3 replies to this topic

[jprocess]

Dear All,
Should we follow API 521 guidelines for blowdown study of slug catchers?
I read in an engineering practise by TECHNIP company that for special case of slug catcher we should follow the procedure that often used for "pipeline" depressuring and not the API 521 guideline because it will result in excessive flows to flare nework.
Your valuable comments are appreciated.
Cheers.

[gvdlans]

Are you aware of the Technical Interpretation 521-I-02/04, see http://committees.ap.../tech/reti.html ?

The Question to the API 521 committee was:

"Background: My company is working on the construction of a gas plant. There is an existing finger-type slug catcher in the inlet section of this plant that receives a three-phase gas stream from a 30-inch pipeline. This slug catcher has been designed based on ASME B31.8 but its design pressure is different with the incoming pipeline. I have the following question regarding of depressurization system for this part of the plant.

Question: Is there any requirement to depressurize this system in case of fire detection? If yes, please let me know if the criteria are based on API 521, Section 3.19, or are there other criteria for this case?"

and the formal answer was:

"API 521 does not provide depressurization guidance for specific types of equipment or vessels. It is up to the user to define what equipment is depressured."

In your case you could take a risk based approach and consider other measures to reduce risk of slug catcher rupture due to external fire (e.g. provision of drainage facilities, active and passive fire protection).

[jprocess]

Dear gvdlans,
Thanks a lot for your reply.
But as I see in P&ID, a blowdown valve and a restriction orifice have been considered for slug catcher depressurizing.

[gvdlans]

Dear jprocess,

Is this orifice sized such that the slugcatcher pressure is reduced to 50% of the design pressure or 7 barg (whichever is lower) in 15 minutes? It could be that the orifice is sized for much slower depressurization, e.g. to prepare for maintenance activities...

I worked on a gas plant in The Netherlands some 12 years ago. It had a finger type slugcatcher at the plant inlet to remove slugs that could form in the pipeline. From what I remember, in case of emergencies the slugcatcher was blocked in and not depressurized. However, facilities were there to depressurize the slugcatcher in about 2 hours (and not in 15 minutes).