Continuous Purge Gas Requirement in Flare, and Vent System on Platform 2

[worachai]

Hi friends,

I received the new P&IDs for the new wellhead platform and found that there is no continuous purge gas for flare, and vent facilities. Normally,flaring, and venting on wellhead platform are occasionally occured. However, as I understand purge gas on flare, and vent system has to be provided to protect the system from air ingress which can lead to underiably internal combustion inside the system.

Could you please tell me if the purge gas can be possibly not to be continuously performed in the flare and vent system ?

If the continuous gas purging may not be required, which cases of flare and vent systems that may not require this continuous gas purging ?

If the continuous gas purging is always required, what is the reason that it is required ?

Thank you very much, (^_^)
Worachai

 

[TD2K]

If this is a conventional flare with pilots that will be on all the time, I would definitely want to be purging the flare system all the time. The combination of lit pilots and a mixture of gas/air in the header is not somthing you want to occur. The gas purging is to keep air out of the flare system, there usually some sort of device in the flare line near the tip to minimize the amount of air that can infiltrate back into the flare system (they are commonly called velocity arrestors or air arrestors. Molecular seals used to be also used but I don't think they are common on offshore platforms).

If this is a cold vent system (basically a vent just to the atmosphere with no pilots), then I've seen these without purges. The logic is that a major release will simply push the air out of the vent header given the gas velocities. Talk to the contractor and find out the design basis for the flare and how it is intended to be used. They you'll need to decide if you agree with that logic. If your company's specs don't help you, review with what you typically do on other platforms and whether you need to match that.

 

[MortenA]

Many "Nomally Unmanned Installations" (NUI) in the North Sea are today designed with no flare. They will usually incorporate HIPPS or piping to wellhead shut-in pressure.

Usually there will be an open drain for maintenance and this will usually have a vent. The vent is not purged but could be purged if required.

Best Regards

Morten

 

[worachai]

TD2K and MortenA,

Thank you very much for your answer. It is very helpful for me. (^_^)

Best Regards,
Worachai

 

[DeltaCascade]

Interesting.
Found excellent article on HIPPS (high integrity pressure protection systems) at:

http://www.clarkson.edu/~wilcox/Design/hipps.pdf

I guess it is allowed by ASME as of 1996 or so, so is relatively new... where HIPPS substitutes for need of pressure relief valves and flare system?...

If worachai, or anyone else, uses HIPPS, is it common to also have flare stack as backup? ... where it would be purged with inert gas prior to lighting??

Cheers//

 

[MortenA]

HIPPS is espcially popular i Norway and somewhat in DK. In Norway you have to pay a CO2 tax for the purge gas that you use in the falre. This amount to a deal (and its also bad PR) and therefore "no-flaring" platform are becoming popular.

I think the stack is purged using N2 and that the flare drum doubles as closed drain with a spill over valve and a special quick acting emergency valve with a bursting disc as a back up.

The flare drum "float" on the compressor inlet fressure under normal operation (constant back pressure on your PSV's).

In denmark its mostly in connection with unmanned installtions (platforms). If you dont have shut-in design pressure on your piping/pipeline then HIPPS is required. Then you dont have a flare stack.

You could also read the NORSOk standards:

http://www.nts.no/norsok/p/p00104/p00104.htm

covers HIPPS as the Norwegians see it (and i belive they are the one that knows most about this subject).

Best Regards

Morten