Summary
A hydrogen explosion occurred at a plant, damaging a wall adjacent to the hydrogen storage assembly. The investigation revealed that the explosion was the consequence of deficiencies in components integral to the hydrogen storage assembly, and that this assembly belonged to a supplier contracted to provide hydrogen to the plant. The analysis revealed that had the supplier properly installed and maintained this equipment, this incident would have been prevented. By receiving assurance, on an ongoing basis, that the supplier was properly maintaining this equipment, the company could have also reduced the chance of occurrence of this incident.
Background
A hydrogen supplier was awarded a contract in 1990 to supply the plant with hydrogen as well as to provide on-site hydrogen storage. At that time, six hydrogen tubes, each about 23 feet in length and having a diameter of 24 inches, were installed at the northwest corner of the turbine house. They were arranged in two rows of three tubes each, stacked two high. For the purpose of this report, the tubes occupying the top row were numbered one through three, from north to south. The tubes on the bottom row were similarly numbered four through six. Tubes 1, 2, 4, and 5 were used to supply hydrogen to the station. Tubes 3 and 6 served as reserve storage and were isolated from the four supply vessels.
A valve and piping arrangement was fitted to the east end of the tube six-pack. It included two pressure relief valves: one plumbed into the four supply tubes and the other being shared by the two reserve tubes. Burst discs were fitted to each of the storage tubes at the west end of the six-pack. Vent pipes rose from each burst disc to an elevation of about 10 feet above grade. Five of the six vent pipes were plumbed straight and near vertical, and were each secured to the tube frame by two U-clamps. The vent pipe fitted to the No. 6 hydrogen tube was not straight; rather it contained two 90° bends, and it was secured to the tube frame by only one U-clamp. Concrete-filled steel bollards, located at 6-ft centers, surrounded the hydrogen storage assembly.
Findings
The burst disc fitted to the No. 6 hydrogen storage tube had ruptured, allowing 14,500 cubic feet of hydrogen stored in the two reserve tubes to release to atmosphere. This burst disc was rated at 3,300 to 3,700 psi. However, the tank pressure at the time of the failure was only 2,100 psi. A pressure relief valve, set at 2,450 psi, was available to respond to any over-pressure condition. There was no known cause for, or evidence of, an over-pressure condition being reached. Therefore, the burst disc must have ruptured prematurely.
The remains of the burst disc were retrieved and submitted to a local engineering firm for examination of its fracture surface using a scanning electron microscope. This examination revealed that the burst disc failed from an initial overload imposed on the atmospheric side of the disc. The only known mechanism by which the burst disc could have been loaded in this manner is by the expansion of water as it freezes to ice while in direct contact with the burst disc. Temperature profiles are consistent with the formation of ice (tank temperatures were recorded to be 0° C for 48 hours preceding the incident, and ambient temperatures recorded at a nearby weather station dropped to -6° C on the evening preceding the incident).
Water could only access the burst disc after passing the vent pipe cap and a plastic weather membrane integral to the burst disc valve body. Caps were found on only two of the six burst disc vent pipes (on the pipes venting the No. 1 and 4 hydrogen tubes). Two additional caps were found on the ground under the tube six-pack. The four recovered caps were constructed of rubber and all were markedly degraded; each cap contained large cracks on their side surfaces. Two coats of paint, red over brown, covered the black rubber caps; brown paint was present on the crack surfaces - indicating both coats of paint were applied to the caps after the cracks had formed! The degraded state of these caps would have readily allowed water to enter the vent pipes.
The plastic weather membrane for the No. 6 burst disc valve body had been consumed by the heat of the fire. Examination of the remaining five membranes revealed each to be markedly degraded. If their condition represented that of the weather membrane for the No. 6 burst disc valve body, then water could readily by-pass No. 6 burst disc weather membrane.
The uncontrolled release of hydrogen occurred as a result of the rupture of the No. 6 hydrogen storage tube’s burst disc. This disc failed in response to being overloaded by mechanical stresses developed as water expanded and formed ice while in direct contact with the burst disc. It was the degraded condition of the vent cap (defective equipment) that enabled water to access the burst disc.
- As a corrective action, eliminate burst discs from hydrogen storage assembly. Redesign venting system for the pressure relief valves to prevent or inhibit moisture build up and allow moisture drainage.
The investigation uncovered two instances where the supplier was in possession of information ("safety data") that, if successfully conveyed to plant management and subsequently acted upon, would have prevented or reduced the chance of occurrence of the subject incident. Specifically, the hydrogen supplier found ice in a vent pipe, and was aware that the vent caps were cracked (recall the cracks were painted). Had a requirement existed for this information to be communicated to the plant, then plant management would have had the opportunity to evaluate and potentially influence the supplier's maintenance and operations program.As a corrective action, contract documents for the hydrogen and nitrogen supplies will be modified to stipulate the following:
- Suppliers of potentially hazardous equipment will provide plant management, for acceptance purposes, with written documentation describing the supplier’s preventive maintenance program.
- The supplier shall provide the plant representative with a copy of a preventive maintenance report upon the completion of each PM check performed by the supplier. The supplier shall expeditiously rectify any identified deficiency.
- Plant management will recommend to the Manager of Corporate Safety and Health that the above contract document modifications are implemented corporate wide.