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Cavitation is sometimes the cause of structural damage to propellers, impellers, and pumps. One cause of cavitation can be improper pump submergence.

 

A pump can exhibit signs of low NPSHa even when the calculations have been gone over and checked for accuracy. Elevations, friction losses, vapor pressure charts, and velocity head make up the components of Net Positive Suction Head available (NPSHa) which is then compared to the pump requirement (NPSHr). A margin of NPSHa over NPSHr will usually ensure that cavitation will be avoided.

 

Occasionally, a situation arises with a pump that is making noise and showing unstable flow and current when approaching the right side of the curve. Of course, the right side of the curve is where NPSHr is the highest, so the calculations are reviewed again.

 

One factor often overlooked is submergence. On submersible applications users have experienced the swirl and a break in suction due to low submergence. You can even see it in a basement sump pit under the right conditions. There are calculations for determining the minimum submergence.

 

In the case of an End Suction centrifugal, submergence is sometimes forgotten about.  We know that we can actually lift water with the centrifugal and forget about it if we are pumping from a tank with elevated fluid level.

 

When the level in the tank is only slightly higher than the suction pipe of the pump, the same swirl occurs around the pipe opening as when a pump is submerged in a pit. This causes air to enter the impeller and then the noise and erratic flow begins. As flow decreases, the problem goes away. As the level in the tank increases, the problem may disappear as well.

 

The diameter of the pipe in relation to the flow rate also makes a difference. It is difficult to use the formula to calculate submergence as complex exponents are involved. So a chart is provided below based on flow in gallons per minute and pipe ID.

 

Ensuring the suction pipe has a proper amount of submergence will eliminate the cavitation due to low NPSHa at the inlet of the pump.

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