New method for determining the optimum fluid temperature when heating pressure thick-walled components with openings

A new approximate method of optimum heating cylindrical pressure elements weakened by openings was proposed. Optimum variations in fluid temperature when heating the pressure component were determined from the condition that the total circumferential stress at the edge of the opening, resulting from the thermal load and pressure is equal to the allowable stress. The allowable stress is determined from the Wöhler fatigue diagram for a given number of start-ups and shutdowns of a power unit from the cold state. Optimum temperature changes are difficult to estimate at the beginning of the heating, using both exact analytical and numerical methods. In case of analytical methods, this is due to the very slow convergence of a series for near-zero time in the exact solution. In this paper, the optimum temperature changes of the fluid at the beginning of heating were determined using the heat balance integral method (HBIM). This method makes it possible to determine with high accuracy the temperature of the fluid for times close to zero, i.e., at the beginning of the heating process. In the second stage of heating, the optimum fluid temperature was determined on the assumption of a quasi-steady temperature field in the pressure element. The problem analysed in the article is important to increase flexibility of boilers and steam units. By optimising the heating up of the boiler during boiler start-up, the start-up time of the boiler and the whole unit can be significantly shortened and the power unit can be quickly connected to the power system.