Acoustic Emission Testing

Brief Description

Acoustic Emission (AE) is high frequency sound generated by cracks and other flaws in materials when stressed. Using AE to detect cracks involves recording and evaluating AE signals to identify any damage. This is usually done as the tank or vessel is filled or pressurised. Structurally significant defects produce relatively intense acoustic emission activity when the vessel or pipe is subjected to a test load and this is the basis for detection of cracks or other damage.

AE testing of materials goes back 40 years and its effective use on process industry equipment has developed over the last 20-years. For many applications it is considered a mature technology with several ASTM and ASME standards in place.

More recently, AE has developed as a means of detecting corrosion damage in tank floors. This is a specialised form of on-line monitoring where AE data is evaluated under static conditions to detect active corrosion processes. Such testing allows a tank owner to set tank inspection priorities and schedules and avoid unnecessary tank entries.

Although initially developed as a practical test method for composite (GRP) tanks and vessels, procedures for testing of metal equipment soon followed. AE is most often used to detect service related damage (stress corrosion cracking, wet H2S damage, fatigue etc.) in process equipment.

In metals, AE is generated when the applied load combined with a local strain concentration such as a crack causes deformation at the crack tip. The resulting high frequency stress waves (AE signals) are detected by sensitive acoustic sensors. These AE signals are recorded and processed to determine the approximate location of any damage and give some measure of its severity.

Although an AE test detects defects, it does not determine their type, size or exact position. The main purpose and benefit of an AE test is to determine if there is a structural problem, approximately where it is and give a measure of its severity. A complementary inspection method such as (shear wave) ultrasonics is needed to map out and size any flaws.


Capabilities

  • Detection of cracks and other damage in pressure vessels, tanks and piping.
  • Corrosion damage assessment for tanks.
  • Can monitor equipment continuously to detect damage as it occurs or where a discrete, pressure test is not possible.
  • Suitable for a wide range of metals, alloys and composite materials.
  • Sensitive to process related(environmental)damage.
  • Minimal disruption to normal operation in many cases.
  • Minimal disturbance to insulation.
  • Helps limit inspection to vessels and areas where damage has occurred.


Limitations

  • Qualitative assessment, does not characterise a flaws or corrosion damage in precise (type of flaw, orientation, size, depth) terms.
  • Generally not sensitive to fabrication defects such as porosity, slag inclusions and small lack of fusion defects in operating equipment.
  • Background noise and other properties of a particular vessel may limit detection sensitivity.


Preparation

  • Collect vessel fabrication details, operating conditions and history.
  • Determine the number and location of sensors needed and the required loading.
  • Make provisions for attachment of AE sensors (manlift or scaffold, insulation cut-outs etc.) and preparation for the pressurization or other means of loading of the vessel.
  • Flaking paint or scale must be removed at each sensor location but sensors can usually be mounted on top of paint that is in good condition.


Services Required

  • Means of access for installing, calibrating and removing the AE sensors.
  • 110V power, a site hut may be needed.
  • Arrangements for pressurising, filling or otherwise loading the vessel or pipe.

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