The German Regulation on Health and Safety in the Use of Work Equipment (Betriebssicherheitsverordnung, BetrSichV) requires periodic technical inspection and testing of pressure vessels, tubing, and other pressurised components. The focus in this context is on aspects such as strength or leakage as applicable as well as interior inspection, i.e. examination of the inside of the column for possible damage mechanisms including cracking and/or corrosive attacks. The more detailed the information that can be gathered about the relevant component, the better maintenance measures can be planned and shut-downs of plants and production losses effectively avoided. These advantages also benefit plant managers who rely on a fast and reliable digital test method, i.e. acoustic emission testing (AT). One example from practice is delivered by the manager of a C4 column. Columns are large-scale vessels of complex design which are used to separate the various fractions from carbon compounds.
A refinery manager decided to rely on AT instead of visual inside examination for the periodic technical inspection (PTI) of the large vessel of its C4 column. Under this method, there was no need to take the system out of operation or clean the column for inspection (Fig. 1). Neither did the method require any other costly or time-consuming measures to ensure occupational health and safety and environmental protection, or cause costs due to production losses from a system shutdown.
The vessel had the following characteristics:
- Material: Fine-grain structural steel (P 355 NH)
- Height: 74.3 metres
- Diameter: 4.44 metres
- Volume: 1,160 m3
- Wall thickness: 22 to 26 millimetres
- Service life: 14 years
The refinery manager commissioned the scaffolding of the column. At the intended positions of the piezoelectric sensors for AT, the vessel insulation was removed over an area of 20 cm2.
Overall, a total of 88 AT sensors were used on the column. These sensors were distributed across the entire outside surface of the vessel according to a layout plan, and then attached with the help of magnetic holders and couplant. The number of sensors was relatively small in relation to the size of the vessel, but sufficient to create a digital twin of the entire structure including its various installations and valve trays, which in turn enabled reliable results to be obtained from testing and inspection.
Table 1: Signal and measures to be taken
|Assignment to risk classes
||Method and action
||No actions required
||Visual examination or other subsequent inspection and evaluation
||Highly active source
||Test interruption or termination, pressure relief, visual examination, other subsequent inspection and evaluation prior to return to service.
Producing test pressure and recording sound waves
AT requires a specific test pressure. The refinery manager pressurised the column to this test pressure, controlling it from the control centre. To replace visual examination of the inside of the vessel, the test pressure must be at least 1.1 times the maximum service pressure over the last twelve months. Pressurising the column, including online monitoring, took around twelve hours. AT analyses ultrasonic waves at high frequencies inaudible to the human ear.
These waves are caused by active discontinuity, i. e. defects such as cracks in the material, which grow minimally under the applied pressure, causing short-term displacements. These sudden motions set the surrounding material structure in vibration, causing 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. These signals are recorded by a test computer, presented graphically in the digital twin and finally interpreted by experienced test engineers.
This method enables discontinuities to be detected in the material structure before they can cause critical states. In many cases, AT enables far more accurate statements to be made than conventional visual examinations or pressure tests. This also applies to the evaluation of non-critical inhomogeneities or micro-cracking that do not propagate in operation and thus can be left unchanged. Based on the recorded data measured on the C4 column, TÜV SÜD recommended subsequent dedicated inspection of suspicious spots on welds using the ultrasonic testing method phased array. This enables flaws to be clarified and the scope of repair to be defined.
Assessment of signals and measures
The acoustic signals are assigned to three risk classes depending on factors such as number, activity, intensity and location (Table 1). These risk classes allow better planning and prioritisation of any follow-up actions derived therefrom. Ideally, the plant manager and the inspection organisation define the evaluation criteria before the actual test and document them in the test instructions.
Digital testing and continuous monitoring
The enormous rise in computing power over recent years has also benefited AT. Faster processors and user-friendly software produce real-time visualisation of several hundreds of localisations per second. The pace 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, complementing statutory tests and inspections. This supplies valuable information for forward planning of maintenance and turnaround intervals. This information can also be transferred via data network (online monitoring), enabling realisation as a cloud solution. Further, non-destructive tests can be completed or extended using additional non-destructive testing methods. One example of this is a “permanent ultra-sonic monitoring” for the wall thickness of vessels.