So you need a pump, but don’t know how to calculate what you need? Here is some pump terminology one needs to understand when choosing and using a pump on one’s smallholding.

Flow rate

This is the volume of water, usually expressed in litres per minute, that the pump will deliver directly from its outlet. In other words, this is the maximum amount of water that the pump is designed to handle.

One can expect a pump with a bigger discharge diameter to have a greater flow rate than a smaller one, and this is indeed the case. Common domestic water pumps are sized in standard discharge diameters of 1”, 2” or 3” (25mm, 50mm and 75mm).

While flow rate is a useful term, and is related to the volume of the pump chamber and the speed at which the impeller revolves, (and thus to the size of the motor driving the pump), it bears no relation to the actual amount the pump will give you out in the field or up a water tower.


This is the vertical distance from where the inlet pipe ends to where the outlet pipe ends, usually expressed in metres. This is a critical measurement because the greater the distance, (the closer one gets to the pump’s “maximum head”) the less water the pump will deliver.

Clearly, there comes a point as one approaches this height, that a pump becomes too weak to achieve a desired outcome.

Maximum suction depth

This is a critical vertical measurement for surface-mounted pumps sucking water from a source at a lower point, because although a pump’s total head may be well within the unit’s range, the suction depth may be such that it renders the pump inefficient. That’s because a pump usually sucks less efficiently than it blows.


This is usually not critical in low-pressure installations such as domestic booster pumps, dewatering pumps, small borehole pumps, and even those used to supply water into a house under pressure. Conventional 6bar LDPE pipe and fittings should be adequate in handling the load without bursting or leaking. The term is important, however, when talking of high-pressure pumps, and those driving spraying systems of any kind. Too low a pressure and the delivery distance of the water leaving the spray nozzle will be compromised.

So, there is a relationship between flow rate, head and pressure. For example, the three may be such that one can operate a single rotary impact sprayer with adequate delivery volume and radius, but adding a second sprayer may result in inadequate delivery to both.

Other factors will, of course, affect this relationship.

One is the diameter of the pipework delivering the water to the end-point. In reality, on an established smallholding, one will find that the pipework is often a mixture of sizes ~ 32mm connected to 40mm etc depending on what purpose the pipe was intended for in the past. And inevitably there will be points along the pipe where the flow is further constricted by connectors, joins, elbows, etc, as repairs to leaks become necessary.

Another is the nature of the inner surface of the pipe. If you feel inside a length of standard black irrigation pipe it will sometimes be rough to the touch rather than smooth. This roughness, as much as the constrictions caused by connectors etc, will impede the flow of the water because of friction from the pipe wall.

Finally, there’s a term you should know if you feel your system is costing you excessively in electricity consumption:


Air in the system is the biggest cause of electricity-wastage in an automatic pumping system. That’s because air is (much) less dense than water, and is thus more compressible. In a water system air acts like a pillow: apply pressure to it and it shrinks; release the pressure and it expands again. When air ~ even a few small bubbles ~ is trapped in the pipework, with all the taps and valves closed, the pressure sensor notices this ever-fluctuating pressure and tries to compensate by switching the pump on for a few seconds to increase the pressure, before switching off again. And this cycle repeats endlessly as long as the sensor detects changes in pressure. This switch-on, switch-off routine is called ranging. But why does it waste electricity? Because any electric motor (eg pump) draws its maximum current on start-up. So every time the pump motor switches on there is a surge in current drawn and this, on a pump ranging every ten or 15 seconds or so, will materially affect your electricity bill.

This is Part Two in a five-part series on Pumps. To read the rest of series, click here.

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