Chemical plants are high hazard by nature: flammable liquids, pressurised gases, exothermic reactions and explosive atmospheres. FM Global DS 7-14 covers sprinkler, foam, gaseous suppression and explosion management for these processes.
Hazard Classification
- Class I flammable liquid (flash point below 38 °C): petrol, ethanol, acetone — foam-water sprinklers
- Class II (38–60 °C): kerosene, jet fuel — foam or HC-3 sprinklers
- Class IIIA (60–93 °C): diesel and some solvents — HC-3
- Class IIIB (above 93 °C): oil derivatives — HC-2
- Flammable gases: CNG, LPG and hydrogen — dedicated detection and suppression
Sprinkler Design
- General process areas: HC-3 at 10.2 mm/min over 372 m²
- Solvent storage: foam-water sprinklers at a defined application rate
- Reactors and distillation columns: water spray or low flow, high pressure sprinklers
- Tank farms: foam application per NFPA 11 plus cooling sprinklers
- Loading and unloading: foam-water with foam monitors
Foam-Water Systems
In a flammable liquid fire, foam forms a barrier on the surface and cuts off oxygen. Concentrate types:
- AFFF: widely used on hydrocarbons, though PFAS content is driving a transition
- Alcohol-resistant AFFF: for polar solvents such as ethanol and acetone
- Fluorine-free (F3): the PFAS-free alternative, increasingly accepted
- Proportioning: typically 3 to 6 percent, metered against the water flow
Explosive Atmospheres and Explosion Prevention
- Zone classification: whether a flammable atmosphere is continuous, occasional or rare, which sets the equipment category
- Mechanical ventilation: a high air change rate to prevent gas accumulation
- Explosion relief: relief panels in the roof or side walls directing the energy safely
- Inert blanketing: nitrogen in the tank vapour space so no flammable mixture forms
- Lightning protection: bonding, earthing and air terminals, critical on chemical sites
BLEVE Prevention
A boiling liquid expanding vapour explosion occurs when external heat ruptures a pressurised flammable liquid vessel. Prevention measures:
- Tank surface cooling sprinklers at a defined application rate
- Separation distances in line with NFPA 30
- Correctly sized pressure relief valves
- Passive fire protection insulation on the vessel
Quick Checklist
- Flammable liquid class correctly assigned
- HC-3 sprinkler or foam-water selection made
- Tank farm foam plus cooling sprinklers provided
- Hazardous area zoning and mechanical ventilation in place
- Explosion relief provided
- BLEVE prevention: cooling sprinklers and relief valves
- Transition plan from AFFF to fluorine-free foam where applicable
Frequently Asked Questions
How are flammable liquids classified for fire protection design?
By flash point. Class I liquids flash below 38 degrees C and are the most hazardous, requiring foam-water protection. Class II covers 38 to 60 degrees C, Class IIIA 60 to 93 degrees C and Class IIIB above 93 degrees C, with the protection reducing in severity accordingly. The class drives whether foam is required at all.
Why is foam used instead of water alone on flammable liquids?
Water alone sinks through or spreads a hydrocarbon fire rather than extinguishing it, and can scatter burning liquid. Foam forms a blanket on the liquid surface that excludes oxygen and suppresses vapour release, which is what actually stops the fire. Alcohol-resistant foam is needed for polar solvents that would otherwise break the blanket.
What is a BLEVE and how is it prevented?
A boiling liquid expanding vapour explosion happens when external fire heats a pressurised liquid vessel until it ruptures catastrophically. Prevention rests on cooling the vessel surface with sprinklers so it never reaches failure temperature, adequate separation distances, correctly sized relief valves and passive fire protection insulation.
Why is the industry moving away from AFFF?
AFFF contains PFAS compounds that are persistent in the environment and are increasingly restricted by regulation. Fluorine-free foams have matured to the point where they are accepted for many applications, so new projects generally specify F3 and existing sites plan a transition rather than restocking AFFF.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 7-14 (Fire Protection for Chemical Plants); NFPA 11 (Low-, Medium- and High-Expansion Foam); NFPA 30 (Flammable and Combustible Liquids Code).