My self-priming pump does not self-prime

Mar 27, 2015

One of the many misleading terms used in the pump industry is “self-priming” This eludes to most of the general public that the pump will such water up and start pumping. Nothing further from the truth could be said.

First of all “self-priming pumps” must be first of all primed.

What does priming a pump involve?

This involves filling the pump body with water. Once full and if there are no air leaks, then the pump will self-prime in the future, but remember it must always be primed first.

Why does my centrifugal pump not such. Well first of all pumps do not such. They expel the water put in them when priming takes place. As that water is expelled through the discharge of the pump a vacuum is created within the pump body. This then allows atmospheric pressure (1 Bar or 100 kpa of pressure) then forces more water (or whatever is around the suction pipe) up the pipe and into the pump. This continues to occur and we have a pumping action taking place. At no stage has the pump sucked water, all the time the action is atmospheric pressure pushing water in and the pump increasing the pressure on discharge. If an air leak is present in our suction line above water level, then this will allow air into the pump and we will lose the ability to pump. Why, well air is to light and has no effect from centrifugal action so the pump cannot create a vacuum.

Another fallacy is “my mate next door suck’s water up 20 metres with his pump. This is impossible to do. Atmospheric pressure is a total of 10 meters, so the maximum you can lift water is 10 meters at sea level. As you go up in altitude, the ability for atmospheric pressure to work decreases as does the reduced atmospheric pressure.

The most common way of getting water up more than 10 meters is to put the pump at or near the bottom and have it push the water up. This is almost boundless in its ability to push up (depending on its design). The other method is to have a second pipe acting as a drive line from the pump that blasts more water down to bring more back with it. This method is inefficient and can be difficult but popular method out on farms.

• So the lessons learned are
• Pumps do not such water
• You cannot lift water more than 10 meters at sea level
• Air leaks on a suction line will totally stop the pump from working
• Your mate next door is probably wrong most of the time

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