NPSHR / NPSHA A = GUAGE OR ATM.. 3

[jamesbanda]

Hello,

i know this is a daft question but i always get this confused.. is nPSHAVAILABLE and NPHSA required in guage or atm units..

eg.
suction head 10 meters,
fluid vapour pressure = 5 meters
atmospheric pressure tank
line loss = 1 meters
NPSHA = 10-5-1+10 ? = 14 meters
or
NPSHA = 10-5-1=4 meters

sorry daft question of the day..

 

[JJPellin]

Net Positive Suction Head is in absolute units. For an atmospheric tank, you would have to add in the atmospheric pressure. In your example, it would be 14 meters available.

Since the units are absolute, the values should always be positive. However, there is one odd exception. Some manufacturers of vertical turbine pumps state their NPSHr with a reference of the discharge flange at the surface rather than at the impeller center-line in the bottom of the pit. If the pit was sufficiently deep, they could provide an NPSHr that was a negative value. But, as long as you calculate the NPSHa at the same reference, the comparison is still valid. It really would be more appropriate to relate both NPSHa and NPSHr at the impeller so that all values are positive.


Johnny Pellin

 

[Pumpsonly]

Johnny,
For the NPSHR of vertical can (VS6) pump to be reference to the suction flange center line is not the choice of the manufacturer.
The purchaser's data sheet will always stated the NPSHA value as at the pump grade/ foundation level. It will be easy for the engineer to evaluate reference to this position.Otherwise he will have to work it out The distance of the first stage impeller below the suction flange center line is usually not given by the manufacturer unless specifically requested.