Air Entrainment Or Pump Cavitation?

Air entrainment and pump cavitation have similar symptoms. In a centrifugal pump, they both have the ability to produce a rumbling, rattling noise, and cause

above base line vibration. If left unchecked, each can cause the impeller to become pitted and damaged. Though the effects of entrained air and pump cavitation are remarkably similar, their causes are quite different. Before troubleshooting the issue, understand which one you’re dealing with.

WHAT IS AIR ENTRAINMENT?

Air entrainment occurs when air or vapor bubbles are already in the fluid before reaching the pump. When the fluid reaches the eye of the impeller, they are subjected to higher pressures and implode on the surface of the impeller, just as they would where cavitation issues are present.

Air entrainment is common where:

There is free falling discharge into a pit or tank near the pump’s suction line
A leaky suction line
Pump whose minimum submergence is not being met
The product has air pockets by nature (paper stock, fermenting liquids, or foaming agents)

Entrained air can also occur in instances where the product being pumped is close to the boiling point, like condensate.

WHAT IS PUMP CAVITATION?

Cavitation is the formation of bubbles or cavities in liquid, developed in areas of relatively low pressure around an impeller. The imploding or collapsing of these bubbles cause intense shockwaves inside the pump leading to damage to the impeller and pump housing.

It occurs when there is not enough NPSH_A, instances like:

Clogged filters or strainers on the suction side
Suction side blockages in the pipe
Poor piping design
Poor suction conditions

HOW TO TELL IF THE PROBLEM IS AIR ENTRAINMENT OR PUMP CAVITATION

Cavitation and air entrainment, though sharing similar symptoms, have different paths to resolution.

A simple test can be conducted by throttling the discharge valve to help you determine if the problem is pump cavitation or entrained air. When this is done, one of 3 things will happen:

The noise and vibration worsen. If this occurs, you may have an issue with recirculation (a topic to cover in another post).
Noise and vibration lessen or go away completely. Cavitation was most likely the issue. Throttling down the valve makes the pump operate back on performance curve thus requiring less NPSH_A.
Noise and vibration remain the same. Flow rate has no effect on the problem, so entrained air is likely the culprit.

Cavitation and air entrainment are problems caused by the system or improper pump selection. The right diagnosis is key to solving the problem for good. Not doing so can cost you in the long run as the pump will experience premature wear and pump failure.

Not sure if you’ve got entrained air or a cavitation problem? Ask us about it! We gladly provide technical assistance to businesses in Wisconsin or upper Michigan.