flow calculation 2

[ris2]

ris2 (Chemical) Mar 4, 2004
I want to calculate the flow rate from a vertical vessel to a closed tank. I know the pressure in the vertical vessel and the line size to the tank. I know also the pressure in the closed tank. What other information do I need and what calculation can I use? There is a level valve downstream of the vertical vessel which maintains the level in the vessel.

 

[TD2K]

Are you asking about a vapor blowby case to your tank?

 

[ris2]

TD2K - No I am not referring to a blowby case. We do not have a flowmeter on the line to the tank and I wish to calculate the flow.

 

[sshep]

There are several options including:

1) Use your info about the pressure drop across the valve and % valve opening- lookup the CV and calculate the flow.
2) Have operator briefly close the level valve and calculate bottoms flow based on level rise and diameter of the vertical vessel. This is a very reliable way.
3) Do calculation #2 using the tank instead if other ins and outs are known or can be closed.
4) Do an energy balance around the tank temperature if other inlet flows and temps to the tank are known.
5) Do a composition balance at the tank based on sample data of a key component.
6) Presumably the vertical vessel is seperating liquid and vapor. Use the feed flow, temp, and composition to do a flash calc to find liquid flow.
7) Use an clamp-on type external ultrasonic flow meter.

In practice do several of these methods and if they come close then you can be confident that your ansewer is good.

 

[Buchi]

Hi Ris2,

in addition to the suggestions of sshep, you can do a sort of "reverse line sizing". By this i mean you should use a line sizing spreadsheet, and throw in the pipe length, fittings,elbows, reducers and the valves, inlet and outlet pressures etc. that exist along the line between both vessels. then by trial and error, see what flowrate will give you the diameter you already have existing now. That is the flowrate you are looking for.

I hope that helps.

Buchi

 

[quark]

Why can't you get it from Bernoulli's equation? The maximum flow rate can, easiliy, be calculated by that equation. If the pressure in the two tanks is higher when compared to the liquid column height, the equation will give you good approximation of flow rates at all times.

However, I faintly remember a derivation that I came through, about balancing tanks. I will go through my books and let you know if I get it.

Regards,


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