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# Pump size,

## Pump size,

(OP)
I need to calculate how large a hydraulic pump ( GPM @ 3000 psi ) I can run with a 75 BHP engine... what's the formula?

Also, would like to know how to caculate the BHP for hydraulic motors given cu.in./rev.,psi,gpm,rpm, and torque (in/lb).

Hope you can help, thanx bunches,
webbfoot

### RE: Pump size,

webfoot, I take it you're using a rotary positve displacement pump? If so, the equation is as follows:

bhp=Q*P/1714*E

where:
Q= flow rate
P=disch. press.
E=pump eff. as decimal

As for the motor appl. check with motor manufacturers via the web.

Hope this helps.
saxon

### RE: Pump size,

For centrifugal pump or other rotation equipment, bhp and shaft torque relation:

bhp=2*pi*N*T/33000

where pi=3.1415926
N=shaft rotation rpm
T=torque ft-lbs

saxon's formula also applys to other pumps but that the pressure (psi) is normally replaced by head (ft) multiplied by specific gravity of fluid so that:

bhp=Q*H*sp/3960*E

### RE: Pump size,

Theoretical formulas (eff not accounted for) for hyd motors are:
D=(2Pi.T)/P
P=(2Pi.T)/D
T=(PD)/2Pi.
Where D = Displacement in in3
T = Torque in lbâ€“ins
P = Pressure in PSI
Pi = 3.1416

Or HP (output) = (Flow x PSI x et)/ 1714
Et = total eff which is mechanical eff. plus volumetric eff.
Ev varies with:
Higher with low pressures and falls with increase pressure
Highest at max speed
Em varies with:
Higher at low speed because of less friction
Higher at higher pressures because of better lubrication

Starting torque may be as high as 60% of running torque

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