Cavitation problems in a sump pump

[Charlesi]

Hi guys

I have a sump pump that is probably experincing cavitation. I say probably because the impeller is coated in a plastic cover (some type of hard epoxy???) and is showing signs of wear. Im not familure with the mechanics of cavitaion but is it possible that a plastic covered impeller could be eaten through by cavitation. I was under the impression that cavitation seldomly effected plastics. Once this plastic cover is removed it takes no time for the impeller to be eaten away. We are replacing it every few months. Here is some more info

Fluid: Saline solution with no particulate at 30 degress c
PH: 5.95

Thanks a lot

 

[yogibear1]

Is it really cavitation - the collapse of vapor bubbles on the impeller? Does it sound like marbles? Solutions to cavitation problems should focus on increasing the fluid pressure - like raising the float level which shuts off the pump; or perhaps removing entrapped air in the solution being pumped - water with bubbles has a lower density and vapor pressure. If the pump inlet is too restricted such that there are large entrance losses, this reduces the fluid pressure and leads to cavitation. Elevated fluid temperature also has an effect upon cavitation - though cooling the saline solution is probably not a preferred option in your case.

 

[Charlesi]

I guess my main question was that if a pump in saline solution is cavitating violently enough could it erode an epoxy cover.

I think what is happening is that once the plastic erodes from the cavitation and there is a little metal exposed to the solution the saline solution vapourises from the cavitation (and produces HCL) and starts to wear the surface away very quickly. Any ideas...

 

[gnordo]

There is no question that cavitation will erode any epoxy even faster than a steel, stainless steel/bronze,etc. impellar alone. Cavitation is a violent occurrence on a very small scale, that by repeated repetition will destroy almost any material I know of.


One possible easy way out may be to reduce the pump discharge flow by throttling the flow with a VALVE ON THE DISCHARGE SIDE OF THE PUMP. This will reduce the inlet flow, and all the pressure drops associated with the inlet piping and inlet condition. There is no question that the flow can be throttled sufficiently to STOP THE CAVIATION; the question is will that reduced flow level be satisfactory to empty the sump.


If cavitation is occurring, correct it as others have said, by altering the inlet condition sufficently to reduce the pressure drop of the piping and or inlet conditions.

Possibilities- make inlet piping larger diameter

Shorten inlet piping.

Reduce pressure drop through any filters/strainers by either enlarging the filter area, or increasing the strainer mesh size,etc.

If possible, get rid of any check valves on the inlet side. Move them to the pressure side of the pump.

Replace any high pressure drop throttling valves located on the suction with full flow valves. If throttling is needed, relocate throttle/balance valve to discharge side of pump.

Locate the pump lower than its present position relative to the lowest fluid level of the sump from which it is pumpin. This will increase both the static, and dynamic suction head.

Raise the lowest level of the sump fluid to increase suction head.

With severe caviation, it can be heard, like as someone said, it sounds like marbles/pebbles rolling around in the pump. Now it still may be cavitating, even though one can not hear it. So if one sees erosion of impellar material, one can presume it is cavitation induced, presuming no chemical incompatabilty is eating the impellar up.

One last consideration, any fluid that is near its boiling point is a major candidate for caviation. If this is part of the problem, perhaps some means of additional fluid subcooling might provide the best answer.

One additional comment- May I suggest installation of a pressure gage on the suction side of the pump, as close to the inlet as possible. If the pump has a tapped boss at the inlet, use it to tap off for the suction pressure gage.

If there are any strainers on the inlet, I like to do a suction pressure manifold in front of and behind the strainer/filter such that the SAME GAGE IS USED TO MEASURE THE PRESSURES. USING THE SAME GAGE ELIMINATES ANY GAGE ERROR IN THE DIFFERENTIAL READINGS.

Good luck Gnordo