If the hose lines are short, or discharge is too free, partly close the water outlet valves, thus throwing an extra back pressure on the pump equivalent to that which would be produced through a greater length of hose.
During this trial it is preferable to discharge the water through lines of 21⁄2-inch cotton rubber-lined hose, preferably each 150 feet long, each connected directly to the hose outlets on the pump, and each line having a 11⁄8-inch smooth nozzle at its outer end. Two lines should be connected for a 500-gallon pump, three for a 750, and so on, having as many lines as rating of pump requires.
A hose line 150 feet long, with an inside surface of average smoothness, and with a 11⁄8-inch nozzle attached, will require about 80 pounds pressure at the pump to discharge 250 gallons per minute, and the nozzle pressure will be about 45 pounds. Therefore, with lines attached as above, a pressure at the pump of about 80 pounds should represent a discharge about equal to the rated capacity of the pump, and would ordinarily correspond with the rated full speed revolutions.
If the pump runs smoothly under these conditions, it is well to open the throttle somewhat further, and bring the pressure at the pump up to 100 pounds. This will give a discharge of about 280 gallons per stream, or about 12 per cent. in excess of the rated capacity. The revolutions will, of course, correspondingly increase, and under all ordinary conditions a pump should run smoothly at this higher capacity though a little more vibration and pounding would be expected than when running simply at its rated speed.
After cushion valves are adjusted there should be no noteworthy water hammer or valve-slam. Sometimes valve-slam is not the fault of the pump, but arises from an obstructed suction pipe. It is objectionable to doctor water hammer in a pump by “snifting” air into the suction, as this cuts down the efficiency and is a poor expedient.
The quietness of that part of the hose near the pump, or its freedom from rubbing back and forth crosswise an inch or more with each pulsation of the pump, is a good index of the pump maker’s skill in securing uniform delivery. Bad pulsation quickly wears holes in the hose, and to reveal this is the object of testing with hose connected directly to the pump.
49. Test of the Internal Friction.
a. This is shown by the reading of steam gauge compared with water pressure gauge at air chamber.
Tests have generally run about as follows, for pumps running at full rated speed: