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DOW
STRESSES FLEET SAFETY WITH ACOUSTIC EMISSION
TESTING
When it comes to quality assurance, Dow Chemical Company superintendent of North American rail services Floyd Bengston, Jr. always looks for the best technology. That's why Dow is a major advocate of acoustic emission (AE) testing. This ultra-sensitive test monitors elastic strain waves induced into an object to locate even the smallest irregularities. Union Tank Car was a pioneer in adapting the procedure to examine tank cars, and is still the only railcar manufacturer or lessor to offer in-house AE testing. The demand for AE testing has been moderate so far, but Bengston predicts that things will change very soon. "Under the new Federal Railroad Administration (FRA) rule changes, in-service tank cars must receive an approved under-the-jacket tank inspection, as early as 1998 in some cases," he explains. "The only alternatives to AE testing for existing fleets involve stripping the jackets and removing the insulation. AE is not only an effective test, but it allows the car to remain intact." AE examination can be used for testing weld and structural integrity by applying pressure to a tank, and by jacking to stress the sill and other frame areas. In the tank pressure test, the car begins at an "at rest" state, then is gradually pressurized with water to a prescribed level and held for 30 minutes. This examination can detect even the tiniest irregularities in a tank. In the sill-twist test, jacks placed at the body bolsters put rotational stress in one direction, then the other. "It's a bit like holding a pencil between your hands and twisting it, but on a very large scale," explains Union Tank Car project engineer Tom DeLafosse. "It is an excellent way to confirm weld quality." About 30 electronic sensors placed around the car record the effect of elastic strain waves in an AE test. During the course of the examination ( about eight hours for complete pressure, jacking and sill-twist tests ) a technician monitors the ultra-sensitive equipment on a computer terminal. "We've detected the impact of sand, rain, even snow-flakes on a tank with AE equipment," reports DeLafosse. "It's incredibly responsive." A defect is graded on the com-puter at levels of severity progressing from A to D. Level C and D defects are traditionally regarded as the only ones requiring corrective action. However, Union Tank Car reviews test data even at levels A and B as an extra precaution. The accuracy of AE has proven superior to virtually any other examination, including the most exhaustive visual inspection. "We had ten cars examined by the best visual inspectors available, and then AE tested," DeLafosse says. "AE found more imperfections; and some of them were so small that we had to actually heat up welds to get them to separate a bit. Those types of defects just aren't visible to the naked eye." And unlike most inspection procedures, an AE test can locate a potential problem on the inside of a tank in a component that is hidden from view. It also provides a permanent computer record of all data collected, which can be reviewed at any time in the future. This helps AE even better serve the new FRA rules, which require a well-documented quality assurance program. Safety plus savings Sweeping rule changes ushered in by FRA docket HM-175A/201 (see Spring 1996 Trans-Action) will add another benefit to AE testing: cost savings. In answer to the FRA requirement of an acceptable under-the-jacket inspection, AE offers the advantage of not having to cut into a tank car's outer jacket and insulation. According to Dow's Bengston, that adds up to some serious savings. "We've used AE for sill-twist examinations on 31 chlorine cars, and we estimate that we've saved close to $200,000 on that test alone," he maintains. "When you combine the cost savings of not having to clean the car and remove components, as well as extra time out of service, we figure that the ratio of savings between AE and a visual test is at least 4 or 5 to 1." The FRA has officially approved AE for stub sill twist tests, and has issued Union Tank Car an exemption to use AE tank tests to comply with HM-201 (CFR49, part 180.509) for structural integrity inspections. Union Tank Car is working with shippers, including Dow, and industry organizations such as the Chlorine Institute and the Association of American Railroads (AAR), to develop some of the finite element analysis requirements for AE testing of particular types of tank cars. "Once this is in place, I think the floodgate will open for AE as the preferred inspection method for in-service cars, particularly insulated ones," DeLafosse predicts. "We're looking at requiring AE tests on all new tank cars we lease or buy," says Bengston. "From a quality standpoint, it just makes sense." Some major rail shippers such as Monsanto and Shell Oil already require AE testing on some types of new tank cars. The biggest challenge to AE test facilities soon may be keeping up with the demand. Union Tank Car has bolstered its capacity to include AE testing at ten locations, plus a mobile unit. But as the company's welding and non-destructive examination coordinator Alan Giffin points out, "Becoming certified as an AE inspector is an extremely rigorous process. We find that it usually takes about two years for a technician to acquire the knowledge and experience in specific application to tank cars and proper data analysis. You can't just open up shop one day and begin AE testing." That point is certainly not lost on Dow, which owns or leases about 7,300 tank cars. "I think that as the FRA compliance dates get closer, facilities that can do AE testing are going to be swamped with work," he forecasts. "We intend to be the first in line, not the last."
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