|TWI has been instrumental in the success of a collaborative project that sought to develop inspection techniques for welded joints in thermoplastic storage tanks.
The PolyTank project has developed ultrasonic non-destructive examination (NDE) procedures, techniques and equipment for the volumetric examination of welded joints in thermoplastic tanks of diameters up to ten metres.
Now drawing to a close, the two-year project saw eight organisations from five different countries combine their expertise to provide a solution to the longstanding problem of thermoplastic tank integrity.
Thermoplastic tanks are an attractive alternative to metal tanks for many products, including hazardous chemicals (eg hydrofluoric acid, caustic soda, sodium hypochlorite), due to their outstanding chemical resistance. However, while owners and operators of metallic storage tanks have well-established requirements for their inspection, thermoplastic equivalents are often installed with the erroneous belief that they will not deteriorate, and therefore do not need to be inspected in order to assess their overall integrity for continued operational service.
Furthermore, until now there have been no established procedures for inspecting thermoplastic storage tank welds, neither at the manufacturing stage nor during service, and consequently tanks could have been used without due consideration of their welds’ condition and fitness for service. Most thermoplastic tanks fail by cracking due to chemical degradation, fabrication flaws or elevated operating temperatures.
The PolyTank development programme was based on the examination of welded joints containing idealised flaws of known size and shape that are representative of the real flaws that can be present in welds in thermoplastic storage tanks in service.
Scientists analysed the generated NDE data to determine the limits of flaw detection for each developed technique and procedure. At the same time, 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 designed and built as part of this project has been assessed under both laboratory and field conditions, using tank samples containing real service flaws.
The development of this volumetric inspection technique will open up new and more demanding applications for thermoplastic tanks and bring their benefits, including resistance to corrosive environments, longer operating life and cheaper installation and maintenance costs, to many industries, including petrochemical, process plant, pharmaceutical, water treatment and food processing.
The PolyTank project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no. 313950.
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TWI Ltd (The Welding Institute)