Do I need to correct NPSHR in pump curve for different S.G.? 1

[Smackavelly]

If my NPSHR curve in the pump curve gives me a value of 10 ft (based on water). Do I need to correct it for a lower specific gravity (S.G) if my pumping liquid has a S.G. of .63. Do I simply do the following (see below)===>

NPSHR in the pump curve based on water (S.G.=1) is 10ft

h=P*2.31/S.G.

10ft=(P)*2.31/1

solve for P

P=10/2.31

Then convert that pressure to head (ft)using the process S.G.=.63

h=P*2.31/S.G.

h=(10/2.31)*2.31/.63= 10/.63=15.87ft

Would 15.87ft be considered NPSHR value of the pump in order to prevent cavitation when pumping the process having S.G=.63?

 

[BigInch]

No. Erase all that.

Pump suction and discharge heads stay do not vary with specific gravity or density of products. The pressures change. Which is why it is much more convenient to work with NPS-Heads, rather than NPS-Equivalent Pressure, especially when you might have multiple products running through the system. Head mesures energy, psi measures force. Even though you can convert from one to the other using SG & density, they each mean very much different things.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[TenPenny]

As eloquently stated above, no, you don't change NPSHr numbers.

But when you calculate NPSHa, make sure you use the correct data for your fluid and piping system. That's where the differences come in.

 

[25362]

If the reduction in specific gravity results from pumping hydrocarbons there is a graph by The Mc Nally Institute allowing a bit of relaxation on the NPSH subject, see quark's link in thread407-152358.
Thread407-155834 may also interest you.

 

[BigInch]

25362, Correct as always, but I'd rather discuss that in the advanced class. thread407-235074

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Smackavelly]

Thanks for the help Big Inch... Note to self: to refer

http://www.pumps.org/content_detail.aspx?id=748

to confirm and address this Rumor of adjusting NPSHR value for different specific gravitys.......

 

[BigInch]

No rumor, it is true, but nobody I know does it unless you absolutely have to. That's when you can't provide NPSHR under any circumstance, like gasoline being pumped from a tank at 45ºC; the pumps were located pretty far outside the tank berm. We even used can pumps and still couldn't get the NPSHR required by the mfgr, but it was close enough that everyone was sure it would work.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/