Explosion at the storage unit of a power plant

A hydrogen explosion occurred at the hydrogen storage unit of a power generation plant. A burst disc opened, releasing approximately 3 tons of hydrogen. The leaked hydrogen formed an explosive mixture of hydrogen and air before igniting. A fire followed the explosion, fed by the hydrogen which continued to escape from the storage tubes.
Internal and external fire-fighting teams intervened by cooling the hydrogen tubes and adjacent gas storage cylinders. The fire burned out approximately 2 hours and 20 minutes.
Damage was limited to area nearby the leak, breaking the wall adjacent to the hydrogen storage assembly. A worker passing nearby was jarred but not knocked down.
The investigation revealed that:
(i) The disc failed at a hydrogen pressure (14.5 MPa, 2100 psi) below its rated pressure ( 24.1 MPa, 3500 psi). The cause of the premature disc failure was damage from the atmosphere side, due to formation of ice. Water had accessed the disc vent pipe because it was not equipped with cap for weather protection.
(ii) The force of the release separated the disc from its vent pipe. Therefore the internal pressure caused the pipe to detach, increasing the probability of combustible mixture formation and ignition.
(iii) If the contract would have properly installed and maintained the equipment, this incident would have been prevented. Even in case of a premature rupture of the burst disc, the hydrogen would have very probably been vented via the disc vent pipe, without ignition.

The INITIATING CAUSE was the degradation and failure of a burst disc due to adverse weather condition (formation of ice), which have to be considered a CONTRIBUTING CAUSE.
ROOT CAUSE
According to the investigation, the explosion was the consequence of deficiencies in components of the hydrogen storage assembly, installed and maintained by a contractor. The deficiencies were partially of a design nature (vent pipe weakly fixed to the frame, one was bent 90 degree), partially by bad inspection and maintenance (missing weather protection cap and damage in vent).
In addition, the plant operator, relied completely on the contractor of the installation safety, and did not consider the necessity to supervise contractor’s compliance with the safety rules and standards.

burst disc, vent pipe

The hydrogen storage had operated without incident since its installation till the date of the incident.
In February 1997, ice was found in the vent for the pressure relief valve coupled to the four hydrogen supply tubes. At this time, the hydrogen supplier changed out the damaged pressure relief valve.

DESCRIPTION of the STORAGE UNIT
The unit had been installed in 1990, in a location just outside the corner of the turbine house. The storage was serving the cooling system of the turbine-generator units. It consisted in six hydrogen tubes, each about 7 m long (23 feet) and with a diameter of 0.6 m (24 inches), arranged in layers of three tubes each, surrounded by steel-concrete posts (bollards). Although the sources do not mention the nominal pressure, a 20 MPa value seems plausible. 4 tubes were directly connected to the supply system, while two tubes were stand alone for reserve. It was the burst disc of one of the two reserve tubes which failed, with consequent emptying of both tubes.
The pressure safety system consisted originally in two pressure relief valves, one for the 4 tubes in sue, one for the two reserve one. Each tube had a burst disc connected to a vent elevated about 3 m (10 feet) above ground.
The unit was later equipped with safety detectors, hydrogen flame detectors, and hydrogen emergency vent valves on the generators.

A worker passing nearby (approximately 20 m) was jarred but not knocked down.
Damage was limited to area nearby the leak, breaking the wall adjacent to the hydrogen storage assembly.

(A) This is an example of incident due to lack of maintenance and incapacity to learn from inspection. Information on degradation of subcomponent was available but it was not acted upon and was not communicated to plat operator.
(B) The plant operator should be commitment to guarantee safety also when all operations are sub-contracted and always supervise/control the compliance of contractor’s intervention plans and their execution to own safety management.

(1) The burst discs shall be eliminated from hydrogen storage assembly.
(2) The venting system for the pressure relief valves shall be redesigned to prevent or inhibit moisture build up and allow moisture drainage.
(3) The contractor shall be contractually bound to provide to the plant operator documentation describing the supplier’s preventive maintenance program and the preventive maintenance reports. These documents will have to be reviewed and accepted by the plant operator.