There are around 300,000 medium-to-large industrial storage tanks in the EU, mainly in the petrochemical, process plant, pharmaceutical and food processing sectors, of which about 60,000 are made of thermoplastic materials. Thermoplastic tanks are an attractive alternative to metal tanks for storing many products, including hazardous chemicals (e.g. hydrofluoric acid, caustic soda and sodium hypochlorite) due to their outstanding chemical resistance, longer design life and cheaper installation costs.
Thermoplastic tanks are often installed with the view that they will not deteriorate and that therefore they do not need to be inspected in order to assess their overall integrity for continued operational service. However, they do fail, usually by cracking due to chemical degradation or fabrication flaws and this is accelerated by elevated operating temperatures. Virtually all medium-to-large thermoplastic storage tanks are fabricated using containment welds between the base and shell of the tank and at inlet/outlet nozzles. These welds are made using manual processes and are therefore more likely to contain defects that threaten the structural integrity of the finished tank either in the short term or during its service life.
Unlike metallic storage tanks, there are no established procedures for inspecting safety critical welds in thermoplastic storage tank welds, either at the manufacturing stage or during service and consequently they are often used without due consideration of their condition and fitness for service. The development of a reliable and repeatable volumetric NDE technique for examining the containment welds in thermoplastic tanks, both at the manufacturing stage and during service, will open up new and more demanding applications for these products.