Study on Permissible Heat Flux of Tube Wall with Consideration on Thermal Stress and Creep Effect

Abstract

Considering rachet mechanism of the combined steady mechanical stress and intermittent thermal stress on a thin tube wall of ductile material, Miller pointed out that, there exists a limit for the strength of thermal stress, above which the tube material shows one directional plastic flow at every start and stop of heating. In this report, the above Miller's theory is extended to the case where creep takes place in tube material, and some consideration on thermal fatigue effect is added to the theory, and upper limits for permissible combination of thermal and mechanical stresses on thin and thick wall tubes are analysed for many cases. By the analysis, the maximum heat flux design and minimum tube wall weight design of heat flux and wall thickness, with given inside diameter and pressure of a tube and given yield strength of its material, are approximately predicted theoretically, within the stress ranges, where plastic flow or plastic hysteresis cycle does not occur even if creep takes place. Some calculated examples show that the heat-flux of this kind will be very important for super high pressure boiler tube in the near future.