Derating centrifugal pump performance for low NPSHa
Derating centrifugal pump performance for low NPSHa
(OP)
Does anyone have a formula or method for predicting the performance for a single stage centrifugal pump based on lower than required NPSH?
I would like to be able to re-draw a pump curve based on the difference between the NPSHr denoted on a pump curve vs. the NPSHa I have in the real world.
This is for a small pump (15hp motor) pumping water.
Thanks!
I would like to be able to re-draw a pump curve based on the difference between the NPSHr denoted on a pump curve vs. the NPSHa I have in the real world.
This is for a small pump (15hp motor) pumping water.
Thanks!





RE: Derating centrifugal pump performance for low NPSHa
But, you can control the flow in some acceptable fashion so that the pump does not pump out far enough on its curve to get into the region where it doesn't have enough npsha.
You can do this with flow control devices, or you can design downstream piping with enough head loss so that the total head loss limits the pump.
I have done both with pumps that had npsha limitations.
You can also change your suction piping so as to minimize restrictions that lower npsha. Upsize the suction piping several sizes so that the head loss due to velocity is minimized. Don't exceed 6 fps, and get below that if practical. Get rid of restrictions like globe valves, (use ball valves if valves are necessary) piping size changes, elbows, etc. Move check valves to the downstream side of the pump if there are any upstream. Use an eccentric reducer at the pump suction to change from the upsized pipe to the pump suction size, and be sure to install it with the flat side up.
Get rid of vapor traps, (horizontal runs) in the suction piping.
In my experience, most npshr curves are relatively flat to a point, at which they begin to tail upward rapidly. The above suggestions are to keep you operating in that flat region. If you can't operate in the region where the npshr curve is flat, then you need a different pump.
rmw
RE: Derating centrifugal pump performance for low NPSHa
thanks again for the prompt response.
RE: Derating centrifugal pump performance for low NPSHa
Have you looked at all the possibilities to reduce the NPSH Required from the pump. There the manufacturer can maybe help. Installing an inducer, lowering the pump speed, increasing the inlet eye of the pump. Pillishing the inlet side of the pump. Lowering the pump position physically. Etc.
What NPSH Available and NPSH required do you have?
Best regards.
Scalleke
RE: Derating centrifugal pump performance for low NPSHa
The one thing I had not considered was turning out the impeller eye.
The existing eye diameter is 2.293". If I leave .030 (per side) for material integrity (SWAG), I could increase the eye diameter to a max of 2.550.
Do you have a method for predicting the performance change, or is this a "try it and see" proposition? If nothing else, decreasing the angle of incidence at the leading edge of the eye would certainly relieve the low pressure area at the leading edge of the eye. Would you consider this amount of change worth the effort?
RE: Derating centrifugal pump performance for low NPSHa
From the inlet flange to the time that the liquid enters the impeller there is quite a bit of cast surface on the sides that will cause quite a bit of friction. Polishing this will reduce friction and you will see a decrease in NPSH requirement of the pump.
Do not increase the impeller inlet eye. NPSH Requirement of the pump will go up.
Do you have the possibility of lowering the installed pump. It sounds like you have only a small cavitation problem as you still have some pump performance. Lowering the pump might give you what you need. I once requested to remove the 1 foot high cement base under a pump to solve a problem (I'm glad to say that it worked).
Can you share some more info on the installation.
Best Regards. scalleke
RE: Derating centrifugal pump performance for low NPSHa
As it exists now I have about 100% excess hp online. As the project exists I have about a 10 ft NPSHR and have about 8 available. The pump is cavitating like crazy but it moved enough water to just get the job done. It is primarily a badly "engineered" suction side design. Once I am allowed to correct the suction situation I can actually more than double the capacity using the same pump. I will have a 16 ft NPSHR and will have 24' available (that's how bad the suction train was).
I basically knew better, but there is always a chance of learning something new, and your input helped me change the minds that needed changing. For that I am quite grateful.
Thanks guys.