Acoustic Emission Testing (AET) | In-Service Pressure Vessel & Pipeline Monitoring

Advanced NDT Method Guide · Updated February 2026

Unlike RT or UT which inspect one location at a time, AET monitors the entire structure simultaneously — detecting active defects, corrosion, and leaks from a fixed sensor array while equipment remains in service.

A complete AET system comprises four main components working together to detect, amplify, digitise, and analyse acoustic emission signals from the structure under test.

AET data interpretation relies on the correlation of signal parameter patterns with source mechanisms. Each parameter carries specific diagnostic information about the nature and severity of the emission source.

A refinery in the Middle East had a 40-metre diameter atmospheric storage tank storing crude oil. The API 653 10-year inspection interval was approaching, which would normally require: emptying the tank (3–4 weeks), cleaning and degassing (2 weeks), confined space entry for internal inspection including ultrasonic thickness scanning of the floor (1 week), and refilling. Total cost estimate: $450,000–$600,000 including product loss and inspection.

Instead, AET was performed with the tank in service and at operating level. Twenty-four sensors were installed on the outer shell, near the bottom annular area, with a calibrated sensor spacing to achieve the required floor coverage. Monitoring was conducted over a 12-hour period. The results showed: two localised areas of elevated AE activity consistent with active corrosion, clustered near the south quadrant; no indications consistent with through-floor leakage; background signal levels consistent with low general corrosion activity across the remaining floor area.

Based on the AET results, the operator extended the interval by 2 years for a focused internal inspection limited to the AET-flagged south quadrant area. Total saving: approximately $380,000 over the original full inspection plan.

Atlantis NDT provides ASNT Level III AET consulting services — procedure development per ASME Section V Article 12, sensor placement design, data interpretation review, and AET program integration with API 510/570/653 inspection intervals. Contact our team to discuss your specific pressure vessel, tank, or pipeline application.

Our ASNT Level III engineers design AET programs for pressure vessels, storage tanks, and pipelines per ASME Section V and API requirements.

What this page covers

What Is Acoustic Emission Testing and How Does It Work?
Key Applications of Acoustic Emission Testing
AET Equipment and System Components
AET Signal Parameters
AET Standards and Codes
AET Advantages and Limitations
Case Study: Tank Floor AET — Avoiding Internal Entry
Related NDT Methods & Resources
Acoustic Emission Testing — Frequently Asked Questions
AET Consulting & Program Design
AET vs Conventional NDT — Key Distinction
Outcome and Cost Comparison
On This Page
AET Standards

Key points covered

How is AET different from conventional UT for pressure vessel inspection?
AET during hydrostatic pressure testing per ASME Section V Article 12. Detects active flaws (cracks, weld defects) during initial or requalification pressure tests without the need to scan every square inch of vessel wall. Sensors are placed at calculated spacings to achieve the required detection sensitivity.
AET from outside the tank shell while the tank remains in service — an alternative to internal entry for API 653 intermediate inspections. Sensors detect corrosion activity, cracking, and pitting on the tank floor. Risk-ranks floor condition to guide targeted inspection during scheduled internal entry.
Continuous AET monitoring for seeping leaks in pressurised pipelines. High-frequency turbulent flow signals from leaks are detected and located by sensor arrays. Effective for buried, insulated, or difficult-access pipelines where conventional inspection is costly.
AET is particularly valuable for composite-overwrapped pressure vessels (COPVs) used in aerospace and hydrogen storage. Matrix cracking, fibre breakage, and delamination all generate characteristic AE signals. Identifies vessels with progressive damage during proof testing.
Continuous or periodic AET monitoring of steel bridges, railway structures, and civil infrastructure for fatigue crack growth, weld cracking, and corrosion. Provides early warning of structural deterioration without requiring direct access to inspection areas.
AET for bearing condition monitoring, gear wear detection, and cavitation detection in pumps, turbines, and compressors. AE signals from bearing defects appear before vibration analysis detects problems — providing earlier warning for condition-based maintenance programs.
Peak signal voltage. Indicates source intensity. Logged in dB above threshold (e.g., 60 dB).
Threshold crossing count per hit. Related to signal duration and repetition.
Integral of squared voltage over hit duration. Best indicator of cumulative damage.
Time from first to last threshold crossing. Long duration = extended source activity.
Time to peak amplitude. Short rise time = sudden fracture; long = friction/corrosion.
Continuous emission level. Used for leak detection and background monitoring.
Rate of threshold crossings. Increasing rate indicates growing/active defect.
Acoustic emission monitoring of structures during controlled stimulation
Resonant or broadband piezoelectric sensors are coupled to the structure surface using a couplant (vacuum grease, petroleum jelly). Resonant sensors (150 kHz, 300 kHz, 60 kHz) provide high sensitivity at their resonant frequency — suitable for most structural and vessel monitoring. Broadband sensors (wideband, 100 kHz–1 MHz) capture the full frequency spectrum — used for source characterisation and material research. Sensors are secured with magnetic hold-downs, adhesive, or mechanical clamps. ASTM E650 governs sensor mounting practices.
Each sensor connects to a dedicated preamplifier that boosts the raw AE signal (typically 1–100 mV) before transmission to the main data acquisition system. Standard gain settings are 40 dB or 60 dB. Preamplifiers are positioned close to the sensor (integral or line-powered) to maximise signal-to-noise ratio before signal transmission over longer cable runs to the data acquisition chassis.
The data acquisition system receives amplified signals from all preamplifiers simultaneously, applies programmable threshold detection, digitises hit waveforms and extracts feature parameters (amplitude, counts, energy, duration, rise time, frequency centroid), timestamps each hit, and transmits data to software for real-time display and analysis. Systems may have 16–128 channels or more for large structure monitoring. Manufacturers include Mistras Group (formerly PAC), Vallen Systeme, and KRAUTKRAMER.
Real-time software (e.g., AEwin by Mistras/PAC, VisualAE by Vallen) displays live activity maps, amplitude distributions, hit rate trends, and source location plots. Post-processing tools identify clusters of high-activity emission sources, filter noise using parametric criteria (frequency, rise time), and generate reports compliant with ASME Section V Article 12 or ASTM E569 requirements.
Susceptible to acoustic noise — expertise required for noise filtering
Cannot detect all defect types (e.g. existing stable cracks may not emit)