Explosion during hydrogen delivery in a power plant

A hydrogen explosion occurred at a 585-MW coal-fired supercritical unit and caused one fatality, injuries to 10 other people, and significant damage to several buildings.
The explosion occurred during a routine delivery of hydrogen when a hydrogen relief device failed, which allowed the contents of the hydrogen tank to escape and be ignited by an unknown source.

The truck driver was delivering hydrogen gas to fill two 6500 cubic feet storage cylinders at the plant. The hydrogen gas was used to cool the generator at the power plant.
At approximately 9:22 a.m., the employer, who was working alone at the time, was in the final stages of filling the cylinders. At this time witnesses heard a loud noise, like high pressure gas venting through a relief valve, and within 15 to 20 seconds, the hydrogen gas exploded, killing the employee and injuring eight co-workers.
The investigation revealed that the hydrogen gas over pressurized a rupture disc attached to one of the hydrogen cylinders. As the set pressure of the rupture disc was between 3500 (24 MPa) and 4000 psi (28 MPa), and the maximum pressure that could have be put on the system during filling was 2600 psi (18 MPa), the rupture disc failed well below its designed pressure.
The release of hydrogen through the relief vent piping, which terminated outside above the roof, also caused the copper vent tubing to fail and burst apart at a 45 degree elbow below the roof, therefore releasing hydrogen gas under the roof. This allowed the gas to accumulate into a large cloud that subsequently ignited and exploded. The force of the explosion killed the employee and injured nine AEP workers in an adjacent building.

INITIATING cause was the failure of a rupture disc well below its nominal rupture pressure
Possible Root Cause was the replacement of rupture disc with another type.
Contributing cause can be found in the possible ill design of the pipe supposed to release the gas above the roof, which failed with the consequence that the gas was released under the roof.

hydrogen storage system, hydrogen trailer

The rupture disc on the storage tanks had been replaced by the hydrogen vendor several months prior, when the vendor was on-site to make repairs related to an apparent leak.
The previous device was a fusible bismuth plug holding a coin-shaped disc in place until temperatures exceed 180 degrees. The replacement relief device assembly did not have a fusible plug to support the disc.

DESCRIPTION OF THE FACILITY
Hydrogen was used at the coal-fired power plant to cool the unit generators.
The on site total storage capacity was of 35 kg.

The storage cylinders were filled once of twice per week by external contractor with a hydrogen trailer.

The trailer used for this delivery carried ten cylinders filled with hydrogen at 2500 psi (17.2 MPa).

The released hydrogen accumulated under an overhanging roof. The gas cloud ignited, creating an overpressure that resulted in substantial structural damages and the fatality of the transport truck driver, who was moving around the truck to stop the delviery after the rupture of the disc.

Nothing is known regarding the emergency action, it is to be assumed that the evetn was so quick that no action would ahve been possible.

OSHA brought enforcement actions against the involved entities as a result of the findings from its investigation of the incident. Those actions initially consisted of 18 citations, nine each against the hydrogen vendor and Ohio Power Co. After an informal conference, the number of citations against each company was reduced to eight. Most of the citations were directed at the design and construction of the hydrogen system.

After depressurisation by gas flare, the equipment was purged with nitrogen, which caused several successive and sudden rises in pressure within the gas flare network along with the valve release.

(from WHA report)
Use good housekeeping and maintenance practices (proper maintenance and management of changes could have improved the integrity of the rupture disc and vent piping).
Design vent stacks to withstand pressure and thrust forces associated with a release ( appropriate materials and design could have prevented the failure in the vent piping).
Design vents to prevent water ingress (best design practices could have prevented corrosion and weakening of the vent piping).
Design weather awnings to prevent accumulation (Proper awning design could have allowed the hydrogen to escape harmlessly into the atmosphere).
Ensure personnel are trained and aware of hydrogen-specific hazards (From initial system design to installation, ongoing use, and maintenance, hydrogen safety training helps defend against hydrogen fires and explosions, protecting lives, property, and business operations).

CONCLUSION of the LEGAL INVESTIGATION (OHSA)
Most of the OHSA conclusion were directed at the design and construction of the hydrogen system.
In addition, the OHSA concluded that
"The employer did not furnish employment and a place of employment, which were free from recognized hazards that were causing or likely to cause death or serious physical harm to employees.
Precautions were not taken to prevent the ignition due to:
(A) Failure to use non-sparking tools (crescent wrench) to open the hydrogen tube trailer main valve;
(B) Failure to shut off the engine of the tractor delivering the tube trailer.
A feasible and acceptable method of abatement would be to remove all potential ignition sources while transferring hydrogen gas from trailers to customer storage cylinders, including:
1. Use non-sparking tools while working on hydrogen gas equipment;
2. Shut down the engine of the tractor before initiating the transfer of hydrogen gas from the tube trailer to the storage cylinders."