Acoustic emission testing: How to listen to pressure equipment
Pressure equipment maintenance activities in chemical plants need to comply with statutory inspection intervals and ensure regular condition monitoring. TÜV SÜD shows how this can be done cost-effectively using acoustic emission testing (AT), a procedure which also benefits predictive maintenance.
The chemical-pharmaceutical industry delivers essential goods such as medicinal products and sanitation articles or is involved in their supply chains and has thus also played a special role in the coronavirus pandemic. As a basic rule, detailed information about a component facilitates the planning of servicing and maintenance. This fact also benefits users of acoustic emission testing (AT), a particularly sophisticated digital test method. A column at a refinery is one possible use case. The column at the refinery is a large-sized vessel used in process engineering. The high, slim columns contain numerous installations and valve trays and predominate in the typical appearance of a chemical plant. They are used to break down mixtures into their constituent components.
Large-sized columns at a refinery
In the case on hand, the large-sized column was to be checked using AT instead of interior inspection, i e. examination from the inside. This involved the advantages that the system did not have to be taken out of service and the column did not need to be cleaned for inspection. Further, the method did not require any actions to ensure occupational health and safety or incur costs caused by the shutdown of the plant.
Key data of the vessel:
- Material: Fine-grain structural steel (P 355 NH)
- Service period: 14 years to date
- Height: 74.3 metres
- Diameter: 4.44 metres
- Volume 1,160 m3
- Wall thickness: Between 22 and 26 millimetres
Proceeding according to a layout plan, TÜV SÜD Industrie Service distributed 88 piezoelectric sensors across the outside wall of the scaffolded column. The number of sensors was sufficient to ensure easy and reliable inspection of the entire structure, including its complex geometries and poorly accessible installations. To be able to affix the sensors directly to the metal using a couplant, the experts had temporarily removed 20 square centimetres of insulation at the points at which the sensors were affixed.
Equipment under test pressure
The test pressure required for AT – which must be at least 1.1 times the maximum service pressure – was controlled by the plant manager via the control centre using the fluid in the vessel. In this case, the online AT process took around 12 hours. AT analyses acoustic ultrasonic waves at high frequencies inaudible to the human ear.
These waves are caused when active defects, such as cracks in the material, expand minimally under the applied pressure. The resulting sudden mechanical motions set their environment into vibration, resulting in a transient elastic acoustic wave. This wave propagates from its point of origin to the sensor’s piezoelectric crystal, which transforms it into electric signals. The signals are then presented graphically by a test computer and interpreted by experienced test engineers.
This method enables discontinuities to be identified in the steel structure before they can cause critical states. In most cases, AT enables far more accurate statements to be made than conventional visual examinations or pressure tests. This also applies to the assessment of non-critical inhomogeneities or microcracking that do not propagate in operation and can thus be left unchanged. In the case discussed here, TÜV SÜD recommended subsequent dedicated inspection of some spots on welds using the UT phased-array method.
Assessment of signals
Acoustic signals are grouped into three risk classes depending on their number, activity, intensity and location (Table 1). This categorisation allows for better planning and prioritisation of any follow-up actions that may be necessary. Ideally, the plant manager works with the inspection organisation to document the quantitative criteria of assessment before the actual inspection.
Background information on health and safety in the use of work equipment
Legal regulations require pressure vessels, piping and other pressurised plant components to undergo periodic technical inspection (PTI). The relevant requirements are laid down in the German Regulation on Health and Safety in the Use of Work Equipment (BetrSichV), which targets all employers. PTI focuses not only on the leak-tightness of pressure equipment, but also on possible cracking or corrosive attacks on walls. Generally, PTI requires examination of the pressure equipment from the inside. According to the German BetrSichV, employers (previously pressure-equipment operators) are permitted to use alternative non-destructive test methods such as AT or the UT phased-array method for this purpose, provided an authorised inspection agency (AIA) confirms that the assessment of plant safety delivered by the test concept is of equivalent quality.
DIN EN 13554 lays down the general approach to AT. The harmonised standard DIN EN 14584 governs the test method for metallic pressure equipment using proof testing with planar location of acoustic emission sources. According to DIN EN ISO 9712, testing must be performed by qualified and certified personnel. DIN EN 13477-2 describes the requirements to be fulfilled by test equipment, which also needs to undergo regular verification of its operating characteristics.
Outlook: digital monitoring – continuous monitoring
Recent years have seen exponential growth in computing power, which has also benefited AT. Faster processors and user-friendly software produce real-time visualisation of several hundreds of localisations per second. The speed at which equipment can detect and analyse potential inhomogeneities or anomalies has increased a thousandfold. Owing to its high level of maturity and real-time capability, AT can also be used for in-service monitoring of plants and systems. It supplies data which are of fundamental importance for forward planning of maintenance and turnaround intervals. This information can also be transferred via data network (also as a cloud solution). Rounded off and complemented by a separate online NDT method (continuous monitoring), these non-destructive test methods may be used for applications such as monitoring of the wall thickness of vessels by UT – information that can likewise be realised via remote data transmission.
Author: Dipl.-Ing. Klaus Michael Fischer, Innovation Manager & Technical Director for Fire and Explosion Prevention, TÜV SÜD Chemie Service GmbH
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