Unit Fire area for relief quantity calculation 1

[reena1957]

While it is clear how to design relief for a single equipment for Fire case, What will be the basis for sizing the relief header from a process unit consisting of only liquid filled vessels and only case is fire case for each of the PSVs? In short, do we take one vessel or multiple vessels under fire for the unit flare header design? There is a reference in literature for considering 2500 to 5000 sqft as fire area. Anybody knows waht Standards say about this OR whatt designers follow for refineries for this case? Thanks in advance.

 

[pleckner]

The literature reference you are referring to is API 521. The fire zone is not a given but without a detailed study of your plant including fire fighting capability, containment, etc., you should follow it since API is considered good engineering practice.

 

[MikeInSWLa]

I am currently involved in a project where we are looking at installing a new flare for emergency relief valves and we are faced with the same question. The practice that we are using to define fire zones is to do it based on drainage within the process unit. We look at high poins of grade, etc, and we define the fire zones tobe those that have common dranage.

Old practices used to be that you would draw a circle of about 25' radius and include all equipment within the resulting area as being in the same fire zone. This type of basis is somewhat arbitrary, and basing it on drainage, which defines the true potential area for a hydrocarbon pool, is more accurate.

My understanding is that API, in its lastest edition of RP 521, is moving more in this direction that using arbitrary circles.

Hope this helps

 

[pleckner]

As I wrote in my post, "The fire zone is not a given but without a detailed study of your plant including fire fighting capability, containment, etc.,you should follow it since API is considered good engineering practice."

A detailed study to reduce the fire zone would be something like being undertaken by @MikeInSWLa. But I would caution that this may still not pass muster with the local Fire Martial or insurance company. It is best to get them to buy in on any deviations from that "suggested" by the "old" rules-of-thumb from API.