I want to completely drain a 1000 litre open tank to a 1200 litre holding tank and then refill.
The holding tank is above the open tank so would like to use a centrifugal pump. Can i use the holding tank extra capacity to create a pump priming circuit? When the open tank is empty (or nearly) to start recirculating the holding tank - keeping the pump primed?
If possible - would be great to use the pump to refill?

do you really have to drain the whole open tank? do you have to drain the entire downstream piping also?  you're going to run the pump dry if you have to, which will tear up your pump seal pretty quickly.  i dont really understand your concept since it sounds like you are going to backflow into the open tank anyway.  why don't you just put a level control on the open tank and shut if off before it sucks air? please explain more.

one option i could see you using like you said in the original post is installing a gate and check valve on the pump suction (make sure you have enough NPSH to prevent cavitation).  somewhere by the bottom of the tank, install a bypass line with a valve and pipe it between the check valve and the pump suction.  right as the tank is about to run empty, crack the bypass valve open and allow fluid to circulate through the pump until the open tank is empty, shut the gate valve, then shut the pump off.  make sure that the bypass line is large enough to supply the pump suction line with enough pressure to prevent cavitation also.

completely draining a tank with a centrifugal pump isnt the best idea, since you will be going through pump seals after each few times you empty the tank if you dont operate it correctly. but i think this will work for you.

an option could be to install a digitsl load monitor, which we use extensively on tanker loading and unloading applications.  It monitors true power absorbed by the pump, and can be set to trip the motor (and in turn protect the pump) once the minimum flow is reached, which will be once the tank runs dry.  (In tanker unloading principles, when the driver is drinking his tea and forgets his pump is running, it automatically trips out).

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