On 28th August 1992, there was a catastrophic failure of a storage tank containing liquefied nitrogen. The failure resulted in the collapse of almost half of the manufacturing site and damage to houses and vehicles within a 400 metre radius. Fragments of the vessel were projected up to 350 metres, the largest of which, a section of the outer shell head was 1.5 metres wide and 8mm thick.
The tank was a double-walled vacuum-insulated ultra-low temperature storage vessel designed to operate at -196°C and 9.3 bar (maximum normal operating pressure).
The inner vessel broke into seven fragments and the outer vessel broke into eleven main fragments and numerous smaller pieces. It was discovered during the course of the investigation that most of the valves on the system were closed including the top liquid inlet, liquid outlet and the isolation valves for the relief valve and bursting disc. The vessel was therefore under completely closed conditions at the time of the accident. The bursting disc was found to be ruptured despite the closed inlet valve, however it was believed that the valve might have been closed after the disc ruptured on a previous occasion. The inner and outer shells ruptured as a result of excessive pressure under closed conditions. It was estimated that the inner shell ruptured at a pressure of 68.7 bar. The pressure reached this level as a result of heat inflow over the sixty days between its final filling and the time of the explosion.
Failures in technical measures
- Isolation valves were fitted below both relief devices without any interlocking system to ensure that one device was always protecting the vessel.
- Relief Systems / Vent Systems: relief valves, bursting discs
- There were no manuals for the operation of the nitrogen vessel. The daily inspections required on the vessel were largely neglected and no safety instructions were given to employees.
- Training: operator training
- Operating Procedures: provision of comprehensive operating procedures
Source: https://www.hse.gov.uk/comah/sragtech/caseliqnitro92.htm
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