PAUT & TOFD Ultrasonic, for Corrosion & Critical Weld Inspection
Swiss Approval Advanced NDT based in UK, is delivering advanced Phased Array inspection services, with use of Total Focusing Method (TFM) Technology, the latest option in this specific UT testing. Our Advanced NDT Laboratory operations, are conformed with ISO 17025 requirements. The advantages of TFM technology in Phased Array inspection activities are:
- Optimal focusing and spatial resolution everywhere
- Direct imaging of a large area for one probe position
- All reachable angles with the array simultaneously
- Defect characterization
- Comprehensive imaging of defect
- 3D imaging
TFM imaging and computational process, is a «must» requirement in advanced UT applications. In Europe & US, TFM, is going to become a standard inspection requirement for Phased Array testing and interpretation of results. With Total Focusing Method (TFM) or Total Electronic Focusing (TEF), we do not see just the waves moving on the monitor, we have inspection results clearly stored and displayed for reporting, either on standard or special materials, like austenitic and high alloy materials as well as in Dissimilar Materials.
- Electronic focusing consists of adapting the receiving delay laws, to focus at many points forming a Grid, after a single pulse which generates a large and/or divergent ultrasonic beam.
- Effective inspection of anisotropic in-homogeneous materials is often a challenge due to the deleterious effects of the material properties on the bulk wave transmission of ultrasound. Reliable inspection capability for these materials must provide high probability of detection whilst minimizing false calls.
- Upon detection of an indication, the sizing and plotting of the indication within the search volume must be sufficiently accurate to support disposition in accordance with applicable codes. Common metallic members of this group of materials, includes austenitic stainless steel welds, coarse grain cast stainless steels and dissimilar metal welds.
PAUT Scanning Solutions
Depending of each particular inspection project, Swiss Approval is using the appropriate Crawler / Scanner, in order to get the best results from scanning options. We propose either, Semi-Automatic PAUT (S-AUT) solutions, as well as, full Automatic PAUT (AUT)applications:
- Multi Probes Crawlers for flat and curved surfaces [S-AUT],
- Specific crawlers for angle welds and joints [S-AUT]
- Specific crawlers for High Diameter Pipe Line inspection, on a fixed route [S-AUT]
- Robotic crawlers for inspection of flat or curved surfaces at height, like tanks shell or similar [AUT]
- Pipe lines and tubes inspection, with robotic or Semi-Automatic crawlers, depending on diameter and inspection position.
PAUT Testing Applications
- PAUT Corrosion mapping to estimate thickness at finite points in pressure vessels, pipes, tanks and structures.
- PAUT on butt welds in pipes and pressure vessels and large structures, Bifurcation lines, Valve bodies, etc
- PAUT scans on complex geometries including painted cylindrical, trapezoidal fuel transport tankers
- Individually tailor development of qualification blocks, configuring rigs for data collection and levels of post-analysis reporting
- 100% weld volume coverage at scanning intervals of 1mm, of high pressure and high temperature piping welds at power station sites using encoded scanning on circumferential and seam welds between 20mm to 75mm wall thickness
- Inspection of small bore tubes – Internal diameter of down to 38mm; having a minimum wall thickness of 5mm
- Inspection of complex geometry branch welds with variable weld profile using encoded scanning for accurate sizing of internal defects
- Turbine and generator components inspections with special focus on areas of high stress concentration such as for tree serrations of turbine blade roots and geometry changes along exciter cooling fans
- Detection of corrosion wastage in wind tower foundation bolts or other similar HOLD DOWN BOLTS with reliable monitoring of corrosion wastage along the length of bolts up to 5 times its diameter
- In-situ inspection of transverse cracks in shafts, axles, spindles, hold down bolts.
- Precise detection, measurement and reporting of smaller cracks for condition monitoring or repair.
Time of Flight Diffraction (TOFD)
Time Of Flight Diffraction (TOFD) is an advanced method of Ultrasonic inspection that leverages on existing UT methodologies, thereby bringing a whole new range of testing and detection capabilities to the NDT forefront. Whilst conventional ultrasonic uses shear/transverse waves as the primary mode of detection, TOFD uses longitudinal waves as the primary detection mode. Ultrasonic sensors are placed on each side of the weld. One sensor transmits the ultrasonic beam into the material and the other sensor receives both reflected and diffracted ultrasound from anomalies and geometric reflectors. Using ultrasonic beam spread, TOFD provides a wide area of coverage for anomaly detection in the material being tested. When the beam impinges on a flaw tip, it is diffracted in all directions. Measurement of the time of flight of this diffracted beam, enables accurate and reliable flaw detection and sizing. This is the case even if the crack is not oriented to the initial beam direction. During typical TOFD inspections, A-scans are collected and used to create B-scan (side view) images of the weld.
- Based on diffraction, so relatively indifferent to weld bevel angles and flaw orientation
- Uses time of arrival of signals received from crack tips for accurate defect positioning and sizing
- Precise sizing capability makes it an ideal flaw monitoring method
- Quick to set up and perform an inspection, as a single beam offers a large area of coverage
- Rapid scanning with imaging and full data recording
- Can also be used for detecting irregularly shaped areas of metal lossduring corrosion inspections
- Required equipment is more economical than phased array, due to conventional nature (single pulser and receiver) and use of conventional probes.
- Highly sensitive to all weld flaw types
Why Advanced NDT is Chosen, over Radiography
Although radiography testing (RT) was the primary NDT method suggested by the manufacturer, SWISS APPROVAL concluded that alternative advanced NDT methods, like ultrasound testing (UTPA), is definitely a more advantageous method.
- RT inspection would have required twice as many inspectors.
- Implementation of a safety zone was deemed too difficult. Inspection activities has to be performed during the night, while other employees are not present.
- It is estimated that the complete project using RT, would take five times longer than UTPA or other advanced UT applications.
- On top of all the extra costs associated with RT, are consumables such as films, photo chemicals, and waste disposal of the lead foils.
- The most important aspect of choosing UTPA or similar over RT, is related to the probability of detection. Lack of side wall fusion, which is a critical flaw to measure, is not well detected by radiography.
High Temperature Phased Arrays Applications
- Phased Array UT presenting ‘A’, ‘B’ and ‘C’ Scans.
- Corrosion Evaluation, Hydrogen Related Condition Monitoring & Weld Testing.
- Performed by Certified Level 2 PA technicians and verified by Level 3 PA (not just UT).
- We apply Phased arrays offline and online up to 200°C actual surface temperature. Continuous water flow ensures that the sensors remain well below their Curie temperature without their operational performance being affected. Thus, accurate evaluation of the structural integrity of the inspected component can be accomplished.
How it is accomplished:
- Our new approach involves a 64 channel PA sensor using water as coupling.
- During operation water flows between the hot surface of the component inspected and sensor array allowing heat transfer to take place thus, reducing actual sensor temperature to normal levels.
- In this way hot measurements are not any longer distorted which means that correction is not needed. This allows straightforward scanning of components t higher temperatures, such as vessels and pipelines, without the need to stop operation and reducing unwanted openings of such equipment during outages reducing significantly maintenance works.
- Acquired C scans are then stitched together providing a meaningful image of the corrosion profile and its extent.
Swiss Approval Laboratory Staff is working consistently to guaranty the maximum efficiency and reliability of our Testing Results. Specific Calibration Blocks for each method, position, for different materials, related to different International Standards, are designed and developed within our Laboratory, in order to build in house our Know-How, necessary for service delivery of excellence. Development of inspection capacity for this category of materials, usually includes many, if not all, of the following:
- Fabrication of mock-ups and samples containing realistic defects,
- Procedure development,
- Validation and subsequent training.
Such development of inspection resources and capacity represents for Swiss Approval Top Management, substantial commitments of time, high qualified personnel and significant capital investment.