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The water pressure that a submersible pump must deliver is determined at the point of use (tap, watering). It is one of the three components of the total dynamic head (TDH).
Useful pressure: what is it?
Useful pressure is the pressure you must have available at the taps or for watering. It is expressed in bar, in metres of water column (mWC) or in kg/cm² (practically equivalent to bar).
Converting bar and metres of water column
To calculate the TDH, all values must be expressed in the same unit: the metre of water column. Reminder: 1 bar = 10 mWC.
| Pressure (bar) | Residual pressure (mWC) |
|---|---|
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
| 3.5 | 35 |
| 4 | 40 |
| 5 | 50 |
What pressure should you plan for?
For a home supplied with running water and watering, the indicative flow rate / pressure table gives the following useful pressures:
| Number of occupants | Useful pressure (bar) |
|---|---|
| 1 to 6 | 3.2 to 3.5 |
| 6 to 10 | 4.0 to 4.5 |
| 10 to 20 | 5.0 |
Indicative values, to be adapted to your installation (length and diameter of pipework, number of water points, watering).
From pressure to TDH
The residual pressure Pr is expressed in mWC and then added to the static head Hr and the head losses Pc: TDH = Hr + Pc + Pr. Example: a pressure of 3.5 bar at the tap gives Pr = 35 mWC.
The steps in practice
- Define the use: number of occupants and area to be watered.
- Choose the useful pressure at the point of use, in bar (see the indicative table).
- Convert it to mWC (1 bar = 10 mWC): this is the residual pressure Pr.
- Calculate the TDH by adding Hr and Pc, then choose the pump from its curve.
Good to know
Combining the pump with a pressure tank limits frequent starts and stabilises the pressure at the tap.