Three common pump calculations: flow from a wet well test, total dynamic head, and horsepower.
Pump Rate from Wet Well Drawdown
Formula
Volume, gal = Area, ft² x Drop, ft x 7.48 Pump rate, gpm = Volume / minutes + inflow, gpm
Inputs used
- Wet well area (ft²)
- Level drop (ft)
- Time for the drop (min)
- Inflow during the test (optional) (gpm)
Worked example (White River WRRF, fictional)
- Volume = 80 x 2 x 7.48 = 1,197 gal
- Rate = 1,197 / 3 = 399.0 gpm
Total Dynamic Head (Hazen-Williams)
Formula
Friction loss, ft = 10.44 x L x Q^1.85 / (C^1.85 x d^4.8655) (Q gpm, d inches, L ft) TDH = Static head + Friction loss + Minor losses
Inputs used
- Static head (lift) (ft)
- Pipe length (equivalent) (ft)
- Pipe inside diameter (in)
- Hazen-Williams C
- Flow (gpm)
- Minor losses (optional) (ft)
Worked example (White River WRRF, fictional)
- Friction = 10.44 x 1,200 x 1,000^1.85 / (120^1.85 x 8^4.8655) = 25.55 ft
- TDH = 35 + 25.55 + 3 = 63.6 ft
Pump Horsepower and Power Cost
Formula
WHP = gpm x TDH, ft x SG / 3,960 BHP = WHP / pump efficiency Motor hp = BHP / motor efficiency kW = Motor hp x 0.746
Inputs used
- Flow (gpm)
- Total dynamic head (ft)
- Pump efficiency (%)
- Motor efficiency (%)
- Specific gravity of liquid
- Run hours per day (optional) (hours)
- Power cost (optional) ($/kWh)
Worked example (White River WRRF, fictional)
- WHP = 1,000 x 60 / 3,960 = 15.15 hp
- BHP = 15.15 / 0.75 = 20.20 hp
- Motor hp = 20.20 / 0.92 = 21.96 hp
- kW = 21.96 x 0.746 = 16.38 kW
- Cost = 16.38 x 24 h x $0.10 = $39.31 per day
Tips
- Hazen-Williams C of 120 to 140 is typical for lined pipe; use lower values for old, rough pipe.
- Minor losses (fittings, valves) are not included; add them to static head if known.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.