The eight metre wall, and why power does not move it
A surface pump does not suck water up. It lowers the pressure at its inlet and lets atmospheric pressure push the water up the pipe. At sea level the atmosphere can support a column of water about 10.3 metres high, and that is a hard ceiling set by physics rather than by the pump.
In the real world you never get the theoretical figure. Friction in the suction pipe, the vapour pressure of the water, any air leak in a joint and altitude all eat into it, which is why the practical limit for a shallow well or jet pump is usually quoted around seven to eight metres of vertical lift from the water surface to the pump. Fitting a bigger motor changes nothing, because the limit is on the inlet side.
Measure to the water level when the pump is running, not to the level when it has been standing all night. Drawdown during pumping lowers the surface, sometimes considerably, and a system that primes happily first thing and loses prime after twenty minutes is almost always one that has pulled the level below what the suction can manage.
| Depth to water while pumping | Pump type | Notes |
|---|---|---|
| Up to about 7 to 8 m | Surface or shallow well pump | Easy to service, must stay primed |
| Beyond about 8 m | Submersible deep well pump | Sits in the bore and pushes, no suction limit |
| Any depth, pressure too low | Booster pump | Raises pressure on an existing supply |
Total head is more than the depth
Head is the total height the pump has to lift against, expressed in metres, and it is made up of three things: the vertical distance from the pumping water level to the outlet, the pressure you want at the outlet, and the friction losses in the pipe along the way. Sizing on the borehole depth alone underestimates all three.
Friction is the part most often missed. A long run of undersized pipe can add several metres of equivalent head, and it does so in proportion to flow, so the shortfall shows up worst exactly when you are drawing the most water. Going up one pipe size is usually cheaper than going up one pump size.
Pressure converts to head at roughly ten metres per bar, so wanting three bar at the tap adds about thirty metres to whatever the physical lift is. Add the three components, then choose a pump whose curve gives the flow you need at that total head rather than one whose headline maximum head merely exceeds it. Maximum head is the point where flow is zero.
What the pressure tank is actually for
A pressure tank is not storage in any meaningful sense. It holds a cushion of air behind a bladder, and its job is to let you draw a small amount of water without the pump starting. The volume you get between the cut-in and cut-out pressures is the drawdown, and it is a fraction of the tank size rather than the whole thing.
That matters because short cycling is what destroys pumps and motors. Every start draws a heavy inrush current and heats the windings, and a pump that starts and stops every few seconds when someone washes their hands will fail far sooner than one that runs for a couple of minutes at a time. A properly sized tank turns a stream of short starts into a few longer runs.
The air charge is a maintenance item. With the system drained the pre-charge should sit slightly below the pump cut-in pressure, and it falls slowly over the years or quickly if the bladder splits. A tank that has lost its charge behaves exactly like no tank at all: the pump hammers on and off, and the usual first sign is a pressure gauge that swings wildly.
Dry running, and the protection worth having
Almost every pump in this class relies on the water it is moving to cool and lubricate it. Run it dry and a submersible can be damaged in minutes, and the failure is usually terminal rather than a matter of a seal. Boreholes that recover slowly, a dropping water table in a dry summer and a stuck float are the common routes to it.
Protection comes in several forms and it is cheap next to the pump. A low level float or probe in the bore cuts the supply before the inlet is uncovered. A pressure switch with a low pressure cut-out catches loss of prime on a surface system. Many submersibles now include a built-in controller that senses the load and shuts down on a dry run.
Fit the pump above the bottom of the bore, not on it, so that silt and sediment are not drawn straight into the inlet. Where the water is sandy, a sleeve or shroud that forces flow past the motor and screens the intake extends life considerably, and it is far easier to fit at installation than to retrofit down a borehole.
