Fire Zone and Fire Scenario Definition 1

[EmmanuelTop]

We are working on the firewater system review for a petrochemical complex, which includes definition of fire zones, fire scenarios, and calculations of the firewater demand. The plant complex consists of multiple process and storage units containing Hydrocarbon liquids and vapors at a wide variety of pressures and temperatures, and size of inventory. For the sake of discussion, we can assume a typical petrochemical facility with Ethylene as feedstock.

The client does not have any specific codes or standards which need to be followed in the process of establishing fire zones and fire scenarios, so I am looking to hear opinions and advices on how to approach this subject. I have been reviewing API 521, API 2030, API 2218, NFPA 13 and NFPA 15, as well as some handbooks on the subject (Handbook of Fire & Explosion Protection Principles, by D. Nollan).

1) How would you define fire zones within a plant/complex, and what factors should be considered? I remember a recent case in which the whole plant area turned out to be a single fire zone, due to incorrectly designed open sewer system.

2) How would you define/develop fire scenarios? Any consequence modeling involved?

Thanks in advance.
Best regards


Dejan IVANOVIC
Process Engineer, MSChE

 

[bimr]

Insurance firms such as Factory Mutual usually do this type of work. The scenarios include a discussion of the economics, business case, and risk.

 

[EmmanuelTop]

Thanks bimr. We are carrying this job ourselves and there is no need to engage an external company at this point. I am simply curious to see how others are approaching this subject, based on their experience and in cases when there are no explicit requirements from the client or in local legislation.

From what I have seen so far, there are two extremes in terms of simplicity and accuracy of fire zoning and fire scenario definition. The simplest method relies on prescribed distances between equipment/units which effectively limits individual fire zones, and assumes the worst fire scenario ("assuming" means that no actual consequence modeling was performed in order to confirm what is the worst scenario, it is chosen purely from experience) for further calculations.

The most sophisticated method starts from looking at all credible fire scenarios. The outcome of consequence analysis calculations provides distance envelopes defining for each case/scenario. The partition of an installation into fire zones is such that the consequences of a flammable gas leak, an explosion or a fire corresponding to the worst credible event likely to occur in the concerned fire zone will not have an impact on other fire zones, to an extent where their integrity could be put at risk. In addition, one would have to look at other factors such as open drain system design and probability of fire propagation, etc. etc.

There is a significant difference between these two approaches (and significant difference in the amount of workload as well). I'd like to hear what experience others have in these and similar situations.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Am no expert on this topic; a few things I know that have tripped up process /safety engineers in the past:

a)Agreed hazardous open drain lines must be segregated between firezones with underground firetraps - firetraps should be designed to enable inspection and periodic flushing

b)Process and utility lines to other firezones must not pass through the affected firezone - this implies that these lines must be in some common safe area plant piperack / track with feeders to the process unit as required.

c)Obvious measure would be that perimeter bunds would be required for each firezone to prevent HC spills from spreading to adjacent zones.

d)For each firezone being depressurised, plant Operations may require the immediately adjacent firezones to be blockedin and depressurised also (either simultaneously or staggered subject to proximity to the affected firezone) - this has to be accounted for in the plant blowdown loads list.

e) For inventories containing large amounts of volatile liquids in these firezones( C2=, C3=, C2, C3 etc), liquid pulldown would be more effective than vapor depressuring.