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Drawdown


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.


R: 0110-DRXX
C: operation, trouble-shooting
F: drawdown


"Make it simple"


Related topic: suction lift
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