Selecting optimal conditions for the turbine warm and hot start-up

The paper discusses optimization issues related to start-ups from intermediate states defined as hot and warm, i.e. start-ups carried out after a few or several hours of downtime. Such start-ups are characterized by a non-uniform field of the initial temperature. For this reason, any simulation of start-up processes should be preceded by simulations of the cooling process to obtain a close-to-real temperature distribution at the moment when the start-up process is to begin. This is of particular importance in the case of elements in contact with the outer environment, such as outer casings and rotors. The simulation results indicate that it is possible to speed up the start-up procedure as recommended by the turbine manufacturer without exceeding allowable stresses in the turbine elements. Naturally, this will have a beneficial impact on the flexibility of the power unit operation. Rotors are characterized by the most difficult operating conditions and it is for them directly that start-ups should be optimized. The maximum start-up rate is determined by the initial temperature field of the turbine element material. For this reason, the paper presents a concept of initial heating of the turbine key elements. Running the process during the boiler heating and start-up might substantially shorten the total time needed to restore the power unit operation. Instead of sequential heating, the boiler heating and the turbine heating processes could be carried out in parallel.