The boreholes were barely a year old; pumps had been installed by one of the leading suppliers. The pumps and motors were a world-renown top-of-the-range model, but three months down the line, 3 out of 6 motors had blown. The pump supplier went to site and inspected the pump controls. Everything was in working condition, so what could have happened to the motors?
The supplier changed the motors, which were on warranty, some with adjusted rating and things went back to normal. But a few months went by and the motors blew up again. The controls were re-inspected and found to be in good order.
Four years earlier, we had installed 4 boreholes for the same client and these were operating without issues since then. The 6 new boreholes had been supervised by another outfit and a blame game had ensued between the supplier and the driller so a third party was called in by the client.
Interference tests returned no evidence and incoming power and controls were sound, so we zeroed in on borehole construction and aquifer dynamics. All the boreholes struck two aquifers, a top water strike and the lower main aquifer. There was a 60-metre elevation difference between the two and the dynamic levels were just above the main aquifer.
The borehole design had screens both in the top and the lower aquifer. This meant that when a borehole was running, water from the top aquifer would drop into the well on to the surface of the water column below. The long drop would generate air bubbles in the water column above the pump and, depending on the depth of the pump, some of the air would eventually find its way into the pump.
In such a situation, the amount of air is not sufficient to cause cavitation of the pump, but enough to stress the pump with resultant overheating of the motor. There’s no electronic control measure for this. Ironically, the pump dry-run protection cannot switch off the motor because it is not running dry anyway.
The 10 boreholes were operated on a 14-hour remote timer so the motors would run this long everyday while heating up each minute. Of note is that the 4 older boreholes which were operating normally only had screens in the main aquifer. This was a classic case of air entrainment of pumps installed in boreholes tapping multi-layered aquifers. While not all cases lead to such serious consequences, it is likely that many a pump life has been shortened by poor casing and screen string design.
This case history illustrates the danger of inserting well-screens in perched aquifers or depleted upper aquifers. It takes some effort to convince some clients that blinding off some aquifers will not adversely affect their well’s yield, but the long years of stress-free operation thereafter is worth it.
In the event that it is absolutely necessary to tap several aquifers, there are construction methods to do that without endangering the pump. Perhaps it’s time to reflect upon a motor failure you may have encountered before?