VGB POWERTECH 10 (2019)

Flexible steam turbine operation in combined cycle power plants VGB PowerTech 10 l 2019
Fig. 10. Application areas of THS’ heating blankets indicated in yellow.
heat transfer coefficients on outer casing)
is applied in the simulation of the 3D model
without blades. The adapted boundary
conditions improve the results, as shown in
F i g u r e 7 c . The differences between the
3D model without blades and the 2D model
without blades originate on the one
hand from the different boundary conditions.
On the other hand, different view
factors due to included flanges in the 3D
model influence the results. The distances
between the inner and outer casing are also
smaller in the 3D model without blades a
fact that influences the heat transfer. Including
the flanges increases the component
volume hence leading to a higher
stored energy. This results in a reduced rate
of cooling in the 3D model without blades.
Comparison of models
All models are able to capture the most important
effects during cool down. The 3D
model without blades performs better due
to the more exact representation of the turbine
geometry. 3D effects should not be
neglected for a detailed analysis. For large
scale optimization approaches, using a 2D
model will be sufficiently accurate. No
model is able to capture the effects at the
beginning of the simulation very accurately,
which is acceptable as the long term
temperature distribution is the main point
of interest.
Application of the HSM
The validated 3D model without blades is
enhanced to additionally include the THS.
The enhanced model is used to investigate
the performance of the HSM.
A load case for the analysis of the warmkeeping
mode is constructed based on the
cool-down load case. The turbine is turned
off and left to cool down. The THS is activated
once the outer casing temperature
reaches a threshold temperature T t .
Different locations of the THS “heating
blankets” are possible. The positioning of
the heating blankets has been exemplarily
investigated for the location shown in F i g -
u r e 10 , where the heating blanket is
mounted on the whole outer casing.
The heating blankets are modeled by including
their respective heat fluxes in the
turbine model. The heat flux does not exceed
a maximum value and the temperature
of the outer casing is defined to be at
maximum 5 K above T t . A PID controller
with temperature feedback loop regulates
the magnitude of the heat flux. It is included
via a user subroutine in Abaqus CAE.
The rotor temperature is the limiting factor
for warm start-up conditions and is hence
chosen as the process variable. F i g u r e 4
shows the location of the considered HP rotor
notch and IP rotor notch. The analysis
of the results shows that the IP rotor notch
temperature is lower than the HP rotor
notch temperature wherefore effectively
the IP rotor notch temperature is the limiting
factor.
The heat flux is the control variable u(t) for
the PID controller, which is computed by
[14]
(10)
where the error value e(t) is the difference
between the actual and the set point temperature
at the IP rotor notch.
F i g u r e 11 shows the temperatures at the
HP rotor notch and IP rotor notch. The temperatures
are normalized by the starting
temperature at the HP rotor notch while
the time is normalized by the time of the
heating blanket activation. The results
show that it is possible to maintain conditions
for warm start-up conditions within
the turbine.
T [-]
1
0.8
0.6
0.4
Cool Down HP
HSM HP
Cool Down IP
HSM IP
0.2
0 0.5 1 1.5 2 2.5 3
Fig. 11. Temperature profiles during cool down
(Cool Down) and in warm-keeping
mode (HSM) at the HP and IP rotor
notches. Temperatures are normalized
by the starting temperature at the high
pressure notch.
Conclusions
The heat transfer throughout the complex
turbine topology has been investigated using
three models with different degrees of
geometry simplification. The analysis
shows that it is possible to capture the most
important aspects using two-dimensional
models with simplified topology. Further
geometry simplifications can be realized by
adapting the thermal behavior of the FEMmodel.
A large part of the heat transfer in the turbine
is realized by radiation. Without modelling
this type of heat transfer, it is not
possible to make an accurate prognosis of
the temperature load and distribution. The
simplified CAE model “2D model without
blades” is able to perform the calculations
faster than models with more geometrical
detail. The negligible change of the accuracy,
with regard to the shortening of the
calculation duration, is therefore fully justifiable.
A controller was further developed
which makes it possible to specify the power
of the HSM within the CAE simulation.
A simulation of the HSM shows that it can
maintain the temperature of the turbine at
a high level. The warm-keeping allows the
customer to preserve the steam turbine
warm start conditions without seal steam
depending on their own commercial targets.
As a result the energy consumption
for long standstill times can be minimized.
Nomenclature
A
F c
r
T
t
cond
conv
real
rad
sub
References
Area
factor for alpha substitution
heat flow
specific heat flow
distance between surfaces
absolute temperature
time
heat transfer coefficient
angle with respect to normal to
the surface
emissivity
thermal conductivity
density
Stefan-Boltzmann constant
normalized time
view factor
conduction
convection
physical value
radiation
substitute value
[1] T. Eisfeld, A. Feldmüller, and F. Roehr,
“CCPP improvements in a business environment
of intermittent power generation,” in
Power-Gen Europe 2017.
[2] A. Feldmüller, T. Zimmerer, and F. Roehr,
“From Base to Cycling Operation – Innovative
Operational Concepts for CCPPs,” in
Power-Gen Europe 2015.
[3] D. Toebben et al., “Numerical Investigation
of the Heat Transferand Flow Phenomena in
an IP Steam Turbine in Warm-Keeping Operation
with Hot Air (GT2017-63555),”
Proceedings ASME Turbo Expo, 2017.
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