Deep Well Pump Sizing Guide: How to Calculate Flow Rate and Total Dynamic Head

Selecting a deep well submersible pump requires more than matching the pump head to the well depth. The correct model must deliver the required flow rate at the system’s actual total dynamic head, or TDH.

If either value is calculated incorrectly, the pump may provide insufficient water, consume excessive energy or operate outside its recommended performance range. Pump sizing should therefore begin with the required duty point: flow rate plus total dynamic head.

Flow rate is the volume of water the pump must deliver within a specified time, normally expressed in m³/h, L/s or GPM.

The required flow should be calculated from the actual application:

• Agricultural irrigation: crop demand, irrigated area and operating hours
• Municipal supply: population, peak demand and storage capacity
• Industrial transfer: process consumption and operating schedule
• Livestock or aquaculture: daily consumption and circulation requirements
• Tank filling: tank volume divided by the required filling time

For example, if a 400 m³ storage tank must be filled in five hours:

Required flow rate = 400 m³ ÷ 5 h = 80 m³/h

The selected flow should also remain within the sustainable yield of the well. A pump that extracts water faster than the well can recover may cause excessive drawdown, unstable operation or dry running.

The static water level is measured when the pump is stopped. During operation, the water level falls to the dynamic water level because of drawdown.

TDH calculations should normally use the dynamic water level, not the total well depth or pump installation depth. Seasonal groundwater changes should also be considered when defining the lowest expected operating water level.

Vertical lift is the elevation difference between the dynamic water level and the final discharge point.

If the dynamic water level is 60 meters below ground and the water is delivered to a tank inlet 12 meters above ground:

Vertical lift = 60 m + 12 m = 72 m

The pump may be installed deeper than 60 meters to maintain sufficient submergence, but that additional installation depth is not automatically added to the required head.

Water loses pressure as it passes through pipes, elbows, valves, check valves and other fittings. Friction loss depends on:

• Flow rate
• Pipe internal diameter
• Total pipe length
• Pipe material and surface condition
• Number and type of fittings
• Water velocity

The U.S. Department of Energy notes that dynamic losses increase approximately with the square of the flow rate. Therefore, increasing flow without reviewing the pipe diameter can significantly raise TDH and energy consumption.

If the system must maintain pressure at the outlet, convert that pressure into meters of water head.

As a practical conversion:

1 bar ≈ 10.2 meters of water head

If the required outlet pressure is 2 bar:

Pressure head = 2 * 10.2 = 20.4 m

Assume the following project conditions:

• Required flow: 80 m³/h
• Dynamic water level: 60 m below ground
• Discharge point: 12 m above ground
• Pipeline friction loss: 8 m
• Required outlet pressure: 2 bar, or 20.4 m

The total dynamic head is:

TDH = Vertical Lift + Friction Loss + Required Pressure Head

TDH = 72 + 8 + 20.4 = 100.4 m

The required duty point is therefore approximately:

80 m³/h at 100 m TDH

After calculating flow and TDH, locate the duty point on the manufacturer’s pump curve. The operating point is determined by the intersection of the pump curve and the system curve, and the selected model should operate within an efficient, stable range.

The final selection should also verify motor power, well diameter, installation depth, voltage, frequency, water quality, material and minimum motor-cooling velocity.

Correct deep well pump sizing depends on both flow rate and total dynamic head. Do not select a pump using well depth or motor power alone. Calculate the required flow, dynamic water level, vertical lift, pipeline loss and outlet pressure, then confirm the duty point against the pump performance curve.

For accurate model selection, provide your required flow, static and dynamic water levels, discharge elevation, pipeline details, outlet pressure, well diameter and electrical supply.