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Normally, a centrifugal pump produces a
relatively low pressure increase in the fluid. This pressure
increase can be anywhere from several dozen to several
hundred psid across a centrifugal
pump with a single stage impeller. The term PSID (Pounds
Force Per Square Inch
Differential)
is equivalent to DP.
In this context, it is the pressure difference between the
suction and discharge of
a pump. PSID can also be used to describe a pressure drop
across a system component
(strainers, filters, heat exchangers, valves, demineralizers,
etc.). When a centrifugal pump
is operating at a constant speed, an increase in the system
back pressure on the flowing stream
causes a reduction in the magnitude of volumetric flow rate
that the centrifugal pump can maintain.
Analysis of the relationship between the
volumetric flow rate ( ) that a
centrifugal ˙V pump
can maintain and the pressure differential
across the pump (DPpump)
is based on various
physical characteristics of the
pump and the system fluid. Variables evaluated
by design engineers to determine this
relationship include the pump efficiency, the
power supplied to the pump, the rotational
speed, the diameter of the impeller and
blading, the fluid density, and the fluid viscosity.
The result of this complicated analysis
for a typical centrifugal pump operating
at one particular speed is illustrated
by the graph below.
Pump head, on the vertical axis, is the
difference between system back
pressure and the inlet pressure of the pump (DPpump).
Volumetric flow rate (V
), on the horizontal axis, is the rate at which fluid is
flowing through the pump. The graph assumes one particular
speed (N) for the pump impeller.
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