Radiography (RT) and phased array (PAUT) are the primary volumetric inspection technologies in use today verifying weld quality. Traditionally, RT has been the dominant method to verify weld quality. However, phased array in lieu of RT has made major inroads in recent years as a preferred method for several reasons. We are going to explore the pros and cons here because we want you to make an informed choice.
To start off, I’ll let you know that we perform both methods because each is capable of detecting discontinuities like cracks, lack of fusion, and porosity with a high probability of detection. Both methods are acceptable per ASME and API codes.
Therefore, the variability as far which method to use comes down mostly to the characteristics of a particular test and situation The situation variable that I see leads to Phased Array in Lieu of RT is the work area. Does the RT test shut down other work? If so PAUT in Lieu of RT’s value benefits become very significant.
Here is a brief examination of each method with some pros and cons.
Conventional radiography using gamma sources is still the workhorse method and has proven its reliability and cost-effectiveness time and time again. When compared with UT, it is more capable with smaller volumetric flaws but less so with planar flaws. It is usable in extremely diverse situations.
Considerations of RT when compared with UT:
· There is a risk of radiation exposure, meaning work around testing sites must stop and each area barricaded until the tests are completed.
· Many inspectors are comfortable reviewing RT Film and Images and are not comfortable reviewing PAUT information.
· Additional licensing/training required.
· Results are not typically not instant – film must be developed or Computed Radiography (CR) plates must be remove and connected to a computer. Digital Radiography (DR) exists which gives immediate images however the cost and fragility have kept it from becoming main stream.
· Film developing involves chemicals and some waste material.
· Re-shots due to film movement or other factors like wrong exposure add to expense.
There is a large available workforce for RT vs. PAUT which has a more limited less available qualified workforce. Ultrasound
PAUT is becoming preferred for more use cases as time goes by due to its portability, cost savings, and availability of qualified personnel.
· Phased array ultrasound is often combined with TOFD (time-of-flight diffraction) since most scanners can perform both methods simultaneously. This results in much higher Probability of Detection (POD). Often, we see that welders take more care when they know that PAUT is the inspection method. I cannot prove this but listening to feedback this is a generally agreed upon statement.
· Allows for defect height, length, depth and orientation measurement vs. just type and length, enabling better decision-making as to the seriousness of discontinuities and remaining service life.
· Can detect stacked flaws in the same circumferential location – which RT cannot do.
· Real-time analysis for instant evaluation and feedback – and issues with scans can be taken care of before leaving the job.
· Digital delivery.
· No hazardous chemicals or radiation.
· Work can continue in immediate vicinity while testing is performed.
As you can see, both methods have their place in the NDT industry, but I think more companies will choose phased array in a greater number of cases as time goes on. The portability, flexibility of scan types (C-scan, S-scan, etc.), high POD, and potential cost savings are compelling.
As mentioned above one important drawback is the comfort level of inspectors reading and evaluating PAUT information. Often inspectors are not comfortable auditing the information from PAUT. They are typically very comfortable auditing or having RT film/images audited. For this reason, we offer a ½ day – ¾ day course to teach “interpretation.”
It’s hard to know which method is best for your specific needs without talking it through with an expert. If you need help determining which method you should employ for your inspection and maintenance plan, we’d be happy to schedule a brief consultation with a specialist who can guide you. Just reach out!
Published By: Darren Billings