Priming is the addition of liquid to a pump casing to displace the air inside, and to
create a liquid seal around it. Air is displaced typically through a vent connection at
the top of casing. If a vent connection were not provided, venting can be done by
filling the casing with liquid while slowly turning the pump shaft to flush out the air.
A self-priming pump is one that develops a vacuum sufficient enough for the
atmospheric pressure to force the liquid through the pump suction nozzle and into
the casing without manually re-priming the pump. Only positive displacement
pumps are truly self-priming but the term has been loosely used to include
self-priming centrifugal pumps.
A self-priming centrifugal pump is especially designed with a large chamber near
its discharge nozzle which acts both as an air separator that separates the air
from the liquid, and as a reservoir that holds a residual liquid to be used for
priming, or re-priming the pump. The pump has to be primed during the initial
start-up but re-priming is done automatically without outside intervention.
Although self-priming is a desirable feature in some centrifugal pump service the
trade-off is that pump efficiency is compromised due to some design constraints.
Self-priming pumps are popular for intermittent service, or service that requires
frequent pump starting and stopping, such as in contractors, drainage, sewage,
and similar applications, but are not widely used in critical service where optimum
reliability and efficiency are of paramount importance, and re-priming is seldom
needed due to continuous pump operation.
C: design, operation
F: pump self-priming
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