The Horsepower required to operate a Positive Displacement Pump has two factors:
The Work Horsepower (WHP) - the actual work done
WHP = Flow(GPM) (X) Pressure(PSI) (/) 1714
The Viscous Horsepower(VHP) - the power required to turn the rotors, gears, etc. inside the viscous fluid. The Viscous Horsepower required is determined by the pump design and speed and is supplied by the pump manufacturer.
HP = WHP + VHP