Where Cavitation Begins – Cooling Tower Pumps and Piping

Last week the R. L. Deppmann Monday Morning Minute defined NPSH_R, and ended with the Hydraulic Institute (HI) definition as the absolute pressure that will cause the total head of the pump to be reduced by 3%, due to flow blockage from cavitation”. Of importance is the fact that it does not say that NPSH_R is where cavitation begins.

The place where cavitation begins is called incipient cavitation and can be from 2 to 20% greater than the NPSH_R reported on the pump curve. In addition, NPSH_R is a tested value, the test is with clear un-aerated water; not dirty, highly aerated, tower water and pumps have manufacturing tolerances. The NPSHR can also vary with the system fluid conditions.

So to reduce the chance of damage to the pump from cavitation, we need to use a larger number than the pump net positive suction head required. A few years ago HI defined some margins to apply to the published pump NPSH_R with different systems and pump types. The margin depended on the application and the SUCTION ENERGY of the pumps. For cooling tower applications, the recommendation was to use a multiplier to NPSH_R of 1.3 for low energy pumps, 1.5 for high energy pumps, and 2.0 for very high energy pumps.

SO HOW DO WE DETERMINE WHICH MARGIN TO USE?

The suction energy of a pump depends on a number of variables but it can be approximated by the formula:

Now you use the result to choose the margin using this chart.

To learn more about NPSH, check out the articles below:

Net Positive Suction Head (NPSH) – Terms and Definitions