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Snail · 06-07-12, 05:26 PM

Hi Daox, Water can only drain faster than the pump can supply if air is able to flow up the pipe against the water flow. Unless the return pipe is over-sized, I am a bit surprised that this would happen at 5gpm flow.

It's over 40 years since I last did any pipe network calculations and I wasn't much good at it then. Even so, I am pretty sure that your second sentence is not right. Assuming that the pump can establish the flow to the point where air ingress does not occur, the top of the circuit can be in suction even as the pump operates.

I surmise that the critical point would be to ensure that the pump must be capable of establishing enough flow to drive the air out of the system before the partial siphon effect can kick in. An alternative might be to have a flow restriction valve at the outlet, that is switched in for a short time after the pump starts, but this is getting complicated.

06-07-12, 05:26 PM	#35
Snail Lurking Renovator Join Date: Jul 2011 Location: Palmerston North Posts: 20 Thanks: 3 Thanked 9 Times in 7 Posts	Hi Daox, Water can only drain faster than the pump can supply if air is able to flow up the pipe against the water flow. Unless the return pipe is over-sized, I am a bit surprised that this would happen at 5gpm flow. It's over 40 years since I last did any pipe network calculations and I wasn't much good at it then. Even so, I am pretty sure that your second sentence is not right. Assuming that the pump can establish the flow to the point where air ingress does not occur, the top of the circuit can be in suction even as the pump operates. I surmise that the critical point would be to ensure that the pump must be capable of establishing enough flow to drive the air out of the system before the partial siphon effect can kick in. An alternative might be to have a flow restriction valve at the outlet, that is switched in for a short time after the pump starts, but this is getting complicated.