Deciphering centrifugal pump suction conditions
If you notice that your centrifugal pump is noisy, performing poorly, breaking shafts or exhibiting any other bad symptoms, the problem is most likely one of suction.
The conditions to look for on the suction side break down into two main categories: NPSH or air.
First, let look at NPSH. The acronym stands for net positive suction head. NPSH is either available (NPSHa) or required (NPSHr). Available refers to what your particular system allows, and required is what the pump needs to operate correctly.
When the required NPSH exceeds the available NPSH, the liquid will turn into vapor in the eye of the impeller. Centrifugal pumps can’t pump vapor, so they either stop running or sound like they’re handling gravel. If they stop pumping completely, it means that the vapor has taken over the entire pump.
The gravelly sound is the noise of small vapor bubbles collapsing as they move deeper into the pump. Each of the collapses can take a small chunk of metal from the impeller.
So how do you recognize an NPSH problem? Call an expert (unless you are one) if you are having pump problems and along with any of these conditions:
- hot liquid
- liquid under vacuum
- volatile liquid
- suction lift on pump
- lots of piping and/or fittings on the suction side
Now, let's look at air. Air or vapor in the pump system is an easier problem to solve. Air or vapor is entrained in the liquid, normally a little retention time in a tank will do the trick. There are several ways in which the system can entrain air into the liquid including:
- Unsubmerged pipes discharging into the tank near the pump suction
- Compressed air lines used to keep a liquid stirred up
- Excessive agitation from mixers or agitators
- Partially submerged suction piping
We saved vortexing for last because it requires a bit more explanation. Vortexing is what the water does when it swirls around a drain. You may have noticed that this does not occur when the sink is full of water. This is because there is enough liquid above the drain to prevent it (submergence). If the drain is small, more submergence is required. We are including a curve which plots feet of submergence against entrance velocity of the liquid needed to prevent vortexing.
Finally, don’t assume there is no vortexing just because you can’t see it. Some of the worst ones from the standpoint of air entertainment aren’t apparent on the liquid surface.