FM Global DS 7-14: Fire Protection for Chemical Plants

In a chemical plant the fire scenario is often not a limited spill but a release of ignitable liquid or gas that can continue for a long time. FM Global DS 7-14 Fire Protection for Chemical Plants (July 2015, interim revision July 2019) therefore recommends a higher level of passive and active protection than the other data sheets covering ignitable liquids (§1.1). This article summarises the clauses that drive the design.

Scope (§1.0–§1.1)

• Fire and explosion hazards at chemical manufacturing and similar processing plants, focusing on ignitable liquid, flammable gas and liquefied flammable gas releases that could cause a long-duration fire, plus reactive chemical hazards.
• In DS 7-14 an “ignitable liquid” is any liquid with a measurable fire point, and “flash point” means the closed-cup value.
• Not covered: ignitable liquid dispensing (DS 7-32), distilleries (DS 7-74), small waste solvent recovery units (DS 7-2), ignitable liquid storage (DS 7-29, DS 7-88), aerosols (DS 7-31), compressed gases (DS 7-50) and LPG (DS 7-55).
• Fire scenarios include pool fires, three-dimensional (3D) spill fires, jet fires and flash fires. FM’s basic recommendation is to limit the amount of liquid that can become involved; since this is often not possible in a chemical plant, the level of protection is raised.

Siting and Construction (§2.1)

• Separation between process units, administration buildings, utilities and tank farms is set by a comprehensive facility siting study; Tables 1a/1b are only a first approximation (§2.1.1).
• Processes using large quantities of ignitable liquids or gases go in open structures with minimal enclosure; where that is not possible and a room or building explosion potential exists, damage-limiting construction (DLC) is used (§2.1.2).
• To limit 3D spill fires, equipment with liquid holdup is located at ground level, and solid, curbed and drained floors are preferred to open grating at upper levels (§2.1.3).
• Containment and emergency drainage around exterior areas with a pool fire potential; alternatively an FM Approved foam-water sprinkler or compressed air foam (CAF) system (§2.1.4).
• Where a 3D spill, long-duration pool or spray fire can occur (e.g. processes using more than 10 m³ of ignitable liquid), load-bearing steel is protected by FM Approved fireproofing rated for at least two hours of hydrocarbon exposure, or by active protection (§2.1.5).
• Safety-critical cables that cannot be routed away from ignitable liquid exposure get a one-hour FM Approved fire wrap extending 6.1 m beyond the exposed area (§2.1.9.2).
• Mechanical ventilation in enclosures where ignitable liquids are processed follows DS 7-32 (§2.2.2).

Indoor Protection (§2.3.2)

• Automatic wet, deluge or preaction sprinklers. A dry system is acceptable only if the operating area is the whole room and water reaches the most remote sprinkler within 60 seconds.
• 12 mm/min (0.3 gpm/ft²) at the roof and under solid floors and mezzanines; 6 mm/min (0.15 gpm/ft²) under open-grated intermediate levels; K ≥ 5.6 (K80), maximum 9.3 m² per sprinkler and 3 m on-line spacing (§2.3.2.2.1).
• Sprinklers below obstructions wider than 0.9 m and larger than 0.9 m², each delivering at least 114 L/min at 0.5 bar (§2.3.2.1C, §2.3.2.2.1G).
• Operating area: 929 m² on every interconnected level where intermediate floors are grated or not liquid-tight, or 929 m² on one level where they are solid and liquid-tight (§2.3.2.3.1). For deluge systems, all systems within 15 m of the extent of fire spread (30.5 m with pneumatic heat-actuated devices) (§2.3.2.3.2).
• A minimum 1,900 L/min (500 gpm) hose allowance and a water supply of at least two hours (§2.3.2.3.4, §2.3.2.4).

Outdoor Protection (§2.3.3)

• Automatic deluge and/or directional water spray; dry pipe systems are not used (§2.3.3.1.1).
• 12 mm/min (0.3 gpm/ft²) per protection level. For water-miscible ignitable liquids (except acetone), a single level of 16 mm/min (0.4 gpm/ft²) below the first mezzanine plus local water spray over equipment above it is an accepted alternative (§2.3.3.2.1).
• Design demand: all systems within 15 m of the extent of fire spread (30.5 m with pneumatic heat-actuated devices); at least a 1,900 L/min hose allowance or two monitors at full capacity; at least two hours (§2.3.3.3–§2.3.3.4).

Vessels, Steel and Other Equipment (§2.3.4–§2.3.7)

• Process vessels: directional water spray of 10 mm/min (0.25 gpm/ft²) over the whole surface using direct impingement or rundown design (§2.3.4.2).
• Inside column skirts 10 mm/min, maximum 4.6 m² per sprinkler (§2.3.5); vertical steel columns 10 mm/min over the wetted area, horizontal members 4 mm/min (§2.3.6).
• Multi-tier pipe racks: 10 mm/min upward at the bottom tier and 6 mm/min at higher tiers (§2.3.7.1); important pumps 20 mm/min (§2.3.7.2); fin-fan coolers 10 mm/min (§2.3.7.3).
• Pressure vessels are protected against fire exposure, unexpected pressure sources and runaway reactions with suitable pressure relief devices (§2.4.2.3, DS 7-49).

Foam-Water Systems (§2.3.8)

• Sprinkler or deluge protection may be supplemented with an FM Approved foam-water or CAF system, which can also serve as an alternative to emergency drainage.
• The concentrate must be compatible with the materials handled; alcohol-resistant (AR) foam is used for polar solvents. Installation follows DS 4-12.
• The sprinkler density is the greater of the data sheet value and the FM Approval listing.
• Foam duration: 10 minutes with fully adequate emergency drainage, 20 minutes with limited or no drainage, and the release duration or 20 minutes, whichever is longer, where a 3D spill fire is possible. Where 3D spill fires can occur, foam is provided at every protection level.

Process Safety and Release Control (§2.3.9, §2.4)

• Safe limits for temperature, pressure, level and flow, with alarms and safety interlocks independent of the process control system (§2.4.1–§2.4.2).
• Where operation in the flammable range is possible: limits and controls or inerting, or alternatively explosion venting, explosion suppression or pressure-resistant design (§2.4.1.2, DS 7-59).
• Isolation valves, depressurisation and dump systems to stop releases; the expected release and fire duration is kept shorter than the fire water supply duration (§2.4.3).
• Fire water mains to hazardous process areas are buried and looped with divisional valves, and risers are located behind pressure-resistant walls or shielded by structural columns (§2.3.9).

Quick Checklist

• Facility siting study completed; Tables 1a/1b used only as a first approximation
• Processes in open structures where possible, damage-limiting construction if enclosed
• Containment and emergency drainage, or a foam-water alternative
• Indoors 12 / 6 mm/min over 929 m²; outdoors 12 mm/min deluge
• 1,900 L/min hose allowance and at least a two-hour water supply
• Local protection for load-bearing steel, vessels, pipe racks and pumps
• AR foam for polar solvents, foam duration set by drainage conditions

Frequently Asked Questions

What sprinkler density does DS 7-14 require in an indoor process area?

Under §2.3.2.2.1, 12 mm/min (0.3 gpm/ft²) at the roof and under solid floors and mezzanines, and 6 mm/min (0.15 gpm/ft²) under open-grated intermediate levels. For wet or preaction systems the operating area is 929 m², applied on every interconnected level where floors are grated (§2.3.2.3.1).

What hose allowance and water supply duration apply?

Both indoor and outdoor areas need at least a 1,900 L/min (500 gpm) hose allowance and a water supply of at least two hours, lasting until the liquid flow is shut off and the area drained or the liquid is consumed (§2.3.2.3.4, §2.3.2.4, §2.3.3.3–§2.3.3.4).

Is a foam-water system mandatory?

No. Under DS 7-14 §2.3.8 a foam-water or CAF system may supplement sprinkler or deluge protection or serve as an alternative to emergency drainage. Where used, the concentrate must be compatible with the materials, alcohol-resistant (AR) foam is needed for polar solvents, and the foam duration is 10 or 20 minutes depending on drainage, or the release duration where a 3D spill fire is possible.

How are process vessels and structural steel protected?

Process vessels get 10 mm/min directional water spray (§2.3.4.2). Load-bearing steel gets FM Approved fireproofing rated for at least two hours of hydrocarbon exposure, or water spray at 10 mm/min on vertical columns and 4 mm/min on horizontal members (§2.1.5, §2.3.6).

What does DS 7-14 recommend against explosion hazards?

Locating processes that use large quantities of ignitable liquids or gases in open structures, or using damage-limiting construction where that is not possible (§2.1.2), and preventing operation in the flammable range with limits, controls or inerting, or else providing explosion venting, suppression or pressure-resistant equipment (§2.4.1.2). Pressure vessels need suitable pressure relief devices (§2.4.2.3).

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