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# How do you size Relief Tanks and its Relief Line?

## How do you size Relief Tanks and its Relief Line?

(OP)
I have a production separator that relieves oil/water/gas to a relief tank.  The tank has a relief line (pipe) from the top, extended 25' horizontally away.  This relief line vents the gas from the tank.  I am trying to calculate the proper size of the relief line.

When the separator exceeds a certain pressure, the oil/water/gas is routed to the relief tank.  Let's say the production stream is 5000 BPD coming into the separator and of that, 20 MMCFD gas.  The maximum pressure is 300 psig and the line from the separator to the tank is 100' 8" nominal pipe.  Therefore, above 300 psig, the fluid screams to the tank from 300 psig to near atmospheric, then vents from near atmospheric to the atmosphere via the relief line.

I assume I need to calculate the flowrate into the tank and the effect of gas expansion inside the tank.  From that I can size the relief line to allow the same rate of gas to escape.  Doe this sound right?

### RE: How do you size Relief Tanks and its Relief Line?

If your dooing this by hand then see if you can get a copy of Cranes "Compressible Flow Manual".

Anyway. The important part is where the flow chokes. So start by assuming choking in vent outlet (not in outlet to tank) and calculate backwards (mass flow is unchanged).

Best Regards

Morten

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