_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.