Assessment of inspection criteria and techniques for recertification of natural gas vehicle (NGV) storage cylinders
Creators: name: Ribarits, S. G.
Publisher: name: University of British Columbia
Year: 2008

description: In-service natural gas vehicle (NGV) storage cylinders are subject to various forms of damage which can degrade structural integrity. In recognition of this, the Canadian Transport Commission (CTC) requires that cylinders be recertified for service every five years. Current standards for NGV cylinder recertification are based on cylinder behaviour during a hydrostatic test (pressurization to 1.67 x service pressure) and specify that a cylinder should be removed from service if either the measured plastic expansion exceeds 10 % of the total cylinder expansion or the cylinder ruptures. These criteria have been established, for the most part, to ensure a minimum cylinder wall thickness during service. A serious deficiency of the current assessment criteria is that they do not specifically address the possibility of sub-critical crack growth that may occur between inspections due to the combined effects of aggressive environmental conditions and cyclic loading. Evidence of sub-critical crack growth in sectioned in-service NGV cylinders however, has raised concerns over the effectiveness of the current recertification criteria and points to the need to determine whether these criteria are adequate to ensure against in-service failure. The objective of this research is to evaluate the current standards for NGV cylinder recertification. In this investigation, various fracture mechanics based methodologies are investigated in view of their applicability to predicting cylinder rupture. Elastic-plastic finite element models of untracked and cracked NGV cylinders are utilized to predict cylinder behaviour (volumetric expansion/crack opening displacement) during a hydrostatic test. These analyses, in conjunction with small scale critical crack tip opening displacement (CTOD) test results and full scale burst test results indicate that a critical CTOD approach, modified by a plastic collapse analysis for small defects can be used to accurately predict cylinder rupture. Analysis of full scale numerical and experimental results indicates that there exists a range of defect sizes which, if present in a cylinder, would not violate the current hydrostatic test acceptance criteria (i.e., volumetric expansion/rupture). This finding, in conjunction with available fatigue crack growth estimates indicates that it is possible for cylinders containing this range of defect sizes to fail in service during the subsequent five year inspection interval. This result indicates that current hydrostatic test criteria are non-conservative. In view of the inability of current retest procedures to reject cylinders containing defects which may lead to in-service failure, the feasibility of utilizing acoustic emission (AE) techniques for NGV cylinder inspection is also investigated. Full scale cylinder AE tests indicate that it is possible for cylinders with similar defect sizes to exhibit markedly different AE characteristics (i.e., hit rate, amplitude distribution, etc.) due to other AE sources such as corrosion. However, the potential for using AE techniques for inspection of cylinders is discussed.