Calculating Pump Horsepower 1

[ghensky]

To calculate the Pump hp I have the following two equations,

Pump hp = (GPM x PSI)/(1714 x Efficiency) (1)
Brake hp (Shaft Power) = (GPM x Head, ft x SG)/(3960 gal/min/ft x Pump % Eff) (2)

With GPM = 22.44, and Head, ft = 38.4, and 60% Pump eff

Equation 1 yields, 1.94 hp
Equation 2 yields, 0.3627 hp

If Head, ft x 2.31 = PSI, and 1714 x 2.31 = 3960 (ignoring rounding off error), how come two different equations to calculate the same thing provide different answers? What am I missing here? Thanks.

 

[btrueblood]

For eq. #1 I calculate: (22.44)(38.4/2.31)/1714/0.6 = 0.363

 

[BigInch]

Ink jet pump?

 

[1gibson]

(Head in ft / 2.31)x specific gravity = psig

Your answers are different by 2.31 x 2.

 

[zdas04]

I get the same value for the two equations as well.

I suggest you lay the equations out with units and do a unit analysis to show you why ft * ft/psi gives you ft^2/lbf/in^2 or (ft^2*in^2)/lbf which is basically length to the fourth power over force.

Maybe you'd be ahead to look at ft / (ft/psi) or psi.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[ghensky]

Thanks guys. The mistake I made was I used Head * 2.31 = PSI, instead of Head/2.31 = PSI.