Thrush OLD - Manual (Page 2)

SUCTION PIPING
Long radius elbows should be used in place of standard elbows wherever possible, because of their superior flow
characteristics. Elbows installed in any position at the suction nozzle have a tendency to distribute the liquid unevenly
in the impeller eye and may cause a reduction in capacity which creates an undesirable thrust condition, or create noisy
operation. Eccentric reducers should be installed directly at the suction nozzle, with the taper at the bottom to prevent
air pockets from forming. Straight taper reducers should never be used in a horizontal suction line because of the air
pocket that is formed at the leg of the reducer and the pipe.
DISCHARGE PIPING
The discharge pipe diameter should be the same as or larger than the discharge nozzle diameter. The size of the
discharge pipe to be used is dependent upon its application.
PROPER PIPING ALIGNMENT IS ESSENTIAL BEFORE CONNECTION IS MADE. PIPING ALIGNMENT SHOULD
NEVER BE ACHIEVED BY FORCE; THIS COULD PRODUCE STRAIN ON THE PIPING AND THE PUMP CASING.
PROPER SUPPORTS SHOULD BE INSTALLED FOR THE PIPING TO KEEP ITS WEIGHT OFF THE PUMP CASING.
NOTE: Centrifugal pumps should never be started dry or run dry. Operating a pump dry will cause scoring of the
mechanical seals, 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 1-A) The liquid source is located above the pump and
all that is necessary to prime the pump is to open the air vent or plug in the pump casing and to crack the isolating
valve in a 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 1-B) The foot valve (located at the
bottom end or foot of the suction piping) functions as a check valve that allows flow in one direction only, toward the
pump. Otherwise, all the liquid may drain from the pump and suction piping 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. The
important thing is to completely fill the suction pipe and pump casing with liquid. When the pump is started, the vacuum
created by pumping the liquid from its source, 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 now trapped in the suciton 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 used.
STARTING THE PUMP
The discharge-isolating 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-isolating 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.