How a borehole pump works, and how to size one
A borehole pump is a multistage submersible that hangs below the water in a drilled bore.
Water enters through a strainer, passes up a stack of impellers, and leaves through the rising main. The motor sits underneath and is cooled by the flow going past it.
That last point decides the sizing. Select against the pumping water level, not the rest level, and against the hole's tested yield, not the flow you want.
The motor sits under the pump, and the water cools it
The motor hangs beneath the pump end, and the only thing cooling it is the water on its way into the strainer above. There is no fan and no ventilation to it.
Read a borehole set from the bottom up and the order is: submersible motor, strainer inlet, the stack of impeller and diffuser stages, a non-return valve, the discharge head, then the rising main up the cased bore to the wellhead. The strainer sits between the motor and the wet end, so every drop the pump moves travels over the outside of the motor on its way in.
It also sets a floor under the duty. The pump has to make the head. It also has to still be moving enough water past its own motor when the borehole is at its worst, and that second condition is the one people forget. Throttle a borehole pump hard, or drop it into a hole that cannot keep up with it, and it dies from heat rather than from wear.
The trade-off comes with the geometry. Nothing to prime, no suction lift to lose, and none of the NPSHa trouble that catches a surface pump on a hot day. Against that, every repair begins by lifting the pump, the rising main and the cable out of the hole. Budget the lifting job at the same time as the pump.
Head comes from stages, because the bore fixes the diameter
A borehole pump cannot get fatter to make more head, so it gets longer. The casing sets the diameter and the hole sets the casing. Most UK boreholes are 4-inch or 6-inch, which is why manufacturers build to those two envelopes: Calpeda's SD range is DN100 for a 4-inch well and DN150 for a 6-inch one.
Each stage is an impeller working into a diffuser that hands the water to the stage above it. Within a family every stage adds roughly the same head at a given flow, so lift is bought by counting stages. Calpeda tabulates 4SDP frames from 5 stages to 48, and 6SDP frames from 2 to 76, and the flow envelope holds across the range while the head climbs with the stack.
So you do not choose a borehole pump against a published maximum head. The bore and the flow pick the family. The lift then picks the number of stages inside it. See the Calpeda SD for how one manufacturer sets that out, and borehole pumps for where the type fits against the alternatives.
What the stage count costs you is length, weight, cable and rising main. A deep-set pump is a heavier lift, a longer column of water to hold up, and more copper to pay for.
Total dynamic head starts at the pumping water level
Measure the static lift from the level the water sits at while the pump is running, not from the level a dipper finds in a resting hole. The gap between the two is the drawdown, and leaving it out is the commonest arithmetic mistake on a borehole.
Rest level is the first figure on the driller's report and the least useful one for sizing. What you want is the pumping level recorded during test pumping, at a stated flow. Take it at the flow you intend to run. A level measured at a gentler test flow will flatter the duty, and the pump will find the real number for you later.
The method runs in five steps, and the order matters: each one depends on the number fixed before it.
Start with the tested yield, not the demand
The hole caps everything downstream of it. Take the sustainable yield from the driller's test data, and if the site demand is higher than the yield, the answer is storage, not a bigger pump. No pump makes water that is not there.
Fix the duty flow underneath the yield
Choose a flow the borehole will hold for the run time you need, and let a storage tank absorb the peaks. This is the step where margin does damage, so fix the number and defend it through the rest of the design.
Take the pumping water level at that flow
Read the drawn-down level from the test data at the flow you have just fixed. The distance from that level up to the surface is your static lift. It is also the level your dry-run protection has to watch.
Add the friction in the rising main
Work out the losses for the actual pipe: length from the pump to the wellhead and on to the discharge, the bore, and every bend, valve and fitting in the run. Do it at the duty flow, because friction climbs with flow. A rising main is long by definition, so bore size stops being a detail: going up a size costs money once, and being one size down costs head for every hour the pump runs.
Add whatever the surface needs
A pressure vessel, a header tank on a roof or a treatment skid all demand head of their own at the top of the rising main. State it in metres so it adds to the rest. Static lift plus friction plus surface demand is the total dynamic head, and that is what you select against.
Oversizing draws the level down onto the pump
An oversized borehole pump pulls the water level down onto its own inlet. Ask for more water than the hole will give and the level falls towards the strainer. The flow past the motor falls with it. The pump loses its cooling before it runs out of water altogether, which is why an oversized borehole pump can fail in a hole that still has water in it.
On a surface installation the penalty for oversizing is mostly economic. You buy a motor size the duty never needed and you pay for it every hour the pump runs. The borehole adds that second, terminal penalty on top of it. It arrives by the usual route, set out in full under pump selection, where each party in the chain adds a little margin for a duty the site might grow into and the pump at the far end is far too big.
Underneath it is the habit we see most often: treating the pump as though the system around it were somebody else's problem. On a borehole that system is the aquifer, the cased hole and the rising main, and none of them are optional. The pump is the part you can order. It is rarely the part that decides the outcome.
Dry running here means the pumping level dropping below the pump, and it is one of the few faults that finishes a pump quickly rather than slowly. The mundane version is worth naming too: somebody shuts a valve and forgets it is shut, then runs the pump. Same result, no design fault involved.
So treat level or dry-run protection as part of the specification rather than an accessory, and where the demand genuinely varies, reach for a storage tank and a variable-speed drive before you reach for a larger pump.
On an inverter, the ramp has to be quick
A variable-speed drive suits a borehole well, because it lets the pump track what the hole will give instead of cycling on and off against it. The drive settings are part of the pump specification, though, not a preference for whoever builds the panel.
Calpeda rates the SD motor for frequency converter duty with a suitable dv/dt filter and holds it between 30 Hz and 60 Hz. The run-up from 0 to 30 Hz and the run-down from 30 to 0 Hz each get one second at most.
The reason is the cooling. Down at the bottom of the speed range the flow past the motor is too weak to carry the heat away, so the drive has to clear that region quickly. A gentle ramp, which is exactly what a soft-start instinct tells you to dial in, holds the motor in the one place it has no cooling. Longer is not kinder here.
Starting arrangements have limits of their own. At rated outputs of 7.5 kW and above, Calpeda requires star/delta, a soft starter, stator impedance or an autotransformer rather than direct on line, which is switchgear and panel space to put in the budget from the start. The figures above are Calpeda's for the SD range. Other manufacturers publish their own, and they are not interchangeable. Get them before the panel is built and hand them to whoever commissions the drive.
What has to come off the driller's report
Six figures specify a borehole pump, and the driller's test data carries most of them.
| Figure | What it fixes |
|---|---|
| Tested yield, and the run time it was held for | Caps the duty flow |
| Pumping water level at that flow, not the rest level | The static lift |
| Setting depth for the pump | Cable and rising main length |
| Rising main bore and length, plus bends, valves and fittings | The friction |
| Head required at the surface, in metres | Whatever the water feeds |
| Water quality, and sand content above all | Pump life |
Sand is the one that decides how long the pump lasts rather than whether it works. Calpeda quotes 150 g/m³ for the 4SDP and 300 g/m³ for the 6SDP, and those are the numbers to hold a driller's development work against. Abrasive water grinds the stages down steadily, so the pump gives up head for months before it gives up altogether. Develop the hole properly first, or you will pay for the pump twice.
On a difficult hole the deepest knowledge of a given pump sits with the manufacturer, and we will take their engineer to site where the duty warrants it. Send the test data and the rising main details and we will size it. Where the honest answer is a storage tank and a smaller pump, that is the answer you will get.
Frequently asked
What is a borehole pump?
A borehole pump is a slim multistage submersible pump set down a drilled borehole, below the water level. It draws water through a strainer, lifts it through a stack of impellers and sends it up a rising main to the surface, and the submerged motor is cooled by the water flowing past it.
How do you size a borehole pump?
Start from the borehole's tested yield and fix a duty flow underneath it. Take the pumping water level at that flow for the static lift, add the friction in the rising main and any head the surface needs, and select against the total.
What is the difference between rest water level and pumping water level?
Rest level is where the water stands in an idle borehole. Pumping level is where it settles while the pump runs at a stated flow, the difference between them is the drawdown, and the pumping level is the lift the pump actually works against.
Why does a borehole pump need water flowing past it?
The submerged motor has no fan and no ventilation. It is cooled by the water being drawn in past its casing, so drawdown onto the pump and a hard-throttled duty are both serious faults rather than inefficiencies.
Can a borehole pump run on a variable-speed drive?
Many can, within published limits. Calpeda rates the SD motor for frequency converter duty with a dv/dt filter between 30 Hz and 60 Hz, and allows one second at most for the run-up from 0 to 30 Hz and the run-down back to zero, because the motor has to clear the low-flow region quickly.