Page 70 - Know and Understand Centrifugal Pumps

Pump Classification

FLOW
Fioure 6-3
In reality, there are small losses in volume delivered as the pressure or
resistance increases, so a more representative PD pump curve appears in
Figure 6-3.
The flow through a PD pump is mostly a function of the speed of the
driver or motor. It is important to note that a pump cannot generate
flow. The flow must be available to the pump suction nozzle. In this
sense the flow in a PD pump is actually energy, called net positive inlet
pressure. The pressure or head that a PD pump can generate is mostly a
function of the thickness of the casing and the strength of the
associated accompanying parts (seals, hoses, gaskets).
Positive displacement pumps normally have some strict tolerance parts.
These parts vary with the type and design of the pump. This strict
tolerance controls the flow, and the pressure that these pumps can
generate. When this tolerance opens or wears by just a few ten
thousandths, these pumps lose almost all their efficiency and ability to
hnction. These strict tolerance parts must be changed with a planned
certain frequency, based on the abrasive nature and lubricity of the
pumped fluid, to maintain the maximum efficiency of the pump.
There is no definite demarcation line, but positive displacement pumps
normally are preferred over centrihgal pumps in applications of:
w Viscous liquids,

w Precise metering, (dosification, pharmaceutical chemistry) and
w Where pressures are high with little flow.

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