Darcy Weisbach equation - units? 1

[eeprom]

All,
If I use the Darcy Weisbach equation for head loss in a pipe, the resulting units are feet. I have always assumed this to be feet of water. But if I am calculating head loss on a fluid with an SG of 0.8, is the head loss still in feet of water? Or is in feet of water/0.8?

Thanks
EE

 

[BigInch]

It is in feet of the fluid you are transporting. I don't see the reason why you think you need to convert it to water, or any other liquid.

Pump head curves are also given feet, which is the same for any fluid being pumped.

 

[eeprom]

So that equation always yields feet of fluid. Thank you very much for clearing that up. Can I also assume that the velocity head V^2/2g also yields feet of the fluid being pumped?

EE

 

[BigInch]

Yes, of the fluid being pumped. Head is measured in feet, but in reality head is not a measure of length. It is a measure of energy contained by 1 unit mass of fluid in the form of height above a datum, so that measure would not vary with specific gravity, as the height of 100m of 1 Kg of mass of one fluid is the same as any other fluid. 10 ft of head lost per unit mass of water is the same as 10ft of head lost per unit mass of gasoline.

Energy loss per unit length of pipe due to friction can vary between fluids, or even with the same fluid, as that is related to the fluid's viscosity and other system parameters, such as velocity and pipe roughness. So 10 ft of head loss pumping water might come at a velocity of 3 ft per second, whereas 10ft of head loss with gasoline might be at 5 ft per second.

This is also why a net positive suction head required for a pump of 22.3ft basically does not change no matter which fluid is being pumped (within reason), but the actual suction pressure required to supply that head can and does vary with the fluid's specific gravity, viscosity and vapour pressure.

 

[eeprom]

BigInch,
Thank you for taking the time to explain that.
EE

 

[BigInch]

Cheers.

Pump differential head curves do not need to be adjusted for fluid either (again within reason). Pumps will lift all fluids to the same head as indicated on the pump curve. What can change with the fluid is likely to be the power required to do so, as volumetric flowrate x mass density x head, is obviously proportional to the density of the fluid. The heavier the fluid, the more power required.