NPSH required AT THE END OF OPERATING CURVE
NPSH required AT THE END OF OPERATING CURVE
(OP)
Dear all,
When evaluating a pump proposal is it realistic to check if the NPSH AVAILABLE is higher than the required by the proposed pump? if yes, what are the reasons and does it have any "written" rules like tha API or other codes.
Regards,
roker
When evaluating a pump proposal is it realistic to check if the NPSH AVAILABLE is higher than the required by the proposed pump? if yes, what are the reasons and does it have any "written" rules like tha API or other codes.
Regards,
roker





RE: NPSH required AT THE END OF OPERATING CURVE
Why do we need to ensure NPSHa is greater - basically to ensure reliable operation. Insufficient NPSHa leads to cavitation, which in turn can cause damage to impellers and casings, reduce flow, and can lead to accelerated bearing and seal failures.
Cheers,
John
RE: NPSH required AT THE END OF OPERATING CURVE
If I see something I don't like about the NPSH characteristics, I don't even bother evaluating the rest of the curve. Why would you?
rmw
RE: NPSH required AT THE END OF OPERATING CURVE
RE: NPSH required AT THE END OF OPERATING CURVE
Johnny Pellin
RE: NPSH required AT THE END OF OPERATING CURVE
RE: NPSH required AT THE END OF OPERATING CURVE
RE: NPSH required AT THE END OF OPERATING CURVE
I was referring only to Net Positive Suction Head in units of absolute pressure. This can only be positive. I understand that some industries use terms like suction head or suction lift and denote these in units of gauge pressure rather than absolute pressure. But, in my industry (refining) I am only accustomed to NPSH in absolute units.
Johnny Pellin
RE: NPSH required AT THE END OF OPERATING CURVE
RE: NPSH required AT THE END OF OPERATING CURVE
For "suction lift"
NPSHa = Ha - Hvpa - Hst - Hfs
For "flooded suction"
NPSHa = Ha - Hvpa + Hst - Hfs
To this you can include your own margins, fudge factors etc.
RE: NPSH required AT THE END OF OPERATING CURVE
Many people associate NPSH requirements with cavitation free operation. This is incorrect.
To perform an NPSHR test at a flow of X gpm, suction pressure is set at some "high" level, and the test loop control valve is set for X gpm. Slowly, the suction pressure is reduced, while monitoring the pump differential pressure. At each reduction, the control valve may need to be readjusted to maintain X gpm flow. When the pump differential begins to fall by an agreed upon amount (usually 1% or 3%), the suction pressure value in absolute units is recorded and NPSHR computed therefrom (considering suction flow velocity, fluid vapor pressure).
This differential head falloff is caused by the cavitation bubbles becoming so numerous that they block the flow passages. Cavitation is by this time quite advanced.
A margin of NPSHA over NPSHR is (1) to make sure that you have some margin over incorrect assumptions and minor upsets, and (2) to make sure that cavitation hasn't become so advanced that significant damage to pump flow passages could occur.
Cavitation damage is related to the heat of vaporization of the fluid involved. The damage is caused when the water boils locally creating a vapor bubble, and the vapor collapses as it's pressure is increased in the impeller. As the fluid changes from vapor to liquid, the heat of vaporization is released as the bubble becomes microsopic. A high speed micro-jet of fluid emmanates from the collapsing bubble. The pressure can be in the high 10's of thousands of psi. Over time, this fatigues the metal surfaces -- kind of like hitting the surfaces lightly with a center-punch.
Since the heat of vaporization is the big factor in damage, petroleum, for example, causes less damage than, say, water.
Just remember, healthy NPSH margins mean less chance of shortened life due to cavitation. Even with healthy margins, almost all pumps in routine commercial service have some incipient cavitation present. They perform thus for years without sifnificant problems.
In some cavitation tests I performed in the past, where there were viewports in the suction eye of the impeller, it took 3 times NPSHR to suppress all cavitation. Trying to do this is excessive and would burn your bank account into oblivion. Earlier posts have suggested some values. Consider those.
RE: NPSH required AT THE END OF OPERATING CURVE
Summing up the situation, unless there are some very compelling reasons, operation well out on the head vs. flow curve should be avoided.
RE: NPSH required AT THE END OF OPERATING CURVE
RE: NPSH required AT THE END OF OPERATING CURVE
roker,
based on the tone of your question i'd guess someone bought a pump with out checking NPSHr vs NPSHa "over the full operating range..."
all above postings are good, but from an applications standpoint API-610 defines the end of the curve at 120% of BEP flow, and defines NPSH margin as "sufficient for all flows"
API-610 10th ed (5.1.10) The Purchaser should consider an appropriate margin...that is sufficient at all flows (from MCSF to max operating flow)
they way interpret this there is some wiggle room if rated flow is back on the curve and it's not practical to size a motor and NPSH all they way to 120% of BEP... you need to determine what flow is considered AT THE END OF OPERATING CURVE
Did you know that 76.4% of all statistics are made up...