Ultrasonics vs. X-Ray
A quick look at the advantages of using UT over X-Ray
Conventional X-Ray NDT methods have been used for inspection purposes for many decades. The method of using radiographs (RT) to get a picture of components that have been manufactured or damaged due to process has not come a very long way, in terms of technological advances, since the beginning. Many inspectors have cut their teeth on RT and used the method for many years are reluctant to give any other inspection technologies a chance to prove their usefulness. Most Digital RT systems still require conventional RT techniques and film shots to be performed before the digitization of the film or shot can occur.
Ultrasonic testing has developed many new methods and has made several technological advances since it was first introduced as an inspection method. Different techniques such as advanced sizing methods, Time of Flight Diffraction, and Phased Arrays have given UT testing incredible abilities to outperform RT inspections on many different levels.
Safety
Safety is first and foremost on everyone’s minds. RT inspections require technicians to be licensed by the state. They must carry RT safety cards and dosimeters to make sure they do not get overexposed to radiation they use. RT trucks carry large vats of dangerous chemicals that are necessary for the development of the film. Inspection areas must be cordoned off or components removed from high traffic areas and placed far away from facilities in order to protect surrounding individuals from unnecessary radiation exposure. This can be difficult to do in small facilities or during Turnaround/Shut Down times when many different personnel are in the same area performing different tasks necessary for overall success. There are also different components in chemical plants and refining facilities that can be damaged or caused to malfunction when exposed to the high doses of radiation used by RT crews and equipment.
Ultrasonic examinations use no radiation and no harmful chemicals. UT technicians and surrounding individuals and equipment are not in danger of harm from the UT equipment they use. Inspections can be performed right next to welding/fabricating personnel as well as other personnel and processes with no side-effects. The equipment is small, portable, and usually handled by one technician when good access to components is feasible.
Quality
RT inspections produce visible images of the insides of components. RT is mostly used for the inspection of welds. RT has the ability to find almost all of the weld-related defects that can form during weld manufacture or through in-service process. It is difficult to see crack and crack-like defects using RT unless positioning of the camera is at the proper angle when the shot is made. Inspections are very problematic and the technician is unsure of the quality of the shot until the film is developed. Several things can go wrong during an RT shot. Not enough/too much exposure time, bad film, wrong camera position, bad chemicals cause image distortion, etc. If the shot is bad for any reason, it must be re-shot. This uses up valuable time and resources, especially if there are surrounding crews waiting to get back to work. Inspection times are long enough due to preparing the area, cordoning off and setting up barricades, placing film and camera into position, taking the shot, breaking down the equipment and area.
UT testing results are immediate and accurate. Almost all of the weld-related defects that can form during weld manufacture or through in-service process can be found using UT inspection. The user friendly equipment allows necessary changes to be made to the inspection to produce better results and images. The technician knows immediately if changes need to be made to a calibration or if different techniques need to be used to enhance the inspection. There is little or no waiting time between inspections. Cracks and planar flaws and slag/porosity or volumetric defects are readily detected by UT inspections. Many advanced techniques like ToFD and Phased Array allow for digital images of the UT signals to be produced and analyzed on the spot. UT also allows for through wall sizing/depth of defects to be presented. RT does not provide through-wall location or depth of defects. Knowing exactly where the flaws is, the length of the flaw, and the depth of the flaw allows manufacturers and repair crews to pinpoint the location and repair the area without disturbing more of the component necessary.
Breakdown
Let's look at a breakdown of the two inspection methods over several different areas.
| RT Inspection | UT Inspection | |
| 1.0 Safety | Radiation Hazard. | No specific hazards. |
| 2.0 Accuracy | Provides location, length, though characterization of defect may be difficult. | Provides location, length, depth, sizing, and characterization of defects. |
| 3.0 Personnel | Require State certification, Two-man crews. | API certification available. |
| 4.0 Cost | From $800.00 to $1,800.00 per day depending on location. | From $600.00 to $2,200.00 per day depending on method and technique used. |
| 5.0 Training | General RT training required. No Advanced training except for Digital RT. | API certification and Advanced training for advanced methods available. |
| 6.0 Inspection Time | Slow setup time, from 4-12 shots per hour if in close proximity. | Short setup, Inspection tims. Immediate results. Up to 20 welds per hour. |
Conclusion
The comparisons between RT and UT Inspections show, in general, that UT is a safer, faster, more reliable inspection method. Costs between the two are comparable and depend largely on the methods used. Thick components such as heavy walled reactors and vessels can be inspected much easier and safer than using RT methods. While there are some applications where RT is a better suited inspection method, nearly every RT inspection can be done using UT methods.
