Knowledge Base

Design Objective & Industry Benchmarks

Clean agent systems extinguish fires without leaving agent residue. Total flooding systems establish a required gas concentration throughout an enclosed space.

Building server rooms, industrial control rooms, and tunnel substations share this basic method. Their fuel hazards, ventilation arrangements, and enclosure boundaries differ.

Confirm that total flooding suits the actual fire hazard. Electrical equipment, cable insulation, oil-filled equipment, and batteries need separate hazard assessment. Clean agents do not automatically control battery thermal runaway.

Halocarbon agents typically require lower concentrations than inert gases. Halocarbons mainly absorb heat; inert gases reduce oxygen concentration. Use agent-specific data rather than transferring concentrations between agents.

A retention period of at least 10 minutes is a common benchmark. Verify the required concentration, protected height, and duration against governing standards.

Step-by-Step Sizing Methodology

1. Establish the design basis

Record fuels, equipment, occupancy, temperature limits, altitude, and required protection duration. Identify the approving authority and applicable editions of both standards.

Select an agent approved for the hazard and permitted locally. Check personnel exposure limits before fixing the design concentration.

2. Define the protected volume

Calculate the net volume served by the system:

V = Σ(L × W × H) − Vd

• V = net protected volume, m³.
• L, W, H = compartment dimensions, m.
• Vd = permitted deductions for permanent, impermeable solids, m³.

Include connected ceiling voids and raised floors requiring protection. Do not deduct ventilated cabinets or racks as solid volumes.

3. Establish the design concentration

Obtain the minimum extinguishing concentration from approved agent and fire-test data. Apply the required hazard-specific safety factor:

Cd = Ce × F

• Cd = design concentration, percent by volume.
• Ce = minimum extinguishing concentration, percent by volume.
• F = required safety factor, dimensionless.

Then check any higher listed minimum for the actual hazard. Energized electrical equipment may require a different concentration basis.

4. Calculate preliminary agent quantity

For halocarbon agents, the basic total-flooding relationship is:

m = (V / s) × Cd / (100 − Cd)

• m = agent mass, kg.
• s = agent-specific vapour volume at minimum enclosure temperature, m³/kg.

Use approved agent data for s. Apply the required altitude correction and any other applicable adjustments separately.

Inert gases use a different displacement relationship. Use the applicable standard and approved calculation method, not this halocarbon equation.

5. Complete hydraulic sizing

Use listed calculation software for the selected system. Model cylinders, pipe sizes, equivalent lengths, elevations, fittings, and nozzle orifices.

Confirm delivered quantity, nozzle coverage, and discharge time. Halocarbon systems generally must reach 95% of minimum design concentration within 10 seconds. Verify the exact criterion for the selected agent and standard.

6. Verify enclosure performance and safety

Specify enclosure integrity testing, commonly using a door fan. Predict retention using leakage distribution and the required protected height.

Calculate pressure-relief vent area using approved agent discharge data and enclosure pressure limits. Check maximum concentration at the highest temperature using the actual installed quantity.

Practical Worked Example

Consider a tunnel substation control room using FK-5-1-12, a halocarbon clean agent.

Assume approved project data establishes a 5.3% design concentration for its assessed hazard. This value is illustrative, not a universal selection.

• Main room: 10 × 8 × 3 = 240 m³.
• Protected raised floor: 40 m³.
• Protected ceiling void: 20 m³.
• Net volume: 300 m³, with no deductions.
• Minimum temperature: 20°C [68°F].
• Site elevation: approximately sea level.
• Approved specific vapour volume: 0.0719 m³/kg at 20°C.

Calculate the preliminary mass:

m = (300 / 0.0719) × 5.3 / 94.7 = 233.5 kg

The enclosure therefore needs approximately 234 kg [516 lb] before final system adjustments. Select listed cylinder fills, then verify delivery hydraulically. Recheck exposure limits using the installed mass and maximum temperature.

Cross-Trade MEP Coordination

• Mechanical engineers define fan shutdowns, damper closures, and pressure-relief discharge routes.
• Electrical engineers coordinate detection, release controls, alarms, and equipment shutdown requirements.
• Structural engineers confirm enclosure pressure limits and cylinder support loads.
• Architectural teams define sealed boundaries, doors, and protected voids.
• Tunnel engineers coordinate room isolation with tunnel smoke-control operation.

Applicable Standards & Codes

Apply NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, where adopted.

Apply ISO 14520, Gaseous fire-extinguishing systems — Physical properties and system design, including the relevant agent-specific part.

Also follow local fire, electrical, environmental, and pressure-vessel requirements. Resolve differences with the approving authority.

Core Rules of Thumb for Practice

These rules of thumb require project verification:

• Establish enclosure boundaries before estimating cylinder quantities.
• Use minimum temperature for quantity and maximum temperature for exposure checks.
• Treat connected voids as protected volume unless properly separated.
• Never select nozzle or pipe sizes from agent mass alone.