Fixed Gas Extinguishing System

What defines this type

A fixed gas extinguishing system protects a whole enclosed space rather than a point source, releasing an agent, most commonly CO2 but also increasingly inert gas blends, that displaces or dilutes oxygen below the level needed for combustion. That mechanism is exactly why it differs so sharply from water-based fixed systems: the space has to be evacuated and sealed before release, because the same agent that suffocates a fire suffocates a person, and CO2 in particular has caused fatalities on ships when crew entered a space during or shortly after discharge.

Main components

Agent storage

A bank of high-pressure cylinders, typically for CO2, or a bulk low-pressure storage tank on larger installations, sized to deliver the design concentration to the largest single protected space.

Release mechanism

A manually operated pull-cable or pneumatic release station outside the protected space, deliberately requiring a human decision rather than being fully automatic on most cargo ship engine room installations, paired with an alarm that sounds inside the space before discharge.

Distribution piping

Fixed pipework from the cylinder bank to discharge nozzles positioned to flood the space evenly, sized so the design concentration is reached within the time class rules require.

Space sealing arrangements

Dampers or shutters that close ventilation openings automatically or manually before release, since the system cannot hold its concentration in a space that is still being ventilated.

Selection and sizing

The extinguishing concentration and quantity of agent are calculated from the protected space's gross volume, with allowances for permanent openings that cannot be closed. CO2 systems are sized to deliver a specified percentage of the space volume within a set number of minutes; inert gas and other clean agent systems use different design concentrations depending on the agent's extinguishing mechanism. The choice between CO2 and an alternative agent often comes down to space size and occupancy pattern, since some alternative agents are approved for use in normally occupied spaces at lower risk than CO2, at higher cost and storage volume for the same protection.

Regulations and class

SOLAS Chapter II-2 and the Fire Safety Systems Code set the required extinguishing concentration, discharge time and alarm sequencing for fixed gas systems by space type. A pre-discharge alarm audible throughout the protected space, with enough delay to allow evacuation before release, is mandatory. Class surveys include a weighing or pressure check of a sample of cylinders to confirm agent quantity has not leaked away, since a cylinder can appear intact while having lost enough gas to fail the design concentration.

Typical faults

What to look for in a supplier

• Type-approved system design matching the actual protected volume, including documented allowance for openings that cannot be sealed
• Cylinder testing and refill service that returns a weight or pressure certificate, not just a visual inspection
• Compatibility of new components with the existing pipework and nozzle layout if only part of the system is being renewed
• Clear documentation of the release sequence and delay timing for crew training, not only for the installation itself

Never treat a CO2 space entry as routine — confirm the release lockout is engaged and the space has been ventilated and gas-tested, every time, regardless of how long ago the system was last discharged.