APPROXIMATION EQUATIONS OF STEAM STATE IN THE LOW-PRESSURE REGION FOR ANALYSIS OF WORK THE LAST STAGES STEAM TURBINE

Abstract

Abstract

The operating conditions of the low-pressure cylinder (LPC) of steam turbines used at thermal and nuclear power plants were analyzed. The LPC stages operate both in the region of superheated steam and in the wet steam region, where phase transition phenomena significantly influence on efficiency and blade erosion. In practical operation, the thermodynamic state of steam is usually determined by using steam tables or complex approximation functions based on tabulated data, which are not always convenient for real-time engineering calculations. Based on the classical ideal gas equation of state and using tabulated thermodynamic properties of water and steam in accordance with IAPWS formulations, approximation equations of state are developed for dry saturated, superheated, and wet steam in the low-pressure range from 3 to 100 kPa. The proposed approach introduces a variable (local) gas constant that accounts the influence of pressure, temperature, and steam quality. Regression equations are obtained for the specific volume of steam as a function of pressure, temperature, and dryness degree. The results demonstrate that, within defined regions of superheat, the thermodynamic behavior of steam approaches to an ideal gas with a practically constant gas constant. For wet steam, simple relations based on steam quality are proposed. The derived approximation equations provide sufficient accuracy for engineering applications and can be effectively used for thermal and energy analysis of LP turbine stages operating under variable load conditions.