_a._ All the suction and discharge valves in any one pump must be of
the same size and interchangeable.
_b._ There must be a clear space around each rubber valve, between it
and the nearest valve, equal to at least one-fourth of the diameter
of the valve, or between it and the wall of the chamber of at least
one-eighth of the diameter of the valve.
_c._ These valves must be of the very best quality of rubber, of medium
temper, with a face as soft as good wearing quality will permit.
They must be double-faced, so they can be reversed when one face is
worn.
The quality of rubber is almost impossible of determination by brief
inspection or by chemical analysis. The relative amount of pure gum
and of cheaper composition may vary, or good material may be injured
by defective vulcanization. The only safe way to secure excellence
and uniformity is for the pump manufacturer to test samples of each
new lot under severe duty (as by a week’s run in a small special
pump, with say 150 pounds pressure and heavy water hammer, or by some
equivalent means) and to furthermore require the rubber manufacturer
to mould a date mark as “(Name of pump manufacturer, lot 201—April 3,
1904.)” on the edge of every valve, by which the pump manufacturer
can keep track of those which prove defective.