The classical symptoms are there; the unique rumbling / rattling noise and excessive vibration. But is the pump really cavitating or is it being subjected to one of the other conditions that share the same symptoms?
Until we know what the problem really is, any corrective action will be just a guess. Consequently, the first thing we have to do is to identify the real problem.
Taking the pump apart and checking the location of the pitting damage on the impeller will give us a good indication, but that’s a lot of work. A simpler method can be used when we consider the general causes of the three conditions.
Cavitation is caused by low suction pressure, and can be cured by increasing the net positive suction head available or running at a lower flow rate that needs less NPSH available.
Air (and vapor) entrainment is most frequently caused by a poor suction design arrangement and can be cured by correcting that layout.
Recirculation is caused by running the pump at too low a flow rate, and can be cured by increasing that flow.
To identify which of the three conditions we are faced with when the common symptoms of noise and vibration are experienced, we can simply throttle the discharge valve. This action reduces the flow and creates three possible scenarios.
1. The noise and vibration will grow quieter.
This means that the pump is now operating at a lower flow that requires a lower level of NPSH, and the quieter, smoother operation identifies that Cavitation was present and is now being eliminated.
2. The noise and vibration will get worse.
This tells us that the pump is moving into a worsening condition of low flow which demonstrates a problem with Recirculation.
3. Little or no difference is experienced.
When only a minimal difference (if any) is experienced, that indicates an Air Entrainment problem that is not immediately susceptible to changes in flow rate.
Every once in a while a situation will exist that will defy this rule of thumb, but that will tend to be the exception rather than the rule. As with any pumping problem, once we’ve defined the problem, the solution is usually quite straightforward.
Author of “The Practical Pumping Handbook”, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260