Steady-state and dynamic heat transfer in steam drums of combined cycle power plants: mathematical and simulation model
by
Stefano Bracco
DIMSET, University of Genoa, Via Montallegro 1, 16145 Genoa, Italy

The present analysis is part of a large study related to the dynamic simulation of energy systems, in particular combined cycle power plants. Nowadays, it’s important to use dynamic simulators which can predict the power plant behaviour in steady-state and transient operating conditions. These simulators are useful, above all, during the design phase in order to reduce future maintenance costs and to improve the power plant operating strategies. In this framework, combined cycle power plants are considered a flexible technology and are more and more subjected to frequent startups and shutdowns in the electricity deregulated market. As a consequence, the most critical components of the plant are subjected to large thermal stresses which determine considerable life consumptions. Particularly, steam drums and superheaters represent the most critical components from this point of view. In order to estimate the life consumption of each component, due to transient operating conditions, it’s necessary to develop simulation models able to calculate thermal stresses in these components. The first step of the analysis is the calculation of the temperature distribution inside the metal parts of steam drums and superheaters. The present analysis is centered around the application of the Fourier’s heat conduction equation to study the heat transfer through the metal and insulation parts of a high pressure steam drum belonging to a heat recovery steam generator of a combined cycle power plant. The paper describes both the mathematical model and the simulator, realized in the Matlab/Simulink environment, that have been developed in order to apply the Fourier’s equation to the heat conduction inside the steam drum.

The Fourier’s partial differential equation analytical solution has been found both for steady-state and the transient conditions. The paper describes these two solutions and the finite difference method that has been developed in order to implement the Fourier equation in the Matlab/Simulink environment, for both the metal wall and the insulation of the steam drum. The simulation model has been validated taking into account results presented in literature and modelling the same problem by means of the software Ansys, using a finite element analysis. Several steady-state and transient simulations have been done and are here described considering steam drums installed in modern combined cycle power plants and characterized by different geometrical dimensions and materials.

Date received: March 17, 2008