High expansion foam systems are one of the most effective — and most misunderstood — tools in the fire suppression engineer’s toolkit. Unlike low-expansion foam agents such as AFFF or FFFP, which form a blanket on the fuel surface, a high expansion foam system works by completely filling an enclosed or semi-enclosed space with foam — displacing oxygen below the level needed to sustain combustion and simultaneously cooling the burning material.
This guide is written for system designers, fire protection engineers, procurement specialists and facility managers who need a thorough understanding of how high expansion foam systems work, what NFPA 11 requires for their design, which spaces they protect, and how to select and size the correct foam concentrate.
Contents
How Does a High Expansion Foam System Work?
A high expansion foam system works on a fundamentally different principle from low-expansion foam systems. Rather than forming a blanket on the surface of burning fuel, it floods the entire protected space with foam — suppressing fire through two simultaneous mechanisms:
Mechanism 1
Oxygen Displacement
As the foam mass fills the compartment, it displaces air — reducing the oxygen concentration from 21% (atmospheric) down below approximately 15%, the minimum level needed to sustain combustion on most flammable materials. At this oxygen level, flames cannot be maintained regardless of fuel or ignition source.
Mechanism 2
Cooling
Water draining from the foam bubble walls continuously cools the burning fuel and surrounding structural steelwork. This cooling reduces re-ignition risk and protects structural integrity — giving firefighters time to approach, investigate and isolate ignition sources before the foam is eventually ventilated.
The foam is generated by a high-expansion foam generator — a blower-type device that draws large volumes of air through a foam-solution-wetted mesh screen, producing foam at between 201 and 1000 times the volume of foam solution input. The foam is discharged into the protected space through fixed ductwork or directly from the generator head. The key word is speed: the system must fill the protected space to the required foam depth within the time limit mandated by the applicable standard (typically 2 minutes for ship engine rooms, 4 minutes for aircraft hangars) before heat and smoke from the fire make the space untenable.
Expansion Ratio — The Key Parameter
The expansion ratio is defined as the volume of finished foam produced divided by the volume of foam solution used. It is the single most important parameter in high expansion foam system design, because it determines how much foam solution (and therefore how much concentrate and water) is needed to flood a given space to the required depth.
Expansion Ratio Formula
Expansion Ratio = Volume of Foam Produced (L) ÷ Volume of Foam Solution Used (L)
CA-FIRE YEG-3 & YEG-6: 201–1000× depending on generator type and airflow
What this means in practice — flood volume at different expansion ratios:
Important: The expansion ratio achieved in practice depends on the generator type, airflow velocity and the specific concentrate used. The 201–1000× range for CA-FIRE YEG-3 and YEG-6 represents the achievable range across different generator configurations. System design calculations must use the verified expansion ratio from the actual generator specification, not a nominal figure.
High Expansion Foam System Components
A complete high expansion foam system consists of the following components. Each must be correctly specified, sized and integrated to meet NFPA 11 requirements:
1
Foam Concentrate Storage
A bladder tank, dedicated concentrate storage tank or IBC tote holding sufficient concentrate for the required system operating duration. Capacity is calculated from the generator flow rate × mixing ratio × operating time per NFPA 11. For marine systems, the storage must be located outside the protected space.
2
Foam Proportioner
An inline proportioner, bladder tank proportioner or balanced pressure system that inductsconcentrate into the water supply at the correct 3% (YEG-3) or 6% (YEG-6) ratio. The proportioner must be calibrated to match the concentrate type and must maintain accurate ratio across the full generator flow range.
3
High-Expansion Foam Generator(s)
The foam generator is the defining component of a high expansion system. It consists of a fan or blower that forces large volumes of air through a mesh screen wetted with foam solution, producing foam at 201–1000× the solution volume. Generators are sized by their foam output rate (m³/min of finished foam) and must collectively produce enough foam to fill the space within the required time limit. Generators are positioned to direct foam into the protected space via fixed ductwork or direct discharge.
4
Detection & Control System
Automatic fire detection (heat detectors, smoke detectors or flame detectors) triggers the system. A control panel manages the sequence: fire alarm confirmation → pre-discharge alarm → time delay → system activation. For attended spaces, NFPA 11 requires a minimum 30-second pre-discharge delay for evacuation. The control panel must also manage generator power supply and interlock with ventilation shutdown.
5
Ventilation Shutdown & Enclosure Dampers
All mechanical ventilation serving the protected space must be shut down automatically on system activation to prevent the foam from being dispersed by airflow. Dampers or actuated louvres close all openings to contain the foam within the protected space. NFPA 11 requires that the enclosure retain sufficient foam to maintain the design oxygen level for the required minimum duration after generator shutdown.
6
Drainage & Water Management
Water draining from the foam must be managed — particularly in below-grade spaces and on vessel machinery spaces where drainage overload can cause flooding. System drainage capacity must be assessed as part of the design. In marine systems, bilge pumps must be capable of handling the foam drainage flow rate without overwhelming the vessel’s stability.
NFPA 11 Design Requirements for High Expansion Foam
NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam is the primary design authority for high expansion foam systems in international and US projects. Chapter 6 of NFPA 11 specifically covers high expansion foam systems. The following table summarises the key design parameters:
Additional NFPA 11 Requirements
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Pre-discharge alarm: A distinct audible alarm must activate before foam discharge. For attended spaces, a minimum 30-second delay between alarm activation and foam discharge must be provided for evacuation.
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Ventilation interlock: All supply and exhaust ventilation must be shut down automatically and ventilation openings closed before or simultaneously with foam discharge to prevent foam dispersal.
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Concentrate supply duration: The foam concentrate supply must be sufficient for the required fill time plus a minimum 12-minute continuous operating period after fill completion (total ≥ fill time + 12 min).
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Manual activation: A manual activation station must be provided, located outside the protected space in a position accessible after fire activation. Automatic activation may be inhibited during personnel occupancy in some system configurations.
Fill Time & Concentrate Volume Calculation
The concentrate volume required for a high expansion foam system is calculated in three steps. The following worked example is for a ship engine room — the most commonly encountered high expansion foam application.
Worked Example — Ship Engine Room
Given:
Engine room volume: 600 m³
Required fill time (NFPA 11): 2 minutes
Generator expansion ratio: 500×
Concentrate mixing ratio: 3% (YEG-3)
Min operating duration after fill: 12 minutes
Step 1 — Required foam output rate:
600 m³ ÷ 2 min = 300 m³/min of finished foam
This is the total foam generator output capacity required to meet the NFPA 11 fill time.
Step 2 — Required foam solution flow rate:
300 m³/min ÷ 500 (expansion ratio) = 0.6 m³/min = 600 L/min
This is the foam solution flow rate (water + concentrate) the generator requires.
Step 3 — Concentrate volume required:
Fill phase (2 min): 600 L/min × 3% × 2 min = 36 L concentrate
Sustain phase (12 min): 600 L/min × 3% × 12 min = 216 L concentrate
Total = 252 L of YEG-3 concentrate
→ Round up to 300 L (1.5 × 200 L drums) for a design safety margin. Verify against your AHJ’s reserve requirement.
Note: This calculation is simplified for illustration. A full NFPA 11 design calculation must also account for foam drainage losses (typically 10–15% additional concentrate), generator efficiency variation, and the actual verified expansion ratio of the selected generator model. CA-FIRE can provide technical support for project-specific concentrate sizing calculations — contact sales@ca-fire.com with your space dimensions and generator specification.
Protected Space Types & Application Examples
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Ship Engine Rooms & Pump Rooms
The most common high expansion foam application. Fixed high-ex systems are required by SOLAS Chapter II-2 on most commercial vessels above 500 GT as an alternative to CO₂ total flooding. High-ex foam is preferred over CO₂ where re-entry after discharge is needed — the foam is non-toxic and can be walked through. NFPA 11 fill time: 2 minutes to full submergence.
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Aircraft Hangars
High expansion foam systems on aircraft hangars are governed by both NFPA 11 and NFPA 409 (Standard on Aircraft Hangars). Group I and Group II hangars require foam coverage to 600 mm depth across the entire floor area within 4 minutes. Hi-ex foam requires far less water than deluge systems, reducing structural loading and water damage risk to aircraft.
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Underground Car Parks
Multi-level underground car parks present significant challenges for sprinkler and water-based systems due to smoke stratification, vehicle density and the increasing prevalence of EV battery fires. High expansion foam fills the entire car park level, suppressing both conventional vehicle fires and EV battery fires while generating significantly less water runoff than sprinkler systems.
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Mine Roadways & Tunnels
Fires in underground mine conveyor galleries, electrical substations and fuel storage chambers are among the most dangerous industrial fire scenarios. High expansion foam completely fills the roadway cross-section, blocking fire spread and oxygen supply. Portable high-expansion generators are also deployed by mine rescue teams for emergency fire suppression in roadways.
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Cable Tunnels & Substations
Power cable tunnels and electrical substation basements present fire risks from cable insulation and transformer oil. High expansion foam fills the tunnel cross-section to suppress cable fires without the collateral damage of water or the toxicity risk of gaseous suppression agents. Particularly effective in inaccessible cable duct runs where manual firefighting is impractical.
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LNG & Cold-Climate Facilities
CA-FIRE YEG-3 and YEG-6 have a freeze point of −20°C and a minimum operating temperature of −15°C, making them suitable for LNG facility pump rooms, offshore platform processing modules in northern waters, and cold-region industrial facilities where many foam types would freeze in storage.
Concentrate Selection — YEG-3 vs YEG-6
CA-FIRE high expansion foam concentrate is available as YEG-3 (3% mixing ratio) and YEG-6 (6% mixing ratio). Both produce the same 201–1000× expansion range and are identical in all performance parameters except mixing ratio. The selection is determined by proportioner calibration — exactly as with AFFF 3% vs 6%.
The 3% model (YEG-3) uses half the concentrate volume per litre of foam solution compared to YEG-6, reducing storage requirements and concentrate cost per system discharge. Modern system designs increasingly prefer 3% for this reason — but the proportioner must be calibrated for 3% to use YEG-3. Always verify the proportioner calibration before ordering.
Life Safety & Evacuation Requirements
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Critical Life Safety Requirement
High expansion foam systems suppress fire by displacing oxygen. The same mechanism that extinguishes fire will prevent people in the protected space from breathing. All high expansion foam systems protecting occupied or potentially occupied spaces must have pre-discharge alarms, evacuation procedures and clearly marked egress routes. Failure to evacuate before foam discharge can be fatal.
NFPA 11 Chapter 6 mandates the following life safety provisions for all high expansion foam systems protecting spaces where personnel may be present:
1
Pre-discharge alarm — minimum 30 seconds before foam discharge
A clearly audible alarm distinctly different from the building fire alarm must activate at least 30 seconds (and ideally 60+ seconds for larger spaces) before foam discharge begins. The alarm must be audible throughout the protected space and adjacent areas where personnel may be present.
2
Clearly marked and illuminated emergency exits
All egress routes from the protected space must be clearly marked with emergency exit signage visible in low-visibility conditions. Exit doors must open from inside without a key and be free of obstruction at all times.
3
Personnel training and emergency procedures
All personnel who work in or near the protected space must receive documented training on the foam system operation, the alarm sequence and the evacuation procedure. Training must be repeated at regular intervals and records maintained.
4
Manual inhibit capability for occupied spaces
Where the protected space is regularly occupied during normal operations, provision for manual inhibit of automatic discharge should be considered — allowing personnel to suppress false alarms without accidental foam discharge. Manual activation remains available at all times for emergency use.
High Expansion Foam vs Alternative Suppression Systems
High expansion foam is particularly preferred over CO₂ in applications where post-discharge personnel re-entry is required — such as ship engine rooms where engineers must immediately investigate the cause of fire and restore propulsion. Unlike CO₂ at suppression concentrations, hi-ex foam is not immediately life-threatening to personnel who cannot evacuate before discharge (though the oxygen-depleted environment is still dangerous for extended periods).
Limitations & Design Cautions
Not suitable for outdoor or open-air applications
Wind rapidly disperses the foam mass in open environments before it can accumulate to the required depth. High expansion foam is designed for enclosed or semi-enclosed spaces with wind-controlled atmospheres. For open-area fuel fires, specify AFFF or FFFP.
Requires dedicated high-expansion foam generator
Standard foam branches, monitors and proportioners used for AFFF or FFFP cannot generate high expansion foam. A dedicated blower-type high-expansion foam generator is a mandatory system component. The generator must be correctly sized for the protected space volume and fill time requirement.
Drainage must be managed to prevent flooding
The water content of the foam blanket — approximately 0.1% of the foam volume at 1000× expansion — will drain as the foam breaks down. In a 500 m³ engine room at 1000× expansion, this represents approximately 500 litres of water drainage in addition to the initial foam solution volume. Bilge and drainage systems must be designed to handle this flow rate.
Enclosure must be sufficiently airtight
The protected space must have sufficient enclosure integrity to retain the foam and maintain low oxygen levels during the suppression period. Spaces with large unsealed openings will require higher foam generation rates to compensate for air inleakage. A leakage assessment should be part of every system design.
📚 Standards & Technical References
→ NFPA 11: Standard for Low-, Medium-, and High-Expansion Foam — Chapter 6 covers high expansion foam system design, fill time requirements, concentrate supply and life safety provisions.
→ NFPA 409: Standard on Aircraft Hangars — High expansion foam system requirements for Group I and Group II aircraft hangars, including the 600 mm foam depth and 4-minute fill time mandate.
→ IMO SOLAS Chapter II-2, Regulation 10 — Fixed fire detection and fire suppression systems for machinery spaces, including high expansion foam as an alternative to CO₂ total flooding.
→ IMO MSC.1/Circ.1432 — Revised Guidelines for the Approval of Fixed Pressure Water-Spraying and Water Mist Fire-Extinguishing Systems — Supplementary marine fire protection design guidance.
→ NFPA 72: National Fire Alarm and Signalling Code — Detection and alarm system requirements for high expansion foam system activation, pre-discharge alarm timing and notification.
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