IMDG Class 3: Flammable Liquids

IMDG Class 3 of the International Maritime Dangerous Goods Code covers liquids and liquid mixtures with a closed-cup flash point of not more than 60°C (or higher when transported at temperatures at or above the flash point). The class is divided into three packing groups by the combination of flash point and initial boiling point: Packing Group I (very dangerous, flash point below 23°C with initial boiling point at or below 35°C), Packing Group II (medium, flash point below 23°C with initial boiling point above 35°C), and Packing Group III (low, flash point at or above 23°C up to 60°C). The class includes most petroleum products (gasoline, kerosene, jet fuel, diesel, naphtha, fuel oil at flash point ≤60°C), most industrial solvents (acetone, methanol, ethanol, toluene, xylene, isopropanol), paints and adhesives, alcoholic beverages above 24% alcohol by volume, and many agrochemicals dissolved in flammable carriers. Stowage and segregation under IMDG Code Chapter 7 is built around the principal hazard of vapour ignition at room temperature: a Class 3 PG I liquid leaking from a drum in a hold can fill the hold with flammable vapour within minutes, requiring immediate ventilation or evacuation. The standard emergency response is the EmS F-E (fire schedule) and S-E (spillage schedule). Class 3 cargoes also frequently carry the marine pollutant designation, attracting MARPOL Annex III obligations alongside IMDG. ShipCalculators.com hosts the principal computational tools that support Class 3 cargo handling: the IMDG segregation calculator, the IMDG packing group calculator, the IMDG packing group from flash point calculator, the IMDG EmS lookup, the IMDG limited quantity calculator, the container IMDG class lookup, the IMDG tank container calculator and the IMO IMDG general calculator. A full listing of related computational tools is available in the calculator catalogue.

Contents

Background

The flash point classification

A liquid is classified as Class 3 if its closed-cup flash point is at or below 60°C. Flash point is the lowest temperature at which the vapour above the liquid forms an ignitable mixture in air; below the flash point the vapour concentration is below the lower flammable limit and ignition cannot sustain itself. Above the flash point a single spark, hot surface or static discharge can ignite the vapour and propagate flame along the liquid surface.

Two flash-point measurement methods are used internationally:

Closed-cup flash point (ASTM D93 Pensky-Martens, ASTM D56 Tag): the standard for Class 3 classification under the IMDG Code. The cup is sealed and a test flame is introduced through a shutter; the closed system gives a more conservative (lower) value.
Open-cup flash point (ASTM D92 Cleveland): the cup is open to ambient air; gives a slightly higher value. Used for some grade-defining specifications but not for IMDG Class 3 boundary.

The 60°C threshold separates Class 3 from non-classified flammable liquids. A liquid with closed-cup flash point above 60°C (e.g., heavy fuel oil HFO at flash point 80-120°C) is not Class 3 and ships outside the IMDG framework as MARPOL Annex I cargo.

The three packing groups

Within Class 3 the packing group determines the rigour of packaging, marking and stowage. Packing group is defined by the combination of flash point and initial boiling point (IBP):

Packing Group I (PG I): flash point any value AND initial boiling point ≤ 35°C. The most volatile category. Examples: diethyl ether, pentane, low-boiling petroleum spirit. Vapour pressure at room temperature is so high that any leak releases substantial vapour quickly.
Packing Group II (PG II): flash point < 23°C AND initial boiling point > 35°C. Examples: gasoline, acetone, ethanol, toluene, methanol. The most commonly shipped category by volume.
Packing Group III (PG III): flash point ≥ 23°C AND ≤ 60°C. Examples: kerosene, diesel, jet fuel A-1, white spirit, paraffin. Lower vapour pressure but still ignitable from external heat sources.

The IMDG packing group from flash point calculator returns the correct PG given the flash point and initial boiling point of a sample.

Distinction from other classes

A liquid that is flammable AND toxic is normally Class 3 (primary) plus Class 6.1 (subsidiary) with the more rigorous of the two stowage requirements. Examples: methanol (Class 3 PG II + Class 6.1 PG II by ingestion), aniline (Class 6.1 PG II + flammable subsidiary).

A liquid that is flammable AND corrosive is Class 8 (primary) plus Class 3 (subsidiary) for the inorganic acids that are also flammable. Most strong acids are not flammable so this combination is uncommon.

A liquid that is flammable AND would otherwise be a marine pollutant carries an additional ‘marine pollutant’ designation (a fish symbol on the placard) and triggers MARPOL Annex III pollution-response procedures alongside the Class 3 fire-response procedures.

Bulk vs packaged

The IMDG Code regulates packaged Class 3 cargo (in drums, IBCs, freight containers, tank containers). Bulk Class 3 cargo is regulated under separate regimes:

MARPOL Annex I: bulk petroleum products carried by tanker (gasoline, gasoil, kerosene, jet fuel, naphtha, condensate). Most ocean-going petroleum trade is bulk under Annex I, not packaged under IMDG.
MARPOL Annex II / IBC Code: bulk noxious liquid substances carried by chemical tanker (methanol, ethanol, MTBE, styrene). The IBC Code sets construction and equipment requirements for the tanker; cargo handling is per the cargo-specific section of the IBC Code.

Packaged Class 3 shipments tend to be:

Specialty grades not justifying a full tanker.
Small quantities for industrial customers.
Hazardous samples, additives or precursors.
Drum or IBC consolidations on container vessels.

Stowage and segregation

Standard stowage requirements

Most Class 3 cargo can be stowed on deck or under deck in approved cargo spaces, subject to:

Away from sources of ignition (engine room bulkheads, exhaust uptakes, electrical equipment, smoking areas).
Protected from direct heating (sunlight on deck for PG I; steam pipes; hot work in adjacent spaces).
Adequate ventilation for under-deck stowage to prevent vapour build-up.
Drip trays for drum stowage to contain minor leaks.
Anti-static earthing for tank container hose connections during loading and discharge.

Bulk lashing standards apply to drum cargo on Ro-Ro vessels and on container vessel decks.

Segregation rules

Class 3 has a moderate segregation profile:

From Class 1 (Explosives): ‘separated longitudinally by an intervening complete compartment or hold from’.
From Class 2.1 (Flammable Gases): ‘separated from’. The combination is incendive but the segregation is at the moderate level because both classes have similar fire response.
From Class 4.1 (Flammable Solids), 4.2 (Spontaneously Combustible), 4.3 (Water-reactive): ‘separated from’. 4.3 in particular requires careful distance because water used to fight Class 4.3 fires would react with the substance to release flammable gas.
From Class 5.1 (Oxidising Substances): ‘separated by complete compartment or hold from’ for most combinations. Mixing flammables with oxidisers produces auto-ignition hazards.
From Class 5.2 (Organic Peroxides): ‘separated longitudinally by an intervening complete compartment or hold from’. 5.2 substances self-decompose exothermically; co-stowing flammables converts the heat release into a vapour cloud explosion.
From Class 6.1 (Toxic): ‘separated from’ for most combinations. Stricter for Class 3 + 6.1 dual-classified cargo where the same drum is both flammable and toxic.
From Class 7 (Radioactive): ‘separated from’ for transport indices below threshold.
From Class 8 (Corrosives): ‘separated from’ for most combinations.

The IMDG segregation calculator implements the full Chapter 7.2 table for any pair of UN entries.

Tank containers

Most Class 3 PG I/II liquids in tank containers use UN T11 portable tanks (general flammable liquid service, 4 bar test pressure, bottom outlet allowed). PG III may use the lighter T7 (1.75 bar test pressure) or the same T11 depending on the cargo specification.

The IMDG tank container calculator returns the required T-code for any UN entry.

Tank container loading at terminals follows the closed-loop loading procedure to minimise vapour release: a vapour-recovery line is connected from the tank to the terminal vapour-treatment unit before the cargo line is opened, so that displaced vapour goes to recovery rather than to atmosphere. Open loading (vapour to atmosphere) is permitted for PG III and small quantities but is increasingly prohibited at modern terminals.

Emergency response

EmS schedules

Most Class 3 entries are assigned:

F-E: standard fire schedule for flammable liquids. Foam is the principal extinguishing agent; water cools containers but does not extinguish the burning liquid (flammable liquid floats on water and continues to burn). Boundary cooling protects adjacent stowage.
S-E: standard spillage schedule. Contain with absorbent (sand, vermiculite, proprietary spill kits), do not flush to overboard or to the engine room bilge, recover into salvage drums for disposal ashore. Use intrinsically safe pumps if any pumping is required.

A few Class 3 entries with secondary hazards have other schedules:

F-D: with combustible packaging additional fire load.
S-D: with corrosive secondary hazard.

The IMDG EmS lookup returns the F-x and S-y for any Class 3 UN entry.

Vapour cloud and BLEVE

The principal Class 3 fire scenario is a vapour cloud explosion (VCE) or, for storage tanks, a boiling liquid expanding vapour explosion (BLEVE):

VCE: a leak releases vapour that mixes with air to form a flammable cloud. An ignition source (spark, hot surface, static) triggers a deflagration that propagates rapidly through the cloud, producing overpressure and thermal radiation. Distance to the ignition source determines whether the cloud reaches the LFL before dispersing.
BLEVE: a tank of flammable liquid is heated by an external fire. Internal pressure rises until the tank ruptures; the liquid flashes to vapour, the vapour ignites, and a fireball forms. BLEVE is the worst-case outcome for tank containers exposed to a deck or hold fire.

The fire-response procedure is therefore to boundary-cool any Class 3 tank containers not already involved, to prevent BLEVE escalation, and to fight the original fire with foam.

Notable casualties

MV Achille Lauro 1994

The cruise ship Achille Lauro caught fire in November 1994 off the coast of Somalia and sank. The fire originated in the engine room, but spread was facilitated by Class 3 paint stores and adhesive consumables stowed near the engine room bulkhead. The vessel was evacuated successfully and no passengers died, but the casualty drove the revised SOLAS Chapter II-2 (fire protection) requirements for stricter separation between Class 3 stores and machinery spaces on passenger vessels.

MV Ever Smart fire 2018

The container vessel Ever Smart suffered a hold fire off Sri Lanka in May 2018 caused by misdeclared Class 3 cargo (an undeclared shipment of charcoal that contained a flammable solvent residue). The fire spread through several adjacent containers and required four days of fire-fighting at sea. The investigation drove industry-wide tightening of the Container Packing Certificate verification and misdeclaration penalties under the IMDG Code Section 5.4.

MV Maersk Honam 2018

The container vessel Maersk Honam suffered a major hold fire in March 2018 in the Arabian Sea attributed to misdeclared dangerous goods including Class 3 substances. Five crew died and large parts of the cargo were destroyed. The aftermath included changes to Maersk’s dangerous goods acceptance procedures, mandatory DG declaration audits on high-risk lanes and the publication of DG misdeclaration loss reports by the IUMI insurance association.

Buncefield depot explosion 2005

While not a maritime casualty, the Buncefield oil storage depot explosion in Hertfordshire, UK, December 2005, demonstrated the worst-case BLEVE / VCE outcome for Class 3 PG II petroleum storage. A gasoline overflow from a tank produced a vapour cloud that detonated, destroying the depot and damaging buildings up to 500 metres away. The incident drove the global revision of storage tank gauging and overfill protection standards adopted in MARPOL and the IGC Code by analogy.

Documentation

Multimodal Dangerous Goods Form (MDGF)

Standard MDGF requirements for Class 3:

UN number and proper shipping name.
Class (3) and packing group (I, II or III).
Flash point in °C (mandatory for Class 3, recorded in the MDGF specifically for masters and emergency responders).
Marine pollutant indicator if applicable.
EmS reference (typically F-E, S-E).
Number and type of packages.
24-hour emergency contact.

Container Packing Certificate

For Class 3 in containers:

Container clean and dry before loading.
Drums on bulkhead-side dunnage to prevent shifting.
Drum rims oriented down where appropriate.
Drip trays under any cargo with seal-leak history.
Marking and placarding correct on all sides.
IMO/CSC plate compliance verified.

Marine pollutant designation

Many Class 3 substances are marine pollutants under MARPOL Annex III by virtue of their environmental persistence and acute aquatic toxicity. The designation triggers:

Additional placard (fish symbol) on packages and on freight containers.
MARPOL Annex III pollution-response procedures alongside the IMDG fire response.
Possible reporting under MARPOL Article 8 if a release occurs at sea.

References

IMO, International Maritime Dangerous Goods Code, 2022 Edition (IMDG 41-22), International Maritime Organization, 2022.
IMO, International Convention for the Prevention of Pollution from Ships (MARPOL), Annex I, II and III, International Maritime Organization, current edition.
IMO, International Bulk Chemical Code (IBC 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.
ASTM International, D93 Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester, ASTM, current edition.
ASTM International, D56 Standard Test Method for Flash Point by Tag Closed Cup Tester, ASTM, current edition.
ICS / OCIMF / IAPH, International Safety Guide for Oil Tankers and Terminals (ISGOTT), 6th edition, Witherby, 2020.
UK Health and Safety Executive, The Buncefield Investigation: Final Report, HSE, 2008.
IUMI, DG Loss Prevention Reports, International Union of Marine Insurance, annual.