How to Calculate Total Dynamic Head for a Submersible Deep Well Pump

When choosing a submersible deep well pump, one of the most common questions is: How much head does the pump actually need? Many buyers simply use the total well depth as the required pump head. For example, if the well is 150 metres deep, they start looking for a pump with a 150-metre head. In most installations, this is not the correct way to calculate it.

The pump does not normally need to lift water from its own installation depth. What matters is the water level while the pump is running, where the water needs to go, how much pressure is required at the destination, and how much pressure is lost through the pipeline. Once these values are understood, calculating the required head becomes much easier. If you are still determining the complete pump specification, our submersible deep well pump buying guide explains the other factors that should be checked together with flow and head.

1. What Does “Pump Head” Actually Mean?

Pump head describes how much energy a pump gives to the water and is normally expressed in metres or feet of liquid. It should not be confused with the physical depth of the pump. A deep well pump may be installed 120 metres below ground, but if the pumping water level is only 50 metres below ground, the pump is not necessarily overcoming a 120-metre vertical lift. The water surrounding the submerged pump already provides pressure at the pump intake, so for a typical open well system the elevation calculation begins from the pumping water level rather than from the pump itself. This distinction is important when selecting a QJ submersible well pump, because selecting the pump according to installation depth can result in substantially more head than the system actually requires.

For most deep well water-supply applications, the required Total Dynamic Head (TDH) can be understood with a simple relationship:

Total Dynamic Head = Vertical Lift + Required Outlet Pressure + Pipe Friction Loss

Each part represents a real resistance that the pump must overcome while delivering the required flow.

2. Start with the Pumping Water Level, Not the Total Well Depth

Before calculating head, it is important to know the difference between total well depth, static water level, pumping water level and pump installation depth. The total well depth tells you how deep the borehole extends. The static water level is the water level when the pump is stopped and the well has recovered. The pumping water level is the lower water level reached while the pump is operating at a particular flow rate. The pump installation depth tells you where the pump is physically positioned inside the well.

For head calculation, the pumping water level is usually the important starting point. Imagine a 150-metre-deep well where the static water level is 35 metres below ground, the water level drops to 55 metres while pumping, and the pump itself is installed at 90 metres. If water is being delivered to a point at ground level, the basic vertical lift is approximately 55 metres, not 90 metres and not 150 metres. The additional depth below the pumping water level mainly keeps the pump submerged and does not simply become additional lifting head. Pumping-test information is therefore much more useful for pump selection than total well depth alone.

3. Add the Elevation Between the Well and the Final Delivery Point

The next step is to look at where the water goes after leaving the well. If the pumping water level is 55 metres below ground and the water is discharged into an open storage tank located 15 metres above the wellhead, the total static vertical lift is approximately 70 metres. If the final tank is below the wellhead, that elevation difference reduces the static lift instead. This is why a site with the same well depth can require a very different pump depending on the terrain and the location of the receiving tank or distribution system.

This point becomes especially important in agricultural irrigation, hillside water supply and long-distance water-transfer projects. A well may not be particularly deep, but if the water must be delivered to an elevated reservoir, the required head can still be high. Conversely, a very deep borehole does not automatically require a very high-head pump if the dynamic water level remains relatively close to the surface. For this reason, borehole submersible pumps for irrigation and water supply should always be selected using the actual operating elevation rather than borehole depth alone.

4. Add the Pressure Required at the Outlet

Not every system simply discharges water into an open tank. Irrigation sprinklers, pressure vessels, industrial equipment, filtration systems and direct water-supply networks often require pressure to remain available after the water reaches the surface. That pressure must also be produced by the pump and therefore has to be included in the total head.

For clean water, 1 bar is approximately equal to 10.2 metres of water head. This means a system requiring 2 bar at the final outlet needs approximately 20.4 metres of additional head, while 3 bar requires roughly 30.6 metres. If a pump delivers water into an open atmospheric tank, there may be little or no additional pressure-head requirement at the tank inlet. If the same pump feeds a sprinkler system requiring 3 bar, however, those additional 30.6 metres cannot be ignored. This is one reason two projects with identical wells can require different pumps even when their flow requirements are the same.

5. Do Not Forget Pipe Friction Loss

Water loses pressure whenever it moves through a pipe. The amount lost depends mainly on flow rate, internal pipe diameter, total pipe length, pipe roughness and the valves and fittings installed in the system. Long pipelines, small pipe diameters and high flow velocities generally increase friction loss. Elbows, check valves, filters, flowmeters and control valves also add resistance.

Friction loss is sometimes underestimated because the vertical lift is easier to see on a site drawing. In a short pipeline the difference may be small, but in a long-distance irrigation or water-supply project it can become a significant part of the total pump head. A smaller discharge pipe may reduce the initial cost of the pipeline, yet the higher friction can force the pump to operate at a higher head and consume more energy throughout its service life. Pipe loss can be calculated using established hydraulic methods such as Darcy-Weisbach, or with pipe-friction tables and engineering software using the actual flow, pipe material and internal diameter. The important point is that the loss should be calculated at the design flow rate, because friction changes as flow changes. Grundfos' pump-sizing guidance likewise treats friction losses through pipe and fittings as part of total dynamic head.

6. A Simple Deep Well Pump Head Calculation Example

Consider a groundwater project where the pumping water level is 55 metres below ground and water must be delivered to a storage and irrigation system located 15 metres above the wellhead. The irrigation equipment requires 2.5 bar of pressure, and the calculated pressure loss through the rising main, horizontal pipeline, valves and fittings is 9 metres.

The calculation is:

Vertical lift = 55 + 15 = 70 m

Required pressure = 2.5 × 10.2 ≈ 25.5 m

Pipeline friction loss = 9 m

Therefore:

Total Dynamic Head = 70 + 25.5 + 9 = 104.5 m

If the system requires 100 m³/h, the required duty point is therefore approximately 100 m³/h at 105 m head. The next step is not to search the catalogue for a pump with a “maximum head of 105 m.” Instead, the performance curve should show that the pump can actually deliver approximately 100 m³/h while producing around 105 metres of head. Pump selection is always a combination of flow and head, not head alone. Grundfos and Xylem both describe pump selection in terms of the system head requirement and the resulting operating point rather than maximum-head figures alone.

7. Why Maximum Head Is Not the Head You Should Buy

A pump catalogue may show a head range or a maximum head, but this number should not be treated as the pump's normal working head. Centrifugal pump head changes with flow. At or near shut-off, the pump may produce its highest head while delivering almost no useful flow. As the flow increases, the available head normally changes according to the pump curve. A buyer who needs 100 m³/h at 105 metres cannot simply choose any pump whose catalogue says “maximum head 105 m,” because at that flow the pump may produce much less than 105 metres.

This is why a complete quotation should include a performance curve with the proposed operating point clearly marked. The curve allows the buyer to confirm whether the required flow and head occur within a suitable operating range. Our submersible pump range includes pumps designed for different combinations of flow and head, while stainless steel QJH deep well pumps are available for applications where water conditions or project specifications require stainless-steel construction. The material or motor size should be considered after the hydraulic duty point has been established, not used as a substitute for calculating it.

8. Calculate Both Normal and Difficult Operating Conditions

A good head calculation should not always be based on one fixed water level. Groundwater levels can change during pumping and may also vary seasonally. Pipeline conditions can change as valves open or close, and a filter can create more resistance as it becomes dirty. For an important water-supply system, it is therefore useful to understand both the normal operating condition and the more demanding condition the pump may experience.

For example, if the pumping water level is normally 50 metres below ground but may fall to 65 metres during the dry season, the system will require approximately 15 metres more static head during that period. This does not mean that an arbitrary safety margin should simply be added to every pump selection. Oversizing also creates problems. Instead, the expected operating range should be discussed with the pump supplier and checked against the performance curve. For projects involving changing demand or pressure, a variable-speed control system may also be considered as part of the overall water supply and irrigation pumping solution.

What Information Should You Send to the Pump Supplier?

If you want a supplier to calculate the head accurately, the most useful information is the required flow, static and pumping water levels, elevation of the final delivery point, required outlet pressure, pipeline length, pipe internal diameter, pipe material and the main valves or fittings in the line. A simple site sketch is often very helpful. If the pumping water level is unknown, it is better to say that it has not yet been measured than to use the total well depth as a replacement.

Once these values are available, the supplier can calculate or verify the system head and select a pump curve around the required duty point. This greatly reduces the risk of buying a pump that produces too little water, excessive pressure or unnecessary energy consumption.

Conclusion

Calculating the head of a submersible deep well pump is much simpler once the different depths are separated correctly. The total depth of the well and the installation depth of the pump are important for installation, but they are not automatically the pump head. For most conventional deep well water-supply systems, the calculation should begin with the pumping water level and then include the elevation to the delivery point, the pressure required by the system and the friction loss through the pipeline.

In practical terms:

Total Dynamic Head = Vertical Lift + Required Pressure Head + Pipe Friction Loss

After the total head has been calculated, it must be combined with the required flow and checked against the pump performance curve. That duty point—not maximum head, motor power or well depth—is what should guide the final pump selection.

If you are selecting a deep well pump and are unsure about the required head, provide the pumping water level, required flow, delivery elevation, outlet pressure and pipeline information. With these values, the operating point can be calculated before choosing the pump model.