Quantitative Nondestructive Evaluation of Railroad Tank Cars

Abstract

A rulemaking issued by the Department of Transportation (DOT) revises Hazardous Materials Regulations (HMR) to replace the hydrostatic pressure test with appropriate nondestructive evaluation (NDE) methods. The rule change is contained in Federal Register 49 Code of Federal Regulations (CFR) Part 180.509, “Requirements for inspection and test of specification tank cars,” paragraph (e) “Structural integrity inspection tests” [1]. The CFR authorizes liquid penetrant (PT), magnetic particle (MT), radiography (RT), ultrasonic (UT), and optically aided visual testing (VT) as allowable NDE methods for structural integrity inspections and tests. Other NDE methods may be allowed under special exemption issued by the Federal Railroad Administration (FRA) Office of Safety. Also included under the requirements of 49 CFR Part 179.7 is the need to qualify not only NDE personnel, but the procedures used to perform NDE reliably. In order to be effective, federal regulations require that the NDE methods have a proven sensitivity and reliability for finding the type and size of flaws likely to cause a tank car failure. In the early 1970s, an internationally accepted quantitative approach that assesses the probability of detection (POD) was developed for the National Aeronautics and Space Association (NASA) and was published in NASA CR-2369, February 1974 [2]. Transportation Technology Center, Inc. (TTCI), under contract with the FRA, and along with industry participation, uses the NASA approach to determine the POD for various NDE methods used in the inspection of railroad tank car circumferential butt welds (girth seam welds), fillet welds, and leak test samples. The emergence of a damage tolerance approach to determine inspection intervals for an engineered structure — in this case a railroad tank car — requires the quantification of the detectable flaw size for the NDE methods used during inspection. Damage tolerance techniques have initiated an evolution in NDE understanding, methods, and requirements. National Transportation Safety Board safety recommendations R-92-21 through R-92-24 address the suggested process of performing reliable inspection of railroad tank cars based on a damage tolerance approach [3]. NDE quantification using the POD approach is a key measure of NDE effectiveness and is integral to damage tolerance requirements. TTCI, working with the FRA, Railroad Tank Car Industry and D&W Enterprises (A NDE consulting company providing expertise in the area of NDE POD), has developed baseline POD curves for the allowed NDE methods. Initial evaluations were performed on the inspection of tank car circumferential butt welds. Subsequent efforts focused on butt welds, longitudinal fillet welds and leak test samples requiring inspection under the CFR. This paper reports quantitative results obtained during this research effort that address system safety and risk analysis during handling and transportation of railroad tank cars carrying hazardous materials.