Q - I recently installed a 3/4 HP deepwell jet pump to pump water to an overhead tank. During its first week of use it took the pump one hour to fill the tank. Soon after that I noticed that it was taking longer, and longer, to fill up the tank - now it takes two hours. I made my observations while there was no water usage when the pump was running.
I went through the vendor's trouble-shooting guide to identify and fix the problem but I have no luck so far. Everything seems fine with the pump - there is no loss of prime, no visible leakage, no unusual noise and vibration. Any ideas?
A - Assuming there is no blockage or leakage in the piping and fittings, and there is nothing else wrong with the pump, the likely reason why it takes twice longer to fill the tank is the drawdown in the water well.
Drawdown is the lowering of water surface level in a well due to pumping usage. If water is being depleted faster than it can be replenished, its surface level will go down. This may not be evident during rainy season when water is plentiful, but is likely to play out during dry months when water is scarce.
If the actual drawdown were more than what were initially expected, or if it were not even considered during the planning stage, the pump would operate with higher suction lift and against a higher head. This would cause the pump to operate back on its curve at a lower flow rate to adjust for the higher head.