Application of Phased Array Ultrasonic Testing (PAUT) and Total Focusing Method (TFM) for Boiler Tube Weld and Flow Area Corrosion Inspection

Introduction to Boiler Tube Inspection

The integrity of boiler tube welds is critical to the safe and reliable operation of power generation and industrial boiler systems. Conventional nondestructive examination (NDE) methods, while widely used, often present limitations when inspecting thin-wall tubing, complex weld geometries, and heat-affected zones. Recent advances in advanced ultrasonic testing, specifically Phased Array Ultrasonic Testing (PAUT) and Full Matrix Capture with Total Focusing Method (FMC/TFM), provide enhanced imaging capabilities and improved detection sensitivity for weld inspection in boiler applications.

Figure 1: Example FMC/TFM and PAUT data from axial cracks induced from erosion.

This article examines the practical application of PAUT and TFM for boiler tube weld inspection, highlighting their advantages for detecting and characterizing common service-related and fabrication defects such as lack of fusion, incomplete penetration, porosity, and cracking. PAUT enables rapid sectorial scanning and precise beam steering, allowing comprehensive coverage of the weld volume and heat-affected zone with a single probe setup. When combined with TFM imaging, inspectors gain high-resolution reconstruction of ultrasonic data, producing detailed images that improve flaw sizing accuracy and reduce interpretation uncertainty.

The implementation of these technologies in accordance with ASME Boiler and Pressure Vessel Code Section V ultrasonic examination requirements provides a robust inspection framework for both fabrication and in-service assessment of boiler tubes. Case studies demonstrate how PAUT and TFM improve probability of detection (POD), reduce false calls, and enhance inspection reliability compared with conventional ultrasonic testing and radiography, particularly in small-diameter tubing and limited access environments.

Figure 2:  Staggered boiler tube welds in various stages of repair.

Description of Boiler Tube Welds for Phased Array Ultrasonic Testing and FMC/TFM

Under the ASME Boiler and Pressure Vessel Code (BPVC) Section I – Rules for Construction of Power Boilers, boiler tubes are pressure-retaining tubular components that form part of the boiler heating surface and contain water, steam, or a water–steam mixture while exposed to heat from combustion gases. These tubes are considered boiler pressure parts because they operate under pressure and contribute directly to steam generation. Typical boiler tube applications include waterwall tubes, generating tubes, superheater tubes, reheater tubes, and economizer tubes. The design, fabrication, and inspection of these tubes are governed primarily by ASME BPVC Section I, with supporting requirements from Section II for materials, Section V for nondestructive examination (NDE), and Section IX for welding procedure and welder qualification.

Boiler tubes are typically manufactured from materials specified in ASME Section II, with common grades including SA-192 and SA-210 carbon steel tubes, as well as SA-213 alloy and stainless steel tubes such as T11, T22, T91, and TP304/TP316 for higher temperature service. Welding of boiler tubes must be performed using procedures qualified in accordance with ASME Section IX, which defines essential variables for welding procedure specifications (WPS) and welder performance qualifications. Typical welding processes used in boiler tube fabrication and repair include gas tungsten arc welding (GTAW/TIG) for root passes, followed by shielded metal arc welding (SMAW) or GTAW for fill and cap passes, although automated orbital GTAW may also be used for small-diameter tubing. Weld preparation generally involves full-penetration bevel joints for tube butt welds and carefully controlled fit-up to ensure proper weld penetration and integrity.

Common weld configurations in boiler construction include tube-to-tube butt welds used during tube replacement or splicing, and tube-to-header welds connecting tubes to drums, headers, or manifolds. These joints are typically designed as full-penetration butt welds, although fillet or seal welds may be used for certain attachments or lower-pressure applications. Inspection of boiler tube welds is performed in accordance with ASME Section V nondestructive examination methods, with visual testing (VT), radiographic testing (RT), ultrasonic testing (UT), and increasingly advanced techniques such as phased array ultrasonic testing (PAUT) used depending on geometry and accessibility. Acceptance criteria and required examination extent are specified by ASME Section I or applicable piping codes such as ASME B31.1, ensuring that boiler tube welds meet the structural integrity and safety requirements necessary for high-temperature, high-pressure service.

Automated and Semi-Automated Boiler Weld Inspection

The Evident (Olympus) COBRA scanner is a compact, encoded phased array ultrasonic testing (PAUT) scanner specifically designed for the inspection of small-diameter pipes and boiler tubes, particularly circumferential welds. It is well suited for applications such as boiler systems, heat exchangers, and process piping, where access is often limited and high inspection accuracy is required. The scanner supports multiple ultrasonic techniques, including phased array UT, TOFD, and dual linear array configurations, allowing for full volumetric weld inspection in a single pass and effective detection of defects such as cracks, lack of fusion, and corrosion.

The COBRA scanner accommodates pipe outer diameters ranging from approximately 21 mm to 114 mm, which aligns with typical boiler tube sizes. Its low-profile design requires minimal clearance—around 12 mm—making it ideal for use in tightly packed tube bundles. It can be installed and operated from one side of the pipe, a key advantage in confined boiler environments. The scanner supports up to two phased array probes with adjustable spacing between 0 and 55 mm, enabling both single-sided and dual-probe inspection setups depending on access conditions and inspection requirements.

Equipped with an integrated encoder providing high positional accuracy (about 32 steps per millimeter), the COBRA scanner ensures precise defect localization and sizing. Its mechanical design includes a spring-loaded clamping system that maintains consistent contact and pressure around the pipe circumference, even on both ferromagnetic and non-ferromagnetic materials. The use of durable urethane wheels allows smooth scanning motion, while the system enables full 360° inspection with reliable ultrasonic coupling.

Example Phased Array Ultrasonic Testing Boiler Tube Weld Data

The data shown below is a typical presentation for double sided boiler tube weld inspection.  In this display, two PAUT groups are setup to control the inspection performed from each side of the weld.   The PAUT A-scan is located on the left hand side of the display.  The sectorial scan (S-scan) from the 90 degree and 270 degree skew are displayed on the top half.  The scan plan selected is close enough to just cover the weld root in the first leg inspection at the higher inspection angles.  The remainder of the weld is covered in the second leg inspection.   

The 90 degree and 270 degree skew C-scan are shown on the bottom half of the screen and are produced from the COBRA scanner encoder.  From left to right numerous low and high amplitude C-scan indications are observed.  Each indication must be sized out in the length direction in the C-scan and high direction in the S-scan.  Results are compared to the ASME boiler tube accept/reject criteria.

https://video.wixstatic.com/video/f11c8f_09d21b8cb6ef43309939744f381ef776/720p/mp4/file.mp4

Figure 3:  Example PAUT boiler tube weld inspection data.

Example FMC/TFM Ultrasonic Testing Boiler Tubes

The FMC/TFM method for weld defects or longitudinally oriented cracks and erosion scars is shown below.  The top view presents a 2-D image of the crack looking downwards on the pipe as shown in the top right and similar to a C-scan.   The side view presents a profile of the crack through the thickness of the tube wall along the blue scan axis similar to a traditional B-scan.   The end view is a slice of the top view and similar to the PAUT S-scan.  In this case, a 2.5” long crack is detected at the ID.

https://video.wixstatic.com/video/f11c8f_5185c64bc8b74bfab0290dc4bb605c4f/1080p/mp4/file.mp4

Figure 4:  Example FMC/TFM boiler tube inspection data.

Summary

This article highlights the use of advanced ultrasonic techniques—Phased Array Ultrasonic Testing (PAUT) and Full Matrix Capture with Total Focusing Method (FMC/TFM)—for improving the inspection of boiler tube welds and detecting flow-related corrosion. Unlike conventional NDE methods, which can struggle with thin-walled tubes and complex weld geometries, PAUT enables rapid, comprehensive weld coverage through beam steering and sectorial scanning, while TFM delivers high-resolution imaging that enhances flaw detection and sizing accuracy. When applied in accordance with ASME standards, these methods significantly improve reliability, probability of detection, and reduce false calls, especially in small-diameter and limited-access environments. The article also underscores the role of automated tools like the COBRA scanner, which enables precise, encoded inspections in tight spaces, and presents real-world data examples demonstrating the effectiveness of PAUT and TFM in identifying defects such as cracks, lack of fusion, and erosion damage-ultimately offering a more accurate and efficient solution for ensuring boiler tube integrity.