Now, let me explain why a condensing steam turbine needs to be evacuated before startup.

　　The exhaust pressure of a condensing steam turbine significantly affects operational economy. The primary factors influencing the vacuum level in the condenser are the inlet temperature of the cooling water and the cooling rate. The former is related to the region where the power plant is located, the season, and the method of water supply. Why is it necessary to remove dust before starting up? Our editor will explain it for you.

　　1. In a condensing steam turbine, before a cold start, there is a large amount of air inside the system. If the vacuum is not evacuated, the following hazards may arise:

　　1. A large amount of steam is required to overcome the frictional resistance in the bearings and the inertial forces of the rotor, which increases the impact of the steam on the blades.

　　2. Due to the presence of air inside the cylinder, the frictional drag from the continuously variable vanes intensifies, causing the exhaust temperature to rise.

　　3. Due to the presence of air in the condenser, heat exchange between steam and water inside the condenser is weakened, leading to a rise in exhaust steam temperature and deformation of the cylinder metal. Additionally, the copper tubes in condensing steam turbines expand and loosen, causing leaks. Since the air does not condense, the exhaust pressure of the turbine rises, triggering the operation of the condenser’s vent valve. For these reasons, condensing steam turbines must undergo vacuum extraction before startup.

　　2. Why is it necessary to supply shaft sealing steam before evacuating the turbine during a high-temperature start-up?

　　When a condensing steam turbine is in the hot state, the temperature of the shaft seal plates at the rotor and shaft seal area is very high. At this point, if the shaft seal is not supplied first, large amounts of cold air will be drawn into the cylinder through the shaft seal, causing the rotor in the shaft seal section to contract rapidly. Therefore, when the steam turbine is starting up from the hot state, the shaft seal should be supplied first, followed by vacuum extraction, to prevent deformation. Additionally, the copper tubes in the condenser may expand and loosen, leading to water leakage.

　　Since there is air inside the condenser, the heat exchange between steam and water within the condenser is weakened, leading to an increase in exhaust temperature. Therefore, before starting a condensing steam turbine, vacuum pumping should be performed.

　　Automatic Main Steam Valve for Condensing Steam Turbines

　　A condensing steam turbine is a type of steam turbine in which all the steam, after doing work in the cylinder, is exhausted into a condenser (under vacuum) and condensed into water.

　　The main steam valve plays a crucial role in rapidly cutting off the steam supply and stopping the turbine once the turbine protection system is activated. Therefore, it serves as a common actuator for protection devices. The structure of an automatic steam valve can be divided into two main parts: the main steam valve itself and the operating mechanism. The operating mechanism is the component that controls the opening and closing of the automatic main steam valve. To ensure proper operation, the automatic main steam valve must act quickly and seal tightly. For condensing turbines, under normal inlet and exhaust parameters, after the automatic main steam valve closes (with the regulating valve fully open), the turbine speed should promptly drop below 1,000 rpm. Typically, the time from the activation of the condensing turbine’s protection system to the complete closure of the main steam valve should not exceed 0.5 to 0.8 seconds.

　　The steam discharged from the low-pressure cylinder of a condensing steam turbine is condensed into water and returned to the boiler for recirculation. The main steam valve creates a high vacuum at the turbine’s exhaust port, allowing the steam to expand to low pressure within the turbine. This increases the available enthalpy drop of the steam in the turbine, thereby improving the thermal efficiency of the cycle.

　　That’s all for today’s explanation on “Why is it necessary to evacuate the vacuum before starting a condensing steam turbine?” Does everyone understand now? We hope the information provided above has been helpful to you.