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Corrosion Surveys of NPS 6 and Smaller Lines with High Production Digital Radiography

By Chris Bishop, CR and DDA Subject Matter Expert at IRISNDT, and Ana Benz, Chief Engineer at IRISNDT. This article appears in the July/August 2021 issue of Inspectioneering Journal.
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Introduction

Piping failures account "for 60% of mechanical integrity losses [1]." Traditionally, we have monitored piping corrosion losses with ultrasonic thickness testing (UTT) based corrosion surveys. While these surveys are valuable, their information for small diameter piping can be challenging to interpret. In contrast, for NPS 6 and smaller lines, digital radiography (DR) based piping corrosion surveys give clearer and much broader information: an image showing wall loss, weld condition, threaded connection integrity, potential problems with insulation, internal product buildup, and often much more.

Using profile radiographic techniques (PRT) for small diameter lines is not a new concept. The API 570 "Piping Inspection Code: In-service Inspection, Rating, Repair and Alteration of Piping Systems" has clear guidelines for PRT of small-bore piping. This article illustrates the significant value of digital radiography as an efficient tool to visualize corrosion and other small bore piping anomalies without disturbing insulation. It also highlights the importance of properly interpreting radiography images and using PRT in conjunction with UTT to survey small diameter lines.

Some Key Benefits of Profile Digital Radiography

Profile radiography is described in detail in API RP 583 "Corrosion Under Insulation and Fireproofing." As stated in the RP, "The exposure source is usually iridium-192, with cobalt-60 being used for heavier wall piping … can also be done using X-ray sources." A typical exposure setup is illustrated in Figure 1. All DR digital detector array (DDA) images shown in this article were obtained using iridium-192.

Figure 1. Image illustrating a profile radiography setup.
Figure 1. Image illustrating a profile radiography setup.
Figure 2. NPS 2 water service pipe. Image shows wet insulation, electrical and steam tracing, an air pocket in the horizontal pipe, and slight ID corrosion.
Figure 2. NPS 2 water service pipe. Image shows wet insulation, electrical and steam tracing, an air pocket in the horizontal pipe, and slight ID corrosion.
Figure 3. NPS 2 line with severe buildup on the horizontal line and less buildup on the vertical line.
Figure 3. NPS 2 line with severe buildup on the horizontal line and less buildup on the vertical line.

Unlike UTT surveys, profile DR surveys on NPS 6 and smaller lines will also:

  • Identify wet and damaged insulation. Figure 2 shows clear insulation above the pipe and clouded insulation below the pipe. The clouded insulation is wet.
  • Identify product buildup or blockages. Figure 3 shows severe buildup on an NPS 2 line.
  • Identify material loss in welds, fittings, and valves. Figure 4 shows an NPS 4 to NPS 3 joint with insufficient deposited weld metal.
  • Differentiate at once uniform vs. localized corrosion. Figure 5 shows a severe pit in NPS 2 piping.
  • Identify areas not represented with a single point corrosion monitoring location (CML). Some of the radiographed locations may have a lower thickness than those observed with a single point CML. See deposited weld metal and various piping in Figure 4.
  • Identify improper or corroded threaded connections.
  • Obtain contextual information/validation of the equipment's original thickness value.
  • Minimize insulation removal. Less insulation removal can minimize insulation breaches, thus reducing the likelihood of corrosion under insulation ensuing.
Figure 4. Insufficient deposited weld metal on NPS 4 to NPS 3 is indicated with white arrows.
Figure 4. Insufficient deposited weld metal on NPS 4 to NPS 3 is indicated with white arrows.
Figure 5. A severe pit in NPS 2 pipe is indicated with a white arrow.
Figure 5. A severe pit in NPS 2 pipe is indicated with a white arrow.

DR images give very useful information and can even provide accurate wall thickness readings at a given point along and around the circumference of the pipe. However, a UTT scan is needed to find the exact location of minimum wall thickness if significant corrosion is identified. This statement is discussed in detail below.

Some Key Benefits of UTT 

UTT surveys are fundamental in today's piping mechanical integrity programs. UTT uses short wavelength, high-frequency ultrasonic waves to identify flaws in a material and the distance from the surface to the back wall (thickness) of materials. Thickness testing of piping should always be completed by qualified personnel with equipment that provides an A-Scan display to properly assess the remaining thickness and potential internal flaws (inclusions/laminations) in the pipe. Note: many errors have been made with unqualified personnel using only numerical digital readout thickness testing equipment.

Testing smaller diameter pipe (NPS 6 and smaller) can result in less-than-ideal contact between the probe and the pipe. Nevertheless, as illustrated in Figure 6, qualified personnel using the proper ultrasonic thickness testing equipment give excellent integrity information. The top image in Figure 6 shows the inside diameter of an NPS 6 line on the top side. After cutting the pipe open, the severe pitting reported with UTT was not obvious due to the corroded inside surface. However, after cutting a cross-section in what appeared to be an area of small pits, only 7% of the original pipe thickness was remaining (see the bottom image in Figure 6). The pipe had severe under deposit corrosion as initially identified with the UTT scan [2].

Figure 6. The top image shows the inside diameter of a NPS 6 line - the white arrow points at a small pit. The bottom image shows the Nital etched metallographic cross-section of the pit.
Figure 6. The top image shows the inside diameter of a NPS 6 line - the white arrow points at a small pit. The bottom image shows the Nital etched metallographic cross-section of the pit.

Illustrating the Effectiveness of Combining profile DR and UTT

As stated above, a single profile DR image does not result in an absolute minimum wall thickness measurement for the section of pipe imaged. The true minimum wall thickness of a section of pipe can only be found with a proper 100% UTT thickness scan of the suspected low thickness section of the pipe area. Without a UTT scan, the DR losses that are assessed:

  • Are more generally qualitative and can be classified as slight, moderate, or severe. An example of the challenge with the grading of a DR image is illustrated in Figure 7. The NPS 6 with a 0.432-inch-thick elbow appeared to have severe, almost through wall pitting. However, UTT assessed the pitted area to have 0.290-inch remaining thickness.
  • Depend on the insulation condition. Wet insulation can mask defects, and it can also look like wall loss. These possibilities are illustrated in Figure 8. With wet insulation, a panel placed on top of a horizontal pipe masked slight pits (see the left side of the image). When placing the panel on the side of the pipe, the wet insulation became visible. As well, slight corrosion losses became visible (see right side of the image). UTT followed and clarified the DR information.
  • Depend on how full the line is with liquid.
Figure 7. The NPS 6 with a 0.432-inch-thick elbow appears to have severe, almost through wall pitting. However, UTT assessed a 0.290- inch remaining thickness.
Figure 7. The NPS 6 with a 0.432-inch-thick elbow appears to have severe, almost through wall pitting. However, UTT assessed a 0.290- inch remaining thickness.
Figure 8. With wet insulation, a panel placed on top of the horizontal pipe masked slight pits (see the left side of the image). When placing the panel on the side of the pipe, the wet insulation became visible. The line had slight corrosion losses (see right side of the image).
Figure 8. With wet insulation, a panel placed on top of the horizontal pipe masked slight pits (see the left side of the image). When placing the panel on the side of the pipe, the wet insulation became visible. The line had slight corrosion losses (see right side of the image).

Conclusion

Beyond conventional thickness measurements, today's combination of profile DR and UTT scanning gives corrosion surveys:

  • Key information beyond just the absolute minimum wall thickness of a given section of pipe. One can identify wet and damaged insulation, product buildup or blockages, and material loss in welds, fittings, and valves. This information is not available with UTT alone.
  • An accurate assessment of the condition of the inspected lines.
  • Rapid and effective communication. The losses are illustrated, and the exact location of the area of interest can be conveyed promptly to decision makers to repair, replace, or run the pipe.

Acknowledgements

The authors would like to thank the understanding customers and colleagues who have shared these findings with the industry. We would also like to thank the IRISNDT management for their support. 

References

  1. March 2020, “100 Largest Losses in the Hydrocarbon Industry,” 26th Edition, Marsh JLT. 
  2. Vera, J.R., Daniels, D., and Achour, M.H., 2012, “Under Deposit Corrosion (UDC) in the Oil and Gas Industry: A Review of Mechanisms Testing and Mitigation,” C2012- 0001379, NACE Corrosion.

Comments and Discussion

Posted by Kevin Wieland on September 10, 2021
Great article that emphasizes that not only one... Log in or register to read the rest of this comment.

Posted by Ana Benz on September 10, 2021
Thank you, Kevin. Log in or register to read the rest of this comment.

Posted by UDAYAN PATEL on September 30, 2021
It is a great article. However there are couple... Log in or register to read the rest of this comment.

Posted by Ana Benz on October 1, 2021
Hi, Udayan, We have been using DDA to identify... Log in or register to read the rest of this comment.

Posted by Chris Bishop on October 4, 2021
Hi Udayan, For the second question , this is... Log in or register to read the rest of this comment.

Posted by UDAYAN PATEL on October 19, 2021
Thanks for response. It is vey helpful Log in or register to read the rest of this comment.

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