Page 68 - Know and Understand Centrifugal Pumps

Pump

Classification

Introduction

In Figure 6-1, Pump Classification, we see two principal families of
pumps: Kinetic Energy pumps and Positive Displacement pumps. These
two families are further divided into smaller groups for specific services.
Both pump families complete the same function, that is to add energy
to the liquid, moving it through a pipeline and increasing the pressure,
but they do it differently.

Positive displacement pumps
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Positive Displacement pumps perform work by expanding and then
compressing a cavity, space, or moveable boundary within the pump. In
most cases, these pumps actually capture the liquid and physically
transport it through the pump to the discharge nozzle. Inside the
pump where the cavity expands, a zone of low pressure, or vacuum, is
generated that causes the liquid to enter through the suction nozzle.
Then the pump captures and transports the liquid toward the discharge
nozzle where the expanded cavity compresses. In this sense, because the
available volume of space at any point inside the pump is a constant, we
can say that in theory, these pumps are considered a ‘constant volume
device’ with every revolution or reciprocating cycle.
Theoretically, the curve of a Positive Displacement pump should appear
as in (Figure 6-2).

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