Choosing RT over PAUT/ToFD but should you?

By: Preston Canada??(NDT SME / Client Solutions / Sales) & Jose Aparicio Director Of Technical Services / Co-Founder at MetallurgyNDT, LLC

Choosing RT over PAUT/ToFD but should you?

Using radiography on your code welding project??Most people choose radiography over Phased Array (PAUT) but should you??At first glance radiography is a cheaper service.?Price per hour or shift is generally lower.?What about production values on each? ?What about soft costs due to radiation exclusion zones??What about Info shots with ToFD for large and thick pipe? Are there advantages with PAUT/ToFD?

Radiography

Radiography has been around for over half a century for weld inspection in field applications.?Most applications are still using film as the image medium.?Computerized radiography (CR) plates are commonly used but have limitations in field applications.?Computerized radiography plates are reusable and will scratch in use.?This makes them costly and not always practical in harsher field environments. Film and computerized radiography have similar exposure times and can be considered?equivalent shot times.?

?Piping 1.5” and larger in diameter is often radiographed in opposite wall techniques (contact shot) with an IR-192 isotope source.?When 36” and larger piping is welded and internal access is present a panoramic technique is often used for weld radiography.?Entering a pipe to set up a radiographic exposure is considered a confined space entry event.?Proper training, equipment and procedures are needed.?Below are exposure times for some example diameters of pipe.?Using 1.7 R factor for shot times and 80 Curies of IR-192.?Using extra heavy wall piping due to extended service life.??

The last line with 54” contact shot technique using IR-192 is not practical but included for comparison.?All projects will have finial tie-in welds that cannot be internally accessed and panoramic techniques cannot be used.?CO-60 would be a better choice for radiography even though the image quality of CO-60 is poor and the safe radiation exclusion zones are massive.?PAUT does not have this limitation with thicker materials.?

These exposure times do not seem to be impactful at first glance, but the exposure time is part of the story on the time it takes to radiograph a weld.?Contact shots are relatively quick to set up for a seasoned radiographer, 30-60 seconds between shots when the weld is in easy access.?Panoramic shots on the other hand take time to set up properly.?What needs to be factored in is the time to enter the pipe and accurately place the source in two dimensions.?The source needs to be equal distance from the pipe walls and centered in the middle of the weld.?If the source is not in the center of the pipe diameter the exposure for radiation to the film will be uneven.?Minor variances are acceptable.?Even more critical is centering the source in the middle of the weld root.?Having the source a few degrees of center will place the weld in an offset orientation.?This will introduce a limitation on finding lack fusion, incomplete penetration and other linear indications like cracks.?This condition can go unnoticed on QC of the film quality because image quality indicators (IQI) may still achieve the correct resolution.?The accuracy of the source placement is critical and difficult to repeat.?The time to set up a panoramic exposure can be 30-60 minutes.?Degrees of difficulty on access vary the times.?Due to the complicated set up and set up time many radiography crews will expose the welds and leave radiographic setup in the pipe while they process the film.?This film process is 45-60 minutes.?Start to finish 90 minutes to 120 minutes on a single weld.?PAUT can complete the scan in 10-15 minutes with high levels of probability of detection of defects.?Once the weld is cool enough to PAUT scan the test can be completed and the QC check of the process is completed generally prior to the next weld is completed.?In the radiography process of QC the welds are generally shot during windows of the other personnel not working.?PAUT keeps the welders welding and provides much quicker feedback on process control.?PAUT crews may be more per hour than radiography crews but the loss of production greatly outweighs the minor difference in costs in a single inspection crew.?????????????????

As base metal thicknesses increase and/or diameter the exposure time increases.?This slows the production and increases the radiation exclusion zone.?This zone is where non radiation monitored personnel cannot be present during an exposure.?This results in radiography work being off shift, during windows where all workers leave the area or the welds being moved to a radiography area and then back.?All three of these radiography limitations slow your progress on the welding.?PAUT can be performed in the middle of the action as soon as the weld is cool enough to scan.?None of the other work needs to stop.?This also allows for nearly instant feedback as quality control in the process.?Find a problem and fix it before multiple welds are created with the same condition.?High end PAUT specialists will use Time of Flight Diffraction (ToFD) in conjunction with PAUT on materials that are but not limited to 0.750” thick and thicker.?This allows for the highest probability of detection and sizing.?But an added benefit is ToFD can be performed after the root pass to ensure a solid start to the weld as a QC check.?

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PAUT Production values

?Above is the discussed softer costs to radiography vs PAUT.?What are some of the direct costs on production values? Below is a table with some basic weld inspection estimates per 10 hour shift.?The variance in the amount of welds scanned is based on variances in access.?Welds on spools in a lay down yard are much quicker to inspect than a weld that needs to be accessed off of a man lift or scaffolding.?For the production values in PAUT the technique being assumed is automated PAUT techniques.?This technique would include ToFD for the materials but not limited to 0.75” thick or thicker.

The upper end of the welds of the production values are based on best case scenarios and 2 person crews.?Radiography by law requires a 2 person minimum for safety.?Exposing a radiographic isotope with only one person present is forbidden.?This means that film cannot be processed while the crew is performing radiography without adding a third person to the operation. ?

PAUT is faster than radiography in a head to head comparison.?A single PAUT crew can equal two crews performing radiography and this production advantage increases as the welds become thicker.?Add in the soft costs that have to be considered with radiography and PAUT for your construction projects can save money even with a higher crew cost.?

?PAUT combined with ToFD

Above it was mentioned the PAUT should be used with ToFD for thicker materials.?There are advantages that ToFD can provide as the base material reaches but not limited to 0.75” or thicker.??ToFD is not required by API 1104, the code requirement is covered by PAUT.?But why would some companies insist on using ToFD in conjunction with PAUT??

?ToFD can be added to the PAUT scan set up and does not add any time to the scanning process.?First advantage is that ToFD is very sensitive to porosities, slag, linear, and crack-like indications and laminations.?ToFD is a single pass scan that works very quickly.?High probability of detection and very accurate on sizing of defects.?This when combined with PAUT allows for cross referencing and reduction in oversizing indications and false calls.?ToFD also provides a full axial view of the weld and Heat Affected Zone (HAZ).

?Any downsides to ToFD??Notable downside is that it requires an operator that knows how to properly set up ToFD techniques.?The additional equipment required is generally not the limiting factor.?Upside of ToFD for the welding contractor is information shots “Info Shots” on your weld prior to fully finishing the weld.?ToFD can be used after root pass or hot pass as a QC check, avoiding high cost and time on material removal during repairs.?ToFD can detect HI/LO conditions, root conditions and other information that could save a full weld cutout.?Full weld cutouts on welds that are but not limited to 0.75” or thicker and on larger diameter piping can be costly on money and time.?If you have a PAUT/ToFD crew onsite the added advantage of using ToFD can save serious time and money by ensuring that welding processes have periodical QC checks.????

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Conclusions

?Should you consider using PAUT or PAUT with ToFD on your next project? ?Yes.?Especially if your project has a high volume of welds.?If your project has larger diameter piping.?If your pipe schedules are heavy.?If you are using more than one radiography crew.?If you would like to take advantage of QC checks prior to completion of welds.?If you would like to not have to worry about radiation and whether or not your radiography crew has a hot enough source.?If you would like to have the crews keep welding while the inspection crews perform the inspection then PAUT is the best option.?Save money and reduce hassles.?PAUT should be considered the better option if these considerations are important to your project