A Look at Digital Radiography for Corrosion Inspection
Digital radiography is faster, safer and more efficient than conventional film radiography, thus making it an ideal corrosion inspection tool for complex piping networks.
Imaging technology has revolutionized the pipeline inspection industry, making it possible for inspectors to assess the condition of various types of pipelines with minimal impact on the piping material.
One of the most popular imaging techniques, X-ray inspection, has been used in the oil and gas industry for over 100 years. The constant evolution of this technology has led to a multitude of innovations. One of the latest developments to come out of the advancement of X-ray technology is digital radiography (DR).
What is Digital Radiography?
Digital radiography (DR) refers to a group of advanced radiographic inspection techniques that use digital imaging as opposed to traditional film. This method uses X-ray radiation to produce digital images during pipeline inspections. This contrasts with traditional X-ray, where films are used for image capture and display.
Some of the immediate advantages of digital radiography include:
- The elimination of costly film processing
- The ability to immediately preview inspection images
- Enhanced overall image quality
- Fewer errors due to over- and under-exposure
- The ability to apply post-processing techniques to improve the quality of the image
Types of Digital Radiography
For pipeline corrosion inspections, two of the most common types of digital radiography techniques are computed radiography (CR) and fluoroscopy/real-time radiography (RTR). (Another techniques that doesn't use X-rays are discussed in the articles Using 3D Laser Analysis for Nondestructive Testing and Evaluation of Pipeline Corrosion and Guided Wave Ultrasonic Testing for Non-piggable Pipelines.)
Computed Radiography (CR)
Computed radiography (CR) uses X-ray producing devices similar to those found in conventional X-ray testing; however, instead of film, images are captured on reusable imaging plates coated with a phosphor-based material. The plate is then scanned by a laser scanner, which produces a digital image that can be viewed, edited and shared by a computer.
CR systems can be easily transported to the field for on-site weld and wall thickness profile inspections for both insulated and uninsulated piping structures. If needed, CR images can also be retrofitted onto conventional film-based systems, thereby eliminating the need for costly processing chemicals and equipment.
Fluoroscopy/Real-Time Radiography (RTR)
Fluoroscopy, or real-time radiography (RTR), works by converting invisible X-rays into visible forms of light. X-ray radiation is emitted on one side of the material, where it penetrates the object and is captured by sensors on the other side.
These sensors, which work by fluorescence, convert X-rays into light to produce real-time digital images. Fluoroscopy, which is faster and safer than traditional radiography, can be used to quickly reveal corrosion and other internal pipeline defects. Like computed radiography, this technology is also considered to be nondestructive, making it especially useful for detecting corrosion under insulation (CUI) without having to remove the lining.
Detecting Corrosion with Digital Radiography
Corrosion is one of the greatest issues plaguing industries that rely heavily on piping distribution networks. If left unchecked, corrosion deteriorates the pipe's wall thickness (i.e., wall loss), which can lead to loss of structural integrity, product loss and in extreme cases, ruptures and explosions.
However, what makes pipeline corrosion particularly dangerous is that some defects are not readily visible. Unseen corrosion, such as those occurring inside the pipe or under insulation, can progress for months or years before they manifest as leaks or spills. (For a case study, see INFOGRAPHIC: The El Paso Natural Gas Company Pipeline Explosion.)
Nondestructive digital radiography techniques, like computed radiography and fluoroscopy, provide inspectors with a clear, safe and nondamaging assessment of the pipeline’s interior. Corrosion inspections with digital radiography involve the evaluation of shadow projections. Because the X-rays penetrate the pipeline, digital radiography has the ability to display pipe wall thicknesses.
Automated edge detection and filter algorithms identify inner and outer wall surfaces to produce accurate thickness measurements along the length of the pipe. By analyzing the variations in the wall thickness, inspectors can detect early signs of metal loss and points of weakness, which may be consistent with corrosion degradation.
Applications of Digital Radiography
The nondestructive nature of digital radiography makes it ideal for identifying defects in sensitive pipeline infrastructure such as those found in the oil and gas industry. The level of detail that can be obtained by this type of imaging technology allows it to be used for numerous inspection applications, including detecting and measuring the following:
- Uniform corrosion
- Erosion corrosion
- Corrosion under insulation (CUI)
- Flow-assisted corrosion
- Corrosion under pipe supports
- Welding corrosion
In addition to corrosion, digital radiography can also be a useful tool for identifying:
- Pipeline blockages
- Valve operational issues
- Liquid-vapor interfaces
Benefits of Digital Radiography
One of the main benefits of digital radiography is its ability to produce images of similar contrast to conventional radiography using up to 80% less radiation. This significantly reduces the likelihood that inspectors and other workers will develop health issues due to radiation exposure, thus making digital radiography inherently safer than conventional X-ray inspections.
Overall scanning times are also reduced. Unlike other inspections methods, digital radiography does not require the erection of scaffolding, access platforms and complex logistics. Additionally, no mechanical probe adjustments are needed over wide dimensional ranges. Therefore, digital radiography can result in significantly less operational downtime and also yield faster results. Reduced downtime also translates to decreased cost of the radiography process.
While digital radiography can produce images with similar resolution to conventional X-rays, the post-processing techniques that are applied to the digital image can improve its overall quality. Image editing software can also be used to make other adjustments to the digital X-ray to improve the measuring and inspection capabilities.
Other benefits of digital radiography include:
- Reduced exposure time
- Elimination of film waste
- Improved data management
- Automated image archiving
Early corrosion detection can mean the difference between minor repairs and costly replacement. Digital radiography technology, which is considered to be faster, safer and more efficient than conventional film radiography, can help operators optimize preventative maintenance and preserve the integrity of their infrastructure.
Written by Krystal Nanan | Civil Engineer