This section discusses the accident scenario of leakage from a vehicle carrying hydrogen or from a hydrogen-powered vehicle. It also presents the key safety and failure elements and proposes specific safety recommendations.
Risk-based Regulatory Design for the Safe Use of Hydrogen
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19. Road transport: A hydrogen leak from a hydrogen transport vehicle driving in a built-up area
Abstract
Road transport
If hydrogen is to become a widely used green fuel then road transport should be considered as a crucial and effective mode of transport in the supply chain. This is a topic which at least in some countries raises serious concerns in terms of safety.
Hydrogen is frequently transported by road as compressed gas or as a cryogenic liquid. Pressurised tube or cylinder trailers typically are at 25 Mpa or higher based on the national transportation restrictions, while liquid hydrogen in cryogenic tank is at approximately 21 K. The storage conditions during road transport should be taken into account in risk management.
Existing technical norms
The Carriage of Dangerous Goods by Road (ADR) Regulation is a 1957 United Nations treaty governing national and transnational transport of dangerous or hazardous goods. ADR specifies packaging types, load security, the classification and labelling of dangerous goods and training of drivers. The Economic and Social Council (ECOSOC) Committee of Experts on the Transport of Dangerous Goods organised by the United Nations Economic Commission for Europe (UNECE) develops and updates safety provisions for the transport of hydrogen by all modes of transport, which are included in the UN Model Regulations on the Transport of Dangerous Goods.
Key safety / failure elements
Incidents in road transport and hydrogen mobility mostly involve either hydrogen containment leakage due to equipment failure or inadequate maintenance of components, tank rupture due to overpressurisation or release through a pressure relief device (PRD) caused by external factors, such as fire or impact. Small leaks can be caused by localised corrosion. A catastrophic failure of containment can also be caused by defects in construction particularly poor quality of seam welding. A hydrogen release will form a flammable vapour up to the point it is diluted or dispersed below its lower flammable limit.
Vehicles that transfer hydrogen and hydrogen powered vehicles are equipped with safety systems, like pressure relief devices that will be activated and vent hydrogen in case of pressure increase inside the storage vessels. Hydrogen venting with upwards orientation should be generally preferred due to the buoyant nature of hydrogen, as it would be easily dispersed and diluted provided that the release occurs in open environment. The size of the pressure relief device is another key element to safety. It should be designed as such as to reduce the formation of a flammable cloud, especially in confined spaces, taking into account also the fire resistance time of the storage vessels. Technology advancements in tanks can reduce the risks associated to tank rupture, which is one of the major safety concerns in road transport.
Most of the reported1 hydrogen incidents related to hydrogen transport and hydrogen-powered vehicles were caused by traffic accidents. In 37% cases no release took place, in 31% there was an unignited release and in 32% a fire or an explosion occurred.
Recommendations on key safety elements for hydrogen road transport and hydrogen-powered vehicles
Recommendations focus mainly on road transport but include also few recommendations for hydrogen-powered vehicles on roads (more recommendations for hydrogen FCEVs can be found in Chapter 20).
Design
Limit the maximum size of individual tube containers.
Limit the maximum pressure in tube trailers to not more than 25 Mpa. An exemption can be made taking into consideration the travelling distances and the routes to avoid transporting high pressure vessels close to populated areas and vulnerable areas, like hospitals and schools.
The package securing system in tube trailers should be designed with adequate safety margins to assure that hydrogen cylinder packing remains secured to the transport trailer under adverse conditions.
In hydrogen FCEVs consider the use of new technologies, like TPRD-less (leak-no-burst) tank that would not release hydrogen through TPRD in extreme conditions, like engulfing fire in hydrogen tank. However, the TPRD-less technology should be considered along with the fire resistance of the tank.
Hydrogen transport and hydrogen-powered vehicles should be fitted with warning signs to alert emergency services approaching defective / crashed vehicle.
Safety devices
Pressure relief valves should be effectively connected to vent tubing to route hydrogen to the top of the truck to safely disperse in the atmosphere.
Systems involving more than one PRD should be designed to avoid simultaneous opening of all PRDs to limit the size of a flammable cloud in the event of an incident.
Safety measures in confined spaces
Mechanical ventilation in confined spaces where hydrogen transport and/or hydrogen-powered vehicles are allowed. Ventilation in garages (Lach and Gaathaug, 2021[1]) should achieve at least 10 ACH (Air Changes per Hour).
Practices
Train and educate drivers on the explosive characteristics of hydrogen. Haulage company’s policies should require safe driving practices under all conditions (Hydrogen Tools, 2017[2]).
Train first responders to deal with all safety aspects for a range of hydrogen applications and design emergency plans based on hydrogen safety science and engineering.
In case of an accident involving hydrogen FCEVs, first responders would be able to approach the vehicle, conservatively, approximately two minutes after pressure relief valve activation (hearing the hissing sound). For the safety of the general public, a perimeter of 100 metres is suggested to be set in the accident scene if no hissing sound is heard. However, the perimeter can be reduced to 10 metres once the hissing sound of hydrogen release is observed. The first responders should remain 6 m from the vehicle if there are no signs of hydrogen leakage.
Controls
Regular maintenance of the trailer, fastenings, manifolds and safety devices.
References
[2] Hydrogen Tools (2017), Hydrogen Delivery Truck Roll Over Accident, https://h2tools.org/lessons/hydrogen-delivery-truck-roll-over-accident.
[1] Lach, A. and A. Gaathaug (2021), “Effect of Mechanical Ventilation on Accidental Hydrogen Releases—Large-Scale Experiments”, Energies, Vol. 14, p. 3008.
Note
← 1. Based on a recent review of the HIAD 2.0 and H2tools hydrogen incident databases 73 incidents related to hydrogen transport and hydrogen vehicles were reported (see Part IV: Review on incident database and lessons learnt).