Fixed fire systemsLesson 2 of 8
CO2 systems: how they work, weighing and the 10-year tests
You will know every part of a high-pressure CO2 system, what each check involves, who may do it, and how to work on the system without killing anyone.
18 minutes · Open to everyone · Rules checked 20 October 2026
Start here
In September 2004, on the containership YM People, about 5 tonnes of CO2 escaped into the manifold while the chief engineer was preparing the fixed system for inspection. The master, the chief engineer, the chief officer and the third engineer all died in the CO2 room trying to vent it. In 2016 and 2017, on the ro-ro ships Eddystone and Red Eagle, malfunctioning cylinder valves released CO2 from whole banks of cylinders into the cylinder room. Only the closed engine room distribution valve kept the gas out of the engine room.
The UK Marine Accident Investigation Branch (MAIB) put the problem plainly: the CO2 concentration needed to put out a fire “is more than double that required to kill a human being within a minute”. A CO2 system is the most effective machinery space fire system on most ships, and the most dangerous thing on board to maintain. This lesson takes it apart.
Try it
A high-pressure CO2 system, from release cabinet to nozzle
In plain words
A high-pressure CO2 system is a row of heavy steel bottles full of liquid carbon dioxide, a pipe that joins them, a valve for each space the system protects, and pipes with nozzles in those spaces. When a fire breaks out in the engine room and everyone is out, the crew opens a cabinet on deck. An alarm starts howling in the engine room. They pull the first lever, which opens the valve to the engine room, and then the second, which opens the bottles. After a short delay the gas rushes in, turns from liquid to a cold white cloud, and pushes the oxygen out of the room. The fire goes out because it cannot breathe.
Neither can people. Above about 10% CO2 in the air a person loses consciousness; at 17% and above, survival time is less than a minute. The system puts in 35 to 40%. That is why every part of the system is designed so that gas cannot reach a space by accident, and why every maintenance job starts by making accidental release impossible.
The checks are simple to list: look at it monthly, inspect it yearly, weigh every bottle and blow air through the pipes every two years, open the valves every five, pressure-test the bottles at 10 and 20 years. The crew does the looking; trained technicians do the weighing, testing and refilling.
In detail
High-pressure and low-pressure systems
Most ships have a high-pressure system: seamless steel cylinders, typically 45 kg each (a rule of thumb, sizes vary), filled with liquid CO2 at its vapour pressure, about 57 bar at 20°C. Above the critical temperature of 31.1°C, CO2 cannot stay liquid, and the pressure climbs quickly, which is why cylinder rooms must not overheat.
Large ships, especially car carriers and ro-ro ships with very large spaces, may have a low-pressure system: one or more insulated tanks of liquid CO2 kept at 1.8 to 2.2 N/mm² by two independent refrigerating units, filled to no more than 95% of their volume, with two safety valves, a level indicator, and alarms for high and low pressure, refrigeration failure and low level. The FSS Code, chapter 5, section 2.2.4, gives the requirements.
How release works
The FSS Code requires CO2 systems for spaces where people work or have access (machinery spaces, ro-ro spaces, container holds with integral reefers, spaces with doors or hatches) to have two separate controls in a release box marked for that space: one opens the valve to the space, the second discharges the cylinders, and “positive means” ensure they work only in that order. IACS Unified Interpretation SC252 says that positive means must be a mechanical or electrical interlock, not a procedure. If the box is locked, the key is in a break-glass case next to it. The pre-discharge alarm must start automatically and sound for at least 20 seconds before the gas is released. Conventional cargo holds and small spaces with only a local release need no alarm.
Before you touch it: the safety plan
MSC.1/Circ.1318/Rev.1, section 2, asks for a safety plan before any inspection or maintenance: account for everyone, set up communications between the inspection team and the duty crew, and tell everyone on board before work begins. The first physical step is to make accidental discharge impossible, for example by locking or removing the operating arms from the directional valves, or shutting and locking the system block valve. In 2011, a shore technician testing components of a tug’s CO2 system without first disconnecting the activation pilot line set off six cylinders and was seriously injured.
The schedule
| Interval | What | Who |
|---|---|---|
| Monthly (at least every 30 days) | Stop valves closed; release controls in position and accessible; piping and pilot tubing intact; cylinders in place and secured; alarm devices in place. Low-pressure: pressure gauge and level in range, tank main valve and vapour valve secured open | Crew |
| Annual | Protected space boundaries have no new openings that cannot be closed; cylinders checked for damage, rust, loose mountings (leaking, corroded, dented or bulging cylinders retested or replaced); piping supports and corrosion; nozzles not obstructed; manifold hoses and fittings tight; doors close properly; warning signs and release instructions present | Crew or service technicians, per the maintenance plan |
| Two-yearly (passenger ships, 2 years ± 3 months) or at each intermediate, periodical or renewal survey (cargo ships) | Weigh all high-pressure and pilot cylinders (or verify by other reliable means) and refill any below 90%; check low-pressure tank level against the largest hazard; check hydrostatic test dates; blow dry air or nitrogen through isolated discharge piping and nozzles | Usually a service company, witnessed for the survey |
| Two-yearly (passenger ships) or at each renewal survey (cargo ships) | Test activating heads at full working pressure through the pilot lines (or pressure-test blanked pilot lines); check pull cables and corner pulleys move freely; check pneumatic tubing for leaks; test controls, warning devices and the time delay; return the system to service | Service technicians trained to standards accepted by the Administration |
| Five-yearly | Internal inspection of all control valves | Service technicians |
| 10-yearly and beyond | Hydrostatic test and internal inspection of at least 10% of cylinders at the 10-year inspection; 50% if one fails, all if more fail. All cylinders before the 20-year anniversary and at every 10-year anniversary after that. Flexible hoses renewed at the maker’s interval, at most every 10 years | Approved cylinder test station and service company |
Weighing and level checks
The FSS Code requires a safe way to check the quantity without removing the cylinders: hanging bars above each row for a weighing device, or other means. The technician lifts each cylinder slightly on a hanging scale and compares the gross weight with the tare weight and nominal charge stamped on the cylinder. Ultrasonic liquid-level gauges are “other reliable means” and avoid lifting. A cylinder with less than 90% of its nominal charge must be refilled. When cylinders go ashore for testing, they are replaced so that the quantity on board still meets the FSS Code requirement for the largest space.
You may see 95% in some notices. That figure is in MSC.1/Circ.1432, paragraph 8.1, for fixed gas systems in general, but Circ.1432 itself says it does not cover CO2 systems. For CO2, the specific guideline (MSC.1/Circ.1318/Rev.1) uses 90%. Some flags apply 95% to other gas systems such as clean agents and halon.
The 10 and 20-year cylinder tests
The 2009 circular (MSC.1/Circ.1318) said cylinders should be tested at intervals not exceeding 10 years, but also that only 10% needed testing at the 10-year inspection. Many read that as “the same 10% every 10 years”, leaving 90% of cylinders and valves untested for the ship’s life. After the Eddystone and Red Eagle releases, the MAIB recommended that the UK ask IMO to remove the ambiguity. In May 2021, IMO approved MSC.1/Circ.1318/Rev.1: 10% at 10 years (escalating to 50% and 100% on failure), then all cylinders before the 20-year anniversary and at every 10-year anniversary after that. The test follows ISO 18119:2018, the standard for periodic inspection and testing of seamless gas cylinders. Plan it with the dry-dock schedule, because removing 100% of the cylinders needs replacement cylinders or a period without the system, which needs flag approval.
Records
Circ.1318/Rev.1 includes example service charts: 26 items for a high-pressure system, from securing the release controls, through weighing, pilot line, alarm and blow-through tests, to putting the system back in service and attaching date tags. Keep the service report and the cylinder test certificates with the ship’s records.
What the rules say
- MSC.1/Circ.1318/Rev.1, section 1, says certain maintenance may be done by competent crew members while other work “should be performed by persons specially trained in the maintenance of such systems”, and the onboard maintenance plan should say which.1
- MSC.1/Circ.1318/Rev.1, section 2, requires a safety plan before any work and measures against accidental discharge, such as locking or removing the operating arms of directional valves or shutting and locking the system block valve, as the first step.1
- MSC.1/Circ.1318/Rev.1, section 4, calls for a general visual inspection at least every 30 days, including that stop valves are closed, release controls are in position and accessible, and cylinders are in place and secured.1
- MSC.1/Circ.1318/Rev.1, section 5, lists the annual inspection, including checking that no modification has created uncloseable openings in the protected space and that leaking, corroded, dented or bulging cylinders are retested or replaced.1
- MSC.1/Circ.1318/Rev.1, paragraph 6.1.1, requires all high-pressure and pilot cylinders to be weighed or otherwise verified at least every two years (passenger ships, ± 3 months) or at each intermediate, periodical or renewal survey (cargo ships), and refilled if below 90% of nominal charge.1
- MSC.1/Circ.1318/Rev.1, paragraph 6.1.2, requires 10% of cylinders to be hydrostatically tested at the 10-year inspection (50%, then all, on failure) and all cylinders “before the 20-year anniversary and every 10-year anniversary thereafter”; flexible hoses are replaced at intervals not exceeding 10 years.1
- MSC.1/Circ.1318/Rev.1, paragraph 6.1.3, requires the discharge piping and nozzles to be isolated and blown through with dry air or nitrogen to prove they are not blocked.1
- MSC.1/Circ.1318/Rev.1, paragraph 6.2, requires service technicians or specialists trained to standards accepted by the Administration to test activating heads and pilot lines, check pull cables and pulleys, and confirm that the time delay works.1
- MSC.1/Circ.1318/Rev.1, section 7, requires an internal inspection of all control valves at least once every five years.1
- FSS Code chapter 5, paragraph 2.1.3.2, requires the pre-discharge alarm to operate automatically “in no case less than 20 s before the medium is released”.2
- FSS Code chapter 5, paragraph 2.2.2, requires two separate controls in a release box clearly identified for the space, with positive means so they can only be operated in order; IACS UI SC252 requires a mechanical or electrical interlock.2
- FSS Code chapter 5, paragraph 2.1.1.3, requires a means for the crew to check the CO2 quantity safely without removing the cylinders, such as hanging bars for a weighing device.2
- SOLAS regulation II-2/10.4.1.5 required CO2 systems for machinery spaces and cargo pump-rooms on ships built before 1 July 2002 to have the two separate release controls by the first scheduled dry-docking after 1 January 2010.3
Flag and class notes
- Bahamas (Marine Notice 79, version 2.2) interprets Circ.1318/Rev.1 as: 10% at the 10th anniversary of the initial hydrotest; all cylinders before the 20th anniversary, except those tested at 10 years; all cylinders at every 10-year anniversary after that. The “anniversary” runs from each cylinder’s own hydrotest date, not from the ship’s age or certificate cycle. Ships aged 20 to 30 years in May 2021 should have had all cylinders tested once; extensions to test everything together must be requested through the RO.4
- Marshall Islands (Marine Notice 2-011-14, Rev. Aug/2025) allows the first 10-year hydrostatic test of fixed gas cylinders that were date-stamped before delivery to be harmonised with the dry-docking at the second renewal survey, provided the stamp is no more than 12 months before delivery. It applies Circ.1318/Rev.1 to CO2 and a 95% refill threshold to other gas systems.5
- United Kingdom: before Rev.1, the MCA’s instructions to surveyors already required all CO2 cylinders to be tested within 20 years, as the MAIB report notes.6
- Class: DNV’s class guideline CG-0058 repeats the requirement that CO2 lines and nozzles be blown through with dry air or nitrogen.6 Flags such as the Marshall Islands expect specialist work to be done by a service facility acceptable to the ship’s recognised organisation, so ask the service company for its approval certificate from the ship’s class society or flag before the job.5
Key numbers
| What | Value |
|---|---|
| Crew visual inspection | At least every 30 days1 |
| Minimum charge before refilling | 90% of nominal charge1 |
| Weighing interval, passenger ships | 2 years ± 3 months1 |
| Weighing, cargo ships | At each intermediate, periodical or renewal survey1 |
| Internal inspection of control valves | At least every 5 years1 |
| Hydrostatic test at 10 years | At least 10% (50%, then 100% on failure)1 |
| Hydrostatic test at 20 years and every 10 after | 100% of cylinders1 |
| Flexible hose renewal | Maker’s interval, at most 10 years1 |
| Pre-discharge alarm | At least 20 s before release2 |
| Low-pressure tank working pressure | 1.8 to 2.2 N/mm²2 |
| Low-pressure tank filling limit | 95% of volumetric capacity2 |
| CO2 vapour pressure in a cylinder | About 57 bar at 20°C, 74 bar at 31°C6 |
| CO2 concentration causing unconsciousness | Above 10%; survival under 1 min at 17%6 |
| Recorded CO2 system accidents 1975-2000 | 72 deaths, 145 injuries6 |
What inspectors find
- Leaking cylinder valves and incorrectly set valve screws: on a sister ship of Eddystone, set screws on 124 of 130 cylinder valves had not been torqued to the maker’s specified 11 Nm, and a manifold pressure test discharged a whole bank of 65 cylinders.6
- Systems left in an unsafe state after service, for example pilot lines not reconnected, stop valves open, or operating arms still removed. Circ.1318/Rev.1 paragraph 6.2.3 lists what must be checked before the system is returned to service.1
- Hydrostatic test dates beyond 10 years, or a 20-year test plan that covers only 10% of the cylinders, which is no longer acceptable under Rev.1 and flag notices.4
- In the 2012 Paris and Tokyo MOU fire safety campaign, the test of the automatic pre-discharge alarm failed in 1.4% of inspections, and that failure was treated as a possible ground for detention.7
- CO2 rooms used for storage, missing warning signs at the doors of protected spaces, and release instructions that do not name the space served (all annual inspection items).1
Questions people ask
Can the crew weigh the CO2 cylinders?
The circular does not forbid it, but the onboard maintenance plan must say which work needs trained people, and many flags and class societies expect the two-yearly weighing for the survey to be done or witnessed by an approved service supplier. The crew does the monthly and, if the plan says so, the annual checks.
We found one cylinder at 88%. Must we refill only that one?
Yes. Circ.1318/Rev.1 says cylinders below 90% of nominal charge should be refilled. Find out why it lost charge: a slow leak at the valve can be the start of the kind of release seen on Eddystone and Red Eagle.
Our ship is 22 years old and only 20% of the cylinders have ever been tested. What now?
Under Circ.1318/Rev.1 all cylinders should have been tested before the 20-year anniversary. Plan the remaining tests as soon as possible and contact your flag through your RO; the Bahamas, for example, sets out how it handles such ships and extension requests.
Do we need to discharge CO2 to test the system?
No. The tests use dry air or nitrogen through isolated piping and full working pressure through disconnected or blanked pilot lines. A real discharge into a space is not part of routine maintenance.
Check yourself
6 questions. Pass with 60% to mark the lesson as done.
Sources
- 1 IMO MSC.1/Circ.1318/Rev.1, Revised guidelines for the maintenance and inspections of fixed carbon dioxide fire-extinguishing systems (25 May 2021, supersedes MSC.1/Circ.1318 of 2009), annex, sections 1 to 7 and appendix (example service charts). https://www.mardep.gov.hk/en/msnote/pdf/msin2136anx1.pdf
- 2 International Code for Fire Safety Systems (FSS Code), chapter 5 as replaced by resolution MSC.206(81) and amended by resolution MSC.339(91), paragraphs 2.1.1.3, 2.1.2.4, 2.1.2.5, 2.1.3.2, 2.2.1.6, 2.2.2 and 2.2.4; with IACS Unified Interpretation SC252. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.206(81).pdf
- 3 IMO Resolution MSC.256(84), Amendments to SOLAS 1974 (adopted 16 May 2008, in force 1 January 2010), annex, new regulation II-2/10.4.1.5. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.256(84).pdf
- 4 Bahamas Maritime Authority, Marine Notice 79, Firefighting Equipment, version 2.2 (20 November 2025), section 6. https://www.classnk.or.jp/hp/pdf/activities/statutory/solas/solas_treaty/fire_protection/circular/ci_fi_bahama.pdf
- 5 Republic of the Marshall Islands Maritime Administrator, Marine Notice 2-011-14 (Rev. Aug/2025), sections 5 and 6. https://www.register-iri.com/wp-content/uploads/MN-2-011-14.pdf
- 6 UK Marine Accident Investigation Branch, Report No 16/2018, Unintentional release of carbon dioxide from fixed fire-extinguishing systems on ro-ro vessels Eddystone and Red Eagle (September 2018), sections 1.2, 4.2, 4.3, 5.2 and 5.7. https://assets.publishing.service.gov.uk/media/5b979251e5274a13a00c8f21/2018_-_16_-_Eddystone_and_Red_Eagle.pdf
- 7 Tokyo MOU, Report of the 2012 Concentrated Inspection Campaign (CIC) on Fire Safety Systems (November 2013), section 3, question 4. https://www.tokyo-mou.org/wp/wp-content/uploads/Final-Tokyo-MOU-Report-FSS-CIC-public-web-site.pdf
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