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Why is NPSH can not be determined if the suction is flooded (?)
2

Why is NPSH can not be determined if the suction is flooded (?)

Why is NPSH can not be determined if the suction is flooded (?)

(OP)
Dear everyone,

I want to consult you guys on this. I asked a package vendor (for a chemical injection package) to provide me both Differential Pressure and NPSH for the metering pump. However, the vendor says this is not required because the suction is flooded. See below:

For the differential pressure, vendor says this simply can not be determined because the suction is flooded (sourced tank will be atmospheric). If this is the case, shouldn't the suction pressure be 0 psig? given the discharge pressure - the differential pressure can now be determined. Correct me if I'm wrong.

For NPSH, again this will not be required because as the vendor says the suction is flooded and as long as the sourced tank and pump are mounted on the same level, NPSH is not required. I am puzzled why the vendor can not provide me the value of NPSH. Can you guys enlighten me first?

Regards,
Virata

RE: Why is NPSH can not be determined if the suction is flooded (?)

Are you asking for NPSHr or NPSHa?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why is NPSH can not be determined if the suction is flooded (?)

(OP)
Hi Artisi, it's available. But I would like to also know if this is also the situation with required.

Regards,
Virata

RE: Why is NPSH can not be determined if the suction is flooded (?)

For something like a packaged chemical system, the vendor knows where the tank is, that it is above the pump inlet level and the inlet piping is very short.

Therefore he simply can't be bothered to work out the NPSH because he knows from past experience that it's not an issue.

Metering pumps are usually PD pumps so the differential pressure will be whatever it needs to be to enter your downstream system, up to a maximum as presumably you have stated in the data sheet.

Therefore the differential pressure will vary up to a set maximum when the relief valves lift.

If you're paying for it then the vendor should be a little bit more accommodating.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Why is NPSH can not be determined if the suction is flooded (?)

NPSHa is atmospheric pressure plus height of the supply over the inlet, NPSHr is a pump function and (should) be known to the manufacturer and available.
LittleInch has filled in the other detail.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Why is NPSH can not be determined if the suction is flooded (?)

Ask your vendor to what ambient air temperature and tank level that your pump can safely run.

RE: Why is NPSH can not be determined if the suction is flooded (?)

'flooded suction' doesn't mean there is enough NPSH, without the rest of the data.

It would be like if someone asked, 'How fast are you driving?' and the answer you give is 'I'm on the highway'.

RE: Why is NPSH can not be determined if the suction is flooded (?)

TenPenny, that may generally be the case, but the vendor in this case knows the product being pumped at ambient pressure and temperature. It is difficult to see that NPSHa would be something to be concerned with.

RE: Why is NPSH can not be determined if the suction is flooded (?)

If the vendor knows the fluid data incl. vapor pressure, he will be able to calculate NPSHA. If not, he might not be the right vendor. NPSH might not be an issue in this specific case but the vendor should be able to proof that with a short NPSHA calculation which shows that NPSHA > NPSHR. If the data are know, it will not take more than 2 minutes.

RE: Why is NPSH can not be determined if the suction is flooded (?)

Vendor is not an engineer and has never been asked these foreign questions before. That's ok. You buy his pump skid. Assume that NPSH is not an issue in this instance (which it won't be for a dosing pump), and do you own determination of DP / power calc (and no, it's not just the difference between discharge pressure and "zero"). All good. You are the engineer.

RE: Why is NPSH can not be determined if the suction is flooded (?)

If the vendor can't / won't answer the question, why would you deal with that vendor?

RE: Why is NPSH can not be determined if the suction is flooded (?)

Seems the OP has fled, maybe we are all to dumb to solve his problem, now if you want to keep spinning wheels, let me recommend the post "power drop-off" 😉

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

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