low suction pressure at the pump
low suction pressure at the pump
(OP)
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
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





RE: low suction pressure at the pump
RE: low suction pressure at the pump
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
RE: low suction pressure at the pump
RE: low suction pressure at the pump
RE: low suction pressure at the pump
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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/
RE: low suction pressure at the pump
**********************
"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/
RE: low suction pressure at the pump
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
RE: low suction pressure at the pump
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
RE: low suction pressure at the pump
10 psig is still 23 ft of head (gage).
rmw
RE: low suction pressure at the pump
**********************
"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/
RE: low suction pressure at the pump
Johnny Pellin