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 Never operate a pump for more than a few minutes with the
discharge valve closed; continued operation without water
circulation causes the pump to overheat. Where system
conditions require possible operation of the pump with no
e f f e c t i v e water discharge, provide a recirculation line on the
discharge side of the pump to bypass approximately 10 percent
o f the rated flow. T h i s line discharges back to the suction
t a n k , preferably below water level.
( d ) After the pump is full of water, start the pump with the discharge
valve closed to reduce the starting load on the drive. When the pump is up to
speed, close the vents and slowly open the discharge valve to obtain required
delivery.
(e) Make sure the bearing oil rings, if so fitted, turn freely. They
may be observed through the oil holes in the bearing caps. I f the rings are
not operating properly, shut down the pump and correct as required.
3.3 Normal Operation.
T h e following considerations are important in
operation.
N e v e r control water delivery by throttling the suction valve,
o r cavitation may be the result. Cavitation, which is the
f o r m a t i o n of partial vacuum in a flowing liquid, reduces the
a v a i l a b l e net positive suction head. Also, the sudden
c o n d e n s a t i o n of the vapor bubbles resulting from cavitation
erodes the rotor and casing.
( a ) Output Control. The delivery of centrifugal pumps can be controlled
by changing the speed, or by throttling the discharge. Normally, the pumps
a r e driven at constant speed and the delivery is controlled by throttling the
d i s c h a r g e , either manually or automatically. -
(b) Pump Characteristics. C e n t r i f u g a l pumps deliver a determined flow
at a given pressure (head) when operated at a constant speed
I f the system
head increases abnormally, pump delivery will decrease rapid l y , f o l l o w i n g i t s
c h a r a c t e r i s t i c curve. A clogged line, a defective valve, or wrong operating
procedures are common causes of head increase in a system. Investigate and
correct immediately, if required.
4-21
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