Page 19 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

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suction piping. The fluid must be available to the pump with sufficient
energy so that the pump can work with the fluid’s energy. The pump
cannot suck on or draw the liquid into the pump. The concept of the
fluid being available to the pump is discussed in detail in Chapter 2 of
this book.
Positive displacement (I’D) pumps take the fluid at the suction nozzle
and physically capture and contain the fluid in some kind of moveable
enclosure. The enclosure may be a housing with a pulsing diaphragm,
or between the teeth of rotating gears. There are many designs. The
moveable enclosure expands and generates a low pressure zone, to take
the fluid into the pump. The captured fluid is physically transported
through the pump from the suction nozzle to the discharge nozzle.
Inside the pump, the expanded moveable enclosure then contracts or
the available space compresses. This generates a zone of high pressure
inside the pump, and the fluid is expelled into the discharge piping,
prepared to overcome the resistance or pressure in the system. The flow
that a PD pump can generate is mostly a function of the size of the
pump housing, the speed of the motor or driver, and the tolerances
between the parts in relative motion. The pressure or head that a PD
pump can develop is mostly a function of the thickness of the casing
and the tolerances, and the strength of the pump components.
As the pump performs its duty over time, and fluid passes through the
pump, erosion and abrasive action will cause the close tolerance parts to
wear. These parts may be piston rings, reciprocating rod seals, a flexing
diaphragm, or meshed gear teeth. As these parts wear, the pump will
lose its efficiency and ability to pump. These worn parts must be
changed with a degree of frequency based on time and the abrasive and
lubricating nature of the fluid. Changing these parts should not be