Start hereRules checked 20 October 2026
Portable meters protect the person about to enter a space. Fixed systems watch spaces nobody is in: the cargo pump-room at the bottom of the ship, the ballast tanks and voids wrapped around the cargo, the hold spaces and interbarrier spaces of a gas carrier. A leak there gives no warning until vapour finds an ignition source or a crew member opens the door.
SOLAS answered these risks in two steps: continuous hydrocarbon monitoring for tanker cargo pump-rooms came with the revised chapter II-2 in force from 1 July 2002 (and was required on older tankers by 1 July 2005 at the latest), and fixed detection in the double hulls of new large oil tankers from 1 January 2012. This lesson sets out what each tanker type must have, how it must alarm, and how the crew and service technicians keep it honest.
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In this lesson
- Which rule does each requirement come from?
- In plain words
- In detail
- What the rules say
- Flag and class notes
- Key numbers
- What inspectors find
- Quiz: 5 questions
Try it
Which rule does each requirement come from?
In plain words
Fixed gas detection is a permanent set of sensors or sampling pipes that keeps checking a space and alarms when gas appears. It is required where leaks are likely and nobody is watching:
- Tanker cargo pump-rooms: a continuous hydrocarbon monitor alarms at 10% of the lower flammable limit (LFL), long before the mixture can burn, in the pump-room, the engine and cargo control rooms and on the bridge.1
- Double hulls of large oil tankers built since 2012: pipes from every ballast tank and void next to the cargo lead to an analyser that samples each at least every 30 minutes and alarms at 30% LFL.23
- Gas carriers: detection in cargo machinery spaces, hold and interbarrier spaces, airlocks and more, alarming at 30% LFL.4
- Chemical tankers: instruments for the specific vapours of each cargo.5
A fixed system only helps if it works: it must be easy to test and calibrate, and when it fails the crew measure with portable instruments and record the results.3
In detail
Cargo pump-rooms: SOLAS II-2/4.5.10
Added by resolution MSC.99(73) (adopted 5 December 2000, in force 1 July 2002) for tankers constructed on or after that date, regulation II-2/4.5.10.1 requires in cargo pump-rooms:1
| Para | Requirement |
|---|---|
| .1 | Temperature sensing for bulkhead shaft glands, bearings and pump casings of cargo, ballast and stripping pumps driven through the bulkhead, with a continuous audible and visual alarm in the cargo control room or pump control station |
| .2 | Lighting (except emergency lighting) interlocked with ventilation, so ventilation runs when lights are switched on; ventilation failure must not put the lights out |
| .3 | Continuous monitoring of hydrocarbon gas concentration, sampling points or detector heads placed so dangerous leaks are readily detected, and a continuous audible and visual alarm at a pre-set level not higher than 10% LFL in the pump-room, engine control room, cargo control room and navigation bridge |
| .4 | Bilge level monitoring devices with suitably located alarms |
Tankers built before 1 July 2002 are not exempt. Regulation II-2/1.6.7 required them to fit the temperature sensing of .1, the bilge level monitoring of .4 and continuous hydrocarbon monitoring by the first scheduled dry-docking after 1 July 2002, and not later than 1 July 2005. For these older ships the alarm (at not more than 10% LFL) must sound in the pump-room and cargo control room, and monitoring systems already fitted with a set point not above 30% LFL may be accepted.1
MSC/Circ.1037 (28 May 2002) clarifies three points: pump-rooms used solely for ballast transfer need not comply; bilge high-level alarms are acceptable as level monitoring devices; and where pump-room lighting also serves as emergency lighting, the ventilation interlock must not stop it working on loss of main power.6
Double-hull and double-bottom spaces: SOLAS II-2/4.5.7.3 and FSS Code chapter 16
Resolution MSC.291(87) (adopted 21 May 2010, in force 1 January 2012) requires oil tankers of 20,000 tonnes deadweight and above, constructed on or after 1 January 2012, to have a fixed hydrocarbon gas detection system complying with the FSS Code in all ballast tanks and void spaces of double-hull and double-bottom spaces adjacent to cargo tanks, including the forepeak tank and other tanks and spaces under the bulkhead deck adjacent to cargo tanks. Tankers with constant operative inerting of these spaces are exempt, and pump-rooms covered by 4.5.10 are not included.2 MSC.1/Circ.1527 explains that “adjacent” includes spaces in corner-to-corner (cruciform) contact with cargo tanks, and that slop tanks count as cargo tanks unless used only for oily water.8
The engineering rules are in FSS Code chapter 16 (resolution MSC.292(87)):3
| Item | FSS ch. 16 requirement |
|---|---|
| Layout | Central gas measurement and analysis unit, sampling pipes to every protected space; may be combined with the pump-room system (2.1.2, 2.1.3, 1.2) |
| Sampling lines | No common lines except for each pair of sampling points; conductive if non-metallic; not aluminium (2.2.1.1, 2.2.1.2) |
| Sampling points | Normally two per space: upper point not lower than 1 m from the tank top, lower point above the bottom girder height but at least 0.5 m from the bottom, closable if clogged (2.2.1.3) |
| Exceptions | One sampling location per tank may be allowed below 50,000 dwt; no upper point needed for double-bottom ballast tanks, tanks not intended to be partly filled, and voids (2.2.1.4, 2.2.1.5) |
| Clogging | Compressed air flushing after switching from ballast to loaded mode; alarm for clogged lines (2.2.1.6) |
| Analyser | In a safe space; flame arresters; manual isolating valve on the gas-safe side of each line; equipment in a gas-tight steel cabinet with its own sampling point, shutting down automatically above 30% LFL inside (2.2.2.1) |
| Sampling rate | Each line of each space sampled sequentially at intervals not exceeding 30 minutes (2.2.3.1) |
| Alarm | Audible and visual in the cargo control room, on the bridge and at the analyser at not more than 30% LFL (2.2.3.3) |
| Backup | Means for portable instruments if the system is out of order or for calibration; procedures to monitor with portable instruments and record results while it is out of order (2.2.3.2) |
| Testing | Designed to be readily tested and calibrated (2.2.3.4) |
All tankers, of any size and age, must also have portable instruments for double-hull spaces and, where hoses cannot reach reliably, permanent sampling lines (II-2/4.5.7.2; lesson 2).2
Gas carriers: IGC Code section 13.6
The International Code for the construction and equipment of ships carrying liquefied gases in bulk (IGC Code), as rewritten by resolution MSC.370(93) for gas carriers constructed on or after 1 July 2016, requires a permanently installed system of gas detection with audible and visual alarms in, among others, all enclosed cargo and cargo machinery spaces with gas piping or equipment, other spaces where cargo vapour may gather (including interbarrier spaces and hold spaces of independent tanks other than type C), airlocks, ventilation hoods and gas ducts, inert gas generator supply headers and cargo handling motor rooms (13.6.2).4
| Topic | IGC Code 2016 |
|---|---|
| Type | Continuous detection, capable of immediate response; sampling type accepted where not used for safety shutdowns (13.6.7) |
| Sampling systems | Each sampling head analysed sequentially at intervals not exceeding 30 minutes; individual lines (13.6.8) |
| Alarm locations | Navigation bridge, the control station that records gas levels, and the detector readout (13.6.13) |
| Alarm levels | 30% LFL; in the other spaces of 13.6.2, the gas-fuel safety functions of chapter 16 before 60% LFL; gas-fuelled engine crankcases alarm before 100% LFL (13.6.15, 13.6.17) |
| Inerted spaces | 0 to 100% by volume measurement (13.6.14) |
| Oxygen deficiency | Monitoring in cargo machinery and hold spaces where required for non-flammable products, and in spaces with nitrogen or inert gas generators or nitrogen refrigeration cycles (13.6.4) |
| Testing | Readily testable; tested and calibrated at regular intervals; test equipment on board with permanent connections (13.6.18) |
Gas carriers built before 1 July 2016 follow the earlier edition of the IGC Code that applied when they were built; check the ship’s Certificate of Fitness.
Chemical tankers: IBC Code section 13.2
The IBC Code relies on the cargo’s own entry in chapter 17, column k, to say whether toxic (T) or flammable (F) vapour detection is needed.5
- At least two instruments designed and calibrated for the specific vapours; two separate sets if one cannot cover both toxic and flammable concentrations (13.2.1).
- Fixed or portable; if fixed, at least one portable as well (13.2.2).
- If no toxic-vapour detector exists for a product, the Administration may exempt the ship with an entry on the International Certificate of Fitness, which must also draw attention to extra breathing air (14.2.4 and 16.4.2.2) (13.2.3).
Keeping a fixed system in service
The rules ask for systems that can be tested and calibrated; how often is set by the maker’s manual, the planned maintenance system and class. A sound routine looks like this (practice):
- Daily and on watch: check the panel for faults, flow alarms, clogged lines and inhibited points. A point that is inhibited or a line closed for flushing is a space no longer monitored.
- Regularly, to the maker’s schedule: test gas applied at each detector head or sampling point, confirming the reading and the alarm at every required location (pump-room, ECR, CCR, bridge for 4.5.10); check the sample flow on each line and the response time.
- Calibration: to the maker’s interval with certified test gas, by trained crew or a service technician; keep the certificate or report.
- When it fails: switch to portable monitoring and record the results, as FSS chapter 16 requires for double-hull systems.3
What the rules say
- SOLAS regulation II-2/4.5.10.1.3 requires tankers to have continuous hydrocarbon gas monitoring in cargo pump-rooms, with an automatic continuous audible and visual alarm at a pre-set level “not higher than 10% of the lower flammable limit” in the pump-room, engine control room, cargo control room and navigation bridge.1
- SOLAS regulation II-2/4.5.10.1.1, .2 and .4 require temperature sensing on pumps driven through the bulkhead, lighting interlocked with ventilation, and bilge level monitoring with alarms in cargo pump-rooms.1
- MSC/Circ.1037 says pump-rooms used solely for ballast transfer need not comply with II-2/4.5.10, and that bilge high-level alarms are acceptable as level monitoring devices.6
- SOLAS regulation II-2/4.5.7.3.1 requires oil tankers of 20,000 dwt and above constructed on or after 1 January 2012 to have fixed hydrocarbon gas detection complying with the FSS Code in ballast tanks and voids of double-hull and double-bottom spaces adjacent to cargo tanks, including the forepeak tank.2
- FSS Code chapter 16 paragraph 2.2.3.1 requires each sampling line to be sampled and analysed sequentially at intervals not exceeding 30 minutes, and paragraph 2.2.3.3 requires alarms at not more than 30% LFL.3
- FSS Code chapter 16 paragraph 2.2.3.2 requires procedures for monitoring with portable instruments, and recording the results, when the fixed system is out of order.3
- IGC Code paragraph 13.6.15 requires gas detection alarms at the equivalent of 30% LFL, and 13.6.18 requires testing and calibration at regular intervals with equipment carried on board.4
- IBC Code paragraph 13.2.1 requires at least two instruments designed and calibrated for the specific vapours of toxic or flammable cargoes, and 13.2.2 requires at least one portable instrument where a fixed system is installed.5
Flag and class notes
- MSC.1/Circ.1527 (6 June 2016), an IMO unified interpretation for flags and class societies to apply, defines which spaces count as “adjacent” for II-2/4.5.7.3: it includes ballast pump-rooms, bow thruster rooms and fresh water tanks under the bulkhead deck, spaces in cruciform contact with cargo tanks, and slop tanks other than oily-water-only ones; fuel oil tanks are excluded.8
- IACS Unified Interpretation SC272 (December 2015) ties the same list of double-hull spaces to the inert gas connections of SOLAS II-2/4.5.5.1.4, for ships contracted on or after 1 January 2017.9
- The Marshall Islands Marine Notice 7-041-1 section 4 reminds crews that pump-rooms may only be entered by trained personnel with suitable breathing apparatus and a lifeline when there are cargo leaks into the bilges.7
- The approved plans and the maker’s manual fix the sampling points and alarm settings for your ship; ask your class society how it checks the system at surveys.
Key numbers
| What | Value |
|---|---|
| Pump-room HC alarm, maximum set point | 10% LFL1 |
| Pump-room rules: new tankers / older tankers | built on or after 1 July 2002 / fitted by 1 July 2005 at the latest1 |
| Double-hull fixed detection: oil tankers | ≥ 20,000 dwt, built on or after 1 Jan 20122 |
| Double-hull sampling interval, maximum | 30 min3 |
| Double-hull alarm, maximum | 30% LFL3 |
| Upper sampling point | not lower than 1 m from tank top3 |
| Lower sampling point | at least 0.5 m from the bottom3 |
| IGC alarm / safety functions | 30% LFL / before 60% LFL4 |
| IBC vapour detection instruments | at least 25 |
What inspectors find
- Points inhibited, lines closed or analysers switched off “temporarily” after a fault, with no portable monitoring recorded, which FSS Code chapter 16 2.2.3.2 requires when the system is out of order.3
- Alarms that sound at the panel but not at all the places the rule names: for 4.5.10, pump-room, engine control room, cargo control room and bridge.1
- Pump-room lighting that can be switched on without the ventilation running, contrary to II-2/4.5.10.1.2.1
- Chemical tankers carrying a cargo for which no on-board instrument can detect the toxic vapour and no Certificate of Fitness entry exists, contrary to IBC Code 13.2.3.5
- Gas detection with no record of testing and calibration at regular intervals, or no test equipment on board, contrary to IGC Code 13.6.18.4
FAQ
Does a ballast pump-room need the 4.5.10 gas monitoring?
Not if it is used solely for ballast transfer, according to MSC/Circ.1037. It is still an enclosed space for entry purposes, and on many tankers it is an “adjacent” space for double-hull gas detection.
Can the pump-room and double-hull detection share one analyser?
Yes. FSS Code chapter 16 allows a combined system if each double-hull space is still sampled at least every 30 minutes and the system fully complies with the SOLAS requirements.
Does a fixed system replace testing before entry?
No. MSC.581(110) still asks for testing with portable or fixed equipment at several levels before entry and regularly afterwards. Fixed sampling lines can help reach remote parts of a space, but the entry decision rests on a full pre-entry test.
Quiz5 questions
Sources
- 1
IMO Resolution MSC.99(73), Amendments to SOLAS (adopted 5 December 2000, in force 1 July 2002), Annex, revised chapter II-2, regulation 1 paragraph 6.7 and regulation 4 paragraph 5.10. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.99(73).pdf
- 2
IMO Resolution MSC.291(87), Amendments to SOLAS (adopted 21 May 2010, in force 1 January 2012), Annex, regulation II-2/4 paragraph 5.7. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.291(87).pdf
- 3
IMO Resolution MSC.292(87), Amendments to the International Code for Fire Safety Systems (adopted 21 May 2010), Annex, new chapter 16, Fixed hydrocarbon gas detection systems, paragraphs 1.2 to 2.2.3.4. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.292(87).pdf
- 4
IMO Resolution MSC.370(93), Amendments to the IGC Code (adopted 22 May 2014; applies to gas carriers constructed on or after 1 July 2016), Annex, section 13.6. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.370(93).pdf
- 5
IMO Resolution MSC.176(79), 2004 amendments to the IBC Code (adopted 10 December 2004, in force 1 January 2007), Annex, section 13.2. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.176(79).pdf
- 6
IMO MSC/Circ.1037, Unified interpretations of the revised SOLAS chapter II-2 (28 May 2002), Annex, regulation 4.5.10. Copy published by Traficom: https://www.traficom.fi/files/media/regulation/22659-MSC_1-Circ_1037_-_tulkinta_revised_SOLAS_chapter_II-2.pdf
- 7
Republic of the Marshall Islands Maritime Administrator, Marine Notice 7-041-1, Entering enclosed spaces: safety precautions (Rev. Jan/2026), section 4. https://www.register-iri.com/wp-content/uploads/MN-7-041-1.pdf
- 8
IMO MSC.1/Circ.1527, Unified interpretations of SOLAS chapter II-2 (6 June 2016), Annex, regulation II-2/4.5.7.3.1, as published by the Marine Department of Hong Kong. https://www.mardep.gov.hk/en/msnote/pdf/msin1660anx1.pdf
- 9
IACS Unified Interpretation SC272, Inert gas supply to double-hull spaces (SOLAS II-2/4.5.5.1), December 2015, copy published by Türk Loydu. https://www.turkloydu.org/pdf-files/iacs-karar-ve-csr-degisimleri/iacs-es-yorumlari/UI_SC272.pdf
Rules checked 20 October 2026. VSL Safety is not an approved training provider and doesn't issue certificates. These lessons don't replace STCW training, or the manufacturer's training and certification that service technicians need.
