NPSHA 1

[macmet]

I have a question that I need a simple answer to.

We have a cooling system that has flows up to 75usgpm. The pressure drop through this system is taken at 30ft of head. This cooling system has an expansion tank that branches straight up about ~10ft abouve the pump and it is open to atmosphere. This branch to the exp. tank is located about a foot upstream of the pump inlet.

When I am calculating NPSHA do I add the atmospheric pressure since the tank is open?

I have searched this site and read quite a few useful threads. I think that I should add the atmospheric pressure but I would like someone to confirm this.

 

[Artisi]

For an open system on the inlet side normally called a flooded system use:-

NPSHa= Ha - Hvpa + Hst - Hs

Ha = absolute pres.in ft of liquid acting on the supply level - at sea level use 34ft

Hvpa = vapour pressure in ft at the pumping temp. assuming standard of 65 - 70 F. Hvpa allow 0.8

Hst = static head in ft above the centre of the impeller - 10 ft as per your post

Hs = all losses in ft for the inlet configuration - ie, pipe work, valves and fittings - assume 3 ft or your calculated figure

For a simple installation like yours

NPSHa calc. is 34 - 0.8 + 10 - 3 = 40.2 ft

 

[macmet]

Artisi,

Thank you very much.