Thrush 3500 RPM - Manual (Page 4)

OPERATION
CAUTION: CENTRIFUGAL PUMPS SHOULD NEVER BE STARTED OR RUN DRY.
OPERATING A PUMP DRY WILL CAUSE SCORING OF THE MECHANICAL SEAL,
RESULTING IN PREMATURE SEAL FAILURE. TO PREVENT THE PUMP FROM BEING RUN
DRY, IT SHOULD BE PRIMED BEFORE STARTING.
PRIMING THE PUMP
The pump will not operate satisfactorily until it is primed. All air must be expelled from the suction
piping and pump casing and replaced by the liquid to be pumped. There are several methods of
priming pumps. The one selected will depend on the specific requirements.
FLOODED SUCTION PRIMING
This method of priming a pump is relatively simple. (See Figure 3) The liquid source is located
above the pump and all that is necessary to prime the pump is to open the air vent valve or plug
in the pump casing and to crack open the gate valve in the suction line. The suction line and
pump should be filled slowly until a steady stream of liquid is observed flowing from the air vent.
After the pump is operating, it is recommended that the air vent valve or plug be opened again to
insure that all air has been expelled from the pump casing.
SUCTION LIFT
A foot valve should be used for priming on suction lift applications (See figure 3). The foot valve
located at the bottom or end of the suction piping functions as a check valve, which allows flow in
one direction only – toward the pump. Otherwise, all the liquid may drain from the pump and
suction piping back into the sump after shutdown. Initial priming is accomplished by completely
filling the suction piping and pump casing with the liquid to be pumped. This can be done by
removing the air vent valve or plug at the top of the pump casing and inserting a pipe nipple in the
orifice with an appropriate increaser to accommodate a hose connection. A priming line can also
be inserted in the discharge piping between the check valve and the pump, or the priming can be
done with a bucket and funnel. IMPORTANT: FILL THE SUCTION PIPE AND PUMP CASING
WITH LIQUID.
When the pump is started, the vacuum created by pumping the priming fluid, combined with
atmospheric pressure in the liquid well, forces liquid into the suction piping, thus opening the
valve and keeping it open until the pump is shut down. When the pump is shut down, the liquid
being pumped reverses its flow causing the valve to close. The liquid is trapped in the suction
piping and pump casing, thus maintaining a prime on the pump.
VACUUM PRIMING
Vacuum priming consists of removing air from the pump casing and suction piping and drawing
liquid into them by means of a vacuum-creating device. The types of vacuum equipment range
from a simple hand pump to a complex central priming system. The specific priming
requirements will govern what type of vacuum primer is used.
STARTING THE PUMP
The discharge gate valve should be partially closed when the pump is started in order to avoid
possible water hammer and initial power draw. As soon as the pump is up to operating speed,
the discharge gate valve should be opened to the desired position. The motor should turn
clockwise when viewed from the motor end and counter-clockwise when viewed from the casing
end.