NPSH Value rate

[MrProjectEng]

Hello;

Why NPSH value should be less than reel value. For instance pumps datasheet normal NPSH is shown 7 meter besides "NPSH required must be less than 50% of NPSH available" is noted.
What is the reason??

Thank you...

 

[Artisi]

Safety factor, probably specified by a someone who doesn't know any better- was going to say arse from elbow but thought that might be a bit rude.

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.)

 

[LittleInch]

The reason someone is doing this is that NPSHR is defined /tested as the point at which the differential head reduces by 3%.

It is NOT the level at which the pump starts to cavitate, which can be anywhere from 1-5m higher

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

 

[rotw]

I agree with LittleInch. In addition to the standard 3% drop of head, I would mention that there is in some cases the so called "residual cavitation" which generally depends on the type of metallurgy quoted for the impeller and accounted for by the manufacturer. on the other hand, I deem that setting the NPSH margin is a difficult art. Either you are lucky and its well defined by a code/standard, either it is matter of practical experience and non written rules.

 

[Artisi]

All the theory is great, but will repeat my initial comment and add excessive "safety factor"
The OP has said that NPSHr is 7 metres (have assumed that's what he meant), therefore NPSHa will have to be in excess of 14 metres - excessive or reasonable.



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.)

 

[georgeverghese]

Also note that the vendor should preferably quote the NPSHr for the service fluid and not for water. Else some correction factors will apply, or a factory performance test may be required for some critical applications. Check if a viscosity corr factor is required.

 

[LittleInch]

Could have meant npshA is 7 m.....

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

 

[Artisi]

Could well be, crystal ball time again

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.)