Piping Non-Destructive Testing: Conventional vs. Computed

 

Contributed by Jim Hughes, Director of Construction QA/QC & Welding Technology

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Piping Non-Destructive Testing: Conventional vs. Computed

For over three decades H+M has worked on capital projects of varying sizes and scopes. One common denominator we see is Welding and with pipe welding comes the need for Radiography. Radiography is a way to take a volumetric image (or picture) of the weld to determine its quality. Radiography is one of the oldest, Non-Destructive Testing (NDT) methods used throughout pipe fabrication and maintenance projects. It looks for initial discontinuity and defect detection, corrosion monitoring and product conformity. Two common ways to assess weld quality are Conventional Radiography and Computed Radiography.

Conventional vs Computed

Due to industry norms and what is most commonly requested from clients, we utilize Conventional Radiography most of the time, even with having the ability to do both. Conventional has been challenged as an industry standard by Computed Radiography since it came into play in 1987. This is partly due to how labor-intensive Conventional Radiography can be, but that is not to say that the Computed method does not present its own challenges. Figure 1 shows a standard Conventional Radiography setup.

First used in the medical industry and then the industrial sector, Computed Radiography is becoming more and more prevalent in the Energy and O&G markets. I would say about 80% of NDT companies can perform Computed Radiography. It is just a matter of determining the needs of the industry and what is best for the project in question.

On average, Computed Radiography, over the project lifecycle, can cost as much as 25% less than Conventional and cuts out 50% of the time needed to complete the same process. This is due to a variety of reasons such as shorter shot times, fewer disruptions at the job site, smaller exclusion zones, and faster turnaround. Since Computed records are all digital, it also allows for easier storage and faster transferability. One deterrent from using Computed Radiography is that it tends to have higher price tag right off the bat, and it can be hard to quantify the cost savings due to the intangible benefits it presents.

The most important benefit in my mind is the faster analysis and higher accuracy. NDT companies can now use digitized radiographs created by Computed Radiography. These digitized images have much greater data accuracy than the hard copies we see with Conventional Radiography where the film is much like the X-Ray your doctor shows during an exam. Any profile work done on a pipe by Conventional Radiography can now be performed by Computed Radiography. This work depends on the ability to take in large amounts of information quickly, which is where Computed Radiography steals the shows.

Initially, Conventional might be cheaper but the overall cost savings Computed can achieve due to time saved in the field is something to consider. It is also important to note that using Computed imagery during NDT has many inherent challenges of its own that the seasoned film radiographer must be aware of. If the learning curve is not addressed properly, these conditions can be detrimental to the overall inspection results. The characterization and evaluation of a Computed image requires additional training, over and above current requirements found in ASME Sec. V Article 2, SNT-TC-1A, and CP-189 for the typical industrial radiographer.

Figure 1 – Standard Conventional Radiography Setup

Implementing Computed Radiography

Implementing Computed Radiography begins with a fundamental understanding of the radiographic principles that have been practiced for decades with film radiography and used by the aerospace, automotive, construction, energy, oil and gas, and shipping industries. Certified Level II radiographers are required to understand these principles, along with specific knowledge in the manufacturing and fabrication process to correctly interpret the acquired digital image. Geometric principles attributed to the setup are essential in the evaluation of mechanical or fatigue defects and for assessment of repair work.

Computed Radiography, as defined by ASME and ASTM (photo stimulated luminescence method), is a two-step radiographic imaging process. First, a storage phosphor imaging plate, which replaces standard radiography film, is exposed to ionizing radiation. Second, the luminescence from the plate’s photostimulable luminescent phosphor is detected, digitized and presented on a high-resolution computer screen. See Figure 2.


Figure 2

Managing this process while working to achieve the optimum digital image is the challenge faced by radiographic practitioners. The overall success is based on the digital image acquisition system utilized and variables that are consistent with typical radiographic conditions.

The utilization of the proper Image Quality Indicators ensures technique, contrast sensitivity, and resolution, and is essential to meet industry specifications and code compliance. Development of an operator and company defined QC program is crucial to monitor equipment performance and maintain records of data collected which ensures a stable continuous process. In addition, having a detailed QA auditing program shows compliance with COC and specifications. System and scanner performance tests vary from manufacturer to manufacturer but must include quality control checks specified by the manufacturer and be modeled to meet the technician’s requirements. If, at the time of inspection, significant equipment malfunctions are found, the technician may be required to perform more frequent testing to ensure good image quality.

Final acceptance of the image is the same as film radiography and is the responsibility of the certified Level II technician, and Manufacturer. Most industries are updating reference radiographs for defect severity and comparisons are being updated to digital formats for ease of interpretation. Preserving the raw image is critical and maintaining its origin is a requirement by most industry codes, specifications, and end-users. It is the operator’s responsibility to maintain this data set to ensure recall in the native format as needed. Another benefit of having digital images is that the shelf life is forever. This enhances our ability to turn over information digitally instead of hard copy. Hard film takes up space, can be lost and deteriorates over time and that deterioration can be accelerated by improper storage.

A New Industry Standard?

Despite a higher initial price tag and some additional training needed, Computed Radiography takes most of the disadvantages of Conventional Radiography out of the equation. Shot times are essentially cut in half, there are no chemicals needed to process film, and interpretation can be done on a computer screen instead of having to review the conventional film on a film viewer as seen in Figure1.

As an example, a 20” Sch. 40 weld has 6 views or pieces of film. To review properly it would take a Radiographer around 20 minutes to review that film using conventional tools, such as a viewer, and densitometer. Using Computed Radiography and reviewing film on a computer screen (see Figure 2) you can see all 6 images at one time. Then, by just clicking on each one you can see the image, the Image Quality Indicators, (which takes out the need for a densitometer) and all other information, such as line number, weld number, welder I.D and client information. The time of review now takes about 7 to 10 minutes.

Conventional Radiography is not going away any time soon, but it is important to identify other technologies that are possibly more effective in determining volumetric weld quality. As the industry adapts and changes to the Digital Age, adding Computed Radiography to the discussion is important to present the best options to clients.

References, ASME Sec. V Article 2, SNT-TC-1A, ASNT CP189.

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Jim Hughes, Director of Construction QA/QC & Welding Technology

CWI, ACCP, SNT-TC-1A Level III VT & RT-F

Jim has 27 years of experience working in the Power Market, OGC, LNG, and Midstream, acting as Sr. Quality Manager and Welding SME, on projects ranging from $30 million to $850 million. His last assignment before joining H+M was Corporate Welding Technical Services Manager. His current responsibilities are as Director of QA/QC and overseeing Welding Technology.