Development and Validation of an Automated
Ultrasonic System for the Non-Destructive Evaluation
of Welded Joints in Thermoplastic Storage Tanks
Project number: 313950
Start date: 1 November 2012
Project duration: 2 years
Funded: within the EC - FP7 'Capacities' programme under the call for 'Research for the Benefit of SMEs'.
Plastics, such as high density polyethylene (HDPE), polypropylene (PP), polyvinylidene fluoride (PVDF) and polyvinylchloride (PVC) offer significant advantages over metals for the storage of chemicals such as acids, caustic soda, detergents and other corrosive liquids. They are chemically inert, tough, flexible and lightweight. However, plastics also have relatively low maximum operating temperatures and loads, which vary from plastic to plastic, as does chemical resistance. For this reason it is important that plastics tanks and storage vessels are designed and built to the relevant European standards.
It is estimated that there are over 50,000 plastics tanks in service in Europe, ranging in size from 150 litres to 100m3. Many of these store materials that are classified as hazardous and therefore have the potential to cause injury or death to personnel and damage to property and the environment in the event of a tank failure. Indeed, there have been a number of reported failures of plastics tanks in service.
Plastics tanks are normally designed for a finite life, usually between 15 and 25 years. However, due to economic pressure, many of these tanks are still in operation beyond their design life, often with little or no engineering justification. It is also not uncommon for plastics tanks to be used for storing chemicals that they were not designed to contain. For these reasons it is very important that operators of plastics tanks and vessels inspect them throughout their life. An issue at hand is that there are currently no standards for the in-service inspection of plastics tanks. There is also very limited expertise available on the visual examination of plastics tanks and virtually no use of non-destructive examination (NDE).
The majority of visual inspections are external and can therefore only identify cracks that break the outside surface of the tank. Since many of the cracks initiate from the inside of the tank there is already a leak path through the tank wall if and when the crack is detected. Internal inspections are carried out less frequently, if at all, because they are expensive, potentially dangerous to the inspector, and result in a shut-down because the tank has to be emptied.
The PolyTank project will determine the potential failure mechanisms in plastics tanks and storage vessels and develop ultrasonic NDE procedures, techniques and systems to be able to identify these. An important aim of the project will be to develop an inspection system that is site-rugged and simple to operate.
As part of the project, welded joints representative of those used to fabricate plastics tanks and vessels will be manufactured containing known flaws. These will be inspected and the NDE data analysed to determine the limits of flaw detection. In parallel, the significance of flaw size and quantity will be established in relation to service requirements. This will be achieved by long-term mechanical testing of joints containing known flaws, and comparison with results for welds containing no flaws. The prototype NDE equipment, designed and built as part of this project, will be assessed under both laboratory and field conditions, and the inspection procedures developed will be submitted to CEN as the basis for producing a European standard on this subject.
The PolyTank project will establish the following:
The project funding mechanism will be through the EC - FP7 'Capacities' programme under the call for 'Research for the Benefit of SMEs'.
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