Explosion from a CGH2 tube trailer

A release and ignition of compressed hydrogen occurred a hydrogen trailer transfer/fill facility in Santa Clara, California. The event started during filling of a tube trailer. Due to a leaking valve, the process was interrupted to repair the valve. During this operation, hydrogen was accidentally released from an open pipe because of an unauthorized action and a subsequent miscommunication between the two drivers filling the trailer.
The hydrogen-air mixture ignited and exploded within seconds of the release, followed by a
high-pressure gas jet fire.

Explosion and fire damaged pipes and triggered the activation of temperature-pressure relief devices (TPRD), with further fuelling of the fire, which eventually spread to adjacent vehicles.

Approximately 250 kg of hydrogen was released during this event. Plant operators initiated a shutdown to isolate other trailers and tanks and prevent further releases. The local Fire Department intervened: they ordered the evacuation of the surrounding area, extinguished fires of diesel fuel and tires, and used water streams to protect surrounding hydrogen storage equipment: neighbouring hydrogen gas trailers, a liquid hydrogen tanker, and a stationary liquid hydrogen tank.

The INITIATING CAUSE was a hydrogen leak was caused by either a cracked O-ring or leaking cone and thread fitting on the isolation valve installed on the front module hydrogen.

ROOT CAUSES
According to the report of the Hydrogen safety Panel, the incident were caused by:
(1) Unauthorized maintenance performed by untrained personnel not following proper procedures,
(2) Miscommunication between the two drivers filling the trailer.
An additional, upstream ROOT CAUSE component was a lack of clarity on what to do when repairing interventions were needed during filling and the overall safety competences of the drivers.

CGH2 trailers

At the time of the incident, module piping had been disassembled in an attempt to repair a leak in the hydrogen supply line of the forward-mounted CT-250 modules.

DESCRIPTION OF THE THE FACILITY
The affected facility transfer liquid hydrogen from an stationary storage tank to high-pressure tube trailer. This process implies the pressurisation of the liquid hydrogen, its warming up to ambient temperature and the transfer into the tubes.

THE TUBES
The tubes installed on trailers are type 3 cylinders (aluminium lined, composite cylinders), with a nominal pressure of 7500 psi (518 bar) and a hydrogen storage capacity of 10 kg each.

THE TRAILER
The tubes are assembled into modules (the CT-250 and CT-500). Each tube is equipped with two pressure relief devices (PRDs) at each end. The PRDs consist of a rupture disc, with a burst pressure of 9,500 + 500 psig (655 + 34 bar-g), backed with a fusible metal alloy that melts at 212°F (100°C).

Two of the four employees on-site reported minor recordable injuries: a shoulder injury due to tripping while evacuating and a second a tinnitus from the explosion overpressure.
According to firefighters, the employees were relatively safe because they stayed at proper distance from their trailer during the fuelling process.

The tubes on the incident trailer sustained extensive damage. Two adjacent trailers and the LH2 tanker sustained less damage.
The company temporarily suspended deliveries from its CT-450 and CT-500.

Officials conducted air sampling and thermal imaging to ensure that the air quality and hydrogen did not pose a threat.

According to True Zero, chain of seven of the hydrogen refuelling stations, there are approximately 3,000 to 3,500 hydrogen fuel vehicles in the Bay Area. It was expected that up to 1,000 True Zero customers were affected the week after the explosion. Honda also released a statement following the blast, knowing that many of its Clarity Fuel Cell customers were impacted.

The following is a summary and simplification of the recommendations issued by the H2 Safety Panel:
1. NFPA should consider providing additional guidance on hydrogen system repairs on requirements for training all personnel.
2. Standards organizations should explore improvements in their standards for PRDs intended to actuate and relieve cylinder pressures during fire exposure. Improvements are needed to specify direct fire exposure requirements and vent line.
3. In-depth examination on fire protection of compressed pressure hydrogen cylinders and the requirements for PRDs, aiming at improved quantitative risk assessment comparing risks of cylinder use with and without PRDs.
4. NFPA should consider including recommendations or requirements for hydrogen process and transfer controls.
5. The role played by walls or bulkheads on hydrogen trailers should be clarified in case of release/ignition. Walls can trap hydrogen but walls can also exclude hydrogen accumulating from different sources.
6. The use of flow-limiting devices for inadvertently opened piping during hydrogen transfers and refuelling should be encouraged
7. Guidance on improved fire protection for trailer/tanker transfer stations should be provided in NFPA 2.

Based on their root cause analysis, the company implemented the following measures.
• Improved Training and Retraining of Drivers
• Improved Trailer Filling Procedures
• Equipment Evaluations and Modifications