Submersible Borehole Pumps: Sizing by Head and Flow

Q: What flow rate do I need for a camp water supply?

Multiply the population by the planning figure, 15 litres per person per day as the Sphere minimum or 20 litres per person per day under UNHCR planning, then divide by realistic daily pumping hours. For 10,000 people at 20 litres over an 8 hour pumping window, that is 25 cubic metres per hour.

Tony Miller
11 minutes ago
7 min read

A submersible borehole pump is sized on two numbers and nothing else: the flow you need in cubic metres per hour, and the total dynamic head the pump must produce at that flow. Total dynamic head is the depth to the pumping water level, plus friction in the rising main, plus the height and pressure needed at the delivery point.

What is a duty point, and why does the pump only obey it?

The duty point is the pair of numbers a pump is selected against: flow (Q) and head (H) at that flow. Every centrifugal pump follows a performance curve, so the two are not independent. Ask for more head and you get less flow. That is why "we need a 7.5 kW pump" is not a specification and "25 cubic metres per hour against 72 metres" is.

Two published limits define the safe stretch of that curve. Grundfos states that its SP submersible pumps must not run continuously above 1.3 times nominal flow, because of the risk of upthrust and cavitation, and must not run continuously below 0.1 times nominal flow, because the flow past the motor is what cools it, per the Grundfos SP data booklet. A pump whose real duty point sits inside that window will run for years. A pump chosen on motor size alone will sit at one end of it.

How do you calculate total dynamic head for a borehole?

Total dynamic head (TDH) is a sum of four field measurements, not a rule of thumb:

Static water level. Depth from ground level to the resting water surface, from the borehole completion record.
Drawdown at the duty flow. How far the level falls once you pump at that rate, taken from the step test. This is the number most often skipped.
Friction losses. Head lost in the rising main, check valve and fittings, driven by pipe diameter, length and flow rate. A rising main one size too small taxes every hour the pump runs.
Discharge head. Vertical lift above ground to the tank inlet, plus any pressure needed in the distribution line.

Static water level plus drawdown gives the pumping water level, and that, not the resting level, is what the pump lifts against. A borehole resting at 45 metres that draws down 15 metres at the duty flow is a 60 metre lift before a single metre of pipe friction or tank height is counted. Sizing against the resting level is the most expensive error in borehole specification: the pump is short of head, delivers less than promised, and gets blamed for a fault that was designed in.

Where does the flow number come from?

From two directions, and the smaller one wins.

Demand. The Sphere Handbook sets 15 litres per person per day as the minimum survival quantity for drinking, cooking and hygiene. UNHCR plans displacement water supply on 20 litres per person per day, excluding leakage, and WHO puts the minimum for essential health and hygiene at around 20 litres per capita per day in Technical Note No. 9. Multiply population by the planning figure, then divide by the hours the pump will actually run, not the hours in a day.

Supply. The borehole's own tested yield. Test pumping exists to determine the reliable long-term or safe yield of a borehole and how many people it can supply, and to help fix the depth at which the permanent pump should be installed, per the ICRC guidelines on test pumping in water wells. A step test establishes the yield-to-drawdown relationship, a constant-rate test running from one or two days up to one or two weeks shows whether the borehole holds that rate, and a recovery test checks the result.

If demand exceeds tested yield, the answer is a second borehole, longer pumping hours or more storage, never a larger pump. A pump cannot create water the aquifer will not give.

A worked example: 10,000 people in a South Sudan site

Take 10,000 people at the UNHCR planning figure of 20 litres per person per day. That is 200 cubic metres a day. With a realistic 8 hour pumping window on a generator, the duty flow is 25 cubic metres per hour.

Now the head, using site figures:

Static water level: 45 m
Drawdown at 25 m3/h, from the step test: 15 m
Friction in the rising main and fittings: 4 m
Lift to the tank inlet above ground: 8 m
Total dynamic head: 72 m

The duty point is 25 m3/h at 72 m, and the pump should reach it comfortably inside its curve, not at the far right-hand end. Then apply the reality check: if the constant-rate test returned a safe yield of 20 m3/h, the 25 m3/h duty is not available. Run 10 hours instead of 8, or add a source. Pulling 25 m3/h out of a 20 m3/h borehole moves the failure from the spreadsheet to the aquifer.

Pumping hours are not a free variable in South Sudan. Fuel prices in the country have doubled since March 2026 in a deepening regional fuel crisis that is restricting aid operations, according to OCHA. An 8 hour assumption that becomes 5 hours when fuel does not arrive is a design fault, so size the storage to carry the gap.

Why does oversizing burn the motor?

An oversized pump does not politely deliver extra water. It does three destructive things instead.

It dewaters the borehole. Pump faster than the aquifer recharges and the water level falls to the pump inlet. Grundfos is explicit that neither pump nor motor is protected against dry running once the water table is drawn below the inlet, per the SP data booklet. A submersible motor is cooled by the water flowing past it, so losing submergence and losing cooling are the same event.

It runs off the end of the curve. Franklin Electric notes that when pump head stays below its normal operating range the pump creates upward thrust, and that running continuously in upthrust causes excessive wear on the upthrust bearing, per the Franklin Electric AIM manual. The same manual requires a flow of 7.62 cm/s past 4 inch motors rated 2.2 kW and above, and 15.24 cm/s for 6 and 8 inch motors, in water up to 30 degrees C, with a flow inducer sleeve where the casing is too wide to deliver it.

It cycles. A pump too large for the tank and controls fills, stops, and starts again. Franklin Electric caps three-phase motors of 5.5 to 22 kW at 100 starts per 24 hour period, requires a minimum run of one minute to dissipate starting heat, and asks for 15 minutes between starts on 6 inch and larger motors. Rapid cycling causes motor spline damage, bearing damage and overheating.

Oversizing also bills you every month. The World Bank puts fuel for a typical diesel borehole pump at more than US$5,000 a year, roughly 40% of the initial system cost, every year.

What should you send a supplier to get the duty point right?

Send the data, not the wish: the borehole completion log and casing internal diameter, the step and constant-rate test results with drawdown at each rate, the proposed pump setting depth, the rising main length, diameter and material, the height and distance to the tank, the population served and the planning litres per person per day, the power source and its realistic daily running hours, and any sand or water quality result. Sand matters: Grundfos rates its SP pumps for a maximum sand content of 50 to 150 parts per million depending on the model, and states that more than that will reduce pump life.

Our position is straightforward. SLS supplies and commissions borehole and dewatering pumps in-country rather than shipping a box to a port, so the duty point is checked against your test data before anything is quoted.

Frequently asked questions

How do I calculate total dynamic head for a borehole pump?

Add the static water level, the drawdown at your duty flow, the friction losses in the rising main and fittings, and the lift to the tank inlet. Drawdown comes from the borehole's step test, which is designed to establish the yield-to-drawdown relationship, per the ICRC test pumping guidelines.

What flow rate do I need for a camp water supply?

Multiply population by the planning figure, 15 litres per person per day as the Sphere minimum or 20 litres per person per day under UNHCR planning, then divide by realistic daily pumping hours. For 10,000 people at 20 litres over an 8 hour window, that is 25 cubic metres per hour.

Can a bigger pump fix a weak borehole?

No. If the abstraction rate exceeds the tested safe yield, the water level falls to the pump inlet and the motor loses both submergence and cooling; Grundfos states that neither pump nor motor is protected against dry running in that condition, per the SP data booklet.

How far outside its rated flow can a submersible pump run?

Grundfos specifies that SP pumps must not run continuously above 1.3 times nominal flow, because of upthrust and cavitation, or below 0.1 times nominal flow, because that flow cools the motor, per the SP data booklet.

Does the casing diameter affect pump selection?

Yes. Cooling depends on water velocity past the motor, and Franklin Electric requires 7.62 cm/s past 4 inch motors of 2.2 kW and above and 15.24 cm/s for 6 and 8 inch motors, fitting a flow inducer sleeve where a wide casing cannot deliver it, per the Franklin Electric AIM manual.