Airlock in low psi irrigation systems
My irrigation system is simple – I pump water to a high placed tank and then allow it to run down to the plants or trees which have to be watered. Occasionally, there is a problem – a connection has come undone and the water spews out before reaching its destination (easily diagnosed and easily fixed); or the connections are intact but the water does not emerge from where it should. This is due to a blockage which could be debris, such as algae, or an airlock. If the problem is debris, then its location must be determined and the material ejected. Fairly quickly achieved for if there are 8 bubbler heads and no water emerges from the last four in the line then the blockage is between bubbler three and four.
Thus water from storage tank A to outlet B is held up or slowed down by an airlock at C.
Possible remedies for an airlock
If the pipe from A to B is continuously downsloping there will not be an airlock. So one remedy could be to eliminate the upward slope at C. Another remedy could be to bleed out the air at C either manually each time the airlock forms or with an automatic device, or with a small hole at the highest point in C which would allow first the air and then the water to escape, but this wastes water. With my gravity feed systems all the water at A drains out through the pipes, so each time I began a new watering cycle an airlock will form, which makes manual bleeding tedious. Initially I thought that a bigger diameter pipe from A to B would solve the problem and I increased the diameter from 1” to 1.25” but this did not help and may have made it worse – the bigger the pipe diameter the bigger the airlock (in fact I have seen it suggested that the diameter of the pipe at C should be reduced to minimize the problem).
My airlock solution
Now if the pressure of the water (psi) at C is increased, the higher pressure can eject the bubble out the airlock. So my eventual solution was to: i) raise the height of A by placing it on cinder blocks; ii) straighten out the pipe at C as much as possible; and, iii) if an upward tick is unavoidable make it occur as close to B as possible. The closer C is to A the bigger the problem since the height of the water above C is reduced and therefore there is less pressure. So if air bubbles are a problem begin the inspection first with the piping closest to A. If B is a good few feet below A and C is close to B then when the water flows it should have sufficient pressure to expel the bubble in the airlock.
See also my post on additional thoughts on airlocks dated 5/20/2016