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.
52. Test of Internal Leakage or Slip.
a. Set safety-valves at 115 lbs., shut all water outlets, admit steam enough to give 100 pounds water pressure, then pump will move very slowly under the influence of the leakage past plungers. About one revolution of pump per minute shows a proper accuracy of fit. Anywhere from 1⁄3 to 2 revolutions per minute is satisfactory.
Too tight a fit is bad, as if not exceedingly uniform it induces scoring or cutting of the metals. Moreover, should the pump happen to be run dry for a few minutes before catching its suction a slight warming and expansion of the plunger may cause it to stick and cut.
53. Test with Maximum Working Pressure.
a. For this, alternately shut down the main outlet gate and adjust the hand-wheel of the safety valve, and open up on the throttle as may be required, running pump at say one-half speed (or, in experienced hands, at full rated speed), and note the greatest water pressure which the full boiler pressure (unless boiler pressure is above 85 lbs.) will yield with pump at full speed.
Sometimes it may be necessary to force water through very long lines of hose, or to an unusual height.
Steam fire engines are not infrequently called on to give 200 pounds per square inch water pressure.
To test short hose lines with anywhere near so high a pump-pressure is dangerous, lest nozzle kick and pull itself away from the man holding it and thrash around; but the ability of the pump may be tested by putting this high pressure-delivery mainly through the safety valve, or in part through the partially closed main outlet gate.
It is not advisable to carry this water pressure above 200 pounds in this test at the factory, (although in the shop test the water pressure is carried to 240 pounds) and engine driver should stand with his hand on the throttle.