NPSHR Variations with Changing Speed or Diameter

When dealing with NPSHR, the best practice to determine the variations is to develop a NPSH test. However there are some other methods to predict the trend or approximately calculate the NPSHR in new conditions.

1. Speed Change

According to API 610, only 3% variation in speed is allowed for centrifugal pumps performance test; “Unless otherwise agreed, the test speed shall be within 3% of the rated speed shown on the pump data sheet. Test results shall be converted to anticipated results at the rated speed.” For this conversion ASME PTC 8.2 (Performance Test Code for Centrifugal Pumps) suggests the following formula based on affinity laws

According to HI standard, “if a pump operates at or near its cavitation limit, the limiting NPSHR value may vary other than as the square of the speed”. Another exponent (other than 2) can be suggested by pump manufacturer for the specific pump at hand. Also some pump engineers believe that exponent 2 is accurate enough when the speed is increased but for the speed reduction an exponent equal to 1.7 will give better approximations.

2. Diameter Change

In this case the affinity laws won’t be applicable to predict NPSHR value and the only way to obtain the accurate NPSHR is testing. But yet there are experimentally acquired trends that help in prediction of NPSHR value.

Some experimental studies have shown that for low specific speed pumps the variation of NPSHR is negligible when the impeller diameter is trimmed. It has been shown that when the pump specific speed is low, trimming the impeller diameter even to 70% of full diameter won’t change NPSHR.

As the specific speed increases, NPSHR will be affected by variation of impeller outer diameter. The following figure show that NPSHR will increase for a trimmed impeller diameter and the value of this increase become greater for high specific speed pumps. So it can be concluded that the impeller diameter of mixed flow and axial flow pumps has a strong influence on pump NPSHR.

NPSHR variation with impeller outer diameter and specific speed [Source: Kondo, Yamada, “Dose Impeller Affect NPSHR,” International Pump Users Symposium 1985, pp. 29-35]