_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.
_c._ A light, rustless metallic plate must be interposed between the
bottom of the spring and the rubber valve, and must be the full area of
the valve. This plate must also be formed with a raised bead to guide
the spring at the bottom.
The weight of this plate should be small, for the inertia of the
lifting parts of the valves should be the least possible, to permit
quick action and to avoid pounding.
_d._ For the average condition of a 10 or 15-foot lift, the stiffness
of suction valve springs should be such that a force of about one pound
per square inch of net port area will lift valve 1/4 inch off its seat.
The springs on the delivery valves should ordinarily be from two to
three times as stiff as just specified, but any other reasonable degree
of stiffness which is proved to work well in practice will not be
objected to.
For suction under a head, the greater snap with which water enters
the plunger chamber when thus pushed in by say twice the atmospheric
pressure renders it difficult to avoid water hammer at high speed.
Extra stiff suction valve springs will commonly aid in controlling
this and should be used wherever pumps are to work under a head.
An approved type of indicator water gate on the suction pipe near the
pump, which can be partly closed, will enable the pump to run quietly
at high speed. Such a gate is an extra not included in price of the
pump.