In the nondestructive testing (NDT) field, industrial radiography is, arguably, one of the most expensive methods to perform. With regard to safety, however, it stands alone due to the risk of exposure to ionizing radiation. Many NDT methods share an inherent safety risk due to environmental hazards, but radiography is the only major method with high-dose radiation concerns. Due to the introduction of digital radiography techniques, those concerns are growing smaller with every innovation.
Modern equipment reduces the required overall radiation exposure to fractions of the original dose. This reduction in total dose allows the operation to take place in a much smaller square-footage than ever before, leaving more space for other trades to perform billable work. In fact, the right piece of equipment, though contracted at a higher hourly rate, could save a fabrication shop a considerable amount of money. Choosing the right modality could be the largest key to success.
The top two contenders for digital imaging options are computed radiography (CR) and digital detector array (DDA). Each has its own characteristics which make them effective at dose reduction, but each have limitations that could be critical. CR operations could see a 50% radiation dose reduction while DDA may see a 75%+ reduction for the same inspection.
Digital Detector Array Safety Pros and Cons
If parts for inspection are very small or relatively flat, DDA may be the answer to your problems. The detector array is, however, rigid, and relatively thick which makes it difficult to use in some applications. Through some engineering and modification, a DDA panel could be mounted in one specific location for manufactured parts to pass through the ‘imaging field.’ The panel allows for a direct-type imaging which requires no processing like CR or conventional film.
The DDA panel also affords another luxury to a radiographic installation: a target wall. Since the panel would be rigidly mounted, the floor or wall behind the panel could be shielded heavily in a dedicated location. The source of radiation could then be focused with shielding, or ‘collimated’, into only an appropriately sized beam. This keeps radiation levels down in occupied areas while targeting unoccupied areas for high doses. It also means that the cost of shielding comes down substantially. Instead of four concrete walls, an exposure area could get by with one or two. Most fabricators with this type of installation could also cut the exposure area down to a quarter of what film would require.
DDA does, however, need an installation and exposure plan to be truly effective at time reduction. That installation would likely need to be custom fabricated and the exposure plan made by an NDT professional. Those things cost time and money, but the savings could be huge. A long-seam pipe, that is, a pipe with a welded seam down its length, could take an hour to expose on film, then another half-hour of processing. Exposure with DDA could take minutes and requires no secondary processing. Total throughput could be 10 times shorter in this situation. There are endless custom configurations for DDA, so it’s relevant to many manufactured parts.
Computed Radiography Safety
If your inspection subjects aren’t easily adaptable to DDA, computed radiography may be more appropriate. Some inspection subjects have complex configurations, making them complicated to handle. In the case of process or spool piping, configurations tend to be varied in diameter as well as length. Valves and other castings tend to have varying thicknesses along with a small bore. CR will excel in conforming to these parts. The reduction of the radiation dose is not as drastic as DDA, but a reduction of half is nothing to scoff at.
That same reduction could be made to the dimensions of the exposure area, allowing additional trades to work safely in the saved square footage. For construction sites, space is always at a premium. A smaller barricaded area means other workers won’t be shut down, making more productive hours throughout the site.
CR also has a portability comparable to that of film, so it can completely replace film operations with no real issues. For the time being, film is still the lower cost option. However, that will change in the future as film gets more expensive, harder to find, and less requested.
Chemical-Free Industrial Radiography
One thing that both CR and DDA share is a chemical-free process. A traditional darkroom configuration is centered around processing silver-based film in reactive chemicals. Care is generally taken to ensure no chemicals are lost at a site, however, while the chances for a spill are low, they are never zero. Some radiographers do not follow the necessary steps to prevent spills, so they may leave a spill for the client to clean.
Digital modalities don’t require chemicals for processing. If secondary processing is required, it is achieved through the application of an energy and sensors. CR, for example, requires secondary processing to complete imaging. That processing is achieved by a laser, which stimulates the imaging plate, and a sensor, known as the PMT, rather than chemical processes. DDA requires no secondary processing which speeds up exposures and makes corrections go smoothly and quickly.
This absence of chemicals makes for a cleaner environment, but also eliminates a maintenance step. Chemicals deplete over time which demands replacement. A portable darkroom must travel to an appropriate location for chemical dumping and a stationary unit would need to be changed on-site. This represents, roughly, 300-plus pounds of liquid chemicals requiring containment and secondary containment to avoid spillage. The cost of correct handling can be relatively high, but the incorrect handling may cost thousands in fines.
A Safer and Cost-Effective NDT Operation
There are many reasons to choose CR or DDA over film. Less radiation exposure to the public means that workers, radiographic and non-radiographic, pick up less radiation in the long run. A smaller footprint for radiographic operations allows workers to be safe at shorter distances. Chemical-free processing also helps with hazard exposure on any site. These things all lead to a safer and more cost-effective operation in the end.
Computed Radiography Webinar
Interested in learning more about the transition from traditional film to computed radiography? Click the link below to sign up for a webinar with the author, Wesley Soape.