Acoustic emission testing inspects welds under jacketsAcoustic emission testing inspects welds under jackets

Chicago--Tank cars often transport a variety of hazardous chemicals and gases. To ensure the integrity of such containers, Union Tank Car has developed a nondestructive acoustic emission (AE) test that monitors elastic strain waves induced into a container. The method is so sensitive that the impact of sand, snow or rain, and expansion due to sunlight are easily detected.

Originally, Monsanto applied AE to container inspection in the 1980s as an alternative to hydrotesting. This method was also used to monitor the Alaskan pipeline, and more recently fiberglass cables.

Union Tank took the testing one step further. Engineers standardized procedures to double the sensors used during a stub sill twist test, to help better pinpoint the emission source location. About 30 electronic sensors from Physical Acoustics (Princeton, NJ), operating at 150 kHz, are placed in zones around a tank. The car's welds and structural integrity are stressed during pressure, jack, and sill twist tests. In the pressure examination, the car begins at rest and is gradually pressurized with water. The pressure is maintained for 30 minutes. A finite element model verifies that adequate stress is applied to the welds to generate emission.

A sill twist test applies torsion to the railcar end frames. 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," says Tom DeLafosse, manager of inspection services for Union Tank Car.

The sensors detect the effect of elastic strain waves generated by the movement of a defect. "We measure signal strength, duration, and amplitude. A computer records the data in milliseconds, and a technician filters out any distortion at the end," says DeLafosse. The complete system has a 40-decibel threshold. "We can pick up corrosion, lining failures, and other growing weld defects without removing the contents or taking the car jackets off."

The procedure not only determines if the tank is free of structural defects, but saves time and money. According to Dow Chemical Company Superintendent of North American Rail Services Floyd Bengston Jr., "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. 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 saving between AE and a visual test is at least 4 or 5 to 1."

Union Tank Car works with shippers including Dow Chemical and industry organizations such as the Chlorine Institute and the Association of American Railroads to perfect this test method.

The FRA requires in-service tanks to receive an approved inspection. The AE test is the only FRA approved method where the jacket does not have to be removed. Because of this, AE is now the most cost-effective test for shippers' existing fleets.