Page 254 - Root Cause Failure Analysis

242 Root Cause Failure Analysis

The NPSHA also can be increased by decreasing the temperature of the liquid being
pumped. This decreases the saturation pressure, which increases the NPSH,.
If the head losses in the suction piping can be reduced, the NPSHA 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 NPSHR, which is
not a constant for a given pump under all conditions. Typically, the NPSHR increases
significantly as the pump’s flow rate increases. Therefore, reducing the flow rate by
throttling a discharge valve decreases NPSHR. In addition to flow rate, NPSH,
depends on pump speed. The faster the pump’s impeller rotates, the greater is the
NPSHR. Therefore, if the speed of a variable-speed centrifugal pump is reduced, the
NPSHR of the pump is decreased.

Variations in the Total System Head

Centrifugal-pump performance follows its hydraulic curve (i.e., head versus flow
rate). 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 oper-
ate 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

The symptoms of failure due to variations in TSH include changes in motor speed and
flow rate.

Motor Speed The brake horsepower of the motor that drives a pump is load depen-
dent. 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, which
either increases or decreases depending on the amount of work that the pump must
perform.

Flow Rate The volume of liquid delivered by the pump varies with changes in TSH.
An increase in the total system back pressure results in a decreased flow, while a back
pressure reduction increases the pump’s output.

Solutions
The best solution to problems caused by TSH variations is to prevent the variations.
While it is not possible to completely eliminate them, the operating practices for cen-
trifugal pumps should limit operation to an acceptable range of system demand for
flow and pressure. If system demand exceeds the pump’s capabilities, it may be nec-