Numerical method for determining the allowable medium temperature during the heating operation of a thick-walled boiler element in a supercritical steam power plant

Abstract

SUMMARY The new generation of steam power plants operates at pressures higher than the critical pressure and at very high temperatures. They are called supercritical power plants and their thermal efficiency is improved by increasing their operating pressure and temperature. Such a demanding working environment causes high stresses in the construction, especially during the heating and cooling operations. Additionally, the cyclic character of loading during operations causes material fatigue, known as low-cyclic fatigue. This phenomenon may lead to the formation of fractures. Steam boiler manufacturers make efforts to design pressure elements to meet these high requirements. They make recommendations for conducting start up and shut down operations in order to keep the stresses in the construction elements within acceptable limits and obey the safety regulations. Thus, it is important to find optimum parameters that can ensure proper heating and cooling processes (Struct. Multidiscip. Optim. 2010; 40:529–535, Proceedings of the Congress on Thermal Stresses, 2007; 437–440). Paper (Proceedings of the Congress on Thermal Stresses, 2007; 437–440) presents the method for determining the optimum medium temperature, which ensures that the sum of the thermal stresses and stresses caused by pressure at selected points do not exceed the allowable stresses. The presented optimum medium temperature consists of the initial medium temperature step and later increases in the optimum rate of temperature change. The extended version of the paper (Proceedings of the Congress on Thermal Stresses, 2007; 437–440) was published in 2010 (Int. J. EnergyRes. 2010; 34:20–35). Another paper (Proceedings of the 8th International Congress on Thermal Stresses, 2009; 2:399–402) presents the numerical optimization procedure, based on the Levenberg–Marquardt algorithm that allows the optimum medium temperature to be established. This procedure is based on the assumption that the thermal stresses in the entire construction elements do not exceed the allowable stresses. The aim of this paper is to present the method, which makes it possible to find the optimum parameters, so that the total stresses during the start-up processes are kept at an acceptable level. The maximum absolute stresses, caused by non-uniform temperature distribution and by pressure, are monitored not only at selected points but also in the whole construction element. The described method is of great practical significance and can be applied directly in the industry. It can be utilized in supercritical as well as subcritical power plants. The method proposed can greatly enhance the performance of the power units by reducing the duration of all the transient operations and extending their longevity. The presented heating operation based on the optimum parameters is compared with the German boiler regulation-Technische Regeln fur Dampfkessel 301 (TRD) (Technische Regeln fur Dampfkessel, 1986; 98–138). Copyright © 2011 John Wiley & Sons, Ltd.