control system is used in the Section 5 for simulation analyses of the
key boiler parameters based on the boiler system dynamic simulation
model developed in the next section. In particular, the simulation
scenario corresponds to the case of external disturbance related
to a sudden drop in fuel oil flow entering the burners.
The boiler control system is equipped with a water level PI controller
commanding the control valve relative opening yd, which is
also shown in Fig. 2. The water level controller needs to deal with
various disturbances, such as the pressure variations-related
shrink-and-swell effect, which can cause notable perturbations of
the controlled variable (water level h). In order to achieve favorable
control system behavior, i.e. well-damped control system
response, the tuning of the PI controller has been based herein
on the appropriate control-oriented linear process model, and the
so-called symmetrical optimum criterion (see Appendix A).
3. Mathematical model derivation
The mathematical model of the natural circulation boiler with a
single steam generation drum is derived herein by employing the
basic laws of conservation of mass and energy [7], extended with
the relationships derived from the law of conservation of water
and steam momentum, as suggested in [8]. The aforementioned
physical laws are applied to each part of the boiler, thus resulting
in a completely analytical boiler model. Any reliance on empirical
relationships, such as proposed in [6], has been avoided herein for
the sake of model fidelity, i.e. to achieve favorable model accuracy
over a wide range of operating conditions.
It should be noted here that the furnace tubes represent a somewhat
specific modeling problem due to the fact that evaporation
processes inside them are spatially distributed. For the sake of simplicity,
it has been assumed that the heat transfer through the boiler
walls is uniform. Additionally, in those parts of the boiler where
the fluid flow is present, viscous friction losses due to interaction
between the flowing medium and tube walls occur. As a result,
the fluid pressure decreases along the tube length, which can have
an influence to the steam/water ratio and, accordingly, to the fluid
medium density. In order to account for those losses, pressure
changes due to viscous friction effects and local energy dissipation
are also analyzed.
control system is used in the Section 5 for simulation analyses of the
key boiler parameters based on the boiler system dynamic simulation
model developed in the next section. In particular, the simulation
scenario corresponds to the case of external disturbance related
to a sudden drop in fuel oil flow entering the burners.
The boiler control system is equipped with a water level PI controller
commanding the control valve relative opening yd, which is
also shown in Fig. 2. The water level controller needs to deal with
various disturbances, such as the pressure variations-related
shrink-and-swell effect, which can cause notable perturbations of
the controlled variable (water level h). In order to achieve favorable
control system behavior, i.e. well-damped control system
response, the tuning of the PI controller has been based herein
on the appropriate control-oriented linear process model, and the
so-called symmetrical optimum criterion (see Appendix A).
3. Mathematical model derivation
The mathematical model of the natural circulation boiler with a
single steam generation drum is derived herein by employing the
basic laws of conservation of mass and energy [7], extended with
the relationships derived from the law of conservation of water
and steam momentum, as suggested in [8]. The aforementioned
physical laws are applied to each part of the boiler, thus resulting
in a completely analytical boiler model. Any reliance on empirical
relationships, such as proposed in [6], has been avoided herein for
the sake of model fidelity, i.e. to achieve favorable model accuracy
over a wide range of operating conditions.
It should be noted here that the furnace tubes represent a somewhat
specific modeling problem due to the fact that evaporation
processes inside them are spatially distributed. For the sake of simplicity,
it has been assumed that the heat transfer through the boiler
walls is uniform. Additionally, in those parts of the boiler where
the fluid flow is present, viscous friction losses due to interaction
between the flowing medium and tube walls occur. As a result,
the fluid pressure decreases along the tube length, which can have
an influence to the steam/water ratio and, accordingly, to the fluid
medium density. In order to account for those losses, pressure
changes due to viscous friction effects and local energy dissipation
are also analyzed.
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