Extend of Pool Fire...

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Pool fire is one of the sources of overpressure for a plant facilities which shall be addressed in any design. However, there is a favorite question raised for the extend of the pool fire shall be considered during design phase.

Height
According to API Std 521, Jan 2007, Fifth edition, section 5.15.1.1 "Effect of fire on the wetted surface of a vessel",

To determine vapour generation, it is necessary to recognize only that portion of the vessel that is wetted by its internal liquid and is equal to or less than 7,6 m (25 ft) above the source of flame.

NOTE : Hydrocarbon fires can exceed 40 m (approx. 130 ft) in height; however, experience has shown that it is necessary only to size relief devices on the basis of the averaged heat input up to a height of 7,6 m (25 ft) above the base of a pool fire.

Fire Zone Area
According to API Std 521, Jan 2007, Fifth edition, section 5.20.2.2 "Vapour from fire-heat input", item a)

The extent of an assumed fire zone is a function of the design and installation features that permit confining a fire within a given area (see 4.3.14). Although the size of the assumed fire zone can vary, experience generally indicates that a fire that can be confined to approximately 232 m2 (2 500 ft2) of plot

In general, a circle with diameter of 15.23 m ( 50 ft) can be considered.

Note : Having said that in many occasion, physical separation and fire wall could limit the fire zone area.

Summary
Above API clauses 5.15.1.1 and 5.20.2.2 can be cited in the design in order to define the extend of pool fire in a onshore plant. While determining pressure relieving load under fire contingency and/or fire depressuring, any Pressure relief device (PRD) and/or Blowdown valve / restriction orifice (BDV/RO) within a cylinder with Internal diameter of 15.23 m ( 50 ft) and 7.6 m (25ft) shall be the accumulated.

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