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IMDG Class 2: Gases

IMDG Class 2 of the International Maritime Dangerous Goods Code covers all dangerous goods that are gaseous at ambient conditions or are kept in gaseous form by compression, liquefaction or refrigeration. The class is divided into three sub-classes: Division 2.1 Flammable Gases (LPG, hydrogen, methane, ethylene, propylene, vinyl chloride, ethylene oxide), Division 2.2 Non-flammable Non-toxic Gases (nitrogen, argon, carbon dioxide, helium, oxygen, refrigerant gases R-134a, R-404A), and Division 2.3 Toxic Gases (chlorine, ammonia, hydrogen sulphide, sulphur dioxide, phosgene, methyl bromide). The sub-class drives the stowage rules under IMDG Code Chapter 7, the EmS Emergency Schedules (F-D for flammable gas fires, F-C for non-flammable, S-U for toxic gas spillages), the interaction with the IGC Code (gas carriers) and the IGF Code (gas-fuelled ships), the use of pressure-rated tank containers (UN T50 for liquefied gases, T75 for refrigerated liquefied gases), and the principal containment standards for each gas type. Division 2.3 Toxic Gases receive the strictest treatment because of the toxic-by-inhalation (TIH/PIH) hazard zone: a leak from a chlorine tank container can produce a downwind hazard zone several kilometres long, requiring evacuation of the surrounding harbour. ShipCalculators.com hosts the principal computational tools that support Class 2 cargo handling: the IMDG segregation calculator, the IMDG EmS lookup, the IMDG packing group calculator, the container IMDG class lookup, the IMDG tank container calculator, the IMDG limited quantity calculator and the IMO IMDG general calculator. A full listing of related computational tools is available in the calculator catalogue.

Contents

Background

What Class 2 covers

The IMDG Code defines a Class 2 substance as one that is in any of the following physical states at 50°C or below 20°C reference conditions:

  • Compressed gas: a gas that is entirely gaseous at -50°C and that is packaged under pressure for transport (oxygen, nitrogen, argon, helium).
  • Liquefied gas: a gas that is partially liquid at the transport temperature, with the vapour above the liquid at the saturation pressure (LPG, anhydrous ammonia, chlorine).
  • Refrigerated liquefied gas: a gas that is liquid at low temperature in an insulated container (LNG, liquid nitrogen, liquid oxygen).
  • Dissolved gas: a gas dissolved in a solvent under pressure (acetylene dissolved in acetone within a porous mass).

The class also covers mixtures of two or more gases and gases used as aerosol propellants (excluded from the main class but regulated under UN 1950 Aerosols).

Why Class 2 has three sub-classes

The three sub-classes correspond to the three principal hazard mechanisms for gases:

  • Division 2.1 Flammable Gases: fire and explosion hazard. Propagation is rapid because gas mixes freely with air, finds an ignition source and burns instantly. Vapour cloud explosions (VCE) and boiling liquid expanding vapour explosions (BLEVE) are characteristic large-scale outcomes.
  • Division 2.2 Non-flammable Non-toxic Gases: physical hazards (high pressure, asphyxiation by displacement of air, cold burns from refrigerated gases) but no chemical reactivity or toxicity. The main risks are mechanical (cylinder rupture) and physiological (oxygen deficiency in confined spaces).
  • Division 2.3 Toxic Gases: lethal at concentrations measured in parts per million. A single tank container leak can produce a kilometre-scale downwind hazard zone requiring immediate evacuation. Toxic-by-inhalation (TIH) gases are the most operationally constrained.

The three sub-classes therefore have different segregation distances, different emergency response procedures, different documentation requirements and different routeing constraints.

Distinction from IGC/IGF code regimes

The IMDG Code regulates packaged Class 2 cargo carried on general cargo, container or specialised vessels. Two adjacent regimes regulate bulk gas carriage:

  • The IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) governs purpose-built gas carriers (LNG carriers, LPG carriers, ethylene carriers, ammonia carriers). Class 2 cargo carried in IGC tanks is not subject to the IMDG Code.
  • The IGF Code (International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels) governs gas-fuelled ships (LNG-fuelled bulk carriers, LPG-fuelled tankers, methanol-fuelled containerships). Bunker tanks of these vessels carry Class 2 cargo but as fuel, not cargo, and IGF rules apply.

A vessel may simultaneously be carrying:

  • Class 2 packaged goods in containers on deck (IMDG Code).
  • Refrigerated cargo in IGC-classified tanks (IGC Code).
  • Bunker fuel in IGF-classified tanks (IGF Code).

The three regimes interact at the segregation level (a package of Class 2.3 toxic gas must be stowed at a specific distance from the IGC-tank manifold even if the IGC tank is empty).

The three sub-classes in detail

Division 2.1: Flammable Gases

A gas is classified as Division 2.1 if it has any of:

  • A flammable range with air at 20°C and 101.3 kPa of at least 12 percentage points.
  • A lower flammable limit (LFL) of 13% or less by volume.

Common Division 2.1 entries:

  • UN 1011 Butane.
  • UN 1049 Hydrogen, compressed.
  • UN 1075 Petroleum gases, liquefied (LPG).
  • UN 1965 Hydrocarbon gas mixture, liquefied, n.o.s.
  • UN 1971 Methane, compressed (or natural gas, compressed).
  • UN 1972 Methane, refrigerated liquid (or LNG).
  • UN 1978 Propane.
  • UN 2034 Hydrogen and methane mixture, compressed.

Stowage requires:

  • Away from sources of heat (engine room bulkheads, exhaust uptakes, accommodation portholes).
  • Protected from direct sunlight (covered with deck tarpaulins or stowed in shaded positions, particularly for LPG and propylene where vapour pressure is sensitive to temperature).
  • Vertical orientation for many entries (cylinders must stand upright with the valve at the top).
  • Pressure relief valve (PRV) functionality verified before loading (corroded or jammed PRVs cannot vent overpressure and risk cylinder rupture).

Division 2.2: Non-flammable Non-toxic Gases

A gas is Division 2.2 if it is neither flammable (per the 2.1 criteria) nor toxic (per the 2.3 criteria). Sub-categories include:

  • Asphyxiant gases: nitrogen, argon, helium, carbon dioxide. Displace breathable air in confined spaces; cause unconsciousness and death within minutes at high concentrations.
  • Oxidising gases: oxygen (UN 1072) and oxygen-rich mixtures. Promote combustion of organic materials but are not themselves flammable.
  • Refrigerant gases: hydrofluorocarbons (R-134a UN 3159, R-404A UN 3337), some hydrocarbons (R-290 propane is 2.1).
  • Refrigerated liquefied gases: liquid nitrogen UN 1977, liquid argon UN 1951, liquid oxygen UN 1073 (also oxidising).

Stowage requirements are the lightest of the three sub-classes, but cylinders must still be:

  • Secured against movement.
  • Protected from impact.
  • Stowed on deck or in well-ventilated holds.
  • Marked with the appropriate Division 2.2 placard (green diamond with cylinder symbol).

The principal operational risk is asphyxiation in confined spaces (cargo holds, cofferdams, tank entries) where leaked gas displaces breathable air. Crew entering such spaces must use oxygen monitors (not just gas detectors, which look for combustible or toxic gas, not for oxygen depletion).

Division 2.3: Toxic Gases

A gas is Division 2.3 if it has any of:

  • An LC50 (median lethal concentration) for acute toxicity by inhalation of 5,000 ml/m3 or less.
  • Known to be so toxic to humans as to pose a hazard to health during transport.

Common Division 2.3 entries:

  • UN 1005 Ammonia, anhydrous.
  • UN 1017 Chlorine.
  • UN 1040 Ethylene oxide (also 2.1 - dual classification, primary 2.3).
  • UN 1052 Hydrogen fluoride, anhydrous.
  • UN 1053 Hydrogen sulphide.
  • UN 1062 Methyl bromide.
  • UN 1067 Dinitrogen tetroxide.
  • UN 1076 Phosgene.
  • UN 1079 Sulphur dioxide.
  • UN 2188 Arsine.
  • UN 2199 Phosphine.

Division 2.3 receives the strictest treatment of any IMDG sub-class:

  • Toxic-by-inhalation (TIH) zone classification (Zone A through Zone D, decreasing severity).
  • Mandatory pre-arrival notification to all receiving ports, often 96 hours in advance.
  • Designated anchorage away from population centres.
  • Pilotage and tug escort for harbour transit.
  • Wind-direction stowage (tank container valve oriented away from accommodation).
  • Crew evacuation drills specific to the cargo and the prevailing weather.
  • Strict packing and tank container standards (UN T50 with specific seal materials compatible with the cargo).

Some Division 2.3 cargoes have national or regional restrictions: chlorine and phosgene are routinely refused entry to certain ports; methyl bromide is restricted under the Montreal Protocol for ozone-depletion reasons.

Stowage and segregation

Tank containers

Most Division 2.1 and 2.3 liquefied gases are shipped in UN T50 tank containers: pressure-rated portable tanks designed for liquefied gases at the saturation pressure of the contents. T50 specifications include:

  • Test pressure 22 bar minimum (or higher per the UN entry).
  • Bottom outlet not permitted for many entries (top discharge only via dip pipe).
  • Pressure relief valve plus emergency relief valve.
  • Compatible seal materials (NBR, EPDM, PTFE depending on cargo).
  • Insulation for some entries (LPG service tanks for solar-load reduction).

UN T75 tank containers are vacuum-insulated cryogenic tanks for refrigerated liquefied gases (LNG, liquid nitrogen, liquid oxygen, liquid argon). T75 has lower pressure rating but heavy insulation to maintain the low temperature without active refrigeration during transit.

The IMDG tank container calculator returns the required tank instruction (T-code) for any UN entry, with the test pressure and special provisions.

Cylinders and packaged forms

Smaller quantities of Class 2 ship in pressure cylinders (steel or aluminium, with shoulder valve and pressure-relief device). Cylinders are grouped on pallets or in protective racks and stowed in containers or on deck. The IMDG Code specifies:

  • Cylinder construction standards (DOT, ISO 11118).
  • Maximum filling ratios.
  • Inspection intervals (10 years typical for general industrial gases).
  • Marking (gas service code, test pressure, manufacture date).

For Division 2.3 toxic gases in cylinders, the cylinder valve outlet is fitted with a specific connector (CGA standard) to prevent accidental cross-connection with incompatible gases.

Segregation rules

The IMDG Code Chapter 7.2 segregation table requires:

  • From Class 1 (Explosives): ‘separated from’ minimum.
  • From Class 3 (Flammable Liquids): ‘separated from’ for 2.1; ‘separated longitudinally by an intervening complete compartment or hold from’ for 2.3 (toxic gas leak into a flammable liquid hold = catastrophic combination).
  • From Class 4 (Flammable Solids): ‘separated from’ for most combinations; ‘separated by complete compartment’ for 2.3.
  • From Class 5.1 (Oxidising Substances): ‘separated from’ for 2.1 and 2.2; ‘separated longitudinally by an intervening complete compartment’ for some specific 2.3 entries.
  • From Class 6.1 (Toxic Substances): same class category for 2.3, ’no requirement’ for 2.1 and 2.2.
  • From Class 8 (Corrosives): ‘separated from’ for 2.3 (corrosion of cylinder valves is a leak risk); ’no requirement’ for 2.1 and 2.2.
  • From Foodstuffs: ’no requirement’ for most Class 2; ‘separated by complete compartment’ for 2.3.

The IMDG segregation calculator implements the full table for any pair of UN entries.

On-deck vs under-deck

Most Division 2.1 entries can be stowed on deck or under deck in approved cargo spaces. Division 2.3 entries are commonly required to be stowed on deck only, in containers or tanks with the valve oriented to allow easy access for emergency response and the dispersion of any leak directly to atmosphere rather than into the cargo hold ventilation system.

Emergency response

EmS schedules for Class 2

The Emergency Response Procedures for Ships Carrying Dangerous Goods (EmS) assigns each Class 2 UN entry to a Fire schedule and a Spillage schedule:

Fire schedules commonly used:

  • F-A: General fire schedule (some 2.2 entries).
  • F-C: Class 2 non-flammable gas fire (mostly 2.2 entries with combustible packaging).
  • F-D: Class 2 flammable gas fire. Standard procedure: shut off gas source if safe, cool containers with water, do NOT extinguish a flammable gas fire unless the gas source can be isolated (a re-ignition risk produces vapour cloud explosion).
  • F-E: Class 2 with toxic combustion products (some 2.3 entries that combust to chlorine compounds).

Spillage schedules commonly used:

  • S-A: General spillage.
  • S-U: Toxic gas spillage (Division 2.3). Standard procedure: muster crew on the windward side, don breathing apparatus, identify the leak source, attempt isolation only with full PPE and command authorisation, abandon ship if leak cannot be controlled.
  • S-V: Refrigerated liquefied gas spillage. Standard: avoid contact (cold burn risk), allow vapourisation, do not direct water at liquid (causes massive vapour cloud).

The IMDG EmS lookup returns the specific F-x and S-y for any Class 2 UN entry.

TIH zones for Division 2.3

Division 2.3 toxic gases are further classified into toxic-by-inhalation (TIH) zones based on the LC50 (median lethal concentration):

  • Zone A: LC50 ≤ 200 ppm. The most toxic. Examples: phosgene, arsine, hydrogen cyanide.
  • Zone B: 200 < LC50 ≤ 1,000 ppm. Examples: ethylene oxide (2.1+2.3), some organic isocyanates.
  • Zone C: 1,000 < LC50 ≤ 3,000 ppm. Examples: hydrogen sulphide, sulphur dioxide.
  • Zone D: 3,000 < LC50 ≤ 5,000 ppm. Examples: ammonia, methyl bromide.

The TIH zone determines:

  • The separation distance from population centres during incidents (Zone A: several kilometres; Zone D: hundreds of metres).
  • The PPE level required for crew approach to a leaking container (Zone A: SCBA + chemical suit; Zone D: half-mask respirator).
  • The emergency response distance for first responders ashore.

Pre-arrival and routing

Vessels carrying any Division 2.3 cargo must:

  • Pre-notify receiving ports (timeframe varies by port; 24-96 hours typical).
  • Use designated anchorages rather than the main port roadstead.
  • Avoid certain ports entirely if national rules prohibit specific cargoes.
  • Route around sensitive coastal stretches (some pilots refuse to bring chlorine cargoes through narrow channels).
  • Carry specific charts showing emergency response stations along the route.

The vessel’s agent normally coordinates these requirements; failure to comply can result in cargo refusal at the destination port.

Notable casualties

MV Mont-Louis 1984

In August 1984 the Ro-Ro vessel MV Mont-Louis collided with the ferry Olau Britannia off the Belgian coast, holed and sank in 14 metres of water. Cargo included 30 Class 2.3 cylinders of uranium hexafluoride (UN 2978, although Class 7 plus 2.3 - dual). The Belgian, French and German navies recovered all cylinders intact within three weeks; no leak occurred. The incident drove the requirement for stronger securing of Class 2.3 cylinders on Ro-Ro vessels, and for pre-loading risk assessment by the master.

Chlorine tanker incidents (various)

Several chlorine tank container leaks at terminal yards (Singapore 1997, Houston 2002, Mumbai 2012) resulted in evacuations of surrounding port areas. None caused fatalities but each demonstrated the rapid downwind dispersion of chlorine and the difficulty of approach by responders without specialist PPE. Modern chlorine tanks are fitted with emergency shut-off valves triggered by leak detection.

Bohai LPG carrier explosion 1971

The LPG carrier Bohai exploded in dry dock in Japan in 1971, killing 50 dockworkers. The investigation found that LPG residue in a cargo tank had not been fully purged before hot work commenced; a spark ignited the residual flammable mixture causing a vessel-destroying detonation. The disaster drove the gas-free certification procedure now mandatory before any hot work on a tank that has held flammable gas, and the development of the IGC Code’s rigorous purging requirements.

See also

References

  • IMO, International Maritime Dangerous Goods Code, 2022 Edition (IMDG 41-22), International Maritime Organization, 2022.
  • IMO, International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code), International Maritime Organization, current edition.
  • IMO, International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels (IGF Code), International Maritime Organization, current edition.
  • IMO, EmS Guide: Revised Emergency Response Procedures for Ships Carrying Dangerous Goods, International Maritime Organization, current edition.
  • United Nations, Recommendations on the Transport of Dangerous Goods, Model Regulations, 22nd revised edition, United Nations, 2021.
  • ASME, Boiler and Pressure Vessel Code, Section VIII (Pressure Vessels), American Society of Mechanical Engineers, current edition.
  • Society of International Gas Tanker and Terminal Operators (SIGTTO), Liquefied Gas Handling Principles on Ships and in Terminals, 4th edition, 2017.
  • IACS, Recommendation 142: LPG and LNG Cargo Operations, International Association of Classification Societies, 2017.