_c._ Four 2-seat controllable valves, one for each pulsation chamber,
and of the general type illustrated in Fig. 14, must be provided. In
these the inlet of water and outlet of air are simultaneously opened
and closed by the pump operator.
[Illustration: FIG. 14.]
Objection has been raised to this double-seated valve from the
possible difficulty of keeping both seats tight. If desired, the
valve may be fitted with a flange instead of a screw connection, and
the stem between the two seats somewhat enlarged and provided with a
suitable spring, thus giving flexibility between the two seats and
preventing all trouble from uneven wear.
_d._ The hand-wheel of each of these valves must be marked as per Fig.
15, so that the pump operator I may clearly understand their use. The
lettering must be very open, clear and distinct, not liable to be
obscured by grease and dirt, and of a permanent character.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
_e._ There must be provided and fitted to each combined valve a check
and umbrella-top air vent, as shown in Fig. 16. This fitting must have
a clear passageway through it, 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.
This method is preferred to the one using rubber priming checks, as
now and then a rubber valve will stick on its seat and thus prevent
priming of one of the chambers. In this arrangement the pump operator
has absolute control over the priming water into each chamber.
Another advantage is that the connection of the air-vent with the
priming valve ensures that the air-vents will be opened; and further,
by the vigorous spurting out of water as soon as the pump is primed,
the pump operator is reminded that the priming valve should be closed.
Should the pump operator, however, through a mistaken idea of
the proper method of operation, think that the priming should be
continued until all air was exhausted from the suction pipe and the
pump running in normal condition, there would be some by-passing
between chambers, but as there is a free vent for the air, the main
result would be simply to limit the amount of air exhausted per
stroke, from the main suction, by the amount of water which entered
a chamber in this way. The amount of water thus entering, however,
would not be appreciably greater than that which would enter from the
priming-tank with the check-valve arrangement.
If, even in spite of the warning given by the spurting air-vents,
the pump operator should neglect to close the priming-valves when
the pump was running normally, the priming-tank would eventually
be overflowed; but this would not be as serious as the drawing in
of air from an exhausted priming-tank, which would result with the
check-valve method, were the main 2-inch valve similarly neglected.