What Is Phased Array UT?
Phased array ultrasonic testing (PAUT) uses a probe containing multiple piezoelectric elements that are pulsed independently with controlled time delays. By varying these delays, the beam can be steered, focused, and swept electronically—without moving the probe—to produce a cross-sectional image of the weld or material. PAUT generates encoded, permanent records equivalent to or better than radiographic testing, without radiation hazards. It is accepted by ASME, API, and EN codes as an alternative to RT for piping and pipeline weld inspection.
How PAUT Works
A phased array probe contains 16 to 128 individual piezoelectric elements arranged in a linear, matrix, or annular configuration. The instrument fires groups of elements (apertures) with precise electronic time delays (focal laws). These delays control the beam angle, focal depth, and scan direction:
- Sector scan (S-scan): The beam sweeps through a range of angles (e.g., 40-70 degrees) from a fixed probe position, producing a cross-sectional image of the weld.
- Linear scan (L-scan): The aperture moves electronically along the array, creating a B-scan image without mechanical motion.
- Compound scan: Combines sector and linear scans for full volumetric coverage.
PAUT vs Conventional UT vs RT
| Parameter | Conventional UT | PAUT | RT |
|---|---|---|---|
| Probe | Single element | Multi-element (16-128) | N/A (radiation source + film/detector) |
| Beam control | Fixed angle per probe | Electronic steering/focusing | Fixed beam direction |
| Image output | A-scan (amplitude vs time) | S-scan, B-scan, C-scan (cross-section images) | 2D projection image |
| Permanent record | Manual logging (no encoded data) | Full encoded digital data | Film or digital image |
| Crack detection | Good (operator dependent) | Excellent (multi-angle coverage) | Poor for planar defects |
| Porosity detection | Moderate | Good | Excellent |
| Flaw sizing | Moderate accuracy | High accuracy (height + length) | Limited (no through-wall sizing) |
| Radiation hazard | None | None | Yes (exclusion zones required) |
| Inspection speed | Slow (multiple probe angles manually) | Fast (all angles in one scan) | Moderate (exposure + processing) |
| Code acceptance | ASME V Article 4 | ASME V Article 4, Code Case 2235 | ASME V Article 2 |
Key Standards
| Standard | Scope |
|---|---|
| ASME Section V, Article 4 | UT examination of welds (includes PAUT) |
| ASME Code Case 2235 | UT (including PAUT) as alternative to RT for pressure vessels/piping |
| ASME B31.3 | Process piping (permits PAUT per Article 4 procedures) |
| API 1104 Annex S | PAUT procedures for pipeline girth welds |
| EN ISO 13588 | PAUT of welds (European standard) |
| DNV-ST-F101 | Subsea pipeline PAUT requirements |
PAUT Applications in Piping
| Application | Configuration | Advantage Over Conventional |
|---|---|---|
| Shop butt welds | Single-side scan, S-scan | Full weld coverage without RT radiation |
| Pipeline girth welds | Mechanized scanner, zone-focused technique | Replaces RT on cross-country pipelines |
| Nozzle welds | Sector scan from nozzle OD | Access where RT geometry is impossible |
| Dissimilar metal welds | Adapted focal laws for velocity changes | Better penetration in austenitic/Inconel overlays |
| In-service corrosion mapping | C-scan from OD surface | Thickness profiles over large areas |
Procedure Qualification
PAUT procedures require validation through technique qualification on representative test blocks containing known flaws. The qualification demonstrates detection capability, sizing accuracy, and false call rates. ASME V and API 1104 Annex S define minimum requirements:
- Calibration on side-drilled holes (SDH) or notches per Article 4
- Sensitivity set to detect 1 mm SDH or equivalent
- Scan coverage verified by plotting beam angles against weld geometry
- Operator certification: ASNT Level II PAUT or ISO 9712 Level 2 UT (phased array)
PAUT data packages form part of the weld documentation alongside mill test certificates and hydrostatic test records.
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