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In most basic submersible water well pump training for pump sizing, preventing cavitation is rarely prioritized. Most basic manuals do not even mention it.

 

While the cavitation seen in many other types of pumping systems also occurs in water well applications, it is usually only discussed in commercial applications.

 

Residential water wells usually are low flow and have sufficient submergence to prevent any damage from cavitation. However, when maximizing the well production flow or in shallow pump settings, cavitation can also be caused in residential situations.

 

The simplest form of cavitation appears when there is little submergence. This happens when the well has little flow from the bottom of the well and the pump is also fed by water from above. This occurs when the pump is set close to the pumping level of the well. It appears as a swirling tornado much like a kitchen sink when draining. This induces air into the pump which causes air bubbles to implode and damage pump components.

 

Another form of cavitation is much more common and can cause even more serious damage. This is referred to as insufficient NPSHa (“a” for available). NPSHa consists of several components that when totaled are more than the NPSHr (“r” for required) that the pump needs to avoid cavitation. These are made of: atmospheric pressure, height above the pump in fluid, any friction on the inlet side of the pump, water temperature all converted to feet of water.

 

In a deep rock well for instance, water temperature and water height above the pump setting and little inlet friction usually make NPSHa high enough that there is no cavitation problem. When you are pumping in a shallow well with warm water, and particularly when pumping on the far-right side of the curve, the components are not always in favor of having sufficient NPSHa. If this well happens to be at a high elevation, the atmospheric pressure is also reduced.

 

When you have calculated the NPSHa, compare it to the NPSHr found on the pump curve or manufacturer information. If NPSHa is less than NPSHr, you will prevent suction side cavitation damage. This is seen on impellers and castings which may look like they have been pounded by rocks or a BB gun. Pumps will lose performance quickly from this damage.

 

Another type of damage occurs in the discharge of the pump. It is corrosive wear in the diffuser bowls and the discharge head itself. This is most often a result of pinching the flow back to the left side of the recommended flow range. The result is a swirling action as the flow that the impeller is moving cannot exit and then must recirculate at high velocities to the suction side of the impeller. This damages castings, impellers, and wear rings as well. By avoiding pumping more than is required, this type of damage will be minimal.

 

Proper sizing for the need is always the best solution. Pumping above the required flow will cause premature failure and require larger motors at additional costs.

 

Charts are published in numerous sites to provide the data for the calculations above. If you need charts that you can plug into NPSH, contact customer service for assistance.