VGB POWERTECH 3 (2021) - International Journal for Generation and Storage of Electricity and Heat

Relaxation effects in high temperature piping systems VGB PowerTech 3 l 2021
A method for the consideration of
relaxation effects in the assessment of
stresses and bearing loads of high
temperature piping systems
Thomas Schmidt
Kurzfassung
Verfahren für die Ermittlung von
Relaxationseffekten bei der Bewertung
von Spannungen und Stützlasten von
Hochtemperatur-Rohrleitungssystemen
Die kontinuierliche Überwachung von Rohrleitungs-
und Halterungssystemen moderner Kohlekraftwerke
ist essentiell, da aufgrund hoher
Dampfparameter Kriechermüdung und Spannungsrelaxation
infolge von Kriechdeformationen
eine wichtige Rolle spielen (z.B. hinsichtlich
der Rohrspannung) und die Spannungsreserven
gering sind. Zudem sind die Rohrleitungskomponenten
durch eine erhöhte Anzahl an
An- und Abfahrvorgängen zunehmend Wechselerschöpfung
ausgesetzt. Aus diesem Grund muss
für die kritischsten Bauteile eine kontinuierliche
Bewertung der Zeitstand- und Wechselerschöpfungsgrade
erfolgen. Dafür muss u.a. geprüft
werden, ob die Rohrleitung im zulässigen Spannungsbereich
betrieben wird. Letzteres stellt
eine gewisse Herausforderung dar, da das Rohrleitungssystem
während des Betriebes Relaxationseffekten
unterworfen ist. Gemäß den gängigen
Berechnungsrichtlinien wird angenommen,
dass diese während des Betriebes zu einer gleichmäßigen
Absenkung der Rohrleitungsspannungen
über der Zeit führen. Während des Betriebes
durchgeführte Kraftmessungen an starren Unterstützungen
zeigen hingegen, dass die Relaxation
ungleichmäßig über das Rohrleitungssystem
erfolgt und demnach an manchen Stellen
auch eine Lasterhöhung zur Folge haben kann.
Daher wird in diesem Beitrag eine Methode vorgestellt,
mit der Relaxationseffekte in einer
Rohrstatikanalyse auf Basis gemessener Unterstützungskräfte
während des Betriebes berücksichtigt
werden können.
l
Author
Dr.-Ing. Thomas Schmidt
Affiliation, where work has been conducted:
MMEC Mannesmann GmbH
(former Technip Germany GmbH)
Department Consulting/
Technical Calculations
Düsseldorf, Germany
Current affiliation:
SMS group GmbH
R&D Division
Düsseldorf, Germany
Continuous monitoring of the piping and
hanger systems of modern coal-fired power
plants is essential. Because of high steam parameters
creep exhaustion as well as stress
relaxation due to creep deformation play
crucial roles (e.g. with respect to the pipe
stresses) and the stress reserves are low. An
increased number of start-ups and shutdowns
moreover evokes low cycle fatigue of
the piping components. Thus, continuous assessment
of the degrees of exhaustion due to
creep damage and low cycle fatigue has to be
performed for the most critical parts. Therefore,
among other things it must be checked,
whether the pipe stresses are in an admissible
range. This is quite challenging, since during
operation the piping system is subjected to
relaxation effects. According to calculation
standards it is assumed, that these effects
uniformly lower the operating stresses
throughout the pipe over time. Actual operational
measurements of rigid support forces
show however, that the relaxation is nonuniform
throughout the pipe and may thus
at some locations even increase the load.
Therefore, this contribution provides a method
of how these relaxation effects can be reflected
in a pipe statics analysis, based on
actual measurements during operation.
1 Introduction
Modern coal-fired power plants are operated
at high steam parameters, in order to
increase the degree of efficiency, which is
accompanied by increased requirements
on the power plant’s piping and hanger systems.
In detail, live steam lines and hot reheat
lines are operated at temperatures
around 600 °C and pressures up to 300 bar.
Usually, therefor the highly-alloyed ferritic-martensitic
steel X10CrWMoVNb9-2 is
applied, which is e.g. characterized by
higher heat transfer and lower thermal expansion
coefficients as compared to austenitic
steels. At such high temperatures e.g.
creep exhaustion as well as stress relaxation
due to creep deformation play crucial
roles during operation time and the creep
strength is rather low, which results in low
stress reserves. On the other hand, the high
pressures require thick-walled components
leading to a high weight of the piping systems.
The latter fact together with the thermal
expansions of the piping put high demands
on the power plant’s hanger system.
Moreover, especially in Germany an increased
number of start-ups and shutdowns
can be expected in the future associated
with the energy turnaround. Therefore,
also contributions to the degree of
exhaustion through low cycle fatigue become
more relevant, which are more critical
for thick-walled components, since
higher thermal stresses are induced due to
higher wall temperature gradients. Respecting
the above considerations, a continuous
monitoring of the actual piping
behavior (as also recommended by the
VGB [1]) is advantageous, in order to
maintain the operation reliability, establish
maintenance and inspection intervals and
ensure a safe long-term operation. Different
monitoring systems (see e.g. [2], [3],
[4], [5]) are available, which besides the
measurements of temperatures and pressures
involve displacement measurements
at constant hangers as well as force measurements
at rigid supports of the piping
system. By means of the measured temperatures
and pressures a lifetime calculation
according to the TRD directives 301/508
[6], [7] or the more recent DIN EN 12952
[8] is conducted for selected thick-walled
piping components based on the Tresca
equivalent stress. This procedure is justified
whenever the Tresca equivalent stress
is not affected by system loads. The verification
of this boundary condition can be
attained through the monitoring of the
piping system. In advance, usually pipe
statics analyses of the as-build pipe geometry
(including correct weights, constant
hanger loads, etc.) are carried out by the
pipe constructor with help of the CAE software
ROHR2 [9]. Thereby, different load
scenarios as e.g. cold piping conditions and
several variants (with respect to friction,
hanger incline, etc.) for hot piping conditions
are investigated in terms of pipe displacements,
rigid support forces, bearing
loads and pipe stresses providing a predict-
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