The Use of “Fitness for Service” Assessment Procedures to Establish Critical Flaw Sizes in High-Pressure Gas Cylinders

Typical flaws that can occur in high-pressure seamless gas cylinders during service are: corrosion pits, line corrosion, gouges, local thin areas of corrosion, and cracks. The required periodic inspection of seamless cylinders requires that “critical flaw sizes” be established. To establish “critical flaw sizes” an assessment of typical flaws that occur in seamless cylinders was carried out using the analytical procedures described in the API Recommended Practice 579 “Fitness-for-Service.” To verify the API 579 analysis procedures, a number of hydrostatic tests were conducted on selected cylinders with various sizes of flaws to determine the burst pressure of cylinder containing flaws. These results showed that the analysis conducted according to API 579 reliably estimated the actual measured burst pressure of the cylinders for all flaw sizes and types. After the API 579 analysis procedures were verified by these experiments to reliably estimate the burst pressure of cylinders with various types of flaws, the “critical flaw sizes” to cause failure of the cylinders at selected pressures were determined by analysis. This paper presents the results of the analytical and experimental work that was performed on the assessment procedures to establish the “critical flaw sizes” in high-pressure gas cylinders.