IGOR J. KARASSIK
C. J. TULLO
2.455
SECTION 2.4
CENTRIFUGAL PUMP
PRIMING
A centrifugal pump is primed when the passageways of the pump are filled with the liq-
uid to be pumped. The liquid replaces the air, gas, or vapor in the passageways. This may
be done manually or automatically.
When a pump is first put into service, its passageways are filled with air. If the suction
supply is above atmospheric pressure, this air will be trapped in the pump and compressed
somewhat when the suction valve is opened. Priming is accomplished by venting the
entrapped air out of the pump through a valve provided for this purpose.
Unlike a positive displacement pump, a centrifugal pump that takes its suction from
a supply located below the pump, which is under atmospheric pressure, cannot start and
prime itself (unless designed to be self-priming, as described later in this section). At its
rated capacity, a positive displacement pump will develop the necessary pressure to
exhaust air from its chambers and from the suction piping. Centrifugal pumps can also
pump air at their rated capacity, but only at a pressure equivalent to the rated head of the
pump. Because the specific weight of air is approximately
that of water, a centrifugal
pump can produce only
of its rated liquid pressure. For every 1 ft (1 m) water has to
be raised to prime a pump, the pump must produce a discharge head of air of approxi-
mately 800 ft (m). It is therefore apparent that the head required for a conventional cen-
trifugal pump to be self-priming and to lift a large column of liquid (and in some cases to
discharge against an additional static liquid head) when pumping air is considerably
greater than the rating of the pump. Centrifugal pumps that operate with a suction lift
can be primed by providing (1) a foot valve in the suction line, (2) a single-chamber prim-
ing tank in the suction line or a two-chamber priming tank in the suction and discharge
lines, (3) a priming inductor at the inlet of the suction line, or (4) some form of vacuum-
producing de