder must necessarily
be represented by the cubic inches cf space, minus the quantity which
flows back during the time the valves are closing. It will thus be seen
that the actual volume of water discharged is considerably less than
a cylinderful, and the difference, whatever it may prove to be, is
called, and is due to slippage.
[Illustration: FIG. 192.]
Fig. 190 represents the same pump running at a comparatively low speed.
It will be noticed that the valves have not been raised as high as in
Fig. 191, because a longer time being allowed for the discharge of the
water, a smaller orifice is sufficient. It will be seen also that the
piston, moving at a lower velocity, cannot travel as far in Fig. 190
before the valves seat, and consequently a vacuum can be created in
the cylinder earlier in the stroke, and a larger volume of water can
therefore be drawn in during the return stroke. In the latter case it
is evident that the volume of water drawn into the cylinder will be
nearly equal to a cylinderful and consequently the loss by slippage
must be correspondingly less.
In order to reduce the loss by slippage several valves are used instead
of a single valve of equal area. A flat disc valve will rise a distance
equal to one-fourth the diameter of the port or of the opening in the
seat to discharge the same volume of water that can flow through the
port in the same time. In practice the rise exceeds this proportion of
one-fourth a trifle, owing to the friction of the water, and this is
especially true at high speeds.
[Illustration: FIG. 193.]
_Reinforced pump valves._ Where pure gum has been used for pump valves
it has always proved too soft and when it has been compounded with
other substances it has been found too hard to withstand the severe
duty to which it is subjected as a material for pump valves.
In the accompanying Fig. 192 is shown the Braden pump valve, which is
made of _composition of rubber having wire rings embedded in the center
of the disc_. The composition has been removed from a section to show
these rings. A ferrule of composition metal forms a hub around the
center through which the bolt or stud passes to guide the valve and to
prevent excessive wear of the hole.
Its wire coil frame work clothed with rubber maintains a due amount
of stiffness, with a degree of flexibility which prevents its bulging
into the holes in the seats, or sticking therein, and thus impairing
the suction and discharge. Both sides, the upper as well as the lower,
are made available for service. These qualities of stiffness and
flexibility combined, permit this valve to adjust itself to form a
water-tight seat.
[Illustration: FIGS. 194, 195, 198.]
[Illustration: FIGS. 196, 197.]
_Armored pump valves._ As represented in Fig. 193 this is a valve
made by stamping a metal disc out of steel which is then plated with
copper to protect the surface and secure the adhesion of the rubber.
Marginal notches are left on the inside and outside edges of the plate
and rubber is moulded around these, and vulcanized to the required
hardness; a brass or copper plate may be used instead of steel and the
plates may be corrugated radially to increase their stiffness when the
area of the valve is large.
Experience proves that the water valve adopted together with its
location, has a material bearing upon the efficiency of any pump;
easy seating valves are subject to more or less slippage, owing to
tardy seating; the location of water valves should be above the pump
cylinder, inasmuch as in operation the pump is always primed, while if
suction valves are placed below, any wear on the valves or valve seats,
or obstruction under the valves, will cause the water to leak entirely
out of the water cylinder, making it necessary to prime the pump before
it can be started.
NOTE.—The screwed seat is shown in Fig. 194, Stud Fig. 195, Metal
Valve Fig. 196, Spring Fig. 197, and all put together in Fig. 198.
[Illustration: FIG. 199.]
_Valve seats, bolts and springs_ should be of the best composition
or gun-metal; and valves of composition, or hard or soft rubber, to
suit the duty such pump is required to perform. These valve seats
are screwed into the valve plate, and valves may be changed from
composition to rubber by merely removing bolt, and substituting one for
the other without removing the seat. This is of great advantage where
a pump is to be used for hot water after being used for cold water.
[Illustration: FIG. 200.]
[Illustration: FIG. 201.]
_Air chambers_ are placed upon the top of a pump, see Figs. 199 and
200, and contain air for the purpose of introducing an _air cushion_
to counteract the solidity of the water, thus preventing shocks as the
water flows through the valves; and also for the purpose of securing a
steady discharge of water.
The water being under pressure in the discharge chamber, compresses the
air in the air chamber during each stroke of the water piston and, when
the piston stops momentarily at the end of the stroke, the air expands
to a certain extent and tends to produce a gradual stopping of the flow
of water, thus permitting the valves _to seat easily and without shock
or jar_.
_The capacity of the air chamber_ varies in different makes of pumps
from 2 to 3-1/2 times the volume of the water cylinder in single
cylinder pumps, and from 1 to 2-1/2 times the volume of the water
cylinder in the duplex type. _The volume of the water cylinder is
represented by the area of the water piston multiplied by the length of
stroke._
For single-cylinder, boiler-feed pumps and those employed for elevator
and similar service the volume of the air chamber should be 3 times the
volume of the water cylinder, and for duplex pumps, not less than twice
the volume of the water cylinder. High speed pumps, such as fire pumps,
should be provided with air chambers containing from 5 to 6 times the
volume of the water cylinder.
The diameter of the neck should not exceed one-third the diameter of
the chamber. When the pumps work under pressure exceeding 85 or 90
pounds per square inch, it is frequently found that the air gradually
disappears from the air chamber, the air passing off with the water
by absorption. In this case air should be supplied to the air chamber
unless the pump runs at very low speeds, say, from 10 to 20 strokes
for the smaller sizes and from 3 to 5 strokes per minute for pumping
engines. At higher speed and with no air in the air chamber the valves
are apt to seat heavily and cause more or less jar and noise, and the
flow of water will not be uniform. The water level in the air chamber
should be kept down to from one-fourth to one-third the height of the
air chamber for smooth running at medium and high speeds.
NOTE.—In large pumping plants small air pumps are employed for
keeping the air chambers properly charged. In smaller plants an
ordinary bicycle pump and a piece of rubber tubing are used to good
advantage.
_Vacuum chambers_ are shown in Figs. 199, 200 and 201. These devices
are attached to the suction pipe. When the column of water in the
suction pipe of a pump is once set in motion, it is quite important,
especially under high speeds, to keep the water in full motion,
and when it is stopped, to stop it gradually and easily. This is
accomplished by placing a vacuum chamber on the suction pipe, as shown
in the figures.
The location of the vacuum chamber may be varied to suit the
convenience of the engine room arrangements. Fig. 199 represents the
vacuum chamber at the side of the pump, Fig. 200 shows it opposite the
suction and Fig. 201 represents its position at the end of the pump.
[Illustration: FIG. 202.]
[Illustration: FIG. 203.]
_Vacuum chambers_ are practically of two designs, as shown in Figs. 202
and 203. The one shown in Fig. 203 should be placed in such position as
to receive the impact of the column of water in the suction pipe. In
order to do this effectively it should be placed in the position shown
in Figs. 199, 200 or 201. The chamber illustrated in Fig. 202 is placed
in the suction pipe below, but close to the pump.
_The action of the vacuum chamber is practically the reverse of that
of the air chamber._ The object of the vacuum chamber is to facilitate
changing continuous into intermittent motion. The moving column of
water compresses the air in the vacuum chamber at the ends of the
stroke of the piston, and when the piston starts the air expands (thus
creating a partial vacuum above the water) and aids the piston in
setting the column of water in motion again.
Thus the flow of water into the suction chamber of the pump is much
more uniform during each stroke of the piston than without the vacuum
chamber, and consequently the pump can be run at higher speeds without
increasing the loss due to slippage and without “slamming” of the
valves. Vacuum chambers should be slightly larger than the suction pipe
and of considerable length rather than of large diameter and short. The
size of the neck is substantially the same as in the air chamber.
[Illustration: FIG. 204.]