Thermally Stressed State of Headers and Steam Pipelines in the Flowpath of Combined-Cycle Power Plants during Their Heating with Condensed Steam

Abstract

One of the problems arising in start-up modes of combined-cycle power units is the preheating of steam pipelines in the high- and intermediate-pressure steam path whose time has a considerable effect on the total start-up time, especially during cold start-ups. Steam condenses on the inner surface during startups of combined-cycle power units in the process of increasing the pressure of high-pressure steam when the initial temperature of a heated part is below the saturation temperature or when a heated surface with a high initial temperature is exposed to the saturated steam at its current pressure for a long time. The paper considers the effect of heat transfer due to the steam condensation on the heated wall of steam pipelines (or headers) on their thermally stressed state. It has been demonstrated that the level of temperature stresses arising in the wall of the steam pipeline is insignificant with this heating since almost all the temperature difference occurs across the condensate film whose heat conductivity is much lower than that of the heated metal wall. Damages to steam pipelines, usually in the form of cracks in drain or impulse lines near the points of their welding to the steam pipelines, are caused by the explosive nature of the boiling during pressure fluctuations in the steam pipelines and by the impact of liquid drops on the "dry" surface of the steam pipeline. The heating of the steam pipeline with increasing pressure and condensation of steam on its wall does not result in considerable thermal stresses in it during the entire stage of heating with condensed steam. Only after the saturation temperature is attained, when there is no more condensation on the wall surface of the heated steam pipeline, temperature stresses appear, whose values are determined by the temperature difference between the steam and the inner surface of the steam pipeline and convective heat-transfer coefficients which are an order of magnitude lower than those observed during heating by steam condensation.