Thrush GTU - Manual (Page 3)

Priming the Pump
A pump will not operate satisfactorily until it is primed. All air must be expelled from the suction
piping and pump casing and be replaced by the liquid being pumped. There are several methods of
priming pumps. The one selected will depend on the specific requirements of the application.
Flooded Suction Priming
This method of priming a pump is relatively simple
(see Figure 3). The liquid source is located above the
pump. To prime the pump, open the air vent valve or
plug in the pump casing. Then slightly crack the gate
valve in the suction line. The suction line and the
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 the air
has been expelled from the pump casing.
Suction Lift
A foot valve should be used for priming on suction lift
applications (see Figure 4). The foot valve is located at
the bottom end or foot of the suction piping. It functions
as a check valve which allows flow only toward the
pump. It prevents the liquid from draining from the
pump and suction piping back into the sump after pump
shutdown.
Initial priming is accomplished by completely filling the
suction piping and pump casing with the liquid to be
pumped. This can be done most easily by first
removing the air vent valve or plug at the top of the
pump casing. Then a pipe nipple can be inserted in the
orifice with the appropriate increases to accommodate a
hose connection alternatively, a priming line can be insert-
ed in the discharge piping between the check valve and
the pump, or the priming can be done with a bucket and a
funnel. It is important that the suction pipe and pump
casing be completely full of the liquid to be pumped.
When the pump is started, a vacuum is created by the
priming fluid. This vacuum, 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 direction of flow,
causing the valve to close. The liquid then becomes
trapped in the suction piping and pump casing, maintaining the prime on the pump.
It is possible for the pump to lose its prime (see Figure 4). This happens when dirt or debris keeps
the foot valve partially open, allowing fluid to drain from the pump casing and suction piping back
into the sump. For this reason, the prime should always be checked before starting the pump.
Refer to GTU pump performance curve (see Figure 5). For N.P.S.H. requirements.
Starting the Pump
The discharge valve should be partially closed when the pump is initially started. This will prevent
possible water hammer and lower initial power draw. As soon as the pump has reached its
operating speed, the discharge valve may be opened to the desired position.