Lighter Fluids Having Higher NPSHA

[shamrockgg]

Something I'm having a hard understanding while calculating NPSHA is that a lighter fluid has a higher NPSHA, at least for the atmospheric pressure part of the equation. A person would think that the heavier the fluid is the more pressure you have at the pump. For instance...

I'm only talking about the standard atmospheric pressure part of the equation, I'm omitting the rest of the calculation to further clarify my question:

(ATM*2.31/SG=Hp
(14.7)*2.31/1=33.9 (Water)
(14.7)*2.31/0.75=45.2 (Gasoline)

 

[bimr]

The NPSH calculation begins with Standard Atmospheric Pressure, Ha.

You are correct, the heavier the fluid is, the more pressure is at the pump.

However, the atmospheric pressure is fixed at 14.7 psia or 0 psig. This is just a conversion of the atmospheric pressure (head).

14.7 psia of pressure equates to 33.9 feet of water head or 45.2 feet of gasoline head.

 

[TD2K]

http://www.pumpschool.com/applications/NPSH.pdf

See the top of page 3. If you neglect the velocity head of the fluid entering the pump and re-arrange the equation, you get:

NPSHA = the suction pressure at the pump minus the vapor pressure of the fluid.

Water at 40 deg F and 0 psig pump suction gauge pressure has about 34 ft of NPSHA

Water at 212F and 0 psig pump suction pressure has 0 ft of NPSHA

 

[LittleInch]

shamrock,

your mixing up head with pressure.

Converting everything to head is needed to arrive at a NPSHA figure in m or ft. NPSH is not a pressure number , it's a head number.

To obtain the same fixed pressure (e.g. atm pressure) you need a higher column of a lighter fluid.

In your case Hp is in ft, NOT pressure.

The downside of a lighter fluid is normally that at the same temperature, it has a much higher vapour pressure than a heavier liquid so when you convert that vapour pressure to m or ft head it also is bigger than the heavier liquid.

bimr, I don't think there is any such thing as "atmospheric pressure head". There is only atmospheric pressure.

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

 

[shamrockgg]

Thank you everyone for your responses, you guys are helpful. Have a great day!!