Control Properties of Bottom Fired Marine Boilers

This paper focuses on model analysis of a dynamic model of a bottom fired one-pass smoke tube boiler. Linearised versions of the model are analysed to determine how gain, time constants and right half plane zeros (caused by the shrink-and-swell phenomenon) depend on the steam flow load. Furthermore the interactions in the system are inspected to analyse potential benefit from using a multivariable control strategy in favour of the current strategy based on single loop theory. An analysis of the nonlinear model is carried out to further determine the nonlinear characteristics of the boiler system and to verify whether nonlinear control is needed. Finally a controller based on single loop theory is used to analyse if input constraints become active when rejecting transient behaviour from the disturbance steam flow. The model analysis shows large variations in system gains at steady state as function of load whereas gain variations at the crossover frequency are small. Furthermore the interactions in the system prove not to be negligible and a subsequent controller design should take this into account using either Multiple input Multiple output control or Single input Single output controllers supported by a dynamical decoupling. The results indicate that input constraints will become active when the controller responds to transients in the steam flow disturbance. For this reason an MPC (model predictive control) strategy capable of handling constraints on states and control signals should be considered. Furthermore hard constraint on the level variations also supports the choice of this strategy.