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Pump Tank Recirculation Calculation

Pump Tank Recirculation Calculation

Pump Tank Recirculation Calculation

I am sizing a pump for recirculating a tank full of water. Would a system as shown in the image be considered a closed system and therefore static pressure head be ignored? Or would the static head "H" be included in the calculation.

RE: Pump Tank Recirculation Calculation

Yes I think this is closed system. You only need to look at the friction, but remember you need to fill the pipe so the pump will need H, but only once

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Pump Tank Recirculation Calculation

As the return is below the water level, it would be closed and you could ignore H.
A return to atmosphere, breaks the closed fluid path, then you have friction plus H.

Here the tank is open to atmosphere, so you have potential for open system flow, if the siphon fails. As Long as you can keep the siphon going without air bubbles collecting at the high point, it will work and H must be theoretically be less than 10m (33ft), actually 22 ft practically speaking, or you will lose the high water to the tank. Backflow through the pump must also be prevented.

If you want worry free, get a pump that can pump your flow rate to the pipe high point.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Pump Tank Recirculation Calculation

Beware of vacuum at the highest point of siphon:
- even a negligible leak in the pipe segment above the liquid tank level will eventually convert a closed loop to open one, means for leak detection are recommended
- if H > ~9.5m then closed loop is impossible because of water vapor pressure, this limit is much lower if water is saturated with a gas or contains droplets of a light-boiling liquid (e.g. naphtha)
- pipe to be assured totally filled as LI mentioned so no free-falling flow (including downflow segment) is allowed
- pump head to be designed for H (open loop) otherwise manual filling of siphon is required
- be sure that pipe is designed to withstand vacuum in siphon
- a portable vacuum kit might be required for sucking a gas pocket in the highest point of siphon as this gas is able to cause flow restriction or even vacuum release
- it is recommended max water level has enough spare to avoid tank overfilling in case of inadvertent siphon emptying

As per my experience it might be a challenging issue to fill the siphon fully especially when tank is near to overfilling as pump capacity might become not enough to fill the downflow segment.

Siphons are good looking on paper but precarious in operation.

RE: Pump Tank Recirculation Calculation

Yes, it's a close system. Unless you have a way to fill the line at the start, you need to consider the static head to size the pump.
Note: Don'ts forget a check valve at the discharge of the pump.
Good luck

RE: Pump Tank Recirculation Calculation

It's a closed system as long as the tank water level stays above the discharge pipe; make sure you consider all eventualities.

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