Table of Contents
What Is Pump Horsepower?
Pump horsepower refers to the power required to move a fluid at a specific flow rate against a given head (pressure). Understanding pump power is critical for selecting the correct motor size, estimating energy costs, and designing efficient fluid systems. There are three key power levels: water (hydraulic) horsepower, brake horsepower, and motor horsepower.
Water horsepower (WHP) is the theoretical minimum power needed to move the fluid. Brake horsepower (BHP) accounts for pump inefficiency. Motor horsepower adds motor efficiency losses on top of that. Properly sizing a pump motor prevents overloading, overheating, and premature failure.
Pump HP Formulas
Power Types Explained
| Power Type | Accounts For | Typical Range |
|---|---|---|
| Water HP (WHP) | Ideal fluid energy only | Lowest value |
| Brake HP (BHP) | Pump friction & losses | WHP / 0.60-0.90 |
| Motor HP | Motor electrical losses | BHP / 0.90-0.97 |
Typical Pump Efficiencies
| Pump Type | Efficiency Range |
|---|---|
| Centrifugal (large) | 80-93% |
| Centrifugal (small) | 50-70% |
| Positive displacement | 85-95% |
| Submersible | 50-80% |
Frequently Asked Questions
What is the 3960 constant in the pump HP formula?
The constant 3960 converts the product of GPM and feet of head into horsepower. It comes from 1 HP = 33,000 ft-lb/min, and 1 gallon of water weighs 8.33 lb, so 33,000 / 8.33 = 3960. When the fluid is not water, multiply by specific gravity to adjust for density.
How do I choose the right motor size?
Calculate the brake horsepower, then select the next standard motor size above that value. Standard motor sizes are 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, and 100 HP. Always include a safety factor of 10-15% for startup surges and wear.
Does fluid temperature affect pump HP?
Temperature affects fluid viscosity and density. Hot fluids are less viscous, reducing friction losses but also reducing density (lower SG). For most water applications, temperature effects are minor. For viscous fluids like oil, temperature significantly impacts required power.