Page 140 - Know and Understand Centrifugal Pumps

‘“h
The System Curve
FE&I\

PUMPS IN PARALLEL PROVIDE
PUMP CURVE
PUMP CURVE
A AND B
A or E
TWICETHE FLOW
--I-- l E : : ~
oounLEs
0 0 -
0 - 0
0 - GPM GPM
GPM

H
FEET

-

0
0 Q
GPM

Three tips
__ -~ ~ ~ ~~
First, one pump running in a parallel system tends to suffer from
cavitation because operation to the right of the BEP indicates that the
NPSHr of the pump rises drastically. To survive this condition, you
should use dual mechanical seals on these pumps. Dual or double
mechanical seals can withstand cavitation better than a single seal.
There is a discussion on this in the mechanical seal chapter of this book.
Many engineers perceive that parallel pumps are problematic because
they appear to suffer a lot of premature seal failure. Parallel pumps
deserve double seals even if it’s only a cold water system.
The solution is: Parallel pumps should have dual or double seals
installed to withstand cavitation when one pump is running solo.