The process and function of the steam turbine vacuum system

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The turbine vacuum system is an important part of the steam turbine’s thermal cycle; it is primarily used to maintain the vacuum level inside the condenser, thereby improving the efficiency and economic performance of the turbine. The following is a detailed introduction to its main components and operating process: I. Composition of the turbine vacuum system The turbine vacuum system is primarily composed of the following equipment and components; configurations may vary depending on the type of unit (such as condensing turbines, extraction turbines, etc.): 1. Condenser – Function: To receive the exhaust steam from the turbine and condense it into condensed water, thereby creating a vacuum environment. - Structure: It is usually a surface-type heat exchanger, consisting of a tube bundle, shell, water chamber, etc. Circulating cooling water (or seawater) flows within the tube bundle to cool the steam outside the tubes. 2. Vacuum extraction equipment – Function: To extract non-condensable gases (such as air) from the condenser in order to maintain a vacuum. - Common types: – Jet pump: Creates a negative pressure using a high-speed jet to draw out gas (requires a jet pump and a water tank). - Steam ejector: Creates a vacuum using high-pressure steam jets (commonly used in older units). - Vacuum pumps: commonly used in modern units, including liquid ring vacuum pumps, water ring vacuum pumps, dry vacuum pumps, etc. - Liquid ring vacuum pump: A water ring is formed by the rotation of the impeller to draw in gas and compress it for discharge. 3. Condensate water system – Condensate pump: Transports the condensate water from the hot well of the condenser to the deaerator or subsequent systems. - Condensate pipes and valves: including check valves, control valves, etc., used to regulate the flow rate and pressure of condensate. 4. Circulating water system – Circulating water pump: Provides cooling water for the condenser (such as water from cooling towers or river water). - Circulating water pipes and valves: Control the flow rate of cooling water to maintain the heat exchange efficiency of the condenser. 5. Vacuum measurement and control equipment – Vacuum gauges/vacuum transmitters: Monitor the vacuum level inside the condenser in real time. - Vacuum break valve: Opens manually or automatically in emergency situations to break the vacuum and accelerate the shutdown of the turbine. - Pressure control valve: Automatically adjusts the operating condition of the pumping equipment to maintain a stable vacuum level. 6. Auxiliary equipment – Coolers: Used to cool the mixed gas discharged by pumping equipment (such as the vapor-water separator of vacuum pumps). - Waste discharge device: Removes impurities or water-repellent substances from the system. - Pipes and valves: including exhaust pipes, drain pipes, globe valves, ball valves, etc., which form the pathways of the system. II. Operating process of the turbine vacuum system The core process of the vacuum system is to use pumping equipment to remove the non-condensable gases from the condenser, maintain a vacuum level, and recover condensed water and heat. The following is the typical process (taking a liquid ring vacuum pump as an example): 1. Vacuum formation stage – before the turbine starts: The vacuum pump (or water jet ejector) is started, and air inside the condenser is removed through the exhaust pipes to establish an initial vacuum. - The exhaust steam enters the condenser: Once the turbine is operating normally, the exhaust steam enters the outside of the condenser tubes, where it is cooled by the circulating cooling water and condensed into water, resulting in a sharp reduction in volume and thus the creation of a vacuum. 2. Non-condensable gas extraction process – Gas mixing in the condenser: The non-condensable gases (air) within the condenser mix with steam and accumulate in the gaps between the tubes or at the top of the shell. - Vacuum pump suction: The vacuum pump draws the mixed gas into the pump body through the suction port, and uses the water ring (liquid ring) created by the rotation of the impeller to compress the gas, thereby increasing its pressure. - Gas-water separation: The compressed mixed gas enters the gas-water separator, where the steam condenses into water. Non-condensable gases such as air are discharged through the exhaust port, while the condensed water is returned to the vacuum pump or condenser (for reuse). 3. Condensate recovery process – Condensate collection: The condensate in the condenser is collected in the heat well (the bottom water storage area). - Transport by condensate pumps: The condensate pumps draw water from the hot well and send it, through condensate pipes, to low-pressure heaters, deaerators, etc., where it is reused as part of the boiler feedwater. 4. Vacuum maintenance and regulation – Vacuum monitoring: The vacuum level of the condenser is monitored in real time using a vacuum gauge or transmitter; if the vacuum level drops (due to air infiltration, insufficient circulation water volume, etc.), the control system will automatically activate backup pumping equipment or issue an alarm. - Circulating water regulation: Adjust the flow rate of the circulating water pump or activate a backup circulating water pump, based on the load and ambient temperature, to ensure the heat exchange efficiency of the condenser. - Emergency handling: When an emergency shutdown of the turbine is required, the vacuum break valve opens, allowing air to enter the condenser rapidly, thereby breaking the vacuum and reducing the rotor’s coasting time. III. The key role of the vacuum system 1. Improving turbine efficiency: The higher the vacuum level, the lower the exhaust pressure of the turbine, resulting in a greater enthalpy drop of the steam within the turbine and thus greater power generation capacity. 2. Recovery of working fluid: Condensate water is recycled as boiler feed water to reduce steam and water losses. 3. Ensure equipment safety: Prevent the accumulation of non-condensable gases from causing an increase in condenser pressure, thereby avoiding damage to the equipment due to overpressure. IV. Comparison of Common Types of Vacuum Systems Type Advantages Disadvantages Application Scenarios Water jet pump Simple structure, low cost High energy consumption, requires a water tank, lower efficiency Small and medium-sized units or older units Steam jet pump No external power required (depends on steam) High steam consumption, high noise level Early condensing units Liquid ring vacuum pump High efficiency, stable operation, low energy consumption Requires circulating water for cooling, slightly more complex design Modern large-scale units Through the coordinated operation of these components and processes, the turbine vacuum system ensures that the turbine operates in an efficient and safe vacuum environment; it is one of the key elements in thermal systems such as thermal power plants and heat power plants.