Fire on a CGH2 tube trailer

A release of compressed hydrogen and subsequent fire occurred during the transportation of a CGH2 trailer by means of a truck tractor. The tube trailer module contained 25 fully wrapped carbon filler reinforced aluminium-lined tubes, 24 of which were fully loaded with 240 kilograms of compressed hydrogen.
The pressure relief devices actuated on 12 of the tubes and released about 120 kilograms of hydrogen that was likely consumed in the fire.
The Fire Department estimated about 1,400 to 2,000 people were evacuated from the adjacent business district and a nearby residential area. Equipment damages were estimated at $175,000.

The National Transportation Safety Board (NTSB) investigated the event and concluded that the probable cause were
(i) the requalification technician’s installation of an incorrectly rated pressure relief device in cylinder No. 14, which actuated during normal transportation and released high-pressure hydrogen,
(ii) The tube trailer module assembly contractor’s failure to sufficiently tighten compression fittings on the pressure relief device vent lines that disassembled under the pressure of escaping gas allowing a fire to develop inside the module and impinge on adjacent cylinders.
(iii) Contributing to the incident was a lack of a requirement for requalification inspectors to verify the pressure relief device pressure rating and to inspect for vent line assembly securement.

INITIATING cause of the event was the opening of a incorrectly rated pressure relief device, which actuated during normal transportation and released high-pressure hydrogen.

ROOT CAUSE was probably the requalification technician’s installation of the incorrectly device and the tube trailer module assembly contractor’s failure to sufficiently tighten compression fittings.

CONTRIBUTING CAUSE was a lack of a requirement for requalification inspectors.

compressed hydrogen tube trailer, 24 tubes, each 240 kg H2

After delivering a similar tube trailer module that morning, the driver picked up the incident tube trailer module from the hydrogen filling station and conducted a pre-trip inspection. The driver connected the module to the filling stanchion and found that although the cylinders were pressurized to 7,855 psig (temperature-corrected loading pressure, 540 bar), the pressure had reduced to 7,300 psig with cooling (503 bar). The driver concluded the module was already full and then proceeded with his trip to deliver it to a FCEV refuelling station.

TECHNCIAL SPECIFICATIONS OF TtHE TUBES AND MODULE
The CT-250 tube trailer module consisted of 25 horizontal neck-mounted Type 3 model carbon fibre composite cylinders arranged in a 5 x 5 array.
Each cylinder had a nominal capacity of contained 10 kg with a hydrogen capacity of 250 kg for the entire module.
The tubes were certified according to the permit DOT-SP 14576, with these design criteria and operational requirements:
Service pressure limited to 7,500 psig at 70℉ (517 bar at 20C) .
Test pressure of 12,500 psig (862 bar).
Minimum cylinder burst pressure = 25,500 psig (1758 bar), or a safety factor of 3.4.
Cylinder service life = 15 years. The tube trailer cylinders had been in service for 5 years, thus they had 10 years of remaining authorized service life.
Each cylinder requalified once every 5 years using a hydraulic or pneumatic pressure proof test.
Cylinders manifolded in accordance with the requirements of Title 49 Code of Federal Regulations (CFR) 173.301(g).
The gas cylinders were equipped with PRDs at each end, giving a total of 50 PRDs installed on the module. They were CG-5 type devices, i.e. consisting in a combination of rupture disk and fusible plug.

The Fire Department estimated about 1,400 to 2,000 people were evacuated from the adjacent business district and a nearby residential area.
Equipment damages were estimated at $175,000.

The NTSB has made safety recommendations to the Pipeline and Hazardous Materials Safety Administration; the US Department of Energy, Pacific Northwest National Laboratory; and the Compressed Gas Association.

Firefighters’ took up positions within feet of the burning trailer, to douse the fire and cool cylinders with water hoses. The incident commander decided to launch an offensive attack on the fire was made to prevent loss of life should the cylinders catastrophically fail because he recognized there was little chance of a timely evacuation in the highly congested area. Firefighters later established monitor nozzles to cool the hydrogen cylinders from a safer distance.
Soon after first responders arrived, one of the PRDs actuated. The most intense gas venting lasted for about 30 to 45 minutes. With a reduction in venting intensity, responders believed the risk of catastrophic cylinder failure had diminished.
The truck driver remained on the scene to further inform emergency responders until the moment the company site manager and the company recovery team arrived almost 1 hour after the incident began. The manager encouraged the incident commander to continue the cylinder cooling efforts with water spray to prevent the fire from involving additional cylinders.
The incident was managed with a 150-foot exclusion zone and 400–500-foot distance to the command post, the DOT Emergency Response Guidebook (ERG) Guide 115, recommends a 1-mile isolation distance. Expanding the evacuation zone was a major goal of the response, but recommended protective distance was not practical given traffic congestion in the area.

After a 30-minute cycle of water spray cooling followed by thermal imaging inspection, firefighters detected a persistent heat signature in the module that kept this process continuing throughout the evening. Company technicians were unable to access the module until 10:00 p.m., about 9 hours after the incident began.

The company recovery team began a controlled venting procedure for the remaining hydrogen cylinders. To accelerate the process, the team checked pressure in the remaining cylinders and vented them simultaneously. About 3:30 a.m., the scene was secured, and the company towed the empty tube trailer module beck to the storage terminal.

The NTSB investigation found the following:
"The County Fire Department emergency responders lacked familiarity with hydrogen tube trailer modules. Emergency responders did not immediately recognize the presence of hazardous materials, had difficulty estimating the likelihood for severe outcomes, and had not received guidance about appropriate mitigating actions needed to reduce the potential for catastrophic gas cylinder failures."

Findings of the NTSB investigation:
1. Greater first responder training and awareness about hydrogen tube trailer modules transported within their jurisdiction could have helped the first responders to initially respond to this incident more effectively, efficiently, and safely.
2. Although there is ample generic hazard information for compressed hydrogen and vehicle fuel systems, the available guidance lacks critical hazard recognition and firefighting information specific to fuel cell electric vehicle fuelling infrastructure and containers currently used for the bulk transportation of compressed hydrogen.
3. The generic guidance for flammable gases contained in the US Emergency Response Guidebook, Guide 115, does not adequately describe unique hazards associated with compressed hydrogen or best response actions for public safety personnel.
4. None of the incident tube trailer cylinders lost mechanical integrity, and they did not contribute to the cause or perpetuation of the incident.
5. The hydrogen release most likely initiated because a pressure relief device with the wrong pressure rating was installed on cylinder No. 14 and actuated under normal working pressure loads.
6. The hydrogen ignited during the forced separation of the cylinder No. 14 pressure relief device assembly from the cylinder.
7. Although the other cylinder pressure relief devices in the tube trailer module were actuated because of fire exposure, the cylinder No. 14 pressure relief device actuated before the fire began.
8. Had the requalification personnel inspected each vent line compression fitting for tightness, the propagation of the fire to the adjacent hydrogen cylinders may have been avoided.
9. The vent line compression fittings that disassembled during venting were not properly installed at the time the tube trailer module was fabricated and should have been noticed and repaired during the requalification inspection.
10. The lack of a Pipeline and Hazardous Materials Safety Administration special permit and regulatory requirements for verifying that pressure relief devices used on cylinders actuate at the correct pressure and venting equipment has been properly assembled and secured increases the risk of uncontrolled fires on flammable gas tube trailers.
11. The lack of vent system design requirements that consider factors such as the force of venting gas and construction material may leave vent systems for cylinders and tube trailers vulnerable to unexpected failure during an incident.
12. The lack of noticeable marking or other visual indicators, unique design features, and unique model numbering to readily identify pressure relief devices intended for different applications, made them easy to confuse and did not avert the requalification technician from installing four wrong pressure relief devices on the incident tube trailer module.