Overview of Radiography
Radiographic Testing (RT) is a nondestructive examination (NDE) technique that involves the use of either x-rays or gamma rays to view the internal structure of a component. In the petrochemical industry, RT is often used to inspect machinery, such as pressure vessels and valves, to detect for flaws. RT is also used to inspect weld repairs.
Compared to other NDE techniques, radiography has several advantages. It is highly reproducible, can be used on a variety of materials, and the data gathered can be stored for later analysis. Radiography is an effective tool that requires very little surface preparation. Moreover, many radiographic systems are portable, which allows for use in the field and at elevated positions.
Types of Radiography
There are numerous types of RT techniques including conventional radiography and multiple forms digital radiographic testing. Each works slightly differently and has its own set of advantages and disadvantages.
Conventional Radiography
Conventional radiography uses a sensitive film which reacts to the emitted radiation to capture an image of the part being tested. This image can then be examined for evidence of damage or flaws. The biggest limitation to this technique is that films can only be used once and they take a long time to process and interpret.
Digital Radiography
Unlike conventional radiography, digital radiography doesn’t require film. Instead, it uses a digital detector to display radiographic images on a computer screen almost instantaneously. It allows for a much shorter exposure time so that the images can be interpreted more quickly. Furthermore, the digital images are much higher quality when compared to conventional radiographic images. With the ability to capture highly quality images, the technology can be utilized to identify flaws in a material, foreign objects in a system, examine weld repairs, and inspect for corrosion under insulation.
The four most commonly utilized digital radiography techniques in the oil & gas and chemical processing industries are computed radiography, direct radiography, real-time radiography, and computed tomography.
1) Computed Radiography
Computed radiography (CR) uses a phosphor imaging plate that replaces film in conventional radiography techniques. This technique is much quicker than film radiography but slower than direct radiography. CR requires several extra steps compared to direct radiography. First, it indirectly captures the image of a component on a phosphor plate, then converts the image into a digital signal that can be visualized on a computer monitor. Image quality is fair but can be enhanced using appropriate tools and techniques (i.e, adjusting contrast, brightness, etc. without compromising integrity). It’s important to know how tools, such as adjusting contrast, effect the image. Care should also be taken to make sure minor defects are not hidden after enhancements are made.
2) Direct Radiography
Direct Radiography (DR) is also a form of digital radiography and very similar to computed radiography. The key difference lies in how the image is captured. In DR, a flat panel detector is used to directly capture an image and display that image on a computer screen. Although this technique is fast and produce higher quality images, it is more costly than computed radiography.
3) Real-Time Radiography
Real-time radiography (RTR), like it’s name suggests, is a form of digital radiography that occurs in real time. RTR works by emitting radiation through an object. These rays then interact with either a special phosphor screen or flat panel detector containing micro-electronic sensors. The interaction between the panel and the radiation creates a digital image that can be viewed and analyzed in real time.
The brighter areas on the image are a result of higher levels of radiation that contact the screen. This corresponds to the thinner or less dense section of the component. Conversely, darker areas are a result of less radiation interacting with the screen and indicate where the component is thicker.
Aside from being able to make the images available more quickly and analyze them in real time, RTR has several other advantages. One being that digital images don’t require physical storage space and thus are easier to store, transfer, and archive than film.
On the other hand, this method has several disadvantages as well. Compared to conventional radiography, RTR has a lower contrast sensitivity and limited image resolution. Images created via RTR often suffer from uneven illumination, limited resolution, a lack of sharpness, and noise. These factors have a major impact on image quality.
4) Computed Tomography
Computed tomography (CT) is a technique that takes hundreds to thousands (depending on the size of the component) of 2D radiography scans and superimposes them to create a 3D radiographic image.
In an industrial setting, CT can be achieved in two ways. In one method, the component to be inspected remains stationary while the radiation source and x-ray detector rotate around the component. This technique is more likely to be utilized for large components. The second method consists of the radiation source and x-ray detector remaining stationary while the component is rotated 360 degrees. This second technique is more useful when the component is small or has complex geometry.
Although this technology is timely, expensive, and requires a large amount of data storage, CT provides highly accurate images, is repeatable and reproducible, and minimizes human error.
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