## Learning to calculate how much head the pump must generate

Learn how to calculate pump head.

Let us start by discussing the pressure parameters that an irrigation pump must overcome to pump out water. An irrigation pump has to overcome four elements of pressure.

These are:

- Pressure needed for application devices such as sprinklers, spray heads or drippers
- Friction loss in the system which includes pipes, screens, valves, elbows, tees, etc
- Elevation lift Suction lift.

For a deep-well pump, such as a submersible or a vertical turbine, another consideration is the drawdown of the static water level.

The static water level is defined as the depth to water when no water is being pumped from the well. As soon as the pump starts pumping, the water level starts to go down. The water level will continue to go down until equilibrium is reached.

That is when the friction loss in the aquifer and the casing screen or meter of friction is the same as the drawdown or meter of head. The dynamic water level is defined as the depth to water when the pump is running at its capacity.

When the total head for a groundwater pump is calculated, two factors that are different from a surface pump are that: There is no suction lift. Instead there is a static water level that may be compared with the suction lift for a dry installed pump. The drawdown has to be added to the elevation lift.

The other components in the calculation are unchanged. Let’s now calculate the head that a pump must produce. For this, let’s assume that: The application devices use 0.5 bar of pressure or 5 m head. The friction loss in the pipes, elbows, valves and tees has been calculated to be 2.5 bar, or 25 m head.

The elevation lift is only 20 m head. Static water level, which corresponds to suction lift for a surface pump, is 50 m. The drawdown in the well is 3 m. Considering these assumptions, the total head requirement from the pump is 5+25+20+50+3 that adds up to 103 m.

In the topic task titled “Calculating Pump Flow” we calculated the flow to be 212m³/h, and we selected the pump type. SP 215 for the requirement. The pump curve shows that a four-stage pump will give the exact flow and head required.

The pump is a submersible pump, type SP215-4 having four stages or bowls with a 75 kW motor. In this example, the performance requirement was right on one of the curves.

Let’s assume that the head requirement had only been 100m. In that case, there is no curve matching the duty point, which is between two curves. In such a situation, the solution is to select one of the closest two curves.

If the upper curve is selected, there is a bit more reserve in the pump; if the lower one is selected, the pump will run marginally for a longer time, as it only produces approximately 197m³/h instead of the required 212m³/h.

In both cases, it has no practical influence on the performance or the efficiency, which can be seen from the efficiency curve.