Guidelines For Nondestructive CUI Detection Techniques For Process Pipelines
Inspecting pipelines for corrosion under insulation (CUI) has long been a challenge for the process industry.
However, advancements in non destructive testing (NDT) techniques along with proven technologies have enabled the industry to conduct cost-effective, comprehensive CUI inspections without requiring the removal of insulation or coatings.
NDT techniques commonly used in the process industry include:
- Profile radiography
- Computed radiography (CR)
- Guided wave ultrasonic technology (UT)
- Infrared (IR) thermography
- Pulsed eddy current (PEC) testing
Profile Radiography
Profile radiography is a simple yet powerful technique for inspecting piping which is vulnerable to corrosion as it is able to detect reduction in internal wall thickness.
It is a convenient technique for the process industry because of the many situations where it is not possible to cut open piping or remove the external insulation of the piping for the purposes of inspection.
One limitation of the technique is that it is used to inspect small bore piping (up to approximately 20cm in diameter) where the source-to-film (SFD) distance covers the entire pipe diameter and where the is sufficient clearance between pipelines.
That said, it is possible to use profile radiography for inspecting wider diameter pipelines for CUI by taking multiple shots of sections of the pipeline to complete the full diameter but this process is time-consuming.
Computed Radiography
Computed Radiography (CR) uses a similar technique to conventional radiography but is a filmless process and enables radiographic images to be viewed from a computer. The computerized images allow easy data sharing and storage of results for future comparisons or audits. Provided the appropriate safety precautions are taken, CR is an effective NDT tool for determining the presence of corrosion under insulation (CUI) and can deliver significant improvements in radiographic inspection productivity as well as faster identification of defects.
Guided wave ultrasonic technology (UT)
Guided wave ultrasonic testing is also commonly referred to as long-range ultrasonic testing (LRUT), and is a highly effective screening tool for rapid testing of long lengths of pipe – particularly in systems that are hard to access.
The advantage for the process industry is that pipeline inspections can be carried out from a single inspection point, and the technology enables 100% coverage of the pipe wall without necessitating removal of any insulation or coating.
The technology works by propagating guided waves into the material (eg an insulated pipe wall) from a ring of equally spaced ultrasound probes. These waves are reflected back when they encounter any discontinuities such as a reduction of wall thickness (which would indicate CUI).
The results from these reflected waves can then be analysed by UT specialists with the help of advanced software.
GWUT works most effectively if the pipeline is straight and can typically scan lengths up to 120m. Bends in the pipeline, flanges, support locations, connections, tees, welded supports, socked welds, and other obstacles will reduce – and even eliminate – the inspection scan and additional inspection arrangements should be made before and after every disruptive feature to guarantee 100% coverage.
That said, UT is still one of the most effective and widely used tools in the process industry to identify CUI in pipelines.
Infrared (IR) thermography
Infrared thermography detects infrared energy emitted from an object, converts it to temperature and displays the result as an image of temperature distribution.
This technique can be used effectively to identify wet insulation in pipelines and because infrared cameras can record thermal images without making direct contact with equipment, pipelines can be scanned from a distance. This saves time and costs and improves safety.
IR thermography is a faster and more sensitive option than conventional moisture gauges and can be used for non-traced insulated pipelines as well as heat-traced ones.
Typically, IR surveys to inspect for wet insulation are conducted two hours after sunset for optimal results because wet insulation retains heat longer than dry insulation. Because the temperature difference between wet and dry insulation isn’t very large, a small temperature span should be used to increase sensitivity and accuracy.
Pulsed eddy current (PEC) testing
PEC is a fast, cost-effective advanced electromagnetic inspection technology used to detect flaws and corrosion in ferrous materials which are typically hidden under layers of coating, fireproofing or insulation.
Because PEC inspections don’t require direct contact with the object under test (unlike conventional eddy current testing), measurements can be made through coatings, insulation materials, weather sheeting and even corrosion products provided they don’t conduct electricity. PEC scans can also be done on pipelines with extremely high operating temperatures (up to 5000C).
That’s why the technique is widely regarded as the front-runner in tackling the problems that pipeline CUI poses to the integrity of assets and plant operations in the process industry.
However, PEC has a relatively low sensitivity compared to radiographic systems and is generally only able to detect flaws that have a diameter around 50% of the thickness of the insulating material. The technology is well-suited for testing for general wall loss and CUI scanning over a larger area, but can’t detect small, isolated defects.
Advanced PEC systems feature dynamic scanning modes and real-time, intuitive C-scan imaging for quick and easy detection of defects. They remove the need to strip cladding and insulation over the component under test in order to perform inspections, making them ideal for fast, cost-effective CUI inspection.
For the process industry, there are number of NDT techniques and systems which offer a fast, cost-effective and safe option for inspecting for CUI without requiring any insulation to be removed.
Ongoing advancements and innovation in the field of NDT enable operations in the process industry to better manage their costs, improve quality and performance, enhance site safety and improve process integrity when inspecting for CUI. It’s also crucial to choose the most appropriate materials, adjust operating parameters accordingly, control the corrosion environments and have systematic procedures in place for proper NDE inspection.
The experience and industry knowledge of a specialist technical equipment supplier like Nexxis can be a valuable asset in achieving the above goals. Over the years, Nexxis has partnered with many organisations in the process industry to identify the most innovative, efficient, economical and flexible solutions that deliver optimum NDT and CUI inspection outcomes.
For more information, please contact the experts at Nexxis.
