Turbine ventilation valve
The Turbine Ventilation Valve (TVV) is used for large steam turbines, which are started up through the intermediate pressure (IP) turbine. While in IP turbine operation mode, the high pressure (HP) turbines require cooling. This cooling is provided by cold steam flow through the HP turbine.
HP Turbine cooling during start-up (Reverse Flow mode) – During the start-up sequence of the power plant, the HP Turbine Control Valves as well as the Turbine Shut-off Valves are closed in order to eliminate any damage to the HP-Turbine caused by improper steam conditions. Meanwhile the HP Turbine Bypass Stations are in operation, leading the steam to the IP/LP Turbine for the startup process (up to approx. 7% load). During this process, the HP turbine requires continuous cooling in order to avoid overheating. Opening the Reverse Flow Valve (RFV) bypasses the Turbine Check Valve. Opening the Turbine Vent Valve (TVV) at the same time forces steam to circulate from the cold reheater line through the HP Turbine to the condenser, providing the required cooling effect.
HP Turbine depressurising at turbine trip or full load rejection (depressurisation mode) – In case of load rejection or turbine trip, the Turbine Safety Valves close immediately and the HP Bypass System takes over the steam flow. To avoid overheating of the still rotating HP turbine, the turbine must be depressurised immediately. The Turbine Control System usually triggers Turbine Safety Valves and the Turbine Vent Valve at the same time. Opening the Turbine Vent Valves will evacuate the trapped steam to the condenser, which leads to an immediate depressurisation of the turbine.