Ultrasonic Testing (UT) and Phased Array Testing of Power Piping in Accordance with Accordance with ASME B31.1 and Section I Appendix A-300

The recommended guidelines to perform ultrasonic(UT) testing and phased array ultrasonic testing (PAUT) of power piping systems are outlined in ASME B31.1 Power Piping [1]. In piping systems governed by ASME B31.1, ultrasonic testing and PAUT are accepted as a substitute testing methods for radiographic testing (RT) when certain conditions are met. The acceptance criteria for flaws identified by UT are crucial to ensure the safety, integrity, and reliability of piping systems operating under pressure.

This article provides a comprehensive overview of UT acceptance criteria according to ASME B31.1, paragraph 136.4.6, which references ASME Section I, Appendix A-300. These guidelines help inspectors and engineers determine which types of weld discontinuities are acceptable or rejectable during construction, maintenance and periodic inspection of power piping systems.

Figure 1:  Example phased array test data from power piping weld showing incomplete penetration.

Ultrasonic Testing of Power Piping

The mandatory minimum nondestructive examinations for pressure welds or welds to pressure-retaining components is outlined in ASME B31.1 Table 136.4.1 (2020).  Ultrasonic testing is required for butt welds and welded branch connections for systems that operate at above 750F regardless of pressure. 

Table 1: Summary of ASME B31.1 non-destructive testing requirements

Ultrasonic testing of welds per ASME B31.1 is outlined in Chapter VI Inspection, Examination, and Testing Section 136.4.6, Ultrasonic Examination.  The section defers to ASME BPVC, Section V, Article 4 regarding ultrasonic testing performance specifications.  However, firm acceptance criteria are provided in terms of indication amplitude compared to the reference standard amplitude, type of defect detected, and length of defect. 

Chapter VI Inspection, Examination, and Testing Section 136.4.6, Ultrasonic Examination (c) offers fracture mechanics based rules in Mandatory Appendix O Use of Alternative Ultrasonic Acceptance Criteria.  In some situations, especially when using more advanced UT techniques like Phased Array Ultrasonic Testing (PAUT), additional parameters such as flaw height-to-thickness ratio (h/t) and length (ℓ) are used for more refined evaluations.  Appendix O considers surface and subsurface indications with height-to-thickness (h/t) and length (ℓ) criteria. Surface indications are those that originate or break at the material or weld surface. These can be especially critical since surface-breaking flaws may more readily initiate crack propagation under cyclic or fatigue loading. Subsurface indications, by contrast, are entirely embedded within the weld or base material. They are generally less critical unless they are volumetrically large or oriented in a way that threatens structural integrity.  ASME B31.1 Mandatory Appendix O provides specific acceptance criteria for weld thicknesses less than 1” and for weld thicknesses 1” and greater.  Separate criteria are provided for surface and subsurface flaws.  The acceptance criteria for weld thickness < 1.0 in. (25 mm) are:

·       Surface flaws: Acceptable if h/t ≤ 0.100 and length ℓ ≤ 0.25 in. (6.4 mm)

·       Subsurface flaws: Acceptable if h/t ≤ 0.286 and length ℓ ≤ 0.25 in. (6.4 mm)

This means that for thinner welds, a relatively small surface flaw must also have a proportionally small depth (height) to be accepted. Subsurface flaws can be slightly deeper due to their lower risk of immediate crack initiation.

Conclusion

Ultrasonic Testing is a reliable and sensitive method for detecting and evaluating flaws in piping welds. The acceptance of detected flaws depends on their type, size, and the wall thickness of the material. ASME B31.1, along with Appendix A-300 of ASME Section I, provides a clear framework for determining whether a discontinuity is acceptable.

Automatic rejection of critical flaw types like cracks and lack of fusion ensures safety, while length-based criteria for non-critical flaws allow for practical decision-making in the field. As inspection technology advances, evaluators can apply more sophisticated criteria such as h/t ratios to improve reliability and reduce unnecessary repairs.

Understanding and correctly applying these acceptance criteria ensures that piping systems operate safely under pressure, minimizing the risk of failure and costly downtime. 

References

1. ASME Boiler and Pressure Vessel Code, Section V Non-destructive Examination, 2023

2. ASME B31.1 Power Piping, 2023

3. ASME B31.3 Process Piping, 2023

4. ASME BPVC Rules for Construction of Pressure Vessels Section VIII Division 1, 2023

5. ASTM E2700-20 Standard Practice for Contact Ultrasonic Testing of Welds Using Phased Arrays