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.
Fig. 15.
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.