Effect of Steam Flow Volumetric Charge on Condensation and Steam Turbine Efficiency

 

Prof. Tarelin A.A., and Dr. Sklyarov V.P.

 

The A.N. Podgorny Institute for Mechanical Engineering Problems of the NASU, Kharkiv, Ukraine

 

Recent investigations that have been carried out by the research workers of Institute for Mechanical Engineering Problems (IMEP) of the NAS of Ukraine, as well as joint investigations with American (Sonoma Research Company, EPRI and STI) and Russian (Moscow Power Engineering Institute)) specialists have shown that a wet steam flow in turbines always carries a volumetric electric charge.

Electrization of wet steam produces a significant effect on the heat and mass transfer processes as well as on the performance of the turbine plant as a whole. The appearance of moisture in the turbine’s phase transition zone is always accompanied by electrization of the steam flow. As the steam moisture content increases, so does the volumetric density of the electrostatic charges, which value is often limited only by the breakdown intensity of the electric field in the working medium. The polarity of the charges in the turbine exhaust depends on the chemical composition of the condensing moisture and the material of the working surfaces that are in contact with the steam flow. Under conditions of a standard water regime, the droplets breaking away from rotor blades made of stainless steel are charged positively. When droplets break away from carbon steel surfaces, they acquire a negative charge. The density of the charges in the flow depends on the velocity, moisture content and chemical composition of the feed water. As the charge density increases, the process of volumetric condensation becomes more intensive to make the moisture content grow that results in a pressure drop in the condenser. Investigations on the turbine Т-250/300-240 have demonstrated that an increase in charge density up to 1.6x10-5 C/m3 leads to a decrease of the pressure in the condenser by 100 to 150 Pa. In the setting and after the last turbine stage, where the charge density is maximal and may achieve up to ~10-4 C/m3, the electric field of the volumetric charge has a negative affect on the gas dynamics of the flow.

The process of electrization of a wet steam flow can be controlled effectively by changing the feed water chemical composition or with the help of special neutralisers introduced into the turbine exhaust hood.

Effective control of the processes of steam flow electrization and neutralisation makes it possible to increase the turbine plant efficiency by 0.5…1.5%.