Near the end of the process of filling a gaseous hydrogen tube trailer at a liquid hydrogen transfilling station, a safety pressure-relief device (PRD) rupture disc on one of the tube trailer’s ten tubes burst and vented hydrogen gas. The PRD vent tube directed gas to the top of the trailer where the hydrogen vented and ignited, blowing a flame straight up in the air. The operator filling the tube trailer heard a loud explosion from the sudden release of hydrogen gas and saw flames immediately. The operator closed the main fill valve on the tube trailer, stopping the hydrogen fill; however, the ten cylinders on the tube trailer were almost full (2500 psig/173 bar). The tube trailer involved in this incident was one of two tube trailers being filled simultaneously and was second in a line up of five tube trailers parked adjacent to one another at this location. The ten PRDs on the tube trailer (one for each hydrogen tube) are located at the front of the trailer (king pin end) and each PRD has a vent pipe to direct vented hydrogen to a safe location on top of the tube trailer (see Figures 1 – 5).
Emergency responders were dispatched to the scene. The facility deluge system was turned on. This system covers the trailer fill aisle with water and includes nozzles at the rear of the trailer and a fire cannon directed to the front. When the emergency responders arrived, they immediately began spraying the adjacent trailers to ensure that they stayed cool. The HazMat crews closed the 10 individual tube fill isolation valves located at the rear of the tube trailer and extinguished the fire. Total time to control the incident was less than 10 minutes and there was no property damage from this event. Minor first aid was required for a mechanic working under the tube trailer during the filling process who bumped into the trailer several times during a hurried exit when the loud noise was heard.
The safety systems on the tube trailer worked as designed. The hydrogen tube trailer involved in this incident was doing its first fill after requalification, where all the PRDs had just been replaced. The PRD rupture disc designed for 3500 psig (241 bar) failed at about 1000 psig (69 bar) below rated pressure. The hydrogen tube trailer was grounded per procedure during the filling operation. Subsequent follow-up examination of the PDR rupture disc lot by the PDR manufacturer found that all of the lot conformed to specification.
- Specific response drills/exercises need to be conducted yearly. In this case, all safety systems worked as they should and outside emergency responders were not needed.
- Performing other tasks while filling hydrogen tube trailers, such as mechanic work, should be avoided. Most premature failures of hydrogen tube trailer PRD burst discs occur during the fill process.
- Grounding, as was done in the incident, should always be done during hydrogen filling. However, even when the fill vessel is grounded, it is not unusual for a hydrogen release to immediately ignite.
- The facility safety deluge water system should be checked periodically for coverage. In this case, a water cannon was a little off target from the last time it was operated and has now be repositioned and stabilized to ensure that it does not move in the future.
- Emergency responders assumed that adjacent tube trailers were heating up from single-cylinder vent flare as a 300°F (149°C) reading was obtained with a thermal device. This slightly delayed the closing of the cylinder isolation valves on the tube trailer. After-incident investigation found no paint discolored or burnt, so the temperature taken by the emergency responders was likely near the flaming vent discharge point.
- Securing hydrogen fill valve(s) at the back of the tube trailer was not dependent on the temperature at the vent stack, as this area was covered by deluge nozzles and located 40 feet (12.2 meters) away from the vent stack.
- Media involvement and resulting speculation can portray a situation as being much worse than it actually is.