This paper characterizes the performances of once-through heat recovery steam generators in combined cycle applications. Although the concept of once-through boilers has been extensively used for fired boilers, this circulation mode is rather new in combined cycles. Once-through circulation is required at very high pressures, including supercritical conditions, for which there is a possible advantage in terms of the plant efficiency. The concept is also certainly the best solution for high subcritical pressures, such as those required by repowering applications. In addition to cycle performance, this paper describes the advantages and disadvantages of the once-through in heat recovery applications. The operational behaviour of such heat recovery steam generators is discussed, together with the main design implications. A prototype plant running in supercritical steam conditions is also described, as well as some of its possible applications.
Heat Recovery Steam Generator start-ups are most often thought of as being a delicate step in the pressurization of the boiler, as are trips and shutdowns. One reason for this is the so-called swell effect and the problems which can arise when this event occurs while another might be the different controls that have to be monitored at the same time. A dynamic software is actually used by Cockerill Mechanical Industries (C.M.I.) to simulate the behaviour of all significant thermodynamic variables of the system as well as the temperatures of the materials, and typical control variables versus time. During the development of this software, special care has been given to the study of the influence of the vaporizer pressure drop on the temperature and pressure gradient at the inlet of the drum, where critical thermal and mechanical stresses might appear. The circulation pump specifications have been taken into account. A discussion is presented in order to measure the influence of this pressure drop on the highest observed stresses during the swell effect as well as the pressure gradient used during the pressurization on the fatigue analysis of the drum and its nozzles. Their design is then briefly overviewed using two methods of calculations.
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