As the lifting force depends upon the “head” of water outside the rising main, it follows that the maximum height to which the water can be raised depends upon the depth to which the air pipe and rising main are submerged below the standing level of the water in the bore-hole. In other words, the greater the lift, the greater the depth to which the air pipe must be carried before releasing the air into the rising main.
Experience shows that the water pipe should be submerged 18 ins. for every 1 ft. lift above the water level in the bore-hole, and allowance must be made for the “depression” of the water level in the bore-hole, which will probably take place when pumping is in progress. This depression will vary according to the water bearing capacity of the strata, in which the hole has been bored, hence it is necessary to go carefully into the conditions before boring the hole. If available, data should be studied concerning the standing water level, and the pumping depression in other bore-holes in the immediate neighbourhood. Also tests should be made before the boring machinery is removed because, although the initial depth of bore-hole may be satisfactory on the basis of standing level calculations, it may be found that when pumping the depression is so great that the bore-hole has to be carried to a greater depth.
The air is supplied at a pressure suitable for the conditions, and can be carried down a separate tube and connected to the rising main at the correct depth (Fig. [33]), or, as is often done, one pipe may be lowered and the rising main supported centrally inside the casing tube, the annular space between the two being used as the air pipe (see Fig. [34]).
The amount of free air required is from 0·6 to 1·0 cu. ft. per gallon of water raised per min., provided that all the details have been studied carefully and the design of the plant worked out accordingly.
If the air pipe is too small the air will bubble slowly through the water, while if it is too large it will blow out with great force, spraying and losing the water: the ratio between the cross-sectional area of the air and water pipes is about 1½ to 4.
Advantages of air-lift pumping are that a greater amount of water can be obtained from a hole of given size than by ordinary pumping; and that one compressing plant can deal with several wells instead of needing a separate pump to each well.
Fig. 33.—Air Pipe Outside Riser.
Fig. 34.—Air between Casing and Riser.