Volumetric Ultrasonic Examinations: ASME Code Compliant vs. In-Service Evaluations

Which Is Right for You?

As a means to promote and regulate welding quality and workmanship, Section V Article 4 of the ASME code provides requirements for performing ultrasonic examinations for weld acceptance. As part of managing aging assets, and without any other guidelines, many times these requirements are referenced for examining welds that have been in service for many years. Additionally, and aside from ASME Section XI, no other code sections actually require or detail examinations during the service life of the component; this includes Section I, Section VIII and B31.1. It’s important to understand the difference between ASME code compliant UT exams and the Structural Integrity examination approach as it relates to component serviceability.

Code Compliant Examination

As it matters to the fabrication of pressure vessels, the ASME code is the governing body and the applicable ASME code section is dependent on the particular component to be fabricated.  For example, B31.1 of the code addresses the fabrication, examination and acceptance standards for power piping and Table 136.4 of B31.1 details what components must be examined and how they are to be examined. Typically these examinations are required on welds joining two components where either a surface examination or a volumetric examination is performed. The acceptance criteria are based on workmanship and the examinations are conducted with the intention of providing assurance that the component will be serviceable.

Purpose

Until recently, the required method for performing a volumetric examination was radiography; however, recent editions of the code now only stipulate that a volumetric examination shall be conducted which can be either radiography or ultrasonic examination. Regardless of which method is chosen, ASME Section V is the governing code section detailing how the examinations are to be conducted. The techniques and methods established in Section V are designed to address the fabrication of the components. Specifically, the acceptance criteria found in Section I, VIII and B31.1 address new construction / fabrication related flaws such as incomplete penetration, slag, porosity, etc.

Examination

Article 4 of Section V contains the specific calibration requirements for the ultrasonic system to perform a code required examination. Additionally, the calibration blocks to be used are specified to contain either side drilled holes and/or notches, of specific sizes based on component thickness.

Once the calibration is established the code gives details on how the weld is to be examined. For welds, scans must be performed to detect indications parallel to the weld and perpendicular to the weld (e.g., transverse cracking). To achieve this, the weld must be inspected from four directions which occasionally require that the component surfaces be contoured to support the probe orientations. Any indications found are evaluated based on permissible lengths and the amplitude of the UT signal as compared to the calibration holes or notches.

In-Service Evaluations

As mentioned previously, the codes do not address performing any NDE examinations after the component is placed in service. Recent editions of B31.1 now require that a program for monitoring the piping system during service is in place, but it is not specific on monitoring practices.  As a result, many equipment owners defer to and rely on criteria established in the code rather than implementing examination practices specifically for the detection of service-induced damage.

Purpose

In general, there are several failure modes that can occur to welds and components during their service life. The type of failure will primarily depend on the service pattern(s) that the component is exposed to during its life. For instance, high temperature components are subject to creep damage and/or creep fatigue while components operating below the creep temperature range may be subject to thermal fatigue damage.

With the lack of any guidance and the failures that began to appear in the early 1980’s, the need for some guidelines and applied practices for detecting damage mechanisms became apparent. Through work with EPRI and the MPC (Materials Property Council), we developed best practice techniques for the detection of service damage specific to component types and failure modes.

While the basis of our UT procedures is derived from ASME Section V, we deviate substantially from the recommended calibration practices.  By utilizing instrument focusing capabilities and creating calibration standards to increase sensitivity and resolution, our service procedures greatly increase the likelihood of detecting damage in its earliest stages.  In addition to higher calibration sensitivities, we have incorporated the latest technologies available for time of flight diffraction (TOFD) and linear phased array (LPA) ultrasonic techniques to provide even greater accuracy and confidence in our ability to detect and characterize damage.

Examination

It is the intent of Structural Integrity’s examination process to provide a full interrogation of the weld volume when looking for service-related damage.  As part of the inspection protocol, the volumetric examinations are complemented with wet fluorescent magnetic particle (WFMT) surface inspection and metallographic replication. Generally speaking, service damage associated with girth welds tends to initiate in the primary (circumferential) weld axis either on the ID/OD surfaces or subsurface and extends radially around the weld volume such that the primary scan axis is perpendicular to the weld. Conversely, longitudinally welded seams tend to show damage in the same volumetric planes but are oriented parallel to the pipe axis and extend axially down the component length.

At the sensitivities used for these examinations, it is commonplace to see small original manufacturing flaws which may or may not have been reported in the original weld acceptance examination-most likely radiography. These fabrication flaws are usually not significant enough to be a concern to continued operation and will not propagate to failure under normal service conditions.

There are some fabrication related flaws, such as lack of fusion or cracking from the welding process, which could propagate to failure. These are routinely detected and documented as part of our in-service inspection practice.  Once documented, the inspection data can then be further analyzed to enable a decision on whether to repair the flaw or re-inspect the weld at a later date.

Recently, there have been a number of transverse cracks (perpendicular to the weld axis) found in welds. Based on our experiences, these cracks are fabrication related (with a greater propensity to be found in submerged arc type welds) and do not normally propagate to failure. In many cases, these indications can be found at the OD surface and properly conducted surface examinations will detect them.  At which point, all appropriate measures would be taken to ultrasonically evaluate the flaw and determine depth values, if significant.

At their greatest extent, transverse cracks pose a potential leak risk so they should be repaired if found; however, they do not pose a threat to the serviceability of the weld itself or create conditions associated with catastrophic failures. Due to the fact that most weld caps are left in the as-welded condition, which is permissible by code, it is very difficult to perform a meaningful examination looking for transverse cracks without conditioning the weld cap (flat-topping) and modifying the probe/wedge assembly to secure contact with the part. As previously mentioned, the ASME and B31.1 codes require this type of examination as part of the weld acceptance practices for new construction.  Since transverse cracks are considered to be fabrication related, and significant depths occur infrequently, it is not part of our normal UT serviceability examination practice to look for this form of cracking.

Conclusion

While it is good practice to use the code requirements as a foundation for developing procedures, it has been proven that the calibration requirements of the code are not sufficient to detect service-related damage at the earliest stages possible.  Structural Integrity, on the other hand, has compiled numerous examples and experiences confirming the effectiveness of its service inspection strategies through decades of continued implementation and development.  Put simply, as it relates to the continued operation of these aging components and the safety of employees working near them, any examination being conducted to detect service-related damage should be performed utilizing the best techniques and practices available.

Therefore, when seeking inspection support to perform service evaluations, requiring that the examinations and acceptance criteria be in accordance with the applicable code section is not recommended.  Alternatively, consider requesting that the examinations be conducted in accordance with EPRI standards and/or MPC recommendations. Coupling these expectations with the utilization of qualified technicians will provide the best possible inspection scenario for evaluating your high energy piping.

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