Power Level Optimization for Reducing Shaft Movement during On-line Test of HP Turbine Control Valves

Most high pressure turbines in nuclear power plants in Korea use the partial arc admission control. It sequentially opens each valve, which is connected to each arc consisting of the HP turbine nozzle when the steam flow rate should be increased. Generally, a nozzle of the HP turbine is divided into four arcs. All turbine valves including the control valves must be tested periodically to confirm that the valves function reliably. During the test, the movement of the center of the HP turbine shaft as well as the rotor vibration increase as the steam flow abruptly changes by the opening and closing of each control valve in a few seconds. Turbines may trip because of high vibration if the amount of steam flow change is too large. The change in the steam flow increases as the electrical output of the plant increases; thus the output level should be lowered before performing the test. However, if the output level is significantly lowered during the test, the loss of power generation will increase. This paper presents the determination of the output level for the control valve test through an analysis of the characteristics of the partial arc admission control and the movement of the center of the HP turbine shaft for an efficient test.