Page 230 - Intro Predictive Maintenance

Process Parameters 221

How to Eliminate

Several design or operational changes may be necessary to stop centrifugal pump cav-
itation. Increasing the available net positive suction head (NPSH A) above that required
(NPSH R) is one way to stop it. The NPSH required to prevent cavitation is determined
through testing by the pump manufacturer. It depends on several factors, including
type of impeller inlet, impeller design, impeller rotational speed, pump flowrate, and
the type of liquid being pumped. The manufacturer typically supplies curves of NPSH R
as a function of flowrate for a particular liquid (usually water) in the pump’s manual.

One way to increase the NPSH A is to increase the pump’s suction pressure. If a pump
is fed from an enclosed tank, either raising the level of the liquid in the tank or increas-
ing the pressure in the gas space above the liquid can increase suction pressure. It is
also possible to increase the NPSH A by decreasing the temperature of the liquid being
pumped. This decreases the saturation pressure, which increases NPSH A.

If the head losses in the suction piping can be reduced, the NPSH A will be
increased. Methods for reducing head losses include increasing the pipe diameter;
reducing the number of elbows, valves, and fittings in the pipe; and decreasing the
pipe length.

It also may be possible to stop cavitation by reducing the pump’s NPSH R, which is
not a constant for a given pump under all conditions. Typically, the NPSH R increases
significantly as the pump’s flowrate increases. Therefore, reducing the flowrate by
throttling a discharge valve decreases NPSH R. In addition to flowrate, NPSH R depends
on pump speed. The faster the pump’s impeller rotates, the greater the NPSH R. There-
fore, if the speed of a variable-speed centrifugal pump is reduced, the NPSH R of the
pump is decreased.

Variations in Total System Head

Centrifugal pump performance follows its hydraulic curve (i.e., head versus flowrate).
Therefore, any variation in the total back-pressure of the system causes a change in
the pump’s flow or output. Because pumps are designed to operate at their best effi-
ciency point (BEP), they become more and more unstable as they are forced to operate
at any other point because of changes in total system pressure, or head (TSH). This
instability has a direct impact on centrifugal pump performance, reliability, operating
costs, and required maintenance.

Symptoms of Changed Conditions
The symptoms of failure caused by variations in TSH include changes in motor speed
and flowrate.

Motor Speed. The brake horsepower of the motor that drives a pump is load
dependent. As the pump’s operating point deviates from BEP, the amount of
horsepower required also changes. This causes a change in the pump’s rotating speed,