How does a fixed CO2 flooding fire-extinguishing system work?

A fixed CO2 flooding system is the ship's heaviest firefighting weapon, protecting the machinery spaces, cargo holds, pump rooms and similar enclosed compartments. It extinguishes by smothering: flooding the space with carbon dioxide until the oxygen concentration falls below the roughly 15 percent needed to sustain flaming combustion. SOLAS and the FSS Code require enough gas for the largest protected space, nominally 35 to 40 percent of a machinery space's volume, with 85 percent of it discharged within two minutes so the fire is overwhelmed before it can regain heat.

The gas is stored in a bank of high-pressure cylinders, or a refrigerated low-pressure tank, in a dedicated CO2 room outside the protected space. Release is deliberately a two-stage manual operation: opening the release cabinet triggers the CO2 alarm and stops ventilation in the space, then separate controls open the pilot cylinders that open the main stop valve and the cylinder bank. There is no automatic release into normally manned spaces, because CO2 at extinguishing concentration kills within minutes.

Using it is a command decision with a strict sequence. First, the fire must be beyond first-aid and boundary attack. Then:

• Muster and count every person; positively confirm the space is evacuated.
• Stop fuel and oil transfer, trip the quick-closing valves and running machinery.
• Stop fans and close all dampers, skylights, doors and other openings to seal the space.
• Release the calculated number of cylinders for that space.

After release, patience is everything. The space stays sealed while boundary temperatures are monitored from outside, often for many hours, because opening up early admits oxygen to a hot fire and it reflashes, and there is only one full charge of gas aboard. Re-entry is made by a breathing-apparatus team through a small opening only when boundaries are cool, followed by thorough ventilation and atmosphere testing before the space is declared safe.

Crew duties include weekly checks of the CO2 room, keeping release cabinets sealed but accessible, knowing which valves serve which space, and periodic level checks and servicing of cylinders by shore specialists. Accidental release has killed people in CO2 rooms and protected spaces, so the room is treated with the respect of an enclosed space and any alarm from it is investigated immediately.