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Acoustic Emission Testing

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Introduction

ACOUSTIC EMISSIONS are stress waves produced by sudden movement in stressed materials. The classic sources of acoustic emissions are defect-related deformation processes such as crack growth and plastic deformation. Sudden movement at the source produces a stress wave, which radiates out into the structure and excites a sensitive piezoelectric transducer. As the stress in the material is raised, many of these emissions are generated. The signals from one or more sensors are amplified and measured to produce data for display and interpretation

Comparison with Other NDT Methods

1. Detects movement of defects
2. Requires stress
3. Each loading is unique
4. More material-sensitive than other NDT methods
5. Less geometry-sensitive than other NDT methods
6. Less intrusive on plant/process than other NDT methods
7. Requires access only at sensors positions
8. Tests whole structure at once
9. Main problems: noise related

Requirements

In most cases for the AET Customer must provide pressure increase on 10 -30% above maximum work level, but less than design pressure, in accordance with a predetermined program.
The direct access to the test object surface is needed – pits in the ground or cut of insulation with notches. The surface of the object should be cleaned from insulation and 50 mm areas cleaned with a file.
In case or road crossing shall be provided measures to protect the data cable from the vehicle, such as fixed metal bath or metal tube. Provided by Customer.
For typical road or river crossing it is needed 4 sensors – 4 pits. Duration of typical test – 6-8 hours including setup, calibration, test and removing the equipment.
During the AET noises level should be minimalized. Very often the test should be conducted at nighttime.
The accuracy of AET increased in case of lower distance between sensors, lower level of noises and higher level of test pressure.

Limitations and requirements of AET

Limitations and requirements of the AET Corrosion, including cracking of corrosion products or local yielding, stress corrosion cracking, certain metallurgical changes, including yielding and crystal dislocations, embrittlement, pits and gouges, incomplete fusion, lack of penetration, Undercuts, Voids and porosity, Inclusions, Contaminations.

1. AET as part of assessment of petrochemical equipment

2. Ensuring the safety of pneumatic tests

3. Corrosion state and leaks on the bottoms of AGST

4. Liquid Gas Station

5. Main and interplant pipelines

6. Different types of crossing

Crack at a heat exchange nozzle & Crack in a girth weld of process pipeline​

Results: crack in the heat affected zone of nozzle D250 mm on depth ~5 mm, length 20 mm, height up to 2 mm.

Results: crack from the internal side of a girth weld of the process pipeline, length 100 mm, height 5 mm.

Pneumatic tests "turnkey" in accordance with the provisions of international standards

Pneumatic tests "turnkey" with AE of coke chambers and atmospheric column, with the development of the loading system and the procedure of pneumatic tests in accordance with the provisions of Russian standards and taking into account international regulatory documents (ASME and EN). In connection with the unique dimensions (2000-3000 m3) of the tested objects, three AE systems with a total number of channels above 100 were used.
Results: defects in the auxiliary seams.

Leakage at the AGST bottom 

Inner sidewall of Metal Storage Isobutane Tank 160x1 need to be tested be AE method to evaluate the condition of the tank.

Two AE systems - Vallen Systeme AMSY-5, Acoustic Emission (AE) and Physical Acoustic Corporation (PAC) DiSP with summary 88 AE channels were used for the AE test of 37,49 m dia, 26,57 m height isobutene tank.

Access to the tank shell for sensors setup and calibration was by telescoping-boom crane. Because of this tank is completely insulated, waveguides with 150 kHz piezoelectric sensors were mounted around the tank wall circumference.

Active (D) and Intense (E) AE sources in the material and welded joints of the body of the tested vessel were not found. It is recommended to make a future inspection in the zones of “C” grade channels – 18 (DiSP) (zone 1).

Corrosion state of AGST bottom

Method is experience-based and is capable of determining the overall floor condition. The tank evaluation can be "good” (A Grade - no further action required), "intermediate” (B and C Grades - either overall activity or potential leak sources), or "poor” (D grade - requiring internal inspection and repair). The method helps to determine if further (internal) inspection and repairs are required and is therefore an effective maintenance planning tool, especially for tank farms.

Assessment of main and interunit pipelines 

Over 500 km of main pipelines are tested by AE – bridge lines, spiral weld pipes, pipelines with welded joints, made with backing rings.

Results: worm-hole in an annular welded joint of the interplant 4“ammonia pipeline.

Leakage at the AGST bottom 

The AE method is used to assess the condition of pipelines for various types of crossings - via highways, railways, water barriers. Results: activity in the zone of sliding base.