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[Zauberberg]

We are about to commission offshore facilities and LNG trains within the next couple of months. At present, I am focused on offshore platforms and verifying critical design parameters and operating manuals, control philosophy etc. There are couple of issues related to the offshore flare system that I'm not much familiar with.

The flare is with a sonic tip, designed to release full blowdown flow @ backpressure of 8 barg. The system design pressure (flare collection header, KO drum, and line to the flare tip) is 15 barg. Maximum relief flow is 49 MMSCFD of gas, and the line size from KO drum to the flare tip is 6". The choked flow pressure for design relief flow is about 1.8 bara (6" line) if I remember well.

The system has been designed with the following philosophy:

- In case of total platform blowdown, there will be a shock wave progressing through the 6" line toward the flare tip (due to choked flow), causing accumulation of gas and pressure buildup. Design relief flow can be achieved only when the flare tip backpressure reaches 8 barg.

- This dynamic event is envisaged to last very short, perhaps a few seconds - when the system will be exposed to sonic velocities. After reaching 8 barg upstream of the flare tip, the flare system will achieve its design relief capacity (49 MMSCFD), and the flow regime will be well below sonic conditions (Mach = 0.3 to 0.5, depending on location in the system and according to API rules).

My concern here is - and perhaps I'm just guessing because I am not familiar with sonic flares - whether the choked flow conditions and sonic velocities through the system (particularly in the 6" line) will last for a sustained/extended period of time, as opposite to what has been calculated and written in the Flare Design Basis. What happens when backpressure reaches 8 barg? The flare starts to transmit full relief flow, but at what conditions and what Mach number upstream of the flare tip? Also, as the blowdown continues and flow rate drops, the backpressure should drop as well. Does this cause flow regime to approach sonic conditions again? I would appreciate if someone experienced with sonic flare systems can explain the sequence of events, from the beginning to the end of blowdown - accompanied by (qualitative of course) description of flow regimes during blowdown.

Thanks in advance,

[Zauberberg]

Here are the two files that could assist in understanding the concept better:

Attached Files

•  Flaring Description.pdf   61.56KB   95 downloads
•  Flare Tip Backpressure vs. Flowrate.pdf   28.2KB   88 downloads