2.4 Class 3 Flammable Liquids and Class 4 Flammable Solids & Reactives
Key Takeaways
Class 3 generally covers flammable liquids with flash point not exceeding 60°C, plus specified hot-material and desensitised-explosive cases and the diesel exception.
Flash point is the lowest temperature at which liquid releases sufficient vapor to form an ignitable mixture with air, whereas auto-ignition temperature is the temperature at which vapor ignites spontaneously without an external spark.
Ordinary Class 3 packing groups use boiling point and flash point criteria, with specific exceptions and classification provisions to check: PG I (initial boiling point <= 35°C), PG II (flash point < 23°C and initial boiling point > 35°C), and PG III (flash point between 23°C and 60°C).
Flammable liquid vapors are heavier than air, accumulate in low areas, and can travel long distances along the ground to distant ignition sources; static electricity generation requires strict earthing and bonding during transfer operations.
Class 4 is split into three divisions: 4.1 Flammable solids and self-reactives, 4.2 Spontaneously combustible (pyrophoric and self-heating), and 4.3 Substances which in contact with water emit flammable gases, where water must NEVER be used for firefighting.
Class 3 and Class 4: Flammable Liquids, Solids, and Reactive Substances
Core Concept: Flammable materials represent the most frequent dangerous goods consignments transported on European roads. Understanding the exact combustion triggers—including flash point thresholds, vapor density, spontaneous heating, and hazardous water reactivity—is fundamental to preventing catastrophic cargo fires.
1. Class 3: Flammable Liquids
According to ADR 2.2.3.1.1, Class 3 covers substances and articles containing substances which:
- Are liquids at 50°C with a vapor pressure of not more than 300 kPa (3 bar), and are not completely gaseous at 20°C at standard pressure (101.3 kPa); and
- Have a flash point of not more than 60°C; or
- The class also includes liquids or molten solids with flash point above 60°C carried at or above their flash point (UN 3256), and separately liquid desensitised explosives. Hot liquid below its flash point may instead meet Class 9 elevated-temperature criteria; 100°C alone does not make bitumen Class 3.
Note on Diesel Fuel / Gas Oil: Although diesel fuel, gas oil, and light heating oil frequently have flash points between 55°C and 75°C, ADR explicitly classifies them as UN 1202 in Class 3, Packing Group III, even if their flash point slightly exceeds 60°C (up to 100°C).
Understanding Combustion Parameters
Drivers must clearly differentiate between three critical thermal benchmarks:
- Flash Point: The lowest liquid temperature at which its vapors form an ignitable mixture with air near the surface of the liquid, capable of flashing when an external ignition source (spark, flame) is introduced. (e.g., Petrol/Gasoline has a flash point of approximately -40°C; Diesel has a flash point of >= 55°C).
- Boiling Point (Initial Boiling Point): The temperature at which the vapor pressure of the liquid equals atmospheric pressure. Liquids with low boiling points vaporize profusely and build rapid internal pressure inside containers.
- Auto-Ignition Temperature: The lowest temperature at which a vapor-air mixture will ignite spontaneously without an external spark or open flame. For instance, carbon disulfide ignites spontaneously on contact with a hot steam pipe or vehicle exhaust manifold at only 90°C.
Packing Group Allocation for Class 3
Class 3 substances (with the exception of liquid desensitized explosives) are assigned to Packing Groups I, II, or III based on their degree of flammability and volatility:
| Packing Group | Danger Level | Flash Point Criteria | Initial Boiling Point Criteria | Representative Commodities |
|---|---|---|---|---|
| PG I | High danger | Any flash point | <= 35°C | Diethyl ether (initial boiling point about 35°C) |
| PG II | Medium danger | < 23°C | > 35°C | Petrol / Motor spirit (UN 1203), Acetone (UN 1090), Ethanol (UN 1170, pure) |
| PG III | Low danger | >= 23°C and <= 60°C | > 35°C | Diesel fuel (UN 1202), Kerosene / Jet fuel (UN 1223), White spirit (UN 1300) |
2. Vapor Accumulation and Electrostatic Hazards
Flammable liquid vapors exhibit specific behaviors that dictate strict operational handling rules:
- Vapor Density (Heavier Than Air): Virtually all hydrocarbon and chemical solvent vapors are significantly denser than atmospheric air (vapor density > 1.0). If a spill or vent leak occurs, vapors flow downward, hugging the ground and pooling in depressions, gutters, wheel arches, and basement drains. A vapor trail can spread dozens of meters from the vehicle until reaching an ignition source (e.g., a cigarette, electrical switch, or hot brake disc), igniting and flashing back instantaneously to the source drum or tanker.
- Static Electricity and Bonding/Earthing: When non-conductive flammable liquids flow through hoses, pipes, or pumps, electrostatic charges generate continuously through internal fluid friction. Without dissipation, static voltage accumulates until a high-energy spark jumps to a nearby metal ground, igniting the flammable vapor space. ADR mandates that during loading, filling, or discharging of Class 3 liquids with a flash point <= 60°C, the vehicle chassis, tanks, and delivery piping must be subject to the applicable earthing provisions, including 7.5.10 for flammable gases and liquids with flash point not exceeding 60°C. Establish the required good electrical connection before tank filling or emptying and follow the authorised site procedure.
3. Class 4: Flammable Solids and Reactive Substances
Class 4 addresses flammable hazards presented by solid materials and substances displaying hazardous chemical reactivity:
| Division | Name & Hazard Label | Reaction Mechanism | Key Characteristics & Examples |
|---|---|---|---|
| Class 4.1 | Flammable solids, self-reactive substances, polymerizing substances, and solid desensitized explosives (Striped red & white diamond) | Readily combustible solids ignited by friction, spark, or flame, burning fiercely. Also includes self-reactive chemicals that undergo exothermic decomposition without oxygen. | Matches (UN 1331), Sulfur (UN 1350), Naphthalene, crude or refined (UN 1334), Azodicarbonamide (self-reactive). |
| Class 4.2 | Substances liable to spontaneous combustion (Half white top, half red bottom diamond) | Pyrophoric substances: ignite within 5 minutes of contact with air. Self-heating substances: react with atmospheric oxygen over hours or days, generating internal heat that cannot dissipate, culminating in spontaneous ignition. | White or yellow phosphorus (UN 1381, pyrophoric); Activated carbon/charcoal (UN 1362, self-heating), fish meal (UN 2216), oily rags/cotton waste. |
| Class 4.3 | Substances which in contact with water emit flammable gases (Solid blue diamond with flame) | React violently with liquid water, rain, or atmospheric humidity to release dangerous quantities of flammable gas (such as hydrogen, acetylene, or methane). | Metallic sodium (UN 1428), Potassium (UN 2257), Calcium carbide (UN 1402, releases acetylene gas), Zinc powder (UN 1436). |
Critical Firefighting Rule for Class 4.3 (Dangerous When Wet)
Strict Fire-Fighting Prohibition: Under NO circumstances should water or water-based extinguishing agents (such as foam or water mist) be applied to a Class 4.3 fire or leaking cargo. Water reacts exothermically with Class 4.3 goods, accelerating the generation of highly explosive gases (like hydrogen or acetylene) and triggering violent explosive eruptions. Drivers must keep away from a dangerous goods cargo fire. Specialist responders select a compatible agent; ordinary ABC powder, carbon dioxide and dry sand are not universal solutions for all water-reactive materials.
Source check: 17 October 2026 — ADR 2025, English text published by the Slovak Ministry of Transport, 2.2.3, 2.2.41–2.2.43 and 7.5.10.
What is the primary difference between a flammable liquid assigned to Packing Group I versus one assigned to Packing Group II in Class 3?
Packing Group I liquids require an open flame to ignite whereas Packing Group II liquids ignite spontaneously
Packing Group I liquids have a flash point above 60°C whereas Packing Group II liquids have a flash point below 23°C
Packing Group I liquids can only be carried in refrigerated tank-containers
Packing Group I liquids have an initial boiling point of 35°C or less, indicating extreme volatility
Which Class 4 division encompasses pyrophoric substances that ignite spontaneously within five minutes of exposure to atmospheric air?
Class 4.2 Substances liable to spontaneous combustion
Class 4.1 Flammable solids
Class 4.3 Substances which in contact with water emit flammable gases
Class 4.1 Solid desensitized explosives
If a transport unit carrying drums of Calcium Carbide (UN 1402, Class 4.3) is involved in an incident with spilled material, what extinguishing agent is strictly prohibited from being used?
Dry sand
Water or water-based foam
Class D dry powder
Non-reactive mineral granules
Before filling or emptying a tank with a flammable liquid of flash point not exceeding 60°C, why does ADR 7.5.10 require a good electrical connection to earth?
To power the vehicle's internal tachograph and lighting systems
To prevent electric shock to the driver from the vehicle starter battery
To dissipate static electric charges generated by fluid friction that could otherwise produce an ignition spark
To ensure that the transport documentation electronically syncs with the carrier database
Sections you finish are checked off in the contents.