Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

566 Centrifugal Pumps: Design and Application
at less than about 50% of Best Efficiency Point (BEP) flow. Vibration
and noise may increase markedly at less man this minimum stable flow.
The result is a decrease in seal life. Resolution of this problem may require
hydraulic modification by the pump manufacturer or a by-pass line
to artifically increase flowrate.
At still lower flows (less than 20% of BEP), the liquid passing through
the pump may be significantly raised in temperature. According to API
610, the minimum continuous thermal flow is "the lowest flow at which
the pump can operate and still maintain the pumped liquid temperature
below that at which net positive suction head available equals net positive
suction head required."
No Flow. In spite of assurances to the contrary by the operating crew,
sometimes pumps have obviously been running "dry." There are many
ways this can happen—a broken level controller, a plugged suction
strainer, a broken check valve or a discharge valve that was never
opened. All of these and more can prevent liquid from entering or leaving
the pump.
Some batch processes actually are designed so that the pump runs dry
as it pumps out a reactor or supply vessel. Dissimilar pumps operating in
parallel are a classic example of how one pump may force another to run
at no flow.
In addition to the noise and vibration problems described for low flow,
operation with no flow may generate enough heat to vaporize any liquid
trapped in the pump. In particular, seals that are flushed from the pump
discharge or suction may quickly become "gassed up." The obvious solution
is to avoid the no flow situation. If the pump must run under these
conditions, a separate flushing system may be required for the seal.
Cavitation. Cavitation has been rightly and wrongly blamed for many
ills in both pumps and seals. Certainly cavitation increases pump vibration
and vibration reduces seal life. Recent studies have shown that the
simple 3% head loss rule that is used to define NPSHR may not adequately
define the onset of cavitation problems. Also, operation at low
flow sometimes produces symptoms similar to cavitation.
Cavitation problems are sometimes difficult to solve. Frequently, increasing
the available NPSH is prohibitively expensive. Reducing the required
NPSH is sometimes possible with special impellers or inducers.
For these reasons, cavitation problems must be carefully addressed during
the initial specification of the pumps.