by Allan Budris
One way that the Best-of-Class users reduce the Life Cycle Costs of their pump installations is by selecting pumps and designing pump systems that will avoid cavitation damage within their pumps. Cavitation damage can drastically shorten the life of the pump impeller, mechanical seals, bearings and possibly other pump components. In other words it will reduce the mean-time-between-failure (MTBF), which will increase maintenance costs and pump down time.
There has been a lot written on the dangers of pump cavitation damage when adequate suction pressure (Net Positive Suction Head Available - NPSHA) is not provided; when pumps are operated at low flow rates (below the start of suction recirculation); and/or when sufficient straight runs of piping are not provided upstream of the pump suction. It can require NPSHA values of four times the published Net Positive Suction Head Required (NPSHR), or more, to avoid all cavitation in a pump. The start of Suction Recirculation, which occurs in all centrifugal pumps at some reduced flow rate, can be as high as 85% or more of the pump best efficiency flow rate (bep). The required minimum straight length of pipe, up stream of the pump suction, may be as high as 15 times the pipe diameter or higher.
However, experienced pump users also know that most smaller pumps do not experience cavitation damage, even when operated under these unfavorable conditions. So the question is, specifically when must pump users take special precautions to avoid cavitation damage, and the associated increase in maintenance costs?
Well, based my extensive experience with several major pump manufacturers, I was able to developed a method that predicts when pumps are susceptible to cavitation noise, vibration, and/or damage; and when they are free of these damaging affects. The method is called “Suction Energy”, with specific gating values of “Suction Energy” identified for different pump types. The amount of energy in a pumped fluid which flashes into vapor and then collapses back into a liquid in the high pressure areas of the impeller determines the amount of noise and/or damage from cavitation.
Suction energy is another term for the liquid momentum in the suction eye of a pump impeller, which means that it is a function of the mass and velocity of the liquid in the inlet. Suction energy is defined as: