POWER March 2022 - 28

OPERATIONS & MAINTENANCE
Monitoring of Power Station
Components: Predictive Maintenance
in Flexible Plant Operation
An innovative software program, coupled with an experienced team of engineers,
allows plant operators to evaluate creep and fatigue stress in components exposed
to more frequent cycling, thereby identifying potential problems, facilitating
operational changes, and reducing the risk of unexpected failure.
Dipl.-Ing. Franz Binder
lexibility of operation is becoming
ever more essential for thermal power
stations. However, this flexibility
is accompanied by increased exposure of
metal components to cyclic loads caused
by changes in pressure and temperature.
The risk of material fatigue grows with
the length of service life, and may lead
to defects in the plant as well as production
losses. This issue calls for a method
supporting quick and realistic analysis of
component condition, to allow timely intervention
and optimum planning of maintenance
measures to be undertaken.
TÜV SÜD Industrie Service offers a
F
proprietary service that caters to this
need. At its core is TSE (temperature
stress exhaustion), a software program
designed especially for this purpose that
can be applied to all plant components
exposed to cyclic loads. It enables the
condition of a component to be determined
with a high level of efficiency,
thereby limiting maintenance measures
to the absolute minimum necessary.
Energy from renewable sources is characterized
by volatile production patterns.
Given this, significant fluctuations in the
amount of energy fed into the grid may
have to be balanced to ensure grid stability.
This impacts on the operation of thermal
power stations. They often must be
run in a significantly more flexible manner
and outside their design parameters.
Originally designed for baseload operation,
in the future these power stations
will increasingly have to operate dynamically
at medium- and peak-load-quite
frequently including minimum load.
New Modes of Operation Leave Their
Mark
These new modes of operation were not
generally considered in the original design
of the plants. Flexibility in terms of
28
capacity and the associated changes in
the modes of operation cause a growing
number of more pronounced operational
transients. These, in turn, result in a higher
risk of fatigue damage in components.
Most prone to this type of damage are
fluid-conveying components and fittings
in the water-steam cycle, such as those
installed in boilers, coolers, collectors,
drums, or superheaters.
Also important to note is that more flexible
plant operation with modified startup
and shutdown transients, and changed
points of operation, frequently causes
secondary load transients of a type that
had likewise not been incorporated in
the original design calculations. As an example,
unfavorable control parameters in
steam cooling, particularly at minimum
load conditions, may lead to load cycles
with higher frequencies. If this phenomenon
goes unnoticed, it may result relatively
quickly in incipient fatigue cracking
on the component's inner surface.
These
developments
lend
new
weight to the monitoring of component
fatigue in thermal power stations. On
the one hand, realistic knowledge of
the state of creep fatigue plays an important
role in comprehensive damage
prevention; on the other, however, plant
managers and operators must also be
able to make full use of the service life
of their plant components. Monitoring
thus makes good sense because the
results provide reliable information for
condition-based maintenance.
Exhaustion Mechanisms
Important degradation mechanisms acting
on pressurized components of the water
and steam cycle include creep and fatigue,
which are also referred to in technical literature
as
" high-temperature
progressive
deformation " and " progressive and
localized structural damage under cyclic
www.powermag.com
1. Workflow of offline calculation of service
life. Courtesy: TÜV SÜD
loading, " respectively. The combination
of these two degradation mechanisms is
known as creep fatigue and referred to in
this article as " state of exhaustion. "
While creep is caused by static loads
and depends on temperature and internal
pressure, fatigue is caused by cyclic
loads-in other words, changes in
pressure and temperature. In the case
of changes in temperature, fatigue increases
in parallel to the rate of temperature
changes. This is due to the following
mechanism: As the thermal conductivity
of the material is limited, steep temperature
transients result in increasingly high
temperature differences inside the component
wall, which in turn give rise to
mechanical stresses.
Initially, the impacts of these stresses
are virtually undetectable because
they first become apparent at the component's
inner surface, and there above
all at points of change in wall thickness,
edges, and anomalies-all areas where
they are difficult to identify by non-destructive
test (NDT) methods. Identification
is only possible if the component
is already affected by incipient cracking.
The result is the occurrence of failure
events, even though the responsible
parties assumed they had taken reliable
precautions in the form of NDT. Many
of these defects and failures could have
been prevented by needs-based calculation
of component life.
POWER | March 2022
http://www.powermag.com
POWER March 2022

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