_f._ Four rubber check valves, one for each pulsation chamber, and
similar to ordinary pump valves, must be provided. The chambers for
these should preferably be made as a part of the pump cylinder, thus
securing a compact arrangement.
Figure 12 shows this arrangement in outline.
_g._ The valve seat should have three ribs to the central hub,
supporting the rubber valve. The net port area through the valve should
be not less than 1-1/2 square inches.
This valve seat should rest in an inverted position, and can be so
fitted up as to be readily removed. The valve stems can be of the
removable type screwing into the seat, but must be made long enough
to receive a check nut on the opposite side of seat. This will
effectually lock the stem in place.
_h._ Care must be taken to arrange the water passages through and about
these priming checks, so as to avoid all air pockets and so as to
reduce to a minimum the possibility of the valves becoming choked up by
refuse.
_i._ The valve seats, stems and all parts must be of composition and of
strong rugged design, so fitted up that there is the least chance for
the rubber valves to stick, and with all parts securely put together
the valves must be readily accessible.
_j._ The valve springs must have only sufficient strength to keep the
valves on their seats, so that they will freely open even with the low
head of priming water often existing.
[Illustration: FIG. 17.]
_k._ There must be provided, and attached to the top of each plunger
chamber, a brass check valve and air cock with umbrella top, as shown
in Fig. 17. This cock and valve must have a clear passageway through
them—the full equivalent of a 1/2-inch bore.
The check valve is to permit the outflow of air, but to prevent the
influx when the plunger is sucking. Cocks with lever handles are
used, as these show clearly whether they are open or shut.
_l._ There must also be provided a 2-inch brass gate valve for the
general control of the water to the four-check valves. The hand wheel
of this valve must be marked as per Fig. 15. The lettering must be very
clear, open, and distinct, not liable to be obscured by grease, and of
a permanent character.
It is essential for a properly working pump that the main 2-inch
priming-valve should be closed as soon as the pump is primed.
Otherwise, water will be drawn from the priming tank, lessening the
lifting power of the pump through the main suction, and if this is
continued the priming tank will often be exhausted and air drawn into
the pump, interfering with its proper action. It is for this reason
that the marking on the priming valve is required.
For all average situations, either method of priming permits of
getting the pump under way in a very few minutes, but, for cases
where the suction pipe is over 300 or 400 feet in length, or
sometimes where the lift is over 18 feet, or where there is a
combination of long length and lift within these limits, so much
time is consumed in exhausting the air from the suction pipe that it
becomes desirable to supplement this method.
For such situations, a steam ejector connected to the suction pipe
near the pump is advised, and may be required in addition to the
regular priming pipes and tank. The size of the ejector should be
sufficient to exhaust the suction pipe within about three minutes.
Such ejectors will be considered as extras not included in the
ordinary pump fittings.
For cases where pump can only take its suction under a head, if
absolutely certain that the level of the suction water will never
fall below level of center of pump, these priming pipes may be
omitted, but openings for them into the pump shell must be provided
and capped or plugged.
_A foot valve on a fire-pump suction is not advised_ except in very
rare cases, as with a lift of 18 feet or a suction pipe 500 feet or
more long. A foot valve is not needed when there is a good efficient
set of priming arrangements as described above and it is commonly
found this gives a false sense of security, and that with a fire-pump
left standing several days the water will often be found to have
leaked back, so that it is no better than if no foot valve had been
used.
A foot valve must of necessity generally be located where it is
inaccessible for quick repairs, and as they grow old, foot valves
are often a source of trouble. Where a suction pipe is exposed even
slightly to frost, a foot valve is specially objectionable.
_A priming tank is provided for the purchaser_ in all cases where
there is ever to be any lift on the suction. It is generally advised
that this tank have a capacity of one-half of what the pump can throw
at full speed in a minute. This means 250 gallons for a 500-gallon
pump and 500 gallons for a 1,000-gallon pump, etc. It is the
intention to make the pump a truly “independent source” of supply,
therefore the need of a special priming tank.