Ideally a borehole owner in an urban area should probably not be pumping more than half the natural hourly yield from his hole. This preserves the resource for himself and his neighbors because he is not putting a dent (or as great a dent) in the water table. The water level never drops as low as the level of the pump. It is a pity that very few borehole owners in urban areas follow this basic principle.
Pumping less than the natural yield has two technical advantages as well. Firstly the pump does not run dry because there is always water above it. Submersible pumps are water cooled and exposing them to air repeatedly will shorten the life of the wet-end and the motor. Secondly pumping the level below a major wet break in a borehole will inevitably create a waterfall. Typically in Harare a pump may be set at 68 metres but the main water is coming from perhaps 38 metres. When the water level falls below the water source, water dropping inside and possibly outside the casing creates a disturbance, so that fines generated will enter the pump. These fines are abrasive and will shorten the life of the pump. They may also find their way into the storage tank and the plumbing.
This article is primarily for the benefit of users of very low yielding holes. Let us say a borehole is yielding 100 litres per hour or a potential 2400 litres per day – so a useful amount of water. However the user wants to preserve his supply so he will only take 1200 litres per day. This gives him 600 litres for the house and another very modest 600 litres for the garden, etc. The pump he is using is 0.5 HP which is the smallest available having the capability to pump to his required head of say 78 metres. The pump can lift 300 l.p.h. from 78 metres but restricting the flow to 50 litres per hour is not practical, so he must limit the number of pumping hours. He can pump his 1200 litres per day by pumping total of 4 hours in 24 hours with sufficient intervals between pumping events to enable the water level to recover.
The most marketed marketed solution to this problem is the Phase Angle Relay (phangle). The majority of submersible pump controllers use this method. When the water is pumped down to pump level the loading changes because there is no water in the wet-end. This causes the pump to shut down but it has already run dry, albeit for a very short period. This can vary from a millisecond to a second or two, depending on the controller used. The product I am looking at has a time switch built in which switches the pump back on again after 10 minutes. If water has accumulated above the pump, pumping will continue.
In our example 17 litres of water has accumulated in the hole during that 10 minute period. That water will be pumped back down to pump level in roughly 5 minutes, then the cycle will repeat. If things are left the way they are the pump will run dry 96 times in a 24 hour period. This will reduce the life if the pump.
A possible solution, but not the best, is to add an external timer switch to the controller, so that the pump only runs perhaps 4 times a day and shuts down before the water level drops to the pump level. In this way the pump never runs dry. One disadvantage is that borehole yields tend to fluctuate with the seasons and low yielding holes especially so. The ON and OFF periods have to be adjusted from time to time so that the pump switches off before the water drops to pump level initiating a dry run. There is also no place for time switches during periods of erratic Zesa supply.
Another disadvantage to phangle type controllers is that a more or less constant voltage is assumed. A Zesa induced voltage fluctuation can produce a dry run signal when the borehole is not dry. The installer sets the controller parameters at a certain voltage. If the voltage changes the controller has to be reset.
Some controllers are used in conjunction with water level indicator probes. These are fixed to the riser pipe above the pump so that the pump is never dry. A lower probe will detect low water and switch off the pump. Another probe higher up the riser pipe will detect that the level has risen to a certain level and switch the pump back on. It may not be necessary to add a timer switch to this arrangement. Water turbulence can be reduced by placing the lower probe quite close below the lowest wet break. This is undoubtedly a better solution for low yielding holes
Water level probes can be mechanical or electronic. The mechanical version is simply a float switch, but owing to the confined space in a borehole the float and the cylinder in which it is housed are small. When the water level drops below the float the contact is broken and the pump is switched off. Because of the confined space the float is susceptible to jamming caused by solids or liming. For this reason electronic probes are preferred. The probe is connected to the controller via a cable which has to be secured to the riser pipe.
The positioning of the probes may take some fine tuning. If the deepest wet break is 40 metres above the bottom of the hole there may be little benefit in placing the pump 2 metres above the bottom or placing the the lower probe just above the pump.
– Richard Bridges – Get Wet Drilling