Irrigation water pumps are usually centrifugal pumps. In essence, they are machines consisting of an impeller (a bladed element that rotates to generate pressure and push the water), a pump body through which the water moved by the impeller flows, and a motor or engine that powers the impeller via a drive shaft.
A water pump can be used to irrigate plants by drawing water from a canal or rainwater reserve. But water pumps are not just for irrigation and agriculture: you can use them to empty – or fill – large bodies of water like a swimming pool or pond. They also help you in emergency situations, such as flooding caused by torrential rain or a leaking pipe.
But what happens if your water pump isn't drawing water? What if it stops sucking or the suction power is reduced? What if water comes out but at low pressure? Let's find out what the cause might be in each of these cases, as well as how to resolve the problem.
When the pump doesn't draw water
If the motor or engine of your Oleo-Mac self-priming water pump is running but no water is being pumped out, the cause might be:
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Suction hose filter clogged by twigs, leaves, stones, bits of plastic etc.: simply remove the debris to unblock the filter.
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Air in the suction hose and/or pump:
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Check that the hose is fully immersed in the water source, firmly attached to the suction connection and in good condition.
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Also check the condition of the connection and, if necessary, replace it with an original spare part.
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All okay so far? Then the problem might be the seal on the pump body, which must be replaced: don’t dismantle the water pump yourself, instead contact your nearest service centre.
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Priming failed: this can happen if the suction hose lets in air, or the pump body does not completely fill with water.
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Excessive static suction head in relation to the power limit of the water pump: the static suction head is the altitude difference between the surface of the water source and the intake port of the water pump. Check in the user and maintenance manual what the maximum static suction head is (for the Oleo-Mac WP 300 it is 7 m).
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Low impeller rotation speed: if you have filled the fuel tank with fresh mixture and the air filter is clean, the problem lies with the water pump’s carburettor, which should be inspected by a service centre technician.
Air can enter the suction hose and pump if the hose itself is split, or if the suction connection is not fully secured and tightened with a clamp, or if it is damaged. The hose may also draw in air if the water pump is poorly positioned and the intake end of the hose is not fully immersed, or is touching the bottom of the water source.
The hose material can also affect the proper functioning of the irrigation water pump. The hose must be of the non-collapsible type, so that it will not kink or pinch inwards on itself and thereby interrupt the flow of water: spiral hoses work well. Be careful to avoid leaving the hose in the sun for too long, as this may cause the material to harden, compromising watertightness at the connection point.
Air can also filter into the pump through the pump body seal, which is a sort of gasket that isolates the pump from the engine/motor whilst allowing the drive shaft to pass through it. The seal must be able to withstand the water pressure and the friction of the shaft, but if it is not in perfect condition it will allow air to enter the pump: a worn seal should be replaced with an original spare part.
In order for the water pump to work properly, there must be no air in the circuit. When you start a self-priming water pump, the self-priming function eliminates air from the suction hose: to ensure this function does its job, however, you must completely fill the pump body with water through the appropriate filling hole.
When the water pump delivers only a trickle
The flow rate is the volume of water, in litres or cubic metres, pumped per second or per minute. If only a trickle of water is coming out of your Oleo-Mac water pump, that is, if the flow rate is poor, the cause could be:
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Suction hose filter clogged: proceed as if no water is being pumped out.
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Suction hose of insufficient diameter: before changing it, check the recommended hose diameter in the water pump’s user and maintenance manual.
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Excessive suction height: proceed as if no water is being pumped out.
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Pump body seal worn: proceed as if no water is being pumped out.
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Impeller obstructed, damaged or worn: in these cases, contact the service centre.
The impeller may be damaged by foreign bodies such as gravel or coarse sand entering the pump together with the water. Otherwise the cause may be cavitation, which is the formation of vapour-filled bubbles near the impeller, at points where the water pressure drops. The bubbles collapse and, in the long run, these micro-implosions can erode the impeller and also ruin the pump components in general. However, if the impeller needs replacing it is important that the spare part is original and the work is carried out by a service centre.
Is water pumping out but at insufficient pressure? The reason might be:
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Air entering the suction hose or pump body: see the solution above to remedy the problem.
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Static suction head disproportionate to the power rating of the water pump: see the solution above to remedy the problem.
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Impeller obstructed, worn or rotating too slowly: see above to remedy the problem.
Here you will find all our articles on using water pumps for the vegetable patch, DIY projects and more: all the possible uses of a water pump, how to water plants with an irrigation pump and self-priming water pumps for irrigation.
