pressure relief system

[wenchop]

Hi, I am working on basic engineering for a pressure relief system, we have decided to install high and low pressure flare headers for high and low pressure relief systems, with high and low pressure flares. The high pressure flare header will relief vessels placed in 300 ANSI class service, and the low pressure flare header will service the tanks. How to determine the pressure threshold to send miscellaneous relief loads to one or another header? I suppose that this would depend on the substances in question, the backpressure that may exist at each point in the header (constant or instant backpressure). Both the low and high pressure flare headers are supposed to be for emergency use only.

A second question is how to calculate the flow of liquid into a flare knockout drum (flare knockout scrubber). The vessels are those found in oil batteries (separators, free water knockout tanks, treaters, etc), so to size them for blocked flow, fire, etc, I think of vapour only exiting the PSV, but how can I estimate the maximum liquid flowrate that may enter the flare knockout drum? I can use a process simulator to see how much gas will condense due to cooling in the header, but this seems to be not a conservative approach. I have seen some people using the inlet liquid flow into the vessel, but this seems excessive and unrealistic, but if high level switches-alarms fail, then the liquid will travel down the flare header.

Any thoughts appreciated.

Regards,

 

[RaRo]

The backpressure will be the determining factor indeed - this encompasses the distance between your equipments and the flares too. Especially if you are averse to balanced-bellows or piloted relief valves.
I would consider that aanything relieving below 10barg should be going to the low pressure flare - but that really depends on your process and products. There is not enough information here for me.

The gas relief condensation is not conservative for me either.
Your maximum water flow would probably come from relieving an overfilling (blocked outlet case and pump pushing). There is an advantage here, your KO pot does not have to behave as an L/G separator in that case.
Some relief may be biphasic, you should check those too.
Finding the right balance is not easy.
T****l standard was to consider the largest liquid inventory in the vessels upstream and use that as the KO pot volume but also to have automated pumping to remove the liquid upon intermediate level detection.