The Use of “Fitness for Service” Assessment Procedures to Establish Allowable Flaw Sizes in Steel Cylinders

Abstract

As part of the U.S. Department of Transportation safety regulations, seamless steel cylinders that are used to transport high-pressure gases are required to be periodically retested during their lifetime [1]. The safety regulations have recently been revised to permit the use of ultrasonic methods for retesting steel cylinders. These ultrasonic test methods permit the quantitative determination of the size of any flaws that are detected in the cylinders. Therefore, to use these ultrasonic test methods it is required that quantitative, “allowable flaw sizes” be established to set acceptance/rejection limits for the cylinders at the time of retesting. Typical flaws that can occur in seamless steel cylinders during service are line corrosion, gouges, local thin areas of corrosion, notches, and cracks. To establish “allowable flaw sizes” for seamless steel cylinders, an assessment of typical flaws that occur in seamless cylinders was first carried out to establish the “critical flaw sizes” (e.g., depth and length or area) for selected types of flaws. The critical flaw size is the size of the flaw that will cause the cylinders to fail at either the designated test pressure or at the marked service pressure. The API Recommended Practice 579 “Fitness-for-Service” was used to calculate the critical flaw sizes for a range of cylinder sizes and strength levels [2]. Several hundred monotonic hydrostatic, flawed-cylinder burst tests were conducted as part of an International Standards Organization (ISO) test program to evaluate the fracture performance of a wide range of steel cylinders [3]. The results of these tests were used to verify the calculated “critical flaw sizes” that were calculated using the API 579 procedures. These results showed that the analysis conducted according to API 579 always underestimated the actual flaw sizes to cause failure at test pressure or at service pressure. Therefore, the “Fitness for Service” assessment procedures can be used reliably to establish the “critical flaw sizes” for cylinders of all sizes and strength levels. After the “critical flaw sizes” to cause failure of the cylinders at both the test pressure and the service were established, the “allowable flaw sizes” were calculated for a wide range of the cylinder types and strength levels. This was done modifying (reducing) the size of the “critical flaw sizes” for each cylinder by adjusting for fatigue crack growth that may occur during the use of the cylinder. This results in the final “allowable flaw size” criteria that are used for defining the acceptance or rejection of the cylinders during retesting. This paper presents the results of the analytical and experimental work that was performed to establish the “critical flaw sizes” and “allowable flaw sizes” for a wide range of high-pressure gas cylinders.