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.