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The total dynamic head (TDH) is the height to which a pump must lift water, including head losses and pressure at the tap. It is the reference value for choosing a submersible pump from its performance curve.
The TDH formula
TDH = Hr + Pc + Pr
| Term | Meaning |
|---|---|
| Hr | Static head: height between the water level and the point of use. |
| Pc | Head losses in the discharge pipework and friction points. |
| Pr | Residual pressure (in mWC): the pressure that must be available at the taps or for watering. |
Unit: all values are expressed in metres of water column (mWC). Reminder: 1 bar = 10 mWC (see Determining water pressure).
Example calculation for a borehole pump
Installation intended to supply running water to a home of 4 people and a garden of 800 m².
| Term | Data | Value |
|---|---|---|
| Hr | Pump submerged at 60 m: depth between the pump intake and the borehole head, at ground level. | 60 mWC |
| Pc | Head losses and friction in the pipework. | 10 mWC |
| Pr | Residual pressure at the tap of 3.5 bar. | 35 mWC |
| TDH | 60 + 10 + 35 | 105 mWC |
Result
You need to choose a pump that delivers 3 m³/h at 105 m (1.50 kW or 2.20 kW depending on the pump’s flow curve). The flow rate of 3 m³/h is given by the indicative flow rate / pressure table.
Choosing the pump from the TDH
- Calculate the TDH (Hr + Pc + Pr), all values in mWC.
- Determine the required flow rate (indicative table, number of water points, watering).
- Read the pump curve: it must pass through the “flow rate / TDH” point of your installation.
- Check the motor power and the pump diameter against that of the borehole.
Common mistakes to avoid
- Forgetting the head losses (Pc) in the discharge pipework.
- Mixing units: adding bar and metres. Everything must be converted to mWC.
- Not adding the residual pressure (Pr) you need at the tap or for watering.