_a._ The relief valve may next be tested by first adjusting it to
“pop” at 100 pounds, then shut the main outlet to pump, and then shut
the hose gates one by one, and thus force all the discharge through
the relief valve, meanwhile opening steam throttle, so as to run pump
_first at two-thirds speed or about 50 revolutions per minute_, and
finally at full speed (70 revolutions). The safety valve (relief valve)
should carry all this and not let the pressure rise above 125 pounds.
The pressure in a quick-moving fire-pump necessarily fluctuates 5
to 15 pounds at different points in stroke, and an air chamber of
reasonable size cannot wholly remove this. Therefore the safety valve
must be set at about 15 pounds higher than the intended average
working pressure; otherwise it will get to jumping with almost every
stroke.