low suction pressure at the pump

[naturalgas01]

Hi all,

We have pump that design for 150 psig suction pressure. In normal operation pump run at this design pressure. But sometimes because of upstream variation suction pressure change from 75 psig to 165 psig. pump manufacturer supplied NPSHr curve. NPSHr for this pump is 10 psig.

My question is : how lower can i go at suction of the pump without cavitating of damaging ? I think i can go up to more than 10 psig + 2-3 psig = 13 psig, is it right ?

thanks

 

[Artisi]

Does the flow rate change with the change in pressure? NPSHr is a function of flow rate.

 

[JJPellin]

I have never seen an NPSHr curve in units of psi. Are you sure that it is not in units of feet. That is more typical. You have not given us enough information to comment. The NPSH(available) needs to be calculated using the standard formula:

NPSHa = Static Pressure - Vapor Pressure + Elevation Head - Flow Losses

All units should be the same and feet are prefered. The fact that the pump was designed for a design suction pressure or rated suction pressure that is higher does not mean that the pump cannot run well with lower suction pressure. Calculate the available NPSH and compare it to the required NPSH. If you ask, you will probably get a lot of different opinions on the necessary margin between these two values. But, I think everyone can agree that the available needs to be well above the required. At our plant, we have a minimum margin of 3 feet for hydrocarbon and 5 feet for water.

Also, keep in mind that NPSHr is always expressed in absolute units. If the NPSHr were 10 psia, it would represent a slight vacuum. If you are reading your suction pressure with a typical gauge, it is probably in gauge units, not absolute. The proper use of units is very important in this analysis.


Johnny Pellin

 

[TenPenny]

Is the supplied NPSHr in feet, or is it NIPR in psi? Is the pump a centrifugal, or positive displacement? Typically, a centrifugal pump will have NPSHr in feet; a positive displacement pumps will ususally have NIPR in psi (net inlet pressure required). Also verify if it's psig or psia.

 

[Artisi]

Good point TenPenny - for some reason I always assume centrifugal - guess it comes from 40 years with centrifugal pumps and 10 minutes with PD pumps.

 

[BigInch]

If its water at 10 psig, that's 23 ft head. Usually not a bad WAG for an NPSHR.


**********************
"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/

 

[BigInch]

psia, rather.

**********************
"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/

 

[wimple]

NaturalGas01,

I assume your talk about centrifugal...To my knowledge in the world of pumps the units are expressed in feet or meters.
When your suction pressure change, check the flowrate and compare the NPSHr at this flowrate to the NPSHa.

I assume that your NPSHr includes the amount required to account for "permanent cavitation". This additionnal amount is function of the metallurgy of the impeller.

Then usually some margin is there between NPSHa and NPSHr. In hydraulic, I used to consider sometimes 3 meters margin especially for overhung design monocellular pump to be on safe side.


What is your type of pumps? what is the flowrate?
Rgds
Wimple

 

[naturalgas01]

thanks to everyone,

it's 10 Psi (not in Psig), and it's multistage centrifugal pump. Pressure increases with flowrate in NPSHr curve

Suction to the pump comes from the storage tank. Storage tank pressure fluctuate as i mentioned between 75 to 165 psig. elevation is appx 10 ft. liquid is hydrocarbon and just at boiling point. there is booster pump ahead of the this pump. booster pump has differential pressure of 5 psid.

i hope this work.

naturalgas01

 

[rmw]

BigInch,

10 psig is still 23 ft of head (gage).

rmw

 

[BigInch]

rmw, yup, but you get to add 15 psia to 10 psig, if you're looking for NPSHa (= 57.7 ft). If its 10 psia, you can't add 15 psia to it (NPSHa = 23ft).



**********************
"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/

 

[JJPellin]

Your situation is still unclear. You state that in the original post that you have an NPSHr of 10 psig. In the next post, you correct that to psi. I assume you mean psia. If the fluid is hydrocarbon at the boiling point, then all of this is irrelevant. If you drop the pressure, the lighter components of the fluid will boil off and the remaining fluid will get heavier. But, if you are still at equilibrium in the vessel, then your suction head is still the same: 10 feet (minus any flow loss). If, on the other hand, you meant to say that the fluid it at the boiling point at atmospheric pressure, then the extra pressure in the head-space of the vessel is over and above vapor pressure. In that case, you need to maintain at least 10 psig above the fluid in order to have an NPSH margin of 10 feet resulting from your elevation head. A margin above that would be desirable if you can hold the pressure higher.

Johnny Pellin