An Analysis of the Effect of Protective Films on the Inside Surface of Headers and Steam Pipelines in Combined-Cycle Power Plants on Their Thermally Stressed State

Abstract

The outlet header of the high-pressure superheater is among the critical components affecting the service life of heat recovery steam generators (HRSG) of combined-cycle power plants (CCPP). Headers, heat-transfer tubes, and steam pipelines are protected against corrosion by running proper water chemistry to form thin protective films on the surfaces. These films formed on the inside surface of heat-transfer tubes protect against corrosion and also reduce thermal stresses in the tube walls during transients. They are formed on the inside of tube systems of HRSGs in CCPPs at temperatures above 230°С and with good deaeration. Results are presented of a numerical analysis of the influence of thin protective films on the state of thermal stresses in headers and steam pipelines of HRSGs used in CCPPs during various transients induced, for example, by startups from different initial thermal state or thermal shocks. Predictions were obtained for different film thicknesses assuming that the film thermal conductivity was less than thermal conductivity of the pipeline metal. The numerical results enabled the determination of the heating-up modes for headers and steam pipelines under which the effect of protective films is the greatest. It is demonstrated that the protective films have a considerable effect on stresses and damage accumulation in the pipeline wall induced basically by large temperature disturbances, such as thermal shocks. The effect of protective films on the thermally stressed state of unheated pipes was assessed by reducing the resourse consumption and accumulation of equivalent operating hours during transients. It is demonstrated that a 50 μm thick protective film alone can increase the service life of an HP steam superheater’s outlet header with a standard size of 426 × 34 mm by 6%.