Explosion Risks in Fire Extinguisher Transportation: Why Are Both Air and Land Transport Restricted?

As mobile pressure vessels sealed with high-pressure gases, fire extinguishers do not pose explosion risks only in extreme scenarios—instead, such risks are potential threats throughout the entire transportation process. Both air transport (conducted at high altitudes) and land transport (conducted on the ground) impose strict restrictions on fire extinguishers. This essentially stems from the “superposition effect” between the environmental characteristics of these two transport scenarios and the explosion risks of fire extinguishers: the enclosed, low-pressure environment of air transport intensifies gas expansion, while the jolts, collisions, and vibrations of land transport easily trigger structural failures. Without targeted controls, explosion risks escalate sharply. This article will examine the root causes of fire extinguisher explosion risks, comparatively analyze the common logic and differentiated reasons behind restrictions on air and land transport, and reveal the safety management logic behind these “dual restrictions” by integrating practical cases and regulatory requirements.

I. Core Triggers of Fire Extinguisher Explosion Risks: Pressure Vessel Attributes and Risk Transmission Chains

To understand the essence of transport restrictions, it is first necessary to break down the mechanism of fire extinguisher explosion risks. Explosions are not caused by a single factor but rather a chain reaction: “pressure vessel structural failure → uncontrolled release of high-pressure gas → energy impact triggering explosion.” Each link in this chain can be amplified in transportation scenarios.

(1) Structural Failure: The Starting Point of Explosion Risks

The sealed structure formed by a fire extinguisher’s metal cylinder and valve serves as the first line of defense against high-pressure gases, yet it is also the most vulnerable link prone to failure:

• Cylinder Weld Cracking: Most fire extinguisher cylinders are manufactured by rolling steel plates and forming circumferential welds, where stress concentration occurs. Sustained vibrations during transportation (e.g., land transport vehicles traveling on rough roads with a vibration frequency of 2–5 Hz) subject these welds to alternating stress. If there are initial minor welding defects (such as pores or incomplete penetration), these defects gradually expand into cracks. When a crack penetrates the full thickness of the cylinder wall (typically 1.5–2 mm), high-pressure gas sprays out at high speed from the crack, causing a “jet recoil.” If the cylinder loses balance and collides with other objects, an explosion may be triggered;
• Valve Failure: Valves are critical components controlling gas release. If the threaded connection between a valve and the cylinder loosens due to vibration, or if the valve core seal ring ages and deteriorates, gas leakage will occur. In the early stages, small leakage volumes may only manifest as a slow pressure drop. However, when the leakage rate exceeds a safety threshold (e.g., >50 g/s for carbon dioxide fire extinguishers), high-speed gas flow generates static electricity through friction. If flammable substances such as dust or oil are present, this can ignite a combustion explosion. In 2023, during the land transport of dry powder fire extinguishers, the aging of valve seal rings caused leakage. The sprayed dry powder mixed with residual engine oil in the vehicle compartment, and static electricity ignited the mixture, resulting in a small explosion that partially burned the compartment.

(2) Pressure Out-of-Control: The Accelerator of Explosion Risks

The gas pressure inside fire extinguishers rises linearly with temperature—a physical property that easily translates into pressure out-of-control risks in transportation scenarios:

• Overpressure Caused by Temperature Rise: According to the ideal gas law (PV = nRT), pressure is proportional to temperature in a fixed-volume cylinder. During air transport, if the cargo hold temperature rises to 50°C due to air conditioning failure or direct sunlight, the pressure of a 4kg carbon dioxide fire extinguisher increases from 5.7 MPa (at 20°C) to 8.2 MPa, approaching the cylinder’s design pressure (9.0 MPa). During land transport, the temperature inside a sun-exposed truck compartment can reach 65°C in summer, at which point the fire extinguisher pressure exceeds the design limit. If the safety valve fails to release pressure normally due to rust or blockage, the cylinder will undergo a “physical explosion,” with fragments flying at speeds exceeding 300 m/s. In 2022, a cargo flight operated by an airline experienced cargo hold temperature 失控 (reaching 58°C), causing two carbon dioxide fire extinguishers to explode due to overpressure. Cylinder fragments pierced the cargo hold wall, forcing the flight to make an emergency diversion;
• Expansion Caused by Low-Pressure Environments: At the cruising altitude of air transport (typically 9,000–12,000 meters), atmospheric pressure is only 25–30% of that at ground level (approximately 25 kPa). The external pressure on the cylinder drops sharply while the internal pressure remains unchanged, creating a situation of “increased internal-external pressure difference.” This pressure difference subjects the cylinder to greater outward expansion forces. If the cylinder has corroded and thinned (e.g., due to contact with acidic substances during previous land transport), it may undergo “instability rupture” in a low-pressure environment, triggering an explosion.

(3) Energy Release: The Ultimate Consequence of Explosion Risks

When structural failure or pressure out-of-control causes rapid gas release, three types of destructive energy are generated, leading to explosive consequences:

• Shock Waves: Shock waves produced by the instantaneous release of high-pressure gas can cause compressive damage to surrounding objects. Experimental data shows that the shock wave from the explosion of a single 4kg carbon dioxide fire extinguisher can deform the compartment of an ordinary truck and shatter glass within a 10-meter radius;
• Fragment Impact: Fragments generated by cylinder rupture (such as cylinder heads or body segments) possess extremely high kinetic energy and can penetrate obstacles like metal plates and walls. In 2021, a dry powder fire extinguisher on a land transport vehicle exploded. A 0.5kg cylinder fragment pierced the vehicle compartment and then penetrated the trunk of a vehicle ahead, narrowly avoiding casualties;
• Secondary Hazards: If an explosion ignites surrounding cargo (e.g., cartons, plastics, or fuel), it will trigger a fire. If gases such as carbon dioxide leak in enclosed spaces (e.g., air cargo holds or truck compartments), oxygen concentrations drop sharply, causing suffocation.

II. Air Transport Restrictions: Dual Risk Superposition of High and Low Pressure

Restrictions on fire extinguishers in air transport are particularly strict—even direct refusal of acceptance in most cases. The core reason lies in the “high-pressure gas + low-pressure external environment” dual characteristics of the air transport environment, which amplify explosion risks to a level far exceeding that of land transport, with accident consequences that are more difficult to control.

(1) Cargo Hold Environment: Enclosed Spaces Intensify Explosion Hazards

The airtightness and spatial limitations of air cargo holds cause the hazards of fire extinguisher explosions to increase exponentially:

• Difficulty in Diffusing Explosion Consequences: During land transport, if a fire extinguisher explodes in an open truck compartment, shock waves and fragments can diffuse in all directions, keeping the hazard scope relatively controllable. In contrast, air cargo holds are enclosed spaces (e.g., the cargo hold volume of a Boeing 777 is approximately 110 m³). Shock waves from explosions reflect and superimpose inside the hold, resulting in greater destructive power—not only damaging the cargo hold structure but also potentially affecting the passenger cabin floor or flight control systems of passenger aircraft, endangering flight safety. In 2019, a fire extinguisher explosion occurred on a Boeing 747 freighter operated by a cargo airline. The shock wave destroyed the cargo hold partition, damaging the adjacent aviation fuel tank and nearly triggering a fuel leak explosion;
• Extreme Difficulty in Emergency Response: During air travel, the cargo hold is in a high-altitude, low-pressure environment, and crew members cannot directly enter the hold to handle initial leaks or fires caused by potential explosions. If a fire extinguisher leaks, monitoring can only be conducted indirectly through cargo hold smoke detectors and temperature sensors. By the time an anomaly is detected, the situation may have already escalated into an explosion. In contrast, during land transport, drivers can stop the vehicle at any time to inspect and use leak-sealing tools or fire extinguishers to address initial risks.

(2) Low-Pressure Environment: Amplifying Structural Failure Risks

The low-pressure environment during air cruise intensifies fire extinguisher structural failure risks in two dimensions:

• Increased Internal-External Pressure Difference Causing Cylinder Expansion: In ground environments, the internal-external pressure difference of a fire extinguisher is 5.7 MPa (internal) – 0.1 MPa (external) = 5.6 MPa. At an altitude of 10,000 meters, the external pressure drops to 0.026 MPa, increasing the internal-external pressure difference to 5.674 MPa. This significantly increases the outward expansion force on the cylinder. If the cylinder is corroded and thinned (e.g., from 2 mm to 1.8 mm in wall thickness), its pressure-bearing capacity decreases by 20%, making it prone to “instability rupture” in the high-altitude, low-pressure environment;
• Gas Expansion Accelerating Leakage: In low-pressure environments, gas molecules inside the cylinder move more vigorously. If there is a small leak gap in the valve, the gas leakage rate can be 3–5 times faster than on the ground. For example, a fire extinguisher leaking at 10 g/h on the ground may leak at 30 g/h at high altitudes. In a short period, this can cause a sudden drop in internal cylinder pressure, leading to structural imbalance, or expand the leak gap due to gas expansion, ultimately triggering an explosion.

(3) Regulatory Constraints: Strict Oversight by International Aviation Organizations

The International Civil Aviation Organization (ICAO) and the International Air Transport Association (IATA) have classified fire extinguishers as “high-risk dangerous goods” through multiple regulations, restricting their air transport from the source:

• ICAO Technical Instructions for the Safe Transport of Dangerous Goods by Air: Explicitly classifies fire extinguishers as “Class 2.2 Non-Flammable, Non-Toxic Gases.” Air transport is only permitted under “special emergency needs” (e.g., for aircraft maintenance) and with a special permit from the civil aviation authority. Additionally, the number of fire extinguishers transported per flight must not exceed 2 units (each weighing ≤ 2 kg);
• IATA Dangerous Goods Regulations: Further refines restrictive conditions, requiring fire extinguishers transported by air to pass a “high-altitude low-pressure adaptability test” (simulating pressure changes in a 12,000-meter high-altitude environment). Packaging must use explosion-proof materials (e.g., stainless steel containers wrapped in shock-absorbing foam), and mixed transport with other dangerous goods is prohibited. These strict requirements lead most enterprises to abandon air transport of fire extinguishers due to excessive costs or insufficient qualifications.

III. Land Transport Restrictions: Risk Amplification from Jolts, Collisions, and Environmental Variables

Compared to air transport, land transport restrictions on fire extinguishers may seem more lenient (transport is permitted with a special permit). However, in essence, factors such as jolts, collisions, temperature fluctuations, and improper operations in land transport scenarios also trigger explosion risks. Moreover, land transport covers a wide range and involves multiple entities, making risk management more challenging.

(1) Jolts and Collisions: The Most Frequent Triggers of Risks

Vehicle vibrations, sudden braking/acceleration, and loading/unloading collisions during land transport are the primary causes of fire extinguisher structural failure:

• Fatigue Damage from Sustained Vibrations: The vibration acceleration of land transport vehicles on ordinary roads can reach 0.5–1.0 g (g = gravitational acceleration). Such sustained vibrations subject the welds of fire extinguisher cylinders to alternating stress. According to mechanical calculations for materials, for Q235 carbon steel under a vibration acceleration of 0.8 g, an initial 0.1 mm weld crack will expand to penetrate the full wall thickness in only 30 hours (equivalent to a transport distance of approximately 1,200 km), ultimately causing cylinder rupture. In 2023, a logistics company transported 20 fire extinguishers using an ordinary truck without anti-vibration measures. After traveling 1,500 km, 6 cylinders developed weld cracks, and 2 of them leaked;
• Instantaneous Impact from Sudden Braking Collisions: During sudden braking in land transport, cargo inertial forces push it forward. If fire extinguishers are not secured or improperly secured, they collide violently with the front wall of the compartment or other cargo. Experiments show that when a truck traveling at 60 km/h brakes suddenly, a 10 kg fire extinguisher generates an impact force of approximately 600 N (equivalent to the weight of a 60 kg object). If the collision point is on the cylinder head weld, it directly causes weld cracking and triggers an explosion. In 2022, a courier company’s truck braked suddenly to avoid a pedestrian. Five unsecured fire extinguishers in the compartment rushed forward; one collided with the front compartment wall and exploded. Cylinder fragments pierced the driver’s cab, injuring the driver’s arm.

(2) Temperature and Environmental Variables: Uncontrollable Risk Triggers

Fire extinguishers in land transport are exposed to more complex environments. Variables such as extreme heat, freezing cold, and severe weather intensify explosion risks in multiple dimensions:

• Overpressure Caused by Extreme Heat: In summer land transport, the temperature inside a sun-exposed truck compartment can reach 65°C. At this temperature, the pressure of a 4 kg carbon dioxide fire extinguisher rises to 9.5 MPa, exceeding the design pressure (9.0 MPa) of most cylinders. If the safety valve becomes stuck due to long-term lack of maintenance and fails to release pressure during overpressure, the cylinder will explode within 10–15 minutes. In 2021, an enterprise transported fire extinguishers using an open-top truck in summer without sun protection measures. After 3 hours, one fire extinguisher exploded due to overpressure, destroying the truck compartment’s side panels;
• Material Brittleness Caused by Freezing Cold: In winter, land transport temperatures in northern regions can drop to -30°C. The impact toughness of carbon steel cylinders decreases from 100 J/cm² (at 20°C) to less than 10 J/cm², entering a “brittleness zone.” At this point, if a fire extinguisher is slightly collided during loading/unloading (e.g., falling from a height of 0.5 meters), the cylinder may undergo “brittle fracture”—a process with no obvious plastic deformation, making early warning difficult. In 2024, a logistics company transported fire extinguishers from Changchun to Dalian without insulation measures. During unloading, 3 fire extinguishers fractured brittlely due to minor collisions. The leaked extinguishing agent froze, disrupting road traffic;
• Risk Intensification from Severe Weather: Heavy rain, floods, and other severe weather can cause trucks to wade through water. If a fire extinguisher cylinder has corroded holes, rainwater seeps into the cylinder and reacts with the extinguishing agent (e.g., dry powder absorb water and clump, blocking the valve). In 2023, a region experienced heavy rain. A truck transporting fire extinguishers waded through floodwaters, and rainwater seeped into 2 dry powder fire extinguisher cylinders, causing the powder to clump. Subsequent use led to valve blockage, and sudden pressure buildup inside the cylinders triggered an explosion.

(3) Regulations and Standards: Full-Process Land Transport Oversight

Regulations such as China’s Regulations on the Administration of Road Transport of Dangerous Goods and the European Union’s Agreement Concerning the International Carriage of Dangerous Goods by Road (ADR) restrict the land transport of fire extinguishers and prevent explosion risks through qualification verification, transport conditions, and operational standards:

• Qualification Verification: Enterprises transporting fire extinguishers must obtain a Road Transport Operation Permit for Dangerous Goods. Vehicles must be dedicated dangerous goods transport vehicles (equipped with ventilation, temperature control, and anti-static devices). Drivers and escorts must hold dangerous goods transport qualification certificates and complete special training on handling fire extinguisher explosion risks;
• Transport Conditions: Fire extinguishers must be packaged on anti-vibration pallets, with a maximum stacking height of 1.5 meters per pallet. The compartment temperature must be controlled between -10°C and 40°C. Sunshades must be equipped in summer, and insulation measures adopted in winter. Mixed transport with acidic substances and flammable/explosive items is prohibited;
• Operational Standards: During transport, stops for inspections must be made every 2 hours to check for cylinder deformation or leakage. Vehicles must be parked away from fire sources and densely populated areas. Unloading must use forklifts or hydraulic pallet trucks; manual rough handling is prohibited.

IV. Common Logic Behind Air and Land Transport Restrictions: Aligned Core Goals of Risk Management

Although the reasons for restrictions on air and land transport differ by scenario, they essentially revolve around the core goal of “explosion risk prevention and control.” By restricting transport conditions, standardizing operational processes, and enhancing emergency capabilities, risks are reduced to an acceptable range. This is reflected in three key areas of common logic:

(1) Risk Anticipation: Identifying High-Risk Links in Advance

Restrictive measures for both transport methods are based on accurate anticipation of high-risk links in explosion risks: air transport focuses on the risk superposition of “low-pressure environments + enclosed spaces,” while land transport focuses on risk triggers from “jolts, collisions + environmental variables.” By restricting transport scenarios (e.g., air transport prohibiting ordinary cargo transport, land transport restricting non-specialized vehicles), high-risk links are avoided.

(2) Source Control