6.5 Fire Protection Systems, Gas Detection & COMAH Emergency Planning

The final line of defense in process safety management combines physical protection systems, automated gas and flame detection networks, and legally mandated emergency response planning. Under European Union Seveso III directives and the UK Control of Major Accident Hazards (COMAH) Regulations 2015, operating companies holding inventories of hazardous substances above defined qualifying thresholds must demonstrate robust emergency preparedness to mitigate on-site personnel risks and off-site societal and environmental impacts.

Passive Fire Protection (PFP) Systems

Passive Fire Protection (PFP) refers to static materials and structural design measures engineered to resist fire exposure without requiring manual activation, electrical power, or mechanical moving parts.

Primary Purpose of PFP

Unprotected structural steel loses approximately 50% of its load-bearing yield strength at $550^\circ ext{C}$. In a hydrocarbon jet fire or pool fire, flame temperatures rapidly exceed $1,000^\circ ext{C}$ within 2 to 5 minutes, leading to catastrophic collapse of pipe racks, vessel supports, and structural decks. The primary objective of PFP is to delay steel temperature rise, maintaining structural integrity for a specified time period (typically 60, 120, or 180 minutes) to allow:

1. Safe emergency evacuation of plant personnel.
2. Orderly execution of Automated Emergency Depressurization (EDP) to dump vessel inventory to flare.
3. Emergency response teams to initiate active firefighting measures.

PFP Material Types

• Intumescent Coatings: Epoxy-based coatings applied onto structural steelwork. Under normal conditions, they form a thin, durable finish (3 to 15 mm). When exposed to high temperatures (> 200°C), the coating undergoes a chemical endothermic reaction, swelling up to 50 times its original thickness to form a thick, insulating carbonaceous char matrix.
• Endothermic and Subliming Materials: Compounds that absorb substantial heat energy through chemical phase changes (releasing bound water vapor) when heated.
• Dense Concrete Casing: Traditional encasement of vessel skirts and lower structural columns in high-density reinforced concrete. Extremely durable against jet fire erosion, though heavy and prone to moisture entrapment.

Active Fire Protection (AFP) Systems

Unlike PFP, Active Fire Protection (AFP) systems require automated detection, manual actuation, or system triggering to deliver fire suppression mediums.

Deluge Systems

Water deluge systems are open-nozzle piping networks installed around high-hazard process equipment (such as LPG storage spheres, crude oil pumps, and distillation columns).

• Cooling Rate Requirements: Designed to deliver high-density water spray to cool vessel surfaces and prevent thermal rupture. API 2030 specifies minimum application densities:
  • Equipment surface cooling: 10.2 L/min/m² of exposed vessel surface area.
  • High-risk jet flame impingement areas / vessel skirts: up to 20.4 L/min/m².
• Hydraulic Activation: Controlled by automatic deluge valves triggered by pneumatic pilot lines, optical flame detectors, or manual pull stations.

Foam Fire Suppression Systems

Water alone is ineffective for suppressing low-density flammable liquid pool fires because hydrocarbons float on top of water, spreading the fire. Firefighting Foams are specified to blanket liquid surfaces:

• Mechanism: Foam solution mixes air, water, and fluorosurfactant concentrate to produce a floating aqueous film and foam blanket. This isolates the liquid fuel from atmospheric oxygen, suppresses flammable vapor emissions, and cools the liquid surface.
• Expansion Ratios:
  • Low Expansion (< 20:1): Heavy, long-throw foam applied via monitors onto large storage tanks.
  • Medium Expansion (20:1 to 200:1): Used for bunded areas and dike protection.
  • High Expansion (> 200:1 to 1000:1): Used to rapidly fill enclosed volumes (such as gas turbine enclosures or warehouse basements).

Gas and Flame Detection Technologies

Early detection of flammable gas leaks before ignition is critical for initiating automatic Emergency Shutdown (ESD) and isolation. Process facilities deploy two principal gas detection technologies:

1. Catalytic Bead (Pellistor) Detectors

Catalytic bead sensors measure the heat generated by the catalytic oxidation of flammable gas on a tiny heated ceramic bead.

• Operating Principle: Contains a active catalytic bead and an inactive reference bead arranged in a Wheatstone bridge circuit. Flammable gas diffusing into the sensor oxidizes on the catalytic surface, raising its temperature and electrical resistance, unbalancing the bridge proportional to gas concentration (0 to 100% LEL).
• Key Limitations:
  • Requires a minimum of 10% Oxygen ($O_2$) by volume to function (cannot detect gas in inerted spaces).
  • Highly vulnerable to Catalytic Poisoning from silicone vapors, sulfur compounds, lead, and halogenated hydrocarbons, which permanently deactivate the catalyst.
  • Point sensor only; gas must physically contact the bead.

2. Infrared (IR) Gas Detectors

Infrared detectors operate on the principle of optical absorption. Hydrocarbon molecules absorb specific infrared light wavelengths (typically 3.3 to 3.4 microns, matching Carbon-Hydrogen C-H bond stretching vibrations).

• Types: Point IR detectors and Open Path IR (OPIR) detectors (which shoot an IR beam over distances up to 100-200 meters to measure integrated gas concentration in % LEL-meter).
• Advantages over Catalytic Beads:
  • Failsafe: Optical degradation or beam blockage triggers a fault signal rather than silent failure.
  • No Oxygen Required: Operates perfectly in 100% Nitrogen inert atmospheres.
  • Poison Proof: Completely immune to silicone or sulfur poisoning.
  • Fast response time (< 3 seconds).

Optical Flame Detectors

Optical flame detectors monitor radiant energy emitted by flames. Modern high-hazard facilities specify Triple Infrared (IR3) or UV/IR flame detectors. IR3 sensors process three distinct IR wavelengths (including the 4.3 micron carbon dioxide combustion peak) and analyze characteristic flame flicker frequencies (1 to 15 Hz) to eliminate false alarms caused by sunlight, hot piping, or arc welding.

COMAH / Seveso III Emergency Planning Framework

Under COMAH Regulations, upper-tier major hazard establishments must establish comprehensive, documented emergency plans to manage major accident scenarios.

1. On-Site Emergency Plan (Internal Emergency Plan - IEP)

The On-Site Emergency Plan is prepared by the Site Operator. Its primary objectives are containing and controlling major incidents within site boundaries, protecting site personnel, and minimizing environmental damage.

• Core Elements:
  • Roles and responsibilities of the Incident Commander and Site Emergency Response Team (SERT).
  • Automated and manual Emergency Shutdown (ESD), blowdown, and depressurization procedures.
  • Primary and secondary Muster Points, personnel accountability headcount systems, and safe escape routes.
  • Site alarms (toxic gas alarm, flammable gas alarm, general fire alarm, all-clear).
  • Integration interface with responding emergency services (Local Authority Fire and Rescue Service).

2. Off-Site Emergency Plan (External Emergency Plan - EEP)

The Off-Site Emergency Plan is prepared by the Local Authority (in consultation with the operator, emergency services, health authorities, and environmental regulators), based on hazard data and consequence modeling supplied by the operator.

• Core Elements:
  • Public warning systems (sirens, automated emergency SMS alerts, local radio broadcasts).
  • Public protection strategies: Sheltering-in-Place (closing doors, windows, turning off HVAC) versus Public Evacuation.
  • Designation of emergency reception centers and traffic control corridors.
  • Off-site environmental monitoring (air sampling, water course protection).

3. Mandatory Review and Testing Frequencies

Under COMAH Regulation 12, emergency plans are living documents subjected to strict legal testing schedules:

• Both On-Site and Off-Site Emergency Plans must be reviewed, tested, and updated at suitable intervals not exceeding 3 years.
• Testing strategies include:
  • Tabletop Exercises: Scenario-based command team walkthroughs (conducted annually).
  • Partial / Modular Drills: Testing specific elements like deluge activation or off-site communication links.
  • Full-Scale Live Exercises: Comprehensive live field exercises involving local emergency services, simulating major releases, fires, or off-site evacuations at least once every 3 years.