Determining the optimum heating or cooling times for thick-walled pressure elements allows a steam boiler to reduce start-up or shut-down times. A new method has been developed to determine the optimum fluid temperature change to shorten the steam boiler start-up and shut-down.
The fluid temperature curve is designated so that the total circumferential stress at the hole edge does not exceed the allowable stress.
The total stress at the hole edge where it is concentrated consists of thermal and pressure stress. The method used was illustrated using the optimum steam boiler drum cooling example. A three-dimensional FEM analysis was carried out for the connection between the drum and the downcomer to determine the transient temperature and stress distributions. It was shown that the total circumferential stress due to pressure and thermal loading at the hole edge did not exceed the allowable stress when optimal fluid temperature changes were used. The method shown in this paper was compared with the boiler standard EN 12952-3, which permits the determination of allowable heating and cooling rates of boiler pressure components as a function of pressure. The comparison shows that the cooling/heating time determined by the new method can be significantly shorter than that determined by the method in EN12952-3.
By using the new method for designating the optimum fluid temperature, the flexibility of the boiler and the power unit can be improved as the heating or cooling of critical pressure elements can be more rapid. It can also be used to accidentally cool PWRs (Pressurised Water Reactors) and BWRs (Boiling Water Reactors) pressure vessels.
BIOGRAPHICAL NOTEJan Taler is a professor of energy engineering at the Cracow University of Technology. His main fields of activity are heat transfer, thermal power plants, dynamics of steam boilers, inverse problems of heat conduction and thermal stresses.
At present, he is the Head of the Energy Department at the Faculty of Environmental Engineering and Energy. He was the recipient of two scholarships: DAAD and Alexander von Humboldt. He is the author of several books and over 350 scientific articles.