On February 11, 2018, about 1:15 p.m. a module of 25 horizontally mounted high-pressure gas cylinders loaded with 240 kilograms (kg) of compressed hydrogen, UN1049, caught fire while being transported. The incident occurred on Golden Springs Drive, near South Brea Canyon Road, in Diamond Bar, California. About 120 kg of the hydrogen was released from 12 of the cylinders that were exposed to the fire. The fire was extinguished, and the trailer was secured around 3:00 a.m. on February 12. No injuries were reported; however, about 500 persons within a 10-block business district were evacuated. Weather at the time of the incident was 69℉, maximum wind gusts were about 14 mph, there was no precipitation, and visibility was 10 miles.
The cylinders were manufactured under US Department of Transportation (DOT) Special Permit 14576. The cylinders were constructed of a carbon fiber composite wrap over an aluminum inner liner. About six weeks prior to the incident, a DOT-approved cylinder requalification facility, conducted a 5-year requalification inspection of the cylinder module. No exceptions with the cylinders or associated equipment were noted during the requalification inspection. However, preliminary inspection by NTSB investigators found:
- Cylinder damage was limited to fire exposure. Twenty of the cylinders exhibited varying degrees of fire exposure, but none of them were breached.
- Pressure relief devices activated on 12 of the cylinders.
- The cylinder manufacturer specifications call for type CG-5 pressure relief devices set at 10,000 psig. However, NTSB investigators and investigation party members found pressure relief devices set at an incorrect pressure rating of 5,833 psig installed in three of the cylinders. Two of these under-rated pressure relief devices had activated. The source of the inaccurate pressure relief devices is under investigation.
- The trailer included tubing attached to each pressure relief device outlet to safely vent relieved gases upward to the outside top of the cylinder module. Seven of the vent tubes became detached from the pressure relief device assemblies and vented gas to the interior of the trailer, which fueled the fire. The separated tubing had not been tightly secured by the compression fittings.
- The incorrectly rated pressure relief devices and unsecured vent tubing were not identified during the cylinder requalification inspections. Responding to initial findings, the carrier inspected its remaining fleet of 13 hydrogen cylinder modules. One trailer undergoing qualification inspection was found to have a cylinder with an underrated pressure relief device. Improperly secured vent tubing was found in about half of all the lines inspected.
CLICK HERE for the full NTSB preliminary report