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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