NPSHR and viscosity
NPSHR and viscosity
(OP)
Has anyone found a correlation of viscosity of the pumped fluid with NPSHR? In particular for liquids having a viscosity above 100 SSU . Is there any significant effect of viscosity on NPSHR ? If there is, what is theory behind it ?
RE: NPSHR and viscosity
For what’s it is worth:
I personally don’t know any direct correlation between viscosity and NPSHr.
Manufactures test NPSHr using fresh water at a fixed temperature. Viscous liquids like oil have a different pressure/temperature relationship than water; therefore, the “required” NPSH will be lower for oil than fresh water.
The other side is that viscous fluids don’t flow as well as water. A manufacture may require you reconsider the intake piping or increase the NPSHr to accommodate the additional viscous drag.
Bottom line is you should contact the pump manufacture to obtain the NPSHr for a given application.
Hope this helps some.
RE: NPSHR and viscosity
There you are.
Where you been?
I was waiting on this thread for a guy like you to post because I never work with viscous sticky junk, but you do.
I like water, clean water in fact, I am no terd herder.
If no one posted I was going to say the same as you did, talk to the manufacturer each time.
I would guess that non-compressible newtonian fluids will be so similar to water that there is no difference, and then as viscosity increases at some point the flow regime within the pump will be affected enough to make a difference.
What do you know 25362?
PUMPDESIGNER
RE: NPSHR and viscosity
K = -0.4776[log(V/V1)]4 + 3.8688[log(V/V1)]3 - 9.875[log(V/V1)]2 + 8.1772[log(V/V1)] + 1
where V1 relates to a liquid with a viscosity of 100 SSU and V relates to the viscosity of the pumped liquid.
Using this formula for a liquid with viscosity 400 SSU (86 cSt) I found it gives a K=2.67 to increase the NPSHr originally intended for a fluid with 200 SSU (43 cSt).
Yedidiah adds, I quote: "At present I know of no experimental data that would confirm or deny the validity of this equation", unquote.
Anybody that could shed some light on this subject is invited to do so.
RE: NPSHR and viscosity
I can see why you want to verify the formula with proof or work on that problem further, really neat.
I always have a few ongoing research problems like that it seems. Have been very excited in the past when I solved one.
Am close to solving one now, how to calculate the amount of energy released from a pipe when pressure drops a specific amount (For PVC, HDPE, etc.). I want that energy stated in ergs and I am very close, but now I realize I will have no experimental data to verify. I have a few friends that could do the work in the field, so I will probably pay them to get it for me.
PUMPDESIGNER
RE: NPSHR and viscosity
23562's equation from Yedidiah is equally entralling. Sounds like that unknown manufacturer ran a multitude of tests at various fluid viscosities. Based on my experience with the variability and non-repeatability of NPSH test results, the coefficient of deterination (R^2) of their curve-fitted
equation needs to be known for a judgment of veracity.
RE: NPSHR and viscosity
RE: NPSHR and viscosity