Page 55 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Type Cause Explanation
Vaporization Operation / Design Vaporization (inadequate NPSHa) is
the result of something that happens
in the suction side of the pump,
strangled valve, clogged filter,
inadequate submergence.
Internal Re-circulation Operation / Design / Internal Re-circulation results from
Ma in tena nce something in the discharge side of
the pump: strangled valve, clogged
filter, over-pressurized header, check-
valve installed backwards.
Vane Pass Syndrome Design / Maintenance Oversized impeller, inadequate
aftermarket parts, repair or rebuild
with incorrect specifications and
measurements.
Air Aspiration Design / Operation / Design, Improper suction pipe, fluid
Ma in tena nce flow too fast. Inadequate flange
torque, operation to the right of BEP.
Turbulence Design Design, Inappropriate suction system
(vessel, piping, connections, fittings)
design.

Be aware that in some cases, you’ll have to live with cavitation. Many
pumps suffer cavitation for reasons of inadequate design. For example,
when operating only one pump in a parallel system, this pump tends to
go into cavitation. Pumps that perform more than one duty through a
valve manifold tend to suffer cavitation. Pumps that fill and drain tanks
from the bottom tend to suffer cavitation. The last pump drawing on a
suction header tends to cavitate. And of course vacuum pumps and
pumps in a high suction lift are candidates for cavitation.
Some solutions may not be practical, or economical, or timely and
consistent with production. You could be forced to live with cavitation
until the next plant shutdown to make the necessary corrections. In the
meantime, the cavitation shock waves and vibrations will travel through
the impeller, down the shaft to the mechanical seal faces, and onto the
shaft bearings. We offer some specific recommendations for surviving
cavitation shock waves and vibrations in Chapters 13 and 14 on
Mechanical Seals.

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