Ultrasonic testing of thermoplastic welding joints
Although thermoplastic tanks are designed for a service life of around 15 to 25 years, many tanks remain in use beyond this time because of economic pressures. Further, they often store chemicals for which they were not designed. Most fail by cracking, but expertise is lacking regarding the visual inspection of tanks. Combined with the danger and time required to empty the tanks and enter them for evaluation of structural integrity, non-destructive evaluation (NDE) methodologies are drastically needed. Companies from different European countries combined their expertise in POLYTANK(opens in new window) (Development and validation of an automated ultrasonic system for the non-destructive evaluation (NDE) of welded joints in thermoplastic storage tanks), determining potential failure mechanisms in plastic tanks and developed ultrasonic NDE procedures, techniques and equipment that can identify them. A newly developed NDE inspection system involved ultrasonic examination of the full weld volume, from the outside surfaces of the tank. This means it is not necessary to open up a tank to prepare the inside for examination. Until now, full volumetric examination of tank welds has not been possible. Project partners produced a range of welded joints containing idealised flaws of known size and shape that are representative of those present in thermoplastic chemical tanks in service. Then they analysed the NDE data to determine the limits of flaw detection for each developed technique and procedure. In parallel, they established the significance of flaw size and quantity in relation to service requirements. This was achieved through comparing the results of long-term mechanical testing of joints containing known flaws with those for welds containing no flaws. The prototype ultrasonic NDE equipment consists mainly of a modular inspection manipulator system, a scanning system and a high-power ultrasonic transducer. In addition, it has control software, data analysis software, and ultrasonic data acquisition and evaluation instrument. The prototype was assessed under laboratory and field conditions, using tank samples containing real service flaws. Data on the inspection procedures developed will be put forward to the European Committee for Standardization (CEN) as the basis for producing a European standard.
Keywords
Ultrasonic testing, welding joints, thermoplastic tanks, non-destructive evaluation