NPSHa calculation of pump

[DanielGong]

hi,
now i working on the NPSHa calculation for the pump. there is a drum with overhead nitrogen seal at the suction side of pump. how to calculate the NPSHa for the pump? how to considerate the pressure of suction side? use the vapor pressure of the liquid may be not suitble in this case.

 

[alisaffari]

Hello Danielgong

NPSHA = system pressure of suction equipment - vapor pressure - static head (from the vessel bottom tangent line) - line friction loss - pressure drop of suction equipments and instrumentation

this formula is applicable for all condition in pump NPSH calculation.
thanks
saffari

 

[786392]

Dear DanielGong,
Not at all going into details.

I have a simple asking query as if what is to be handled by pump?

Since the reply is/would obviously be the process liquids not N2 gas.Therefore the sealing pressure should not govern NPSHa for designing I understand.

Best Regards
Qalander(Chem)

 

[DanielGong]

thanks for your answer.
the pumped flow is HDO/LVGO,that is the feed of Diesel hydrotreating unit. there is a feed surge drum with nitrogen seal at the top.OT is 149ºC, and the vapor pressure is 0.06kg/cm2a.
Does the nitrogen seal have any effect on the calcultion.
nitrogen may be saturated in the liquid.

 

[786392]

In all fairness my answer shall be;No.

However more learned colleagues at the forum may jump-in with helping loads of useful advises indeed!

The general formula equation given above by alisaffari should be helpful for your purpose,I assume.

Best Regards
Qalander(Chem)

 

[DanielGong]

thanks, 786392
"Pumping Liquids Loaded With Dissolved Gas"
i have found some infomation about the dissolved gas affect the vapor pressure of the pumped flow.
may be i do not express clearly!
Thanks again!

 

[JJPellin]

Trust the math. The formula works for all normal circumstances. We have a dozen or more pumps in services like you are describing. Most of them us treated fuel gas for the blanket rather than nitrogen. Gas oil at these temperatures will not tend to dissolve much gas. When evaluating the NPSH to all of our hydrotreaters, we take full credit for the blanket gas pressure and generally, we have had very good results. If you are still experiencing cavitation, I would look for problems with the suction piping, flow rate relative to BEP, level control on the suction vessel or function of the pressure control on the blanket gas. One small correction: the NPSH is comprised of static pressure minus vapor pressure minus flow losses plus elevation head.

Johnny Pellin

 

[DanielGong]

thanks JJpellin