FIRE SAFETY IMPORTANT FACTS, PART -01

FIRE SAFETY ASSESSMENT - SEQUENTIAL STEPS

Determination of loss of containment and fire scenarios

1. Loss of containment scenarios to be evaluated should be restricted to equipment failures that are likely to occur and which involve the leakage of flammable, and harmful products (both people and environment for which the frequency of occurrence and the severity of the consequences have been evaluated. a. Examples of scenarios that meet the above criteria include:

i. small leaks from process equipment and piping, sampling systems, sight glasses, etc. For calculation purposes an equivalent hole size of 6 mm (0.25 in) diameter should be assumed; ii. leaks from failure of an instrument fitting, for calculation purposes an equivalent hole size of typically 10 or 15 mm (0.4 or 0.6 in) in diameter should be assumed; iii. small leaks from flange joints for calculation purposes an equivalent hole size of 1 mm (0.04 in) diameter for ring type joints, 2.5 mm (0.1 in) diameter for spiral wound gaskets and 7 mm (0.28 in) diameter for sheet gaskets, should be assumed; iv. full bore ruptures of process lines up to size DN 20 (NPS 3/4); v. pump and compressor seal failures; for calculation purposes an equivalent hole size of 9 mm (0.35 in) diameter should be assumed; vi. for storage areas, releases into bund areas (e.g., mixer leak, etc.).

Fire scenarios

1. Fire scenarios to be evaluated shall include those that may occur at locations such as tank farms/storage areas (to include atmospheric, refrigerated and pressurised storage), process areas and other associated facilities.

2. Typical credible fire scenarios shall include: a. full surface fire scenarios on fixed roof tank in storage areas; b. vent fires on fixed and internal floating roof tanks; c. fires on the roof (partial or full surface) and fires in the rim seal area for floating roof storage tanks the rim seal area; d. jet fires, pool fires and combined jet/pool fires for process areas; e. flash fires (e.g., involving LPGs); f. confined and/or congested Vapour Cloud Explosions (VCEs); g. boiling liquid expanding vapour explosion (BLEVE) for pressurised vessels.

3. Gas and liquid flow rates from holes shall be calculated using HCM for jet/pool fires. Boil over scenarios, such as escalation of a full surface tank fire incident might need to be included depending the type of tank and location of the storage tank.

Pool fires 1. The assessment shall include the possibility of pool fires for hydrocabons that can form liquid pools. Liquid pools can be formed by all hydrocarbon products containing pentane and heavier components but also by butanes/ butene at ambient temperatures below 0 °C (32 °F). Additionally, propane and lighter refrigerated/cryogenic liquefied gases (e.g., LNG) may also form liquid pools in the event of accidental leakage while they are handled at temperatures at or near their atmospheric boiling points. In particular, this may be the case during a prolonged leakage if the rate of leakage exceeds the rate of vaporization.

JET FIRE

Vapor leakage will by nature disperse as a jet. Pressurized liquefied gases that will not form liquid pools upon accidental leakage will disperse as a vaporizing liquid aerosol jet. Ignition of a liquid or a vapor jet results in a jet fire that may cause impinging flames with high radiation intensities. The length and width of unobstructed jet flames vary as a function of the pressure upstream of the leak, the size and geometry of the hole and the wind speed. Levels of radiant heat may be in the order of 250 kW/m2 (80,000 Btu/h?ft2) – 450 kW/m2 (140,000 Btu/h?ft2). Radiation heat level from high pressure jet fires such as those involving LNG, could be above 1 MW/m2 (3,414,425 Btu/ft2).

Flash fires A flash fire can occur when the combustion of a flammable liquid and vapor results in a flame passing through the mixture at less than sonic velocity. Damaging overpressures are usually negligible, but severe injuries can result to personnel if caught up in the flame. Also, a flash fire may travel back to the source of any release and cause a jet fire if the release is pressurised.

Vapour cloud explosions Vapour cloud explosions can result in damaging overpressures, especially when flammable vapour/air mixtures are ignited in a congested area. Personnel may be killed or injured by blast effects, and buildings, plant and equipment could be damaged or demolished.

Boiling liquid expanding vapour explosion A boiling liquid expanding vapour explosion (BLEVE) is usually a consequence of prolonged heating of a pressurised (normally LPG) vessel by an external fire. The vessel may heat up rapidly and fail, spreading burning fuel as it ruptures. The initiating fire may be a pool or jet fire.

Atmospheric storage tank fires Vent fire A vent fire is a fire in which one or more of the vents in a fixed roof tank or internal floating roof tank has ignited.

Fixed roof tank full surface fire A full surface fire in a fixed roof tank can be caused by static discharge, lightning strike or vapor space explosion. A vapor space explosion can occur if the vapor space is within a flammable range and this is ignited either by hot tank surfaces (fire heated surfaces greater than the auto-ignition temperature) or by a flash back (through tank openings, defective flame arrestor, etc.) If the tank is constructed to have a frangible roof (see API STD 650 for an example of design requirements) then the roof should separate from the tank shell along a weak seam. Depending on the force of the vapor space explosion, the roof may either be partially removed (creating a “fish mouth” opening) or fully removed.

Floating roof tank full surface fire A full surface fire is a fire in which the tank roof has lost its buoyancy and some or the entire surface of liquid in the tank is exposed and involved in the fire. If a roof is well maintained and the tank is correctly operated, the risk of a rim seal fire escalating to a full surface fire is normally low.

Floating roof tank spill–on–roof fire A spill-on-roof fire is a fire in which a hydrocarbon spill on the tank roof is ignited but the roof maintains its buoyancy. In addition, flammable vapours escaping through a tank vent or roof fitting may be ignited. It is difficult to prevent a spill-on-roof fire from escalating to a full surface fire because most firefighting systems are designed for fires in the rim seal area.

Bund fire A bund fire is any type of fire that occurs within the containment area outside a tank shell. This type of fire can range from a small spill incident up to a fire covering the whole bund area. In some cases (such as a fire on a mixer) the resulting fire could incorporate some jet fire characteristics due to the hydrostatic head.

(Adopted from DEP)