_a._ Provide outlets for the water; start the pump slowly, gradually
open steam-throttle to bring the pump to full speed.
The pump should run smoothly at the rated full speed of 70 revolutions
per minute (or 60 revolutions if a 1,500-gallon pump) with full length
of stroke, and meanwhile maintain a water pressure of 100 lbs. per
square inch.
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 2-1/2-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 1-1/8-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 1-1/8-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_.