Photo: Termap AB
|Plastic materials are often associated with long life and high levels of resistance to corrosive and hazardous chemicals. However there is still no reliable data on long-term effects. Consequently NDT is an essential tool for the assessment of remaining lifetime. Inspecta's Pernilla Utterström, together with Andrey Shipsha, technical specialist and Jan Larsson, testing expert, delve deeply into this problem area.
"Often, the same rules apply to plastic materials as to other vessels, but conventional NDT methods are not enough. They are primarily developed for metallic materials and are sometimes totally unsuitable for plastics and composites," states Pernilla Utterström.
Digital radiography, visual examination and ultrasound developed for plastic
Ultrasonic testing, digital radiography and visual inspection are all NDT methods that have been developed to perform skilled and reliable testing of plastics. Pernilla describes the different methods in more detail.
"These NDT methods work well in the field and verify thickness, joints and detect defects even in thicker plastics", she says.
When examining the service life and status assessments of entire facilities, these methods are combined as they complement each other extremely well.
Pernilla Utterström, Inspecta
|"We have received an increasing number of inquiries and requests for NDT inspections to meet specified standards. For example, during installation inspections where a joint standard is stated in the contract," adds Pernilla.
VIDISCO has developed a portable unit for digital imaging which is particularly suitable for plastics. Test results are processed immediately and evaluated in the computer. The equipment is small-scale and convenient to use. The method was then further developed for glass and carbon fibre reinforced epoxy, VE (vinyl ester) and VEN (novolac vinyl ester) composites. Plastic properties mean that a lower level of irradiation energy is required as compared to testing steel.
Ultrasonic inspection is used to detect delamination and corrosion; however the structure of glass fibre reinforced plastic can make the use of ultrasound fairly complicated.
In order to standardise and improve the visual inspection procedure, Inspecta has devised a method to detect the most common defects. This makes it possible to distinguish manufacturing defects from those that have occurred during operations.
"The difficulty with plastic is that you build the component at the same time as you build the material. With visual inspection it can be difficult to know what is a manufacturing defect and what has been caused by load or stress," says Pernilla and continues, "It is vital to learn to tell the difference between these types of defects. This is why we have also invested in extensive training for our engineers."
Using these three NDT methods it is possible to comply with existing regulations, even when components are made of GRP.
"Unfortunately, there have been some nasty accidents where plastic items have cracked because it has not been possible to examine the objects properly. Using these advanced procedures we have every opportunity to ensure that the material is in good shape," asserts Pernilla.
What do the rules say?
The EU Pressure Equipment Directive is clear as to when non-destructive testing is required of plastics, as well as when periodic inspection by the Work Environment Authority is necessary. NDT is also necessary for quality control during lamination. Both the lamination process and the employees carrying it out must, according to the PED, be approved by a third-party organisation. For the final assessment, a visual inspection is the minimum required by the PED.
Facts about regulations
- NDT is required under current regulations for production according to the European Pressure Equipment Directive (PED) (97/23/EC) and according to Swedish regulations for production control (AFS 1999:4 and 2005:2)
- NDT is a requirement for periodic inspection according to AFS 2005:3.
- NDT is necessary for quality control during lamination. Both the lamination process and the employees carrying it out must, according to the PED, be approved by a third-party organisation.
- For the final assessment, a visual inspection is the minimum requirement according to PED (97/23/EC).
Facts - What is GRP?
Glass fibre reinforced plastic (GRP) is a composite material comprising a plastic matrix (resin) with glass or carbon fibre reinforcement. GRP is a lightweight, high-strength material that is optimal for many industrial applications. For example, GRP is used for pulp mills tanks and pipelines, heating plants, gas treatment plants and in steel and mining industry containers for acids, electrolytic processes and water treatment.