POWER March 2022 - 29

OPERATIONS & MAINTENANCE
2. Controlled progression of the state of
exhaustion based on realistic analyses. Courtesy:
TÜV SÜD
Stress-Strain Analysis Using TSE
The TSE software developed by TÜV
SÜD calculates component exhaustion
(fatigue and creep) on the basis of pressure
and temperature curves. TSE uses
algorithms aligned to the specific regulations
and standards in encapsulated
functions and is fully in conformity with
the applicable codes and standards. The
resulting report thus also constitutes a
reliable legal compliance report.
Input data required by the software include
data on component geometry and
materials in addition to the measured
temperature and pressure loads. Within
the scope of the analysis, the program
uses integrated non-stationary calculation
of the temperature field to model
non-linear
distribution
of
temperature
across the component wall for a close
network of interpolation points at any
point in time throughout the analysis period.
The software is also able to process
temperatures measured on the component
exterior. In other words, the TSE
solution eliminates the need to perform
complex temperature measurements on
the interior wall of the component or the
fluid, or even measurements of the temperature
difference including determination
of the temperature inside the wall.
Even temporary measurements can be
included in the calculation.
For plants in Germany, data analysis is
aligned to technical regulations and standards
harmonized across the European
Union. Using offline evaluation of the
measured and saved data, the solution
delivers proof of whether component
exhaustion is below the critical thresholds
defined in DIN EN standards. The
intervals of this type of creep-fatigue life
analysis should be based on the state of
exhaustion determined in the most recent
previous analysis and the mode of
plant operation. In many cases, TÜV SÜD
March 2022 | POWER
recommends annual evaluation cycles to
ensure timely inclusion of new load phenomena
in the evaluation.
Offline evaluation offers significant
benefits, particularly in the as-is analysis
of the stresses acting on the component,
which is part of the first step of the
evaluation. This is because monitoring
over longer continuous periods enables
stress patterns to be identified.
To this end, appropriate pattern recognition
routines have been implemented
in the TSE program. They enable critical
load events to be categorized reliably
and-where necessary-changes in the
mode of operation to be initiated for the
purpose of reducing loads. In step two of
the evaluation, the program determines
the actual state of component exhaustion
(Figure 1). In addition, the program
also informs about the fatigue reserves
of the components available at this
stage. The TSE software thus provides
information about the remaining number
of load cycles and the extent to which
further changes in the mode of operation
can be realized.
In a possible third step, the program
models the progression of exhaustion
resulting from new transient configurations,
and aggregates it with the degree
of creep fatigue from previous operation.
This approach allows experts to
define optimized transients that ensure
consistent use of fatigue reserves while
keeping component exhaustion below
critical thresholds. The software is thus
designed to determine values that are as
realistic as possible, while preventing the
introduction of additional safety margins
that are not justified from a physical perspective
and not required by standards
and regulations.
Realistic Presentation of Fatigue
The calculations performed by the TSE
software do not use the temperature
inside the component wall, but the
physically correct integral mean wall
temperature. If the program used the
temperature inside the component wall,
the calculated Delta T would be excessive,
resulting in 50% higher fatigue values.
This is one example demonstrating
that the phenomenon of " fake fatigue "
can be avoided by consistently relying on
realistic analyses instead of more conservative
methods-a necessary step
for further improving the flexibility of
plant operation (Figure 2).
The objective should not be to keep
component exhaustion to a minimum,
but to achieve controlled component
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exhaustion in line with the available fatigue
reserves. To this end, experts could
also carry out predictive what-if analyses
calculating the respective progress of
component fatigue for various different
operating transients.
Assessing Components in the
Waste-Heat Boiler
The following case study shows how
TSE can be used for assessing components
in power stations. TÜV SÜD was
commissioned to evaluate the condition
of critical components in a waste-heat
boiler. The sensitivity analysis carried out
in advance of the assessment identified
15 components exposed to high stresses.
Using the TSE software, the experts
next measured and recorded the temperature
and pressure at the inner walls
of the components over a six-month period
by affixing sensors at strategic locations
on the components.
The analysis of the measurement results
showed that some components
were exposed to significant cyclic loads
caused by high-frequency changes in
temperature and pressure at short intervals
of 10 to 12 minutes. The overall load
thus fluctuated up and down by a factor
of 10 compared to the baseline of the underlying
load level. More detailed analysis
of the data revealed the high loads were
due to the mixing of superheated steam
and steam condensates in the cooling
process to lower steam temperatures.
The problem was solved when the power
station manager made a small but significant
adjustment to the cooling process,
which reduced the major fluctuations in
the components' load profile and thereby
lengthened the service life of the critical
components in the waste-heat boiler.
The evaluation of component condition
using TSE was subsequently continued
with the aim of extending identification
and monitoring of relevant fluctuations
in pressure and temperature to take in
further phases of plant operation. The results
supplied fundamental information
not only about the state of exhaustion of
the individual components, but also about
the condition of the system and the transients
that are particularly significant for
fatigue processes. This, in turn, gave the
plant manager the opportunity to improve
the station's predictive maintenance procedures
and thereby minimize the risk of
unexpected component failure.■
-Dipl.-Ing. Franz Binder (franz.binder@
tuvsud.com) leads the Piping Systems
and Lifecycle Analyses team at TÜV
SÜD Industrie Service GmbH.
29
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POWER March 2022

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