Optimisation of the cooling of pressurised thick-walled components operating with fluid at saturation temperature

Abstract

Determining the optimum heating or cooling times for thick-walled pressure elements allows a steam boiler to reduce start-up or shutdown times. A new method has been developed to determine the optimum fluid temperature change to shorten the steam boiler start-up and shutdown. 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 example of the optimum cooling of the steam boiler drum illustrated the method used. A three-dimensional finite element method (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 EN 12952-3. 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 accidently cool PWRs (Pressurised Water Reactors) and BWRs (Boiling Water Reactors) pressure vessels.