Fire from a hydrogen storage tank

A hydrogen fuel tank release caused a fire at a hydrogen refuelling station.
A pressure relief valve installed on one of the 18 storage tanks failed at a pressure far below its design opening value. The failure triggered the immediate release of approximately 30 kg of hydrogen in the first minute and a total of approximately 300 kg. This rapid release of hydrogen mixed with air in the vent tube and subsequently ignited producing the loud “boom” reported by eye and ear witnesses. After the pre-mixed gases were consumed the venting hydrogen produced a jet flame emanating from the outlet of the vent system.
The flames from this vent scorched the top of the canopy of the manual dispenser causing combustion of the paint and dust and superficial damage to the canopy.

The fire lasted for more than 2 hours, at which point the local fire department allowed technicians of the stations and experts to enter the station and stop the flow of the gas.
The fire brigades and the local authorities evacuated several businesses and a high school and closed several road intersections.

The INITIATING CAUSE was the accidental activation of a pressure relief valve installed on one of the storage tanks.

The pre-mature failure was the use of materials that were incompatible with hydrogen services (440C steel is affected by embrittlement in presence of hydrogen).

ROOT CAUSE was a design shortcoming. A CONTRIBUTING CAUSE to the escalation of the event was the lack of timely information regarding the status of the system. Another CONTRIBUTING CAUSE was the station design (interaction of canopy and venting system).

pressure relief valve

DESCRIPTION OF THE FACILITY
The compressed hydrogen storage system of the refuelling station had 18 high-pressure storage tanks, which were protected by pressure relief valves designed to open before pressures was high enough to rupture the storage tank itself.

I nadditional t othe comressed gas storage, the hydrogen fuelling station comprised:
(1) a low-pressure liquid hydrogen storage tank,
(2) an electrolyser, and
(3 ) a dispensers for both heavy-duty vehicles (buses) and light-duty vehicles (cars).

The station was capable of fuelling 12 buses in a 24-hours period using hydrogen made on site with an electrolyser (65 kg/day) and liquid hydrogen vaporized into a gas for fuelling. The liquid hydrogen supply was replenished via truck delivery, typically every five or ten days. On average, the station contains 1800 to 1900 kg of hydrogen.

Post-incident analysis of the failed valve demonstrated a poor choice of materials in the valve used for hydrogen service. The type of steel in the failed valve is known to weaken in the presence of high-pressure hydrogen gas. This characteristic was made worse in this case because the steel was excessively hardened.

Post-incident analysis of the responses from AC transit, EFD and Technicians indicate that the EFD response was appropriate given the information available to them at the time of the incident. However, deviations from the District's written procedures, as contained in the Emergency Response Plan, caused delays in providing critical information pertaining to the precise location of the fire and the pressure in hydrogen storage tanks.

Post-incident analysis of the station design revealed that failure of only one of the 18 pressure relief valves allowed for the discharge of the pressurized gas stored in all 18 high-pressure storage tubes.

The cause of the incident was the use of materials in the valve that are incompatible with hydrogen service. Apart from the failed valve, the hydrogen system functioned as designed, venting the hydrogen gas at a safe distance above surrounding structures and keeping the subsequent fire away from personnel and equipment.
The investigation generated the following recommendations:
1) Replace pressure relief valves with devices specifically designed for hydrogen service. This requires an analysis of the materials used in components throughout the system (440c stainless steel should not be used for valves. Type 316 austenitic stainless steel is an example of suitable material and is used extensively in gaseous hydrogen systems).

2) Update communications plan relative to all responsible parties. In particular establish process responsibility or “ownership” to better centralize the flow of information.

3) Update training documentation based on timeline analysis.

4) Perform refresher training and “table-top” drills with key personnel. Incorporate continuous improvement principles.

5) Modify the vent systems to ensure relief vent outlets are sufficiently above and oriented away from vulnerable equipment.

6) Improve process system considering isolation of sub-sets of the storage system.

7) Modify the fire detection systems to identify hydrogen flames on the system.

8) Improve sub-contractor and sub-supplier qualification process for those companies who provide safety critical equipment