Risks with fire-exposed CNG and hydrogen tanks

Hydrogen and compressed natural gas (CNG) tanks are being developed with thte help of rigorous international safety requirements with the aim that these vehicles (ie vehicles powered by CNG or hydrogen) will be at least as safe as conventional vehicles. In the event of a vehicle fire, there are many risks from toxic smoke and projectiles from springs etc, regardless of the fuel used. However, each fuel has particular risks associated with it. When it comes to compressed gas, the gas tanks are equipped with a melt-fuse that in the event of fire should release the gas, which then forms a jet flame. However, the development of new technology is rarely straightforward. For a long time with lots of regulations and inventions along the way, the handling of conventional fuels such as petrol and diesel has become as safe as it is today, even though these fuels are classified as flammable liquids. Because CNG and hydrogen are stored under high pressure, there is in particular a risk of pressure vessel explosion. This has occurred twice in Sweden in the event of a fire. Why the melt-fuse did not release is an example of questions that were intended to be answered with the field trials:

• How is the risk of a pressure vessel explosion affected if the gas tank is exposed to a local fire?
• How is the risk of a pressure vessel explosion affected if the melt-fuse is cooled?
• Are fire-exposed cylinders dangerous after they have cooled down?

To investigate the risk of local fires, field trials were carried out on 8 CNG tanks in 2019. The experiments were funded by TUSC, Tunnel and Underground Safety Center. The results are documented in DiVA through a RISE report and a movie from the fire tests. The field trials confirmed that a pressure vessel explosion is an rare scenario, but that it can occur to a certain type of composite tank if they are exposed to a prolonged (20 min in the experiment) local fire that does not reach the melt-fuse.

Whitin the project BREND 2, field trials were carried out in 2021 on CNG and hydrogen tanks to investigate the risk if the melt-fuse is cooled, for example by a sprinkler system in a tunnel or ro-ro vessel, or by a manual extinguishing operation. Even if the fuse was effectively cooled by the water, this did not lead to a pressure vessel explosion, which is good news for, for example, tunnels with sprinkler systems or the cases where a quick extinguishing operation is important, for example on ro-ro vessels. More info about these tests are avaiable in Swedish.

Our research also confirms previous research that fire-affected gas cylinders regain a margin of safety against bursting when they have cooled down, regardless of whether they are allowed to cool naturally or cooled with water.

From the incidents that have taken place and the research that is being conducted, we at RISE are convinced that the energy carriers of the future will be at least as safe as those of the past! For example, we see a development where composite cylinders begin to release the gas through the composite coating in a controlled and safe way already after 7-8 minutes of fire exposure. However, it will be important to continue to follow developments and ensure that knowledge of new materials and the development of different standards is kept up to date.