Like hydrocarbon fuels, if an accidental release of pressurised hydrogen occurs, there is the potential for a jet fire. Understanding the impact on critical structures and equipment, and importantly the performance of current passive fire protection (PFP) materials, is therefore critical.
DNV has undertaken comprehensive studies and tests at its research and testing centre in Cumbria, UK, to first understand the similarities and differences between hydrogen and hydrocarbon release scenarios and then assess how this might affect the properties of a jet fire.1 Consideration was then made on the impact of hydrogen and natural gas fires on PFP and the potential for the current standard jet fire (ISO 22899-1:2021) test to apply to hydrogen releases.
The aim is to determine if current PFP materials and testing methodologies are adequate for hydrogen jet fires and if urgent changes are required to mitigate this occurrence and ultimately, safeguard lives.
Historically, jet fire hazards have tended to be most prevalent in the oil and gas industries and have related to the accidental release of pressurised hydrocarbons. Such events have been subject to extensive research.
While small-scale hydrogen fires have been widely studied, there is little published data of large-scale releases, particularly where fire proofing may be required as well as the internal characteristics of a large-scale hydrogen jet fire. The largest ignited hydrogen jet fire releases to date were conducted by DNV at its Cumbria facility from large capacity onsite gas storage tanks.2
Smaller scale studies in 2006 and 2007,3,4 hydrogen jet fires were described as having a radiative fraction of about half that of natural gas at a comparable size. However, research also demonstrated that the fraction of heat radiated increases as outflow rate gets bigger2, becoming similar to natural gas.
The factors considered to form the basis of the comparative study are detailed in Table 1 below.
By referencing experimental studies and jet fire modelling the team has shown that, for like-for-like releases (same pressure and hole size), the external flame characteristics (flame length, thermal radiation) are not significantly different, with hydrogen flames being about 10% shorter than like-for-like methane/natural gas flames.
They also identify that a transient release (i.e. from an isolated inventory), like-for-like, inventories of hydrogen and methane/natural gas, produces a resulting hydrogen jet fire that will have a duration of about one third of that of a methane/natural gas jet fire due to the higher volumetric outflow in the case of hydrogen.
The understanding of the internal characteristics of the flame are less certain. Hydrogen jet fire flame temperatures are greater than those in methane/natural gas jet fires. Modelling suggests the temperature difference is at least 150°C, but it could be more. The higher temperature suggests that the convective thermal flux to an object in the flame may be higher in the case of hydrogen compared to methane/natural gas, but there is no confirmatory experimental evidence available at this time.
Modelling also indicates that the gas velocities and densities in the bulk of the flame are not appreciably different between hydrogen and methane/natural gas, suggesting that erosive forces will be similar.
Hydrogen is routinely stored at higher pressures compared to natural gas. If releases at these very high pressures follow a similar behaviour to lower pressure releases, then they will correlate with flame power. In this case, a very high pressure release through a smaller hole size can equate to a lower pressure release through a hole increased in size to give the same power, though it is possible that conditions very close to the release point may vary. This is an area lacking in any large-scale experimental validation, particularly as very high pressures may result in jet flame powers that exceed the range of current hydrocarbon jet fire data.
Impact on PFP materials
The main differences between methane and hydrogen jet fires that are likely to affect PFP performance are increased flame temperature and the potential for higher erosion due to higher pressure at locations that are close to the release point. This combination may have a greater effect on the erosion of reactive PFP systems such as intumescent coatings, than may be seen in hydrocarbon jet fires.
If the jet fire power is within the range of existing hydrocarbon data, then there is no evidence to indicate that the higher pressure will, on its own, have an effect on PFP performance.
However, the combined impact of erosion and higher flame temperatures for hydrogen release may affect the performance of PFP. This effect is likely to be increased for high pressure releases relating to gas.
A safer hydrogen economy
According to a recent report by DNV, global energy professionals identify lack of investment in infrastructure as the joint-highest risk their organisations face in relation to hydrogen – and a significant majority (78%) say repurposing existing infrastructure will be crucial to develop a large-scale hydrogen economy.5
Rising to the Challenge of a Hydrogen Economy draws on a survey of more than 1,100 senior energy professionals and in-depth interviews with industry executives, on emerging hydrogen value chains, from production to consumption. It states that some 84% of senior energy professionals believe that hydrogen has the potential to be a major component of a global, low-carbon, energy system, while three quarters (73%) say Paris Agreement targets will not be possible without a large-scale hydrogen economy.
Hydrogen use is being considered across a range of sectors including energy transportation and storage, land transport, maritime propulsion, domestic heating and ‘hard to de-carbonise’ industries. Therefore, understanding its jet fire properties, particularly flame temperature and convective and total heat flux, is important. This knowledge will enable a broad range of industries to assess whether this affects the performance of certain PFP materials and whether the current ISO22899-1 and high heat flux (HHF) tests, developed for hydrocarbon jet fires, are sufficient to indicate likely PFP performance when subjected to hydrogen jet fires.
There is also the need to define representative hydrogen jet fire scenarios for the various hydrogen applications to understand both the likely size and duration of such fires. It is highly likely that in some instances, ignited hydrogen releases will have short durations that may mean that PFP is not required for mitigation.
Rian KE. 2019. Modelling and numerical simulation of hydrogen jet fires for industrial safety analysis – comparison with large scale experiments. 8th International conference on hydrogen safety (ICHS2019), 2019
Schefer RW., Houf WG., Bourne B., Colton J., 2006. Spatial and radiative properties of an ope flame hydrogen plume. International journal of hydrogen energy 31 (2006) 1332-1340
Schefer RW., Houf WG., Williams TC., Bourne B., Colton J., 2007. Characterisation of high pressure underexpanded hydrogen jet flames. International Journal of Hydrogen Energy 32 2007 2081-2093