Koliadiuk A. Thermal strength of the steam turbine control valve

The thesis is devoted to solution of an actual scientific and technical problem of simulating the steam flow, thermal state and issues of thermal stress state, creep, cyclic fatigue and resource assessment of the steam distribution system elements. An analysis of existing approaches for evaluation of steam flow and thermal stress state of steam turbine steam distribution systems is performed. The main issues in solving the thermal stress state problems of the control valve body are highlighted. The analysis of stationary modes of the control valve operation has been carried out. A mathematical model and methods for solving the coupled problem of steam flow and thermal conductivity of the control valve in a three-dimensional formulation are presented. Finite-element models for chosen stationary modes of the steam turbine operation have been developed. Uneven flow of steam in the flowing part of the valve and the area with separations of flow and vortices of steam flow has been identified. The results of scientific research revealed a significant uneven flow of steam through the sieve. The temperature distribution of the check valve body and steam pressure on its walls have been obtained. Based on computational studies, it is determined that in some nodes of the steam distribution system, such as the saddle, the temperature can drop by more than 100⁰C. This is due to the processes of steam throttling with a small opening of the control valves. The temperature increase by 12⁰C of the sieve and the steam flow that washes it in comparison with the temperature of steam at the inlet to the valve was revealed. This phenomenon is caused by the fact that during the onset of steam flow, which enters from the inlet pipe into the steam receiving chamber, the filter sieve inhibits it. This is the transition of the kinetic energy of the steam flow into potential energy, i.e. thermal energy. The thermal stress state of the body of the shut-off and control valve of the steam distribution system at stationary modes of operation is investigated. The mode of operation of the K-325-23.5 turbine at which the greatest equivalent stresses in the body of the regulating valve are observed is defined. It is shown that the places with the maximum equivalent voltages correspond to the zones of damage occurrence during the operation of the steam distribution system. In the mode of the turbine operation, which corresponds to a capacity of 180 MW, the voltage reaches a value of 190 MPa. The mathematical model used to solve the creep problem is given. It is shown that the places with the maximum creep deformations also correspond to the zones of damage occurrence during the steam distribution system operation. The solution of the creep problem of the valve body showed that at the temperature of the supplied steam 540⁰С creep deformations are 0.136% for 200 thousand hours. When the temperature of the steam supplied to the steam distribution system increased to 565 °C, the maximum creep deformations were 0.32% for 200 thousand operation hours. From the analysis of the obtained dependences of deformations on time it is possible to draw a conclusion that the constant creep takes place after 30 thousand operation hours of the steam distribution system. It is shown that the phenomenon of creep in the stationary mode of operation, which