Maintworld 3/2020

ASSET MANAGEMENT
• Diameter: 4.44 metres
• Volume 1,160 m 3
• 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.
Applicable standards
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.
Classification Assessment Method and actions
Class 1 Insignificant source No actions required
Class 2 Active source Visual examination or other subsequent inspection and evaluation
Class 3 Highly active source Test interruption or termination, pressure relief, visual
examination, other subsequent inspections and evaluation prior to
the return to service.
Table 1: Clarification of signals and actions
50 maintworld 3/2020