5 replies to this topic

[Tran]

Dear all,

 

I am having an issue that need your advice, my story is rather long, so please be patient with me.

 

We are joining to a subsea pipeline system which transfer dehydrated gas and condensate with the terminal locate onshore. Now we are planning to launch the intelligent pig from our platform to such terminal. The problem is in our pipeline there is an amount of Mono ethylene glycol (MEG) remain after commissioning (we export dehydrated gas only hence this amount of MEG remain in the pipeline from our platform to the tie-in point to the main pipeline). And once we launch the pig, this  amount of MEG will come to the terminal and can cause it to shutdown. Hence we want to install a temporary equipment skid to separate the MEG from the drain line of the slug catcher of the terminal and pump it to the tanker as marked up in the attached drawing (the red line).

 

The temporary equipment skid include 1 glove valve to drop pressure, 1 de-gassing vessel (with 2 separate vent line to flare KOD (also used as closed drain vessel). The liquid form de-gassing vessel shall be pumped to the tanker while the globe valve is closed.
The slug catcher design pressure is 145 barg, hence the piping segment from slug catcher to the globe valve will be design for such pressure (ANSI class 900). The de-gassing vessel is design for 10 barg, so the biggest concern is overpressure for this vessel.  Hence 2 separate vent line are provided to back up each other in case one of two get blocked or the valves on such line is closed due to human error (very low possibility). The globe valve shall be sized so that for the worst case (at 100% globe valve open, the liquid come out is condensate, and one vent line blocked), the maximum back pressure (caused by flashed gas) in the de-gassing vessel is below 10 barg (the operation will be in short time i.e 1 day and fully attended so the gas blow by is impossible).

 

My question is: is there any standard that can be used as an argument to prove that my design is safe by providing 2 separate vent line? I referred to API 14C ed. 7, Appendix B,  item A.4.a.5 A.4.c.5 but this is applied for final scrubber only while in my case there are the valves between the de-gassing vessel and the Flare KOD.

 

Appreciate your advice!

Attached Files

•  Slug Catcher.pdf   372.64KB   27 downloads
•  Flare scrubber.pdf   337KB   26 downloads

[Tran]

Any help, please?

[Zauberberg]

The question can as well be: "is there any standard that requires maximum one line from a vessel to the flare system"?

 

I really do not understand your query, but maybe I am missing something here. Have you investigated the design? Has it gone through formal risk assessment? Are there any supporting calculations showing that there are or there aren't any concerns with regards to functionality-as-intended?

 

If you and your team have addressed these questions and found no obstacles to proceed, then there is no real question here. It would be the same as if you are asking can there be two PSV's on a vessel instead of one?

[Tran]

Dear Zauberberg,

 

Thanks for your reply; as I mentioned above, it would be very simple if  there is no valves between the de-gassing vessel and the Flare KOD or the venting line have no potential to be blocked. Then I can use only one line.

 

To process this job, we need to convice the terminal operator company. The two separate vent lines is the outcome from Risk Assessment from our team, but it may be not to convince the terminal operator, so it would be better for us if we have an argument to prove that the design is safe.

[Zauberberg]

There are administrative controls which can be applied in order to keep the valve open at all times - if your concern is that someone can close the valve inadvertently.

 

You can use the Lock Open facility and implement a mandatory audit/checklist each time this system is to be operated. That means a two-stage verification process which can hardly fail.

 

Having the valve Lock-Open and without performing an audit is arguably insufficient to prevent from accident happening, as per several company standards. My previous company was accepting credits for Locked Open valves, the others did not. A lot of things depend on the job and safety culture in the facility you are upgrading.

[Tran]

Dear Zauberberg,

 

It's clear. Thanks so much for your help!