Q. Is the NPSH test for reciprocating pumps the same as for rotodynamic (centrifugal) pumps?
A. For reciprocating pumps, the determining factor for NPSH is a reduction in the rate of flow. The NPSH test should be performed at constant differential pressure (specified pressure unless limited by available power) and speed while measuring rate of flow at different values of the NPSH available. The NPSHA should be incrementally reduced until rate of flow loss reaches 3 percent.
The setup for testing should be in accordance with Figure 6.68.
When in service, the pump must be operated above the NPSHR if pitting, noise, vibration unstable operation and even mechanical failure are to be avoided. The margin of operating NPSHA above NPSHR depends upon the particular pump, liquid and installation.
During the test, caution must be taken to avoid operation in deep cavitation. If that should occur with suction the valve throttling, the discharge valve should be opened to release the pump load simultaneously with the opening of the suction throttle valve.
Additional details for testing reciprocating pumps are provided in Reciprocating Pump Tests, ANSI/HI 6.6-2000.