An explosion risk report written in 2018 — along with the accompanying video — has finally been released by gas distributor SGN after it was ordered to do so by the UK’s information commissioner, following a freedom of information request.

Stay ahead on hydrogen with our free newsletter
Keep up with the latest developments in the international hydrogen industry with the free Accelerate Hydrogen newsletter. Sign up now for an unbiased, clear-sighted view of the fast-growing hydrogen sector.

SGN — which is due to hold a hydrogen heating trial in 300 homes in Fife, Scotland, next year — had declined to publish the full report “because of concerns this would be used out of context and could have been misinterpreted”.

The newly released SGN video showing the test explosions

The gas distributor added that the “key findings were made public in 2021 as part of the Government-commissioned Hy4Heat programme to explore the evidence case for the use of hydrogen in homes”.

Indeed, the recommendation for excess flow valves to be fitted to homes in order to prevent significant leaks of hydrogen was already widely known.

The tests, carried out by consultant Kiwa, were designed to simulate gas escapes of hydrogen into a domestic kitchen using “fire investigation boxes” — basically shipping containers that had been fitted with windows and a door and filled with the contents of a kitchen, along with dummies representing people (and in two cases, pig carcasses) — with sparks introduced at the height of a typical light switch.

The study found that at low and high gas injection rates (16kW and 64kW), similar levels of damage were caused for both methane and hydrogen.

However, it added: “There was evidence that hydrogen transitioned from a deflagration to a detonation-type explosion when the injection rate was around 64kW and hydrogen concentrations [in the air] near the ignitor were above 20%. Localised structural damage and overpressures around three times higher than previous ignitions were observed.”

A deflagration is an explosion slower than the speed of sound that would usually be confined within an enclosed space. A detonation is an explosion faster than the speed of sound that would usually rupture an enclosed space, and is much louder.

Indeed, the 20% hydrogen test — called “Ignition 4” in the report — caused debris from the windows to be thrown up to 40 metres, with the door blown off the hinges and its debris flying 25 metres. The “very audible bang” was heard in buildings 1km away.

At concentrations of 20% hydrogen and 80% air, “it is likely there would have been significant damage to brickwork, and occupants of the house would probably have been severely injured,” the report added.

It continued: “At increasing injection rates (100kW) where very large volumes of hydrogen were injected, and with hydrogen concentrations near the ignitor around 30%, there was severe damage.

“It is recommended that techniques are developed to minimise the risk of high concentrations of hydrogen occurring. Further work is recommended to investigate the feasibility of installing automatic shut off valves and hydrogen detectors.”

The UK government has recommended fitting always-open air vents above hydrogen appliances, to ensure that any leaking hydrogen escapes into the atmosphere, rather than accumulate in enclosed spaces to avoid the risk of a spark causing an explosion. However, this is not a recommendation of the SGN report.

A video that accompanied the study has also been released for the first time and uploaded to YouTube, although it is unlisted, meaning it cannot be found through searching or the SGN main page, and is therefore can only be accessed via this link (or by clicking the video above).

But it does show the impact of the 64kW tests, as well a large detonation in the “stoichiometric test”, when the mix of hydrogen in the air is around 30%. The “stoichiometric mixture” is the ideal ratio of air to fuel to ensure that all the fuel burns with no excess air.

Scenes such as these may have been at least part of the reason why SGN chose not to make the full report and video publicly available.

SGN points out that none of the homes in the Fife trial will have supplies above 40kW and excess flow valves will be fitted to all proprties.