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# Liquid velocity (flow) in pipe during gravitational tank to tank transfer

## Liquid velocity (flow) in pipe during gravitational tank to tank transfer

(OP)
Hello,

I was wondering if someone has a calculation tool (for instance excelsheet) for a quick and dirty calculation of the liquid velocity (or flow) between two tanks if the liquid is transferred from one (high level) to the other (low level) by gravitation.

Pepijn

### RE: Liquid velocity (flow) in pipe during gravitational tank to tank transfer

It is a trial and error procedure - you can use any spreadsheet based on Darcy equation that calculates flow/DP for any liquid with known properties.

Try e.g. http://www.itacanet.org/natural-flow-calculator-sp... - it seems like the kind of spreadsheet you are looking for, but I haven't verified how accurate it is. The best thing is to develop your own spreadsheet and be 100% confident with the results.

Dejan IVANOVIC
Process Engineer, MSChE

### RE: Liquid velocity (flow) in pipe during gravitational tank to tank transfer

Unless your line size is small or tank to tank distance long, you might find initial flow is high. Of course flow will start to fall off straight away as the levels equalise.

There are many Web based or apps which will give you the answer for a given set of inputs.

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

### RE: Liquid velocity (flow) in pipe during gravitational tank to tank transfer

Also consider the implications of high velocity / flow intertank transfer rates ( due to high intertank differential level) on the safety vacuum breaker capacity for the source tank, and the coincident fill rate on the target tank for outbreathing safety breather capacity. Some tamper proof mechanism should be installed on the transfer line if the either of these installed inbreathing / outbreathing rates are exceeded ( a restriction orifice?).

### RE: Liquid velocity (flow) in pipe during gravitational tank to tank transfer

Try to see a good discussion here.

### RE: Liquid velocity (flow) in pipe during gravitational tank to tank transfer

The difficulty with this calculation is that it is an iteration within an iteration. The Darcy-Weisbach equation is formulated to calculate the pressure drop from a known flowrate. But you need to calculate the flowrate from the pressure drop (difference in levels in tanks) and the friction factor depends on the flowrate (which you do not know yet). So you have to calculate the flowrate for any given set of tank levels iteratively, or else use an explicit friction factor approximation that allows you to get the answer in a single step. For an explicit friction factor correlation see the recent work by Goudar, or the classic equations by Churchill or by Zigrang and Sylvester.

The levels in your tanks will be changing continuously so the pressure drop will also be changing continuously and this necessitates the outer level of iteration.

The quickest way to solve this problem is to use one of the explicit friction factor equations to calculate the flowrate for each pressure drop value, and then to copy this equation "many" times into a spreadsheet where the pressure drop can be changed as you go down step-wise. The value of "many" will depend on how accurate you want to be, but it is very easy to copy the equations 100 or even 1000 times and go down in very small steps and get a relatively accurate answer.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

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