Failure analysis of in-service Type-II CNG cylinders from composite part damaged by hexagon flange bolt

초록

This paper investigated the failure of in-service Type-II compressed natural gas (CNG) cylinders whose composite parts were damaged by contact with hexagon flange bolts, i.e., clamp bolts, in buses. Eighteen Type-II CNG cylinders that had been in service for 7-10 years were examined: mechanical tests and chemical analysis were conducted on 12 cylinders; extreme temperature cycling tests on 4 cylinders; ambient temperature cycling tests on 2 cylinders. The results showed that pre-existing damage of composite part increased in size under cyclic pressurization, and cylinders with severe composite defects eventually ruptured. These experiments confirmed that the interference from hexagon flange bolts, leading to damage of the composite part is a critical safety hazard. It is recommended that clamp attachment bolts should be redesigned or installed with sufficient clearance, e.g. a smaller-head hexagon bolt and a washer, to avoid contact with the cylinder, and that periodic re-inspections be mandated to detect composite damage before catastrophic failure occurs.

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