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Protect processing plants against jet fires

11.01.2013  |  Zolfkhani, M.,  Petrofac Engineering , Kuala Lumpur, Malaysia

The impact of jet fires on process equipment is critical, and it is important to examine possible design considerations that can mitigate many of the jet fire consequences.

Keywords: [jet fire] [petrochemicals] [refining] [steel]

When a pressurized piping or vessel that contains hydrocarbon gas leaks to atmosphere, it can catch fire and form a jet fire. The impact of jet fires on process equipment is critical, and it is important to examine possible design considerations that can mitigate jet fire consequences. Material alternatives and wall thickness (Wt) of the vessel exposed to jet fire are two key parameters discussed here. The rupture time of the process vessel in different scenarios is also examined.

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Muhammad Saim
11.26.2013

1- When developing plant layout segregate plant into "units". Critical equipment of adjacent units shall not be located within 37.5 Kw/m2 without considering depressurization i.e. model jet fires at operating pressure.

2- Optimize the initial lay-out (step 1) by calculating the frequency, critical equipment of adjacent units shall not be located within 10-4/yr contour (or what ever value/ALARP limit set out by the company) for jet fire radiation contour of 37.5Kw/m2 after combined isolation and depressurization.

3- Further optimization shall be considered with detailed failure analysis (as summarized in article above) if initial layouts are not acceptable/optimum.

Dave Scott
11.10.2013

Allvery good in theory. But the reality is tougher. Once your plant's on fire, it's written off. Nothing can protect it. Just make sure the people escape.
What makes a difference is prevention of ignition, and Gas Detection. So shutdown and settleout should have been intiated, peole alerted, and pipelines and Slug Catchers isolated, on Conf. Gas. Elimination of hotsurfaces, careful atention to upset conditions when spec'ing ATEX, ban petrol-engined vehicles etc itd will buy more risk reduction than exotic materials at heavy wallthickness.

Vincent ALARY
11.08.2013

Hi,

It is a common misunderstanding that API 521 (a Standard in 2007, fifth edition, and no longer an RP) requires depressurizing to 7 barg in 15 min.

One should read §5.20.1 more thoroughly. API state that depressurizing shall achieve its goal i.e. to prevent premature rupture and potential escalation.

They also state that "for pool fire [...] this generally involves reducing the equipment pressure ... to 50% of the vessels design pressure within approximately 15 min." This is related to pool fire only and associated to a reduction down to 50% of the design pressure.

In separate (p57) they mention depressurizing to 690 kPa (for vessel operating at 1700 kPa or higher) but not necessarily associated to 15 min. 690 kPa is deemed a "safe" pressure to which rupture could occur with "limited" consequences.

In other words, and as uncomfortable it may be, API is not prescriptive but goal setting and "depressurizing all vessels from initial pressure to 7 barg in 15 min" is not complying with API Std 521.

To be noted although how little is said related to jet fire!

Nonetheless this doesn't change any of the conclusions of above paper.

Regards,

Ram Bhargava
11.08.2013

Excellent overview on jet fire protection

Vincent ALARY
11.07.2013

Hi,

It is a common misunderstanding that API 521 (a Standard in 2007, fifth edition, and no longer an RP) requires depressurizing to 7 barg in 15 min.

One should read §5.20.1 more thoroughly. API state that depressurizing shall achieve its goal i.e. to prevent premature rupture and potential escalation.

They also state that "for pool fire [...] this generally involves reducing the equipment pressure ... to 50% of the vessels design pressure within approximately 15 min." This is related to pool fire only and associated to a reduction down to 50% of the design pressure.

In separate (p57) they mention depressurizing to 690 kPa (for vessel operating at 1700 kPa or higher) but not necessarily associated to 15 min. 690 kPa is deemed a "safe" pressure to which rupture could occur with "limited" consequences.

In other words, and as uncomfortable it may be, API is not prescriptive but goal setting and "depressurizing all vessels from initial pressure to 7 barg in 15 min" is not complying with API Std 521.

To be noted although how little is said related to jet fire!

Nonetheless this doesn't change any of the conclusions of above paper.

Regards,

Diego Parra
11.07.2013

Another method to avoid fire jet inpingement that is not mentioned in this article and that has been effectively implemented is to locate certain critical items of the plant (eg. gas pipeline ESD valves) in a pit below ground such that fire jet inpingement from potential sources in the vicinity is not possible.

Some of the conclusions drawn in this article oversimplify the problem, for example when a fire is detected it may not be possible to depressurize all nearby sections without delay, the flare must be sized to take these concurrent loads simultaneously or else a sequence needs to be followed. Similarly the general conclusion that plant depressurization per API RP 521 cannot prevent process equipment rupture when exposed to a jet fire is only based on the specific conditions used in the analysis presented in this article (constant jet heat flux of 350kW/m2, CS construction, high normal operating pressure, etc).

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