34. Delivery Valves.
a. The total lift of delivery valves must not exceed one-half inch.
This is to avoid valve slam, as explained in Art. 33.
b. The aggregate valve-port area should be restricted to about two-thirds the suction-valve area.
A small restriction of water-way through the delivery valves steadies the action of the pump and tends to prevent undue pulsations of pressure in the delivery pipe or fire hose. Fewer delivery valves than suction valves are, therefore, preferred, and if extra holes in the delivery deck are cast for shop purposes these had better be plugged than fitted with valves.
The suction valves require more generous port-circumference and port-area than delivery valves because when a pump has to suck its supply through a considerable height or through a long pipe there should be the least practicable waste of the atmospheric pressure in getting the water into the plunger chamber, or in retarding it from following the plunger in full contact. With the water once into the plunger chamber there is plenty of steam pressure available to force it out through the delivery valves.
35. Valve Springs, Guards and Covers.
a. All valve springs must be of the best spring brass wire, and must be coiled on a cylindrical arbor.
Conical valve springs are not approved because the strain is not uniform throughout spring, thereby increasing the liability to breakage and the chance of their getting out of center and becoming “hooked up.”
b. The valve spring must be held centrally at its top by resting in a groove in valve guard, substantially as shown in [Fig. 9].