Dewatering Pump Capacity Estimator Calculator

Inputs

Example Data Table

Formulas Used

• Total design flow: Q = (Q_seep + Q_rain + Q_extra) × (1 + SF).
• Rain runoff: Q_rain = (I × A × C) / 3600, where I is in m/h.
• Simplified Darcy seepage: Q_seep ≈ K × (Perimeter × Saturated thickness) × (Saturated thickness / Flow path).
• Hazen–Williams friction head (SI): h_f = 10.67 × L × Q^1.852 / (C^1.852 × d^4.871).
• Minor losses: h_m = K_total × v² / (2g), where v = Q / A.
• Total Dynamic Head: TDH = Static lift + h_f + h_m.
• Power: P(kW) = ρgQTDH / (η × 1000), with η = η_pump × η_motor.

How to Use This Calculator

1. Enter excavation dimensions and expected water height above the bottom.
2. Choose a seepage method: direct inflow or simplified Darcy estimate.
3. Add rainfall intensity and contributing area if runoff is expected.
4. Set a safety factor to cover uncertainty and peak inflow.
5. Enter discharge lift, pipe length, diameter, and fitting losses.
6. Provide efficiencies to estimate required power realistically.
7. Click Estimate Capacity to view results above.
8. Use the CSV/PDF buttons to export the latest result.

Professional Guide: Dewatering Pump Capacity Planning

Dewatering keeps excavations stable and workable by controlling groundwater and surface water during construction. A dependable estimate starts by identifying all inflow sources, then pairing that flow with the head the pump must overcome. Inflow may include seepage through soil, leakage from utilities, washdown water, and storm runoff. Head includes vertical lift plus friction and fitting losses along hoses, pipes, bends, valves, and check valves.

This estimator supports a practical planning workflow. Start with excavation geometry and the expected water level above the bottom. If you have observations or a temporary test pump, enter a direct base inflow in liters per second. When measured data is not available, the simplified Darcy option provides a screening seepage estimate from hydraulic conductivity, perimeter, saturated thickness, and an assumed flow path length. Treat this result as an initial value and refine it as field information improves.

After seepage is defined, include other sources. Extra inflow can represent intermittent utility leakage or unknown connections. Rain runoff is estimated from rainfall intensity, contributing area, and a runoff coefficient that represents how quickly water reaches the excavation. Apply a safety factor to cover peak conditions, pump wear, partial blockage, and uncertainty in assumptions. The resulting design flow should represent a credible worst-case duty point rather than an average day.

Next, estimate Total Dynamic Head (TDH). Static lift is the elevation difference from the pump to the discharge point. Friction losses rise quickly as flow increases or pipe diameter decreases, so selecting a larger discharge diameter can reduce TDH and required power. Minor losses represent fittings and appurtenances; use a higher total K value when hoses include multiple bends, valves, manifolds, strainers, or quick couplers. Lower TDH typically reduces energy use and improves reliability.

Example data: A 20 m by 12 m excavation with 8 L/s base inflow, 15 mm/h rainfall over 240 m², runoff coefficient 0.6, and a 25% safety factor yields a design flow near 11 to 12 L/s. With 35 m of 75 mm discharge pipe, Hazen–Williams C of 130, total minor-loss coefficient K of 6, and 6 m static lift, TDH commonly falls around 9 to 12 m. With 55% pump efficiency and 90% motor efficiency, required power often lands around 2 to 3 kW.

Finally, confirm practical constraints before procurement. Check suction conditions, priming method, solids handling, available electrical supply, discharge routing, and erosion control. For critical excavations, validate assumptions with field measurements and adjust the safety factor to match risk and consequences.

FAQs

1) What flow rate should I select if inflow varies during the day?

Use the highest expected inflow and apply a safety factor. If peaks are short, consider a staged setup: one duty pump and one assist pump for peak periods.

2) Is the simplified Darcy method accurate for all soils?

No. It is a screening estimate. Soil stratification, anisotropy, partial penetration, and boundary conditions can change seepage significantly. Use field data or a detailed design for sensitive excavations.

3) Why does pipe diameter change power so much?

Friction head rises steeply with smaller diameters at the same flow. Increasing diameter reduces velocity and friction, lowering TDH and power, often improving efficiency and pump reliability.

4) How do I choose the runoff coefficient?

Select based on surface type and slope. Paved areas are high, compacted mixed areas are moderate, and soil or vegetated areas are lower. When uncertain, use a conservative higher value.

5) What should I use for total minor-loss coefficient K?

Add K values for fittings, valves, entrances, and exits, or use an allowance when details are unknown. More bends, valves, and manifolds generally mean a larger total K.

6) Does this include suction limitations and cavitation risk?

Not directly. Always check Net Positive Suction Head requirements, suction hose condition, elevation, and water temperature. Poor suction conditions can reduce capacity even if TDH looks acceptable.

7) Should I size one large pump or multiple smaller pumps?

Multiple pumps improve redundancy, allow flexible staging, and simplify maintenance. One pump can be simpler to manage. Choose based on access, power availability, risk tolerance, and site logistics.