Fire Zone

Definition

Fire zone: Area within the installation where equipment is grouped by nature and/or homogeneous level of risk attached to them. The partition of an installation into fire zones results in a significant reduction of the level of risk. This implies that consequence of fire, flammable gas leak or an explosion corresponding to the credible event likely to occur in the concerned fire zone, shall not impact other fire zones to an extent where their integrity could be put at risk. 

 

Credible event: Incident likely to occur within a concerned area. Used for the definition of fire zones. Consequences corresponding to a credible event of a given fire zone shall not impact other fire zones 

 

General methodology

Hazardous source list/ Consequence analysis

Distance envelope/safety distance or contour limit

 

Specific aspects of fire zones

Fire zones should be as small as possible to increase safety, however the number of fire zones should be reduced in order to minimize the installation complexity.

 

Partitioning Criteria

-        Combination of layout related considerations,

-        Functional independence between fire zones

-        Homogeneity of risks presented by units.

For onshore, separated by safety distances.

 

Relation with safety systems

(1)   FGS/ESD/EDP

To ensure the functional independence between fire zones, the associated logics and the ESD logics shall be grouped by fire zones with corresponding ESDV’s, BVD’s and active FF systems.

 

(2)   Deluge system

A deluge zone coincides exactly with a fire zone. The largest pool fire is about 6000 m2 (???)

The deluge zone can be broken down into “sub-deluge zones” when the reduction of the fire water demand becomes paramount.

 

Fire zone limits

ESDV’s

• A radiation of 20kW/m2[1] (6300 BTU/hr/ft2), solar radiation included.
• An overpressure of 300mbar[2] whichever the largest.

In case of excess of above-mentioned values, fire/blast proofing shall be applied to the valve (actuator and/or body) and/or associated piping.

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[1] Threshold of strength of concrete structures during several hours and corresponding to the threshold hours and of very severe damages on concrete structures.

[2] Pipe support deforms. Slight deformation of a pipe-bridge.

 

 

Fire protection is not required in the following cases [1]: 

·       Fusible link actuators that are designed to close under the fire conditions listed above.

·       Fail-safe type valve and actuators that are designed to close even with the loss of the actuator.

 

Safety distances calculations

General assumptions

Design bases for fire zones

·       Credible event

o   20 mm leak size[1]

·       ESD/EDP/fixed FF systems are out of service.

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[1] Corresponds to ¾ inch fitting rupture scenario (rounded to nearest upper natural number)

 

 

Criteria for Fire zones

Scenario	Criteria fire zone	Damage to Assets
Un-ignited flammable gas/spray cloud dispersion from gas/2-phase or liquid release	LFL
Un-ignited toxic gas/spray cloud dispersion from gas/2-phase or liquid release	NR
Jet fire from gas, 2-phase or liquid release	15.9 kW/m2 (Including solar)	Threshold of prolonged exposure of the structures and corresponding to the threshold of very severe damages on the structures, except on concrete structures. ->  Probable fire transfer to nearby equipment.
		12.5kW/m2: No probable fire transfer to cooled tanks. Minimum energy required for piloted ignition of wood, melting of plastic tubing.
Pool fire in retention basin	NR
Liquid pool vaporization (LPG, LNG or other fluids if relevant)	LFL
Vapor cloud explosion (VCE) in unit	200 mbar	Heavy machines in industrial buildings suffer little damage; steel frame building distorted and pooled away from foundations.
BLEVE	NR
Roof tank explosion	NR
Boil over (2)	NR
Flare flame out (3)	NR
Flare - continuous operation (3) (ignited release)	NR
Flare - emergency operation (3) (ignited release)	NR
Cold vent - emergency operation (un-ignited release) (3)	NR
Cold vent - emergency operation (Ignited release) (3)	NR
Degassing vent - normal operation (un-ignited release)	NR
Degassing vent - normal operation (ignited release)	NR
Blow-out (un-ignited release, during operation phase or for SIMOPS)	NR
Blow-out (ignited release during operation phase or for SIMOPS))	NR

 

Reference

1.     API RP 553 Refinery valves