# Where Cavitation Begins – Cooling Tower Pumps and Piping

Norm Hall
/
April 23, 2012

Last week the R. L. Deppmann Monday Morning Minute defined NPSHR, 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 NPSHR is where cavitation begins.

The place where cavitation begins is called incipient cavitation and can be from 2 to 20% greater than the NPSHR reported on the pump curve. In addition, NPSHR 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 NPSHR 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 NPSHR 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:

 Suction Energy= D x n x S X 10 (-6) Where S = Suction Specific Speed or (RPM X Q(.5) / NPSHR(.75) ) D = Pump suction size (in) n = Speed Q = Flow at Best Efficiency Point (BEP) for the maximum impeller NPSH is at the flow at BEP for maximum impeller also. Note: that for double suction pumps, you use ½ of the flow at BEP

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

 Suction Energy Pump Type Margin for Cooling Towers Less than 150-220 Low Energy 1.3 Between 150-220 and 250-380 High Energy 1.5 Over 250-380 Very High Energy 2.0