Failure Analysis of a Compressed Natural Gas Storage Cylinder

In this work the rupture of an 80 liters capacity CNG storage cylinder installed at a CNG sales station was investigated. It was reported that the cylinder had ruptured only a few months after installation. During the initial investigation, the material of the cylinder was found to be in compliance with the specifications of composition and mechanical properties. However, thorough visual examination of the ruptured surfaces indicated the presence of multiple crack initiation sites within a rusted region on the inner surface of cylinder. This observation indicated the potential for stress corrosion cracking. Further macro-examination of the crack established this feature. Metallographic examination of areas adjacent to the fractured surfaces showed the presence of deep draw-marks almost everywhere and a longitudinal fold of unusual depth. These defects might have acted as stress raisers to assist stress corrosion cracking. It is suggested that the defects were present in the failed cylinder due to improper inspection procedures. (author)