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Hydrogen Safety by Design – Unpacking the hierarchy of risk controls

When considering hydrogen’s role in a low carbon economy, concerns are often raised regarding its safety. Certainly, hydrogen’s reputation has been tarnished by its association with the Hindenburg airship disaster and the hydrogen bomb. Whilst such wariness is understandable, the hazards associated with hydrogen – and means of controlling them – are far from unknown.

As is the case for any substance that can cause a major accident, a thorough understanding of the material being handled and a first-principles approach to managing the hazards is the best place to start.

HYDROGEN – A NEW PLAYER, BUT JUST THE SAME OLD GAME?

Various tried and tested risk management techniques, proven over decades in major hazard industries, can play a crucial role in enabling the safe adoption of hydrogen in a cleaner and more sustainable energy system. One key method is the Hierarchy of Risk Controls (Figure 1).

Figure 1- The Hierarchy of Risk Controls

The Hierarchy of Risk Controls is an invaluable tool in the locker of every engineer and is enshrined in UK health and safety legislation (Ref. 1). It illustrates the preferred order in which risk reduction measures should be applied from most to least effective, following the acronym ERIC-PD (derived from the first letter of each level of defence). Pairing this framework with knowledge of the hazards arising from hydrogen’s well known physical properties can enable hydrogen to play a safe role in the energy transition.

ELIMINATE

Using less hazardous alternatives

The emergence of Liquid Organic Hydrogen Carriers (LOHC), and other carrier chemicals including ammonia, presents an opportunity for hydrogen’s hazards to be eliminated for a portion of its lifecycle. LOHCs are substances which are reacted with hydrogen to contain it within a less hazardous material for storage and transport. The hydrogen is extracted for use and the de-hydrogenated LOHC is reused over many cycles. These substances eliminate – or significantly reduce – risks such as fire and explosion.

Additionally, there may be another less hazardous material that can be used in place of hydrogen. Hydrogen has its benefits but is not appropriate in every application. Substances of this nature are best utilised in controlled environments that are specifically designed to manage the risk of hazards present.

REDUCE

Reducing the number of leak points

Hydrogen molecules are the smallest and lightest molecules in the universe, so can leak through spaces other substances cannot. Leakages are most common through fittings such as valves and flanges. To lower the likelihood of releases, the design of hydrogen systems should minimise the number of fittings, by using welds rather than flanges for example.

Limiting leakage rates and volumes

Hydrogen burns at an extremely high temperature of up to 2,045 ⁰C in air (Ref. 2), but hydrogen flames are often invisible to the eye and have a low radiant heat.  People located adjacent to hydrogen leak points – in vehicle refuelling stations, for example – might not even realise the risk of contact with a hydrogen flame.

Fortunately, there are several means of limiting leakage rates and volumes:

  • Reducing the pressure of hydrogen inventories limits the release rate, the flame length in the event of a jet fire, or the volume of an explosive gas cloud. Additionally, reducing the pipe diameter will limit the largest hole size, to similar effects.
  • Reducing pipe diameter also decreases the volume of isolatable sections of a system, lessening the volume released after successful detection and isolation.  Similarly, including regular automated isolation valves will limit the volume of each isolatable section.

Reducing congestion

If the ignition of a hydrogen release is delayed, the accumulated gas cloud can explode, generating potentially catastrophic overpressures. There are two mechanisms of explosion:

  • Deflagration – the rapid burning of a hydrogen vapour cloud (flame speed of up to the speed of sound, 341 m/s) coupled with a pressure wave that can produce overpressures of up to 8 times the initial atmospheric conditions. Very little energy (0.02 millijoules) is required to initiate a deflagration – an electrostatic discharge would be sufficient (Ref. 3, 4).
  • Detonation – where the speed of the flame could be up to 2,000 m/s and a shock wave is generated resulting in overpressures up to 16-20 times the initial atmospheric conditions. For instant detonation, much more energy is required – around 10,000 Joules, for example high-voltage electrical discharges or lightning strikes (Ref. 3, 4).

In particularly congested areas, a Deflagration-to-Detonation Transition (DDT) can occur. The shear energy created by the flow of a hydrogen release passing around obstacles and against the walls of an enclosure can increase the flame speed from deflagration into detonation conditions through a positive feedback loop. Therefore, congestion of hydrogen facilities must be minimised.  Wherever possible, hydrogen should be stored outside in open environments to allow dispersion of releases.

Gaseous hydrogen is the lightest and least dense element and hence buoyant – exacerbated if a release has low momentum. Storage in an enclosed space requires adequate ventilation, and vents should be located at high points where hydrogen is most likely to accumulate.

ISOLATE

Material selection

Hydrogen can be absorbed into storage materials and make them more prone to cracking, known as hydrogen embrittlement. Reducing leakages requires successful containment – confining hydrogen within the boundary of a pipe or vessel and thus isolating it from the surrounding environment.  This can be achieved by choosing materials that are resistant to hydrogen embrittlement and can safely operate at the expected temperatures and pressures. Rigorous maintenance and inspection are required to ensure that signs of hydrogen-induced degradation are identified as early as possible.

Material selection extends to sealing systems, the origin of many hydrogen leaks. Elastomer seals are widely used in natural gas pipeline systems, but their improved suitability for hydrogen applications is under development (Ref. 5). Seals are particularly vulnerable to pressure changes, and there is a specific risk of “explosive decomposition”.  This occurs when hydrogen under high pressure is absorbed into the material and remains at high pressure as the system is depressurised, causing the seal to burst.

Physical isolation

Hydrogen has a wide flammability range (4 to 75% concentration in air, by volume) compared to other fuels such as methane (5.3 to 15% concentration). Furthermore, its extremely low minimum ignition energy, almost 15 times lower than methane, means that even the friction caused by a release impinging upon an adjacent surface can cause ignition (Ref. 2, 6).

The following isolation measures can help to manage the flammability of hydrogen:

  • Isolating hydrogen inventories from potential ignition sources.
  • Installing equipment suitable for use in areas where flammable hydrogen-air mixtures could form.
  • Requiring workers to only use approved equipment, for instance, electronic devices which will not create a spark.
  • Designing hydrogen systems to prevent releases impinging upon surfaces from which the friction could cause ignition. For example, disturbance of gravel.
  • Locating hydrogen inventories away from people – even the static energy produced by the friction of clothes can ignite hydrogen and isolating people from hazardous areas so far as is reasonably practicable protects them from harm during an incident.

Safe venting of hydrogen

Due to its low density, hydrogen will rise if releases are directed upwards in open, outdoor environments. Therefore, hydrogen vents should be located at high points away from ignition sources so that releases will naturally disperse upwards.

CONTROL

Detection

Hydrogen is colourless, odourless and tasteless, so human detection of unignited leaks is not reliable. Hydrogen applications often require it at very high purity (for example, fuel cells require less than 0.01% impurities), which may not be achievable if a scent is added. Therefore, robust leak detection and alarm, isolation & shutdown systems are required to alert people and prevent escalation. Due to hydrogen burning at such elevated temperatures and at high flame speeds, acoustic and infrared monitoring are appropriate means of detection as well as more traditional leak detection, such as monitoring for pressure drop.

PERSONAL PROTECTIVE EQUIPMENT (PPE)

PPE is the penultimate line of defence in protecting against a hazardous scenario – the focus should be on preventing incidents rather than trying to mitigate the final consequences. Nevertheless, workers in close proximity to hydrogen should be equipped with PPE that protects against hydrogen fires, as well as personal gas detection devices to monitor the presence of hydrogen. Anti-static PPE should be worn to reduce the likelihood of ignition.

DISCIPLINE

The characteristics of hydrogen are unique and can be counterintuitive as they differ from the more familiar hydrocarbons of the energy sector. Wherever hydrogen is used, people need to be educated in its safe handling, as well as in responding to emergencies. Human error is a frequent cause of incidents, so the risk reduction measures further up the Hierarchy of Risk Controls must always be prioritised, with designs taking human error into account.

CONCLUSION

Established risk management techniques, like the Hierarchy of Risk Controls, can play a key role in enabling hydrogen’s part in the energy transition. The tool supports a comprehensive and structured approach to ensuring that the hazards associated with the unique properties of hydrogen are effectively managed.  Transforming our energy networks does not mean having to reinvent the wheel.

References

  1. The Management of Health and Safety at Work Regulations 1999, Schedule 1
  2. https://h2tools.org/hyarc/hydrogen-data/basic-hydrogen-properties
  3. Fundamentals of Hydrogen Safety Engineering II, Vladimir Molkov (2012), ISBN 978-87-403-0279-0
  4. https://www.h2euro.org/hyfacts/wp-content/uploads/sites/2/2014/06/8-Chapter-DM-hydrogen-deflagration-and-detonation-final-1.pdf
  5. https://www.lbf.fraunhofer.de/en/research-projects/elastomer-seals-h2-transport.html
  6. https://hyresponder.eu/wp-content/uploads/2021/06/Lecture-10-slides.pdf

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