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Using the Total Focusing Method (TFM) to Detect H2S Stepwise Cracking Damage


H2S Damage Inspection in Oil Refineries

NDT inspectors are important to oil refineries—there are so many components to inspect, and it is such a critical place to ensure that nothing goes wrong. Because of the number of different damage mechanisms affecting oil refinery parts, inspectors need to have a very good understanding of the overall processes and risks for every component.

One of the common deterioration processes to look for on refinery equipment is known as wet hydrogen sulfide (H2S) cracking. As the name implies, wet H2S cracking can occur in carbon-steel parts when they are exposed to hydrogen sulfide and moisture. Wet H2S cracking incorporates many types of failure mechanisms, including hydrogen-induced cracking (HIC), sulfide stress cracking (SSC), blistering, and stepwise cracking (see Figure 1). This application note will focus on the latter two, which are inextricably linked.

Figure 1—Picture of blisters and stepwise cracking caused by HIC

Figure 1—Picture of blisters and stepwise cracking caused by HIC

Blistering and Stepwise Cracking Progression

Blistering refers to subsurface cavities that are created by the collection of hydrogen gas. Over time, these voids appear as subsurface “blisters.” A buildup of hydrogen gas eventually creates enough pressure within the part that these blisters attempt to expand by connecting. The linking of these blisters is known as stepwise cracking, and it is very detrimental to the structural integrity of the component. If the blisters successfully connect, the metal strength will be considerably weakened and the risk for catastrophic failure is imminent.

Use Case

A 25 mm (1 in.) thick carbon steel component with known H2S blisters and stepwise cracking was used for this application note (Figure 2).

Figure 2—Carbon steel component with known H2S damage

Figure 2—Carbon steel component with known H2S damage

Challenges When Inspecting for Blisters

Ultrasonic inspection (UT) has been used for many years to prove the structural integrity of carbon steel and other material in refineries. There are two main challenges when applying UT to blistering and stepwise cracking inspection. The first one is to differentiate blisters from a total loss of back wall (due to corrosion or material loss). Inspectors may misinterpret the UT data and assume that the signal reflected off the blistering represents the end (back wall) of the component. This can lead to unnecessary rejection and decommissioning of the component.

The second challenge is the high potential to miss detecting the stepwise cracking that has formed between the blisters. Since UT inspection is often performed at zero degrees for the inspection of the base material, the incidence angles—or reflectivity—of the signals with the stepwise cracks may not be optimal (see Figure 3). In such cases, the sound reflected off the stepwise cracks may not be received by the probe.

Figure 3—(a) Sound directed at 0 degrees, reflecting back to the probe from a horizontal reflector and (b) sound directed at 0 degrees, reflecting away from the probe on an angled reflector

Figure 3—(a) Sound directed at 0 degrees, reflecting back to the probe from a horizontal reflector and (b) sound directed at 0 degrees, reflecting away from the probe on an angled reflector

Because of this poor reflectivity, inspectors often use multiple groups at different angles when they suspect that the part contains H2S damage (see Figure 4, where 3 groups are being used). They later merge the data from these groups for analysis, which can be cumbersome and time consuming.

Figure 4—Three groups are created to achieve optimal reflectivity on the defect

Figure 4—Three groups are created to achieve optimal reflectivity on the defect

Solution for Detecting Stepwise Cracking Using the Total Focusing Method (TFM)

The total focusing method (TFM) involves a different firing sequence than conventional UT or conventional phased array UT inspection (refer to “Using the Total Focusing Method to Improve Phased Array Ultrasonic Imaging” for more details on TFM). The firing sequence, known as full matrix capture (FMC), enables a greater reflectivity from angled reflectors, even for a zero-degree contact inspection. The possibility for a focused image throughout the full depth of the component (when using the proper probe) is another advantage of TFM compared to earlier technologies. The image below shows the OmniScan™ X3 flaw detector’s Acoustic Influence Map (AIM) simulator, which was used to select the proper probe and propagation mode for the 25 mm component (see Figure 5). The result is an image where blisters can be identified and stepwise cracking is clearly detected (see Figure 6).

Figure 5—AIM simulator of the OmniScan X3 flaw detector’s onboard scan plan, showing the sensitivity index at 157.16 for the flexible 5 MHz PAUT probe

Figure 5—AIM simulator of the OmniScan X3 flaw detector’s onboard scan plan, showing the sensitivity index at 157.16 for the flexible 5 MHz PAUT probe

Figure 6—TFM imaging showing stepwise cracking linkages

Figure 6—TFM imaging showing stepwise cracking linkages

Limitations of Applying TFM Technology to Stepwise Crack Detection

It is important to keep in mind that the pulsing sequence of the TFM technique might not be ideal for every application. The single-element pulsing may not generate enough energy to penetrate through the entire depth of a given part. For example, the probe used for this 25 mm component application (5 MHz, 64 elements) would not be suitable for a thicker component, since the near field for this probe is estimated at about 26 mm. Trying to exceed the physical limitations of the probe using the TFM technique will result in poor and inaccurate results. Also, make sure to select the proper probe for the application.

Another important consideration when using TFM technology is the exactitude of the values for the part geometry and material sound velocity that are entered for the inspection. This technique offers a multitude of wave sets (modes of propagations), so many reflections from the back wall to the probes are to be expected. The downside of these wave sets (TTT, TTTT, TLT, etc.) is that having the wrong information about the part thickness or sound velocity can exponentially increase the room for sizing errors. The more bounces that are expected at a certain location, the more the software calculations will be off if the reality doesn’t meet the inputted values. Therefore, it is important to use these modes with care and to be mindful of the potential variabilities in the results.

Figure 7—Velocity input: 2.5% difference in value in half-skip (TTT) resulted in a loss of signal by 17.9 dB for a vertical notch

Figure 7—Velocity input: 2.5% difference in value in half-skip (TTT) resulted in a loss of signal by 17.9 dB for a vertical notch

Conclusion

The total focusing method (TFM) can be an effective tool for H2S damage inspection, but using the right probe plays a considerable role in making it successful. The FMC firing sequence that is used for the TFM processing helps improve the reflectivity for defects that are not horizontal to the scanning surface. This results in a greater probability of detection for previously challenging flaws such as stepwise cracking linkages. Additionally, the OmniScan™ X3 scan plan tool with its AIM simulator can help ensure in advance that the probe, wedge, and wave set selected will perform well on the part and targeted defect. See more on this subject in “Selecting the Best Propagation Mode for a Reflector Using the AIM Modeling Tool for TFM (Total Focusing Method) Inspection.”

Olympus IMS
Products used for this application

The OmniScan X3 flaw detector is a complete phased array toolbox. Innovative TFM delivers outstanding images that help inspectors identify flaws with confidence while powerful software features and simple workflows help you get to work fast.
Our innovative flexible phased array probe enables users to undertake new applications. The FlexoFORM™ scanner uses a flexible array probe to perform corrosion inspection on pipe elbows.
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