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How It Works
This calculator handles three pump equations. Water horsepower (WHP = QH/3960) is the theoretical minimum power. Brake horsepower (BHP = QH/(3960η)) is the actual shaft power, always higher because no pump is 100% efficient. Pump efficiency (η = WHP/BHP) indicates how well mechanical energy converts to hydraulic energy. Typical centrifugal pumps run at 50–85% efficiency.
Example Problem
A pump delivers 500 gpm against 120 feet of total dynamic head with 72% efficiency. What brake horsepower is required?
- BHP = Q × H / (3960 × η) = 500 × 120 / (3960 × 0.72)
- BHP = 60,000 / 2,851 = 21.0 hp
A 25 hp motor would be selected to provide a safety margin.
Frequently Asked Questions
What is the difference between water horsepower and brake horsepower?
Water horsepower is the theoretical minimum power to move water at the desired flow and head. Brake horsepower is the actual power needed at the shaft, which is always higher due to pump inefficiencies. BHP = WHP / efficiency.
What is a pump's best efficiency point (BEP)?
The BEP is the operating point where the pump transfers energy most efficiently. Running at or near BEP minimizes energy waste, vibration, and wear. Most pump curves show BEP at about 80–85% of maximum flow capacity.
What is total dynamic head (TDH)?
TDH is the total equivalent height the pump must overcome, including static lift, friction losses in piping, and discharge pressure. A system with 20 ft of static lift and 15 ft of friction loss has a TDH of 35 ft.
Related Calculators
- Darcy-Weisbach Calculator — calculate friction losses that contribute to total dynamic head.
- Minor Losses Calculator — add fitting losses to your TDH estimate.
- Bernoulli Theorem Calculator — full energy balance for pump system analysis.
- Pipe Flow Calculator — compute flow rate and velocity in the piping system.
- Hazen-Williams Calculator — estimate friction losses in water distribution mains.
- Power Unit Converter — convert pump power between watts, kilowatts, and horsepower.