Dynamic Simulation of Combined Cycles Operating in Transient Conditions: an Innovative Approach to Determine the Steam Drums Useful Life Reduction

The present study is relative to the dynamic simulation of heat recovery steam generators. In particular, the study is focused on the development of simulation models able to calculate thermal and mechanical stresses and, as a consequence, the low-cycle fatigue damage of high pressure steam drums in combined cycle systems. The paper describes the mathematical model that the author has developed to estimate the useful life reduction of the high pressure steam drum belonging to a three pressure levels heat recovery steam generator. The aim of the present study is to propose a detailed calculation procedure useful to electrical energy producers that want to predict the steam drums useful life reduction, due to different transient operating conditions, in order to optimize plant load cycles. The steam drum life reduction has been evaluated following the guidelines proposed by the EN 13445 Standard, and subjecting the uniaxial equivalent stress to the “rainflow cycle counting method” reported by the ASTM E 1049 Standard. As described in the paper, one of the innovations introduced by the author to apply the EN 13445 Standard is relative to the analytical method proposed to calculate the allowable number of fatigue cycles without using iterative formulas. Furthermore, it is important to say that the calculation tool can be used to estimate the life consumption of any steam drum of a combined cycle power plant; the model is characterized by low run times and high reliability. In fact, it has been successfully tested and validated taking into account experimental data from real power plants; the paper reports some calculation results that refer to typical transient operating conditions.