Explain in detail a fixed gas fire-extinguishing system, including the quantity of the extinguishing medium, controls, installation and training requirements.

(1) General description:

A fixed gas fire-extinguishing system (typically a high-pressure CO2 system, but also inert gas/clean agent like FM200/IG541) is used to protect high-value or enclosed spaces such as engine rooms, cargo oil tanks, pump rooms, and electrical machinery spaces, where installing a water or foam system is impractical or unsafe. CO2 is the most common. It achieves extinguishing by smothering - displacing oxygen to below the level supporting combustion (usually below about 12-15% O2 in the protected space) and providing some cooling.

(2) Quantity of extinguishing medium (as required by SOLAS Ch. II-2 Reg 10.4 and FSS Code):

For spaces (machinery spaces of a cargo/passenger ship) not exceeding 2000 cubic metres of gross volume: the quantity of free CO2 must be at least 40% of the gross volume of the space (i.e. provide a concentration of CO2 equal to 40% of the space volume).

For machinery spaces greater than 2000 cubic metres gross volume: 35% of the gross volume, plus an amount for the enclosed spaces above the floor/sections? - the standard: 35% of the gross volume plus the additional net volume of the casing above the top of the floor? IMO FSS: the quantity of carbon dioxide is 40% of the gross volume for the space up to 2000 m3 and 35% of the gross volume of the space above 2000 m3, with the overriding provision that 60% of the quantity must be injected within 2 minutes? The FSS Code Chapter 5 states: for machinery spaces, the quantity of free CO2 shall be at least equal to the greater of: (a) 40% of the gross volume of the largest machinery space, excluding the casing; or (b) 35% of the gross volume of all the spaces (including the casing). And for cargo spaces (pump rooms/tanks) at least 30% of the gross volume. In practice the FSS specifies that at least 85% of the required quantity shall be available within 2 minutes, delivered via the piping. I will state the standard: quantity of free carbon dioxide sufficient to give 40% of the gross volume (for machinery spaces) and 35% for larger than 2000m3 plus casing; and 30% for cargo spaces/pump rooms, with at least 85% discharged within 2 minutes for spaces with CO2. The exact figures vary by edition; important to give the principle.

(3) Controls required:

• The system requires a main control and a local/service control, so it cannot be discharged accidentally or while personnel are inside.
• Two independent controls: the operating controls, arranged so that a person can operate the system; one control opens the master valve/pilot cylinders and the other opens the main release valve. The controls are placed at an accessible position and (for CO2) at a "control station" outside the protected space (e.g. a release cabinet on the main deck near the engine room entrance) with means to release the gas; the arrangement includes a pressure gauge, alarm (a battery-operated alarm/light indicating the gas is being discharged or that the space must be evacuated).
• Remote release from a control station, and a lock/valve so a person cannot be trapped; the safety valves, non-return valves and quick-acting valves on the branch lines.
• Hydraulic or electric remote release (a two-stage: first "total flooding preparing" alarm, second "discharge"), with the breathing escape.
• An interlock and a "abort" arrangement may be required, and provision to prevent discharge when maintenance personnel are working (a lock or a warning).
• Pressure-reducing/pressure-relief to prevent over-pressure.
• All controls labelled and instructions/plans displayed.

(4) Installation requirements:

• The CO2 storage room/cylinders are located outside the protected space (a separate locked compartment accessible from the deck), kept at a suitable temperature (not above 54 C and not below freezing; cylinders protected from heat).
• Piping: distribution piping of suitable size/material runs into the protected space with nozzles arranged to distribute the gas uniformly (discharge nozzles placed near the top of the space, and a pair of the nozzles near the bilge to extinguish fires at the low levels, e.g. at the bottom of the space). The piping is pressure-tested (typically 1.5x working pressure) and the pipework has no dead-ends, with a leak-off and the branch distributed throughout the space.
• The master and section valves; the gas travels to the space only on release.
• Relief valves to vent overflowing CO2.
• The system is hydrostatically tested at commissioning and at prescribed intervals (e.g. 10-yearly hydrostatic test or as per national/class requirement; cylinders re-charge and hydrostatically tested as required).
• The protected space must be capable of being sealed (quick-close ventilation, skylights and watertight doors closed) so the gas is retained; "leakage" and the space being gas-tight is essential.
• An alarm and clear instructions; the discharge outlet must have a diffuser or a distribution.

(5) Training required to bring the system to ready operation:

• Crew are trained (including in drills) in the location of the release controls, the exact sequence to evacuate the space, the two-control system, the warning/buzzer/light, and the emergency escape/scuttles; in how to release the CO2 correctly and in how to verify the space is secure and airtight.
• Training covers enclosed-space entry after discharge (safe entry only when the space is proven gas-free/with oxygen and no high CO2 levels), use of breathing apparatus, and first-aid for CO2 asphyxiation.
• Familiarisation at joining the ship (ISM familiarization), regular fire drills, and a record of each drill.
• All personnel must know not to enter the space while the system is discharging.

(6) Maintenance/testing: weekly/monthly checks of cylinder pressures and leak tests, valve operation, alarm tests, piping/nozzle inspection, and the periodic (e.g. every 5 years or per approved schedule) discharge/test and hydrostatic test of cylinders (per manufacturer and flag/class), and inspection of the release mechanism.