Hydrogen truck explodes on way to FCV refueling site [Video]

The February incident prompted an NTSB investigation into safety implications of \"emerging transportation technologies.\"

The National Transportation Safety Board has launched an investigation into a February accident involving a hydrogen transportation truck that caught fire in Diamond Bar, California.

The incident did not receive widespread attention at the time, aside from local news reports. The NTSB disclosed the inquiry as a footnote in its investigation of a battery fire in a fatal Tesla Model S crash earlier this week.

The truck was carrying more than two dozen high-pressure gas cylinders loaded with around 529 pounds of compressed hydrogen destined for a hydrogen fuel-cell refueling station. Around half the total hydrogen haul was released from a dozen cylinders exposed to the fire.

YouTube videos appear to show at least one of the tanks releasing hydrogen in a quick burst, causing a fireball and a modest boom. No injuries were reported, but first responders had evacuated around 500 people within a 10-block business district.

"Hydrogen may form explosive mixtures in air and it is easily ignitable, by static electricity or self-ignition, if a cylinder valve is opened to the air," the NTSB says. "It burns with an invisible flame. Upon exposure to intense heat and flame, cylinders may vent rapidly or rupture violently."

The handful of automakers currently offering FCVs have insisted the cylinders installed in vehicles are strong enough to withstand car accidents without rupturing. Toyota even shot the Mirai's tank with a .50-cal bullet, resulting in a mere surface dent.

After inspecting the hydrogen refueling truck, the NTSB found that several cylinders had been equipped with under-rated pressure relief devices. The valves were set to vent hydrogen at 5,844 psi, below the 7,500 psi of a full tank and far short of the proper 10,000 psi specification.

Further contributing to the fire, the safety relief valves were not effectively connected to vent tubing that would have routed the hydrogen to the top of the truck and safely into the atmosphere. Seven of the vent tubes had detached from the valves, venting gas to the interior and fueling the fire.

"NTSB has a long history of investigating emerging transportation technologies, such as lithium ion battery fires in commercial aviation, as well as a fire involving the lithium ion battery in a Chevrolet Volt in collaboration with the National Highway Traffic Safety Administration," said NTSB chairman Robert Sumwalt. "The goal of these investigations is to understand the impact of these emerging transportation technologies when they are part of a transportation accident."