Accept/reject criteria of high pressure steel and aluminum gas cylinders

摘要

Traditionally recertifying steel and aluminum gas cylinders has been done through visual inspection and hydrostatic testing. Recently, ultrasonic inspection methods have been developed which offer quantitative measurements of wall thickness and flaw sizes, in comparison to the effectively qualitative global condition assessment which traditional inspection methods provide. For the new quantitative measurements to be employed, however, a critical flaw size which represents the maximum allowable flaw size to set acceptance/rejection limits at the time of retesting, must be established. In this structural integrity analysis, NTIAC analyzed data acquired from extensive testing of steel cylinders containing flaws; comparable data for aluminum cylinders were not available. To establish critical flaw size, the procedures outlined in API Recommended Practice 579, Fitness For Service, (API 579) were employed. The critical flaw sizes as found from API 579 were then corrected to account for any time-dependent degradation that can occur in service, e.g. crack growth or stress corrosion. These maximum allowable flaw sizes then set the acceptance and rejection criteria for a wide variety of cylinders.