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I. Function and Composition of the Main Condensate Water System The main function of the main condensate water system is to transport the condensate water from the heat well of the condenser to the deaerator. To ensure the reliable operation of the entire system and improve efficiency, during transportation, the condensate must be desalted and purified, heated, and subject to necessary control adjustments. Meanwhile, water for equipment cooling, sealing, cooling, and control is provided during operation, in addition to making up for the steam and water losses that occur in the thermal cycle. The main condensate water system generally consists of main equipment such as condensate pumps, shaft seal heaters, and low-pressure heaters, along with the pipes that connect them. In subcritical and supercritical parameter units, since the boilers have high requirements for the quality of feedwater (especially in once-through boilers), desalination devices are installed after the condensate pumps. Due to the pressure tolerance limitations of the desalination unit in domestic units, the condensate water is pressurized in two stages; therefore, a condensate water booster pump is also installed after the desalination unit. For large units, the main condensate system also includes a make-up water system composed of a make-up water tank and make-up water pumps. The main condensate water systems of typical units share the following common features: (1) There are two condensate pumps or condensate water booster pumps, each with a capacity of 100%; one operates normally while the other is in standby mode, and the standby pump starts automatically in case the operating pump fails. (2) The low-pressure heater is equipped with a main condensate bypass. The function of the bypass is to allow condensate to flow into the deaerator through the bypass when a heater fails and is taken out of service, thereby preventing an accident involving a heater from disrupting the normal operation of the entire power plant. Each heater is equipped with a bypass, known as a small bypass ； Two or more heaters share one bypass, known as the main bypass. The large bypass offers advantages such as a simpler system, fewer valves, and reduced investment costs. However, when one heater fails, the remaining heaters in this bypass also stop operating, resulting in a significant drop in the condensate temperature. This not only reduces the thermal efficiency of the unit’s operation but also lowers the inlet temperature of the deaerator, leading to unstable operation and poorer deoxygenation performance. The small bypass is the exact opposite of the large bypass. Therefore, the main condensate water system of low-pressure heaters often employs a combination of large and small bypasses. (3) Set the minimum condensate flow recirculation. To prevent cavitation in the condensate pump during startup or at low load, and to ensure that sufficient amount of condensate flows through the shaft seal heater so that any steam leakage from the shaft seal can be completely condensed, thereby maintaining a slight negative pressure in the shaft seal heater, a recirculation pipe for the minimum flow rate of condensate returning to the condenser is installed in the main condensate pipeline downstream of the shaft seal heater. (4) Various cooling water and miscellaneous water pipelines are connected to the outlet of the condensate pump or after the desalination unit. Because these water requirements are for pure pressurized water. (5) Branch pipes leading to the drain ditch are connected at the bottom of the condenser hot well, on the condensate pipeline at the outlet of the last low-pressure heater (in the direction of the condensate flow), and at the bottom of the deaerator tank. These pipes allow for the discharge of unqualified condensate into the drain ditch when flushing the condensate pipelines before the unit is put into operation. (6) Chemical make-up water enters the condenser through the make-up water control valve to compensate for the steam and water losses during the thermal cycle. II. Example of the main condensate water system 1. Condensate pumps and their pipelines: The condensate water is drawn out through a pipeline from the hot well tank of the condenser; this pipeline is connected, using T-joints, to the inlets of two condensate pumps (one in operation and one as a backup). An electric gate valve and a flanged conical filter are installed on the inlet pipe of each pump. Gate valves are used to isolate pumps during maintenance, and filters prevent residues that may accumulate in hot wells from entering the pumps. A pressure differential switch is installed on the filters; when the pressure drop due to clogging of the filter reaches a specified value, it sends an alarm signal to the control room. If it is confirmed that the inside of the hot well is clean, the filter screen can also be removed to reduce flow resistance losses. Check valves and electric gate valves are installed on the outlet pipes of both condensate pumps; the gate valves are equipped with travel switches to facilitate the control and monitoring of the valve’s open or closed state, while the check valves prevent backflow of condensate. The outlet pipes of the two condensate pumps converge into a single main pipe that leads to the chemical desalination unit; a recirculation pipe for the condensate pumps is connected to this pipe. An electric gate valve is installed on the pipeline, which can be operated from the control room. The function of the recirculation pipe is: ① to maintain the level in the hot well during the commissioning and startup of the condensate pumps, thereby preventing cavitation in the pumps ； ②When installing the cover filters one by one, control the amount of condensate water appropriately to protect the cover filters ； ③When cleaning the condenser and condensate pumps, circular cleaning can be carried out through the recirculation pipe. 2. Chemical treatment of condensate: To ensure the quality of boiler feedwater and prevent the presence of saline solids in the condensate due to leaks in the condenser copper tubes or other reasons, a condensate desalination unit is installed after the condensate pump to control the concentration of dissolved solids in the condensate. An electric isolation valve and an electric bypass valve are installed on each of the inlet and outlet pipes of the desalination unit. When starting up the unit with water or when it needs to be shut down due to a fault during operation, the bypass valve opens while the inlet and outlet valves close, allowing the main condensate water to flow through the bypass route. When the desalination unit is put into operation, the inlet and outlet valves are opened, while the bypass valve is closed. 3. Condensate booster pumps and their pipelines: There are two condensate booster pumps, one of which is in operation while the other serves as a backup. Between the inlet of the condensate pump and the inlet of the condensate booster pump, there is a connecting pipe called a \"condensate tee.\" The function of this pipe is as follows: ① It is used for individually testing the condensate booster pump; in this case, water can be taken from the condenser, passed through the condensate booster pump, and then recycled back to the condenser via the minimum condensate flow rate ； ②When the unit is temporarily shut down, the condensate pump can be used to maintain the circulation of condensate in the desalination unit, thereby preventing the filter membrane from being clogged ； ③During unit startup and low-load operation, the amount of condensate water in the constant desalination device is maintained to ensure that the effectiveness of the filters is not affected. This is because the outlet pressure of the cover filter is greater than its design pressure, or when the inlet pressure changes significantly, the cover filter becomes unsafe. A pipe leading from the make-up water tank is connected to the inlet main of the condensate booster pump. The function of this pipe is as follows: ① In cases where, for some reason, the inlet of the condensate booster pump suddenly loses flow, or when the flow rate from the condensate booster pump exceeds that of the condensate pump, causing the pressure at the inlet of the condensate booster pump to drop below 0.04 MPa, the make-up water tank can immediately supply water to the condensate booster pump, thereby temporarily balancing the water volume and preventing the situation from worsening ； ②After the condensate booster pump is installed or overhauled and requires a separate test run, it can be supplied with water from the make-up water tank, and the water returns to the make-up water tank via the high-level drain pipe located between the shaft seal heater and the make-up water tank ； ③When it is necessary to fill the cold furnace and the water tank more quickly (usually by using a make-up water pump), this pipeline can also be used to start the condensate booster pump to carry out the filling process ； ④Before starting the condensate water system, the make-up water tank can be used to fill and purge the condensate water pipeline from the condensate water pump to the inlet of the condensate water booster pump. 4. Recirculation of minimum condensate flow: A recirculation pipe for the minimum condensate flow is installed after the shaft seal heater and before the low-pressure heater H8, leading to the condenser. The pipe is equipped with a condensate minimum flow recirculation device, which consists of a control valve, two isolation valves, and a bypass valve. The signal for the control valve comes from the condensate flow meter located before the shaft seal heater; when the flow rate during operation is below the minimum required by the condensate booster pump and the shaft seal heater, the recirculation circuit is activated automatically to maintain a minimum flow rate in the condensate booster pump and the shaft seal cooler. 5. Deaerator feed water tank level control: The feed water tank level control panel is located behind the shaft seal heater, on the water inlet side of low-pressure heater H8. It consists of a main control valve and an auxiliary control valve. During normal operation, the auxiliary valve is fully open, and the main control valve uses the three signals of the water level in the feed tank, the flow rate of boiler feedwater, and the flow rate of condensate water to automatically adjust and maintain a normal water level in the feed tank. When the unit load is less than 30% MCR, or when the main control valve is under maintenance, the manual auxiliary valve in the control room is used to maintain the water level in the feed tank. The water tank is equipped with a water level protection system; when the water level rises to the high-level warning threshold, the high-level switch activates and triggers an alarm in the control room to alert the operators ； When the water level continues to rise to the high-water-level warning threshold, the high-water-level switch activates; it triggers an alarm and opens the drain valve on the water tank, while closing all water-level control valves on the main condensate pipeline as well as the normal drain valve of the high-pressure heater H3. The automatic recirculation of the minimum condensate flow is then activated ； If the water level rises to the accident water level warning threshold, the accident water level switch activates; it triggers an alarm and simultaneously closes the extraction check valve and the electric isolation valve, while opening the drain valves on both sides to prevent the condensate from the deaerator from entering the turbine through the extraction pipes. When the water level in the water tank drops to the low-level warning line, the low-level switch activates, triggering an alarm in the control room; if necessary, the water flow into the deaerator is increased ； If the water level continues to drop to the lower water level warning line, the feed water pump should be shut down accordingly. Generally, the low-water level warning line is designed to be as low as possible, to avoid unnecessary shutdowns of individual units due to premature disconnection. 6. Low-pressure heaters and their pipelines: There are two low-pressure heaters, H8, each with a capacity of 50%; they are installed in parallel at the neck of the condenser and share a single bypass pipeline. Its inlet and outlet valves as well as the bypass valve are linked to the high level of water in the heater; when the water level in the heater reaches a high value, the bypass valve opens while the inlet and outlet valves close. The condensate then flows through the bypass, thereby preventing water and steam from entering the turbine via the steam extraction pipeline, where no protective valves are present, due to leaks in the U-tubes of the heater. The low-pressure heaters H5, H6, and H7 are each equipped with electric small bypasses. A pressure relief valve is installed on the water side of each heater. A check valve is installed on the main condensate pipe before entering the deaerator, to prevent steam from within the deaerator from flowing back into the condensate system in the event of a load rejection due to an unit malfunction. 7. Make-up water system and condenser water level control: The make-up water for the unit comes from chemically treated water, and its flow into the make-up water tank is automatically controlled by a pneumatic control valve. During unit startup, the make-up water pump is used, along with flow orifice plates, to supply water to the condenser’s hot well, the boiler, for sealing the water pumps during startup, and to fill the pipelines. During normal operation, the bypass valve of the make-up water pump is opened, the make-up water pump is shut down, and self-priming make-up water supply is achieved by relying on the pressure difference between the make-up water tank and the condenser. The water level in the condenser hot well is controlled by a regulating device installed on the make-up water pipeline in front of the condenser; this device is equipped with primary and secondary control valves. During normal operation, the water supply volume is automatically controlled by the hot well level signal through the main control valve, in order to maintain the condenser level. When the water level is low, open the main control valve wide ； As the water level continues to drop and a low-water-level alarm is triggered, the bypass valve is quickly opened in the control room to increase the water supply volume. When the hot well reaches a high water level, reduce the control valve ； As the water level continues to rise, the high-water-level drain control valve located before the shaft seal heater is activated, and the condensate returns to the make-up water tank via the condensate pump, desalination unit, and condensate booster pump. 8. The branch pipes for various cooling water and miscellaneous water supplies, which are connected to the main outlet pipe of the condensate pump, include: the cooling water pipe for the exhaust steam from the low-pressure cylinder of the turbine, the water supply pipe for the vacuum valve seal, the cooling water pipes for the main steam valve and the medium-pressure combined steam valves of the turbine as well as the cat’s paw cooling system, the cooling water pipe for the boiler’s steam and water sampling coolers, and the cooling water pipes for the high- and medium-pressure drain expansion tanks. The water supply branches connected to the main condensate pipeline, after the condensate booster pump but before the shaft seal heater, include the seal water pipes for the feedwater pump, condensate pump, and condensate booster pump; the low-pressure bypass temperature reduction water pipes; the water pipes for the third-stage temperature reducer in the low-pressure bypass condenser; the water spray pipes for the condenser water curtain; the steam seal steam temperature reduction water pipes; the make-up water pipes for the generator’s internal water cooling system; and the auxiliary steam temperature reduction water pipes. 9. A drain pipe is installed at the bottom of the water tank located at the lower part of the heat well of the shell-and-tube condenser, leading to the gutter; this pipe is used for draining water from the condenser after installation or maintenance. A water seal valve that remains closed most of the time is fitted on this pipe to prevent air from entering the condenser and disrupting the vacuum. A condensate drain trench pipe is installed on the outlet pipeline of the condensate pump, so that after starting the condensate pump and its recirculation pipe to clean the condenser and the condensate pump, the water that does not meet the standards can be discharged into the trench. Two water pipes are led off on the outlet pipeline of the low-pressure heater H5, in front of the electric outlet gate valve. The entire path is lined with open cleaning pipes, leading to the gutter. The pipe is equipped with an electric gate valve, which remains locked in the closed position when the unit is operating normally. The drainage pipes are put into use at the beginning of unit startup to perform open cleaning of the low-pressure water pipelines, thereby removing impurities from the equipment and piping systems. The other line is the low-pressure circulation cleaning water pipe, which leads to the condenser; this pipe is also equipped with an electric gate valve. Through this pipe, relatively clean deionized water is used to circulate and clean the low-pressure pipeline system, in order to meet the requirements of subcritical-parameter boilers regarding the quality of condensed water. Once the water quality meets the requirements, close the electric gate valve and open the outlet gate valve of the low-pressure heater H5, allowing the condensate to flow into the deaerator. Main condensate system for imported 300MW units. Its feature is that the chemical desalination unit uses a high-speed mixed-bed reactor, which can withstand high pressures, and it has no special requirements regarding changes in condensate water pressure or flow rate. Therefore, a condensate booster pump is no longer required in the main condensate system, thereby simplifying the system and reducing costs and floor space. Both the H7 and H8 low-pressure heaters are of the built-in type, and they share a single bypass pipe. Cooling water for various purposes and miscellaneous water are taken out after the desalination unit. The water level control device for the deaerator feed water tank consists of a main control valve, a secondary control valve, and an electric bypass valve. When the unit load is less than 30% MCR, the main control valve is fully closed, and the secondary control valve is used to perform single-pulse control of the deaerator water level. When the unit load is greater than 30% MCR, the secondary control valve is fully open, and the main control valve performs three-input control on the deaerator water level. When both the main and auxiliary control valves fail, the operator manually adjusts the bypass valve from the control room. The rest are similar to the main condensate systems of domestic 300MW units. The main condensate system of the 600MW unit is basically the same as the one described above. III. Operation of the main condensate water system: The operation of the main condensate water system will be illustrated below using an example from a domestically produced 300MW power unit. (1) Startup 1. Preparations before startup Before starting the condensate water system, the following conditions must be met: ① The make-up water tank must be filled to the normal water level; the make-up water pipes leading to the heat well of the condenser must also be filled, and the make-up water pump must have been filled with water and be ready for operation ； ②The condensate water system and feed water tank have been flushed; the condensate water system has been filled with water and its air has been removed. The hot wells of the condenser and the feed water tank of the deaerator have been filled to a higher water level ； ③The recirculation electric valve for the condensate pump is open, and automatic control devices such as those for recirculating the minimum flow rate of condensate, controlling the water level in the deaerator tank, and monitoring the water level in the condenser hot well are in operational mode. All control systems are in perfect condition. 2. Starting of the condensate pump and condensate booster pump: When all the above starting conditions are met, first start the make-up water pump to supply seal water to the condensate pump and condensate booster pump, then manually start the condensate pump and use it for recirculation operation. When the inlet pressure of the condensate booster pump reaches a certain value, the condensate booster pump is started manually; the electric valve of the condensate pump recirculation line is closed, and after the shaft seal heater is activated, recirculation of the minimum amount of condensate flow takes place. When the condenser vacuum is sufficient for gravity-fed make-up water, the make-up water pump is shut down, and the seal water for all pumps is supplied by the condensate water. 3. Startup of the low-pressure heater: The startup of the low-pressure heater is generally divided into startup during unit operation and emergency startup. (1) Startup during unit operation. At startup, the low-pressure heater is first gradually opened up to the start-up vent valve of the condenser, in order to discharge the large amount of air that has accumulated in the steam side space while the low-pressure heater was shut down. It is important to monitor the vacuum level of the condenser at this time, to prevent a rapid opening of the heater’s start-up vent valve from causing a drop in the condenser’s vacuum. Then, slowly open the water-side inlet valve of the low-pressure heater to fill it with water, and monitor the level of water in the heater’s drain line to check for any leaks. At the same time, open the vent valve of the water chamber and close it once water starts flowing out. Once the water injection is normal, gradually open the outlet and inlet valves on the water side to supply water to the heater. When adjusting the condensate level using the heater outlet valve, care should be taken to determine the opening degree of the outlet valve based on the level condition. Thereafter, the check valve control system on the extraction steam pipeline was activated to put it into operation. Slowly open the electric isolation valve on the steam extraction pipeline to supply steam to the heater for preheating. After preheating for a period of time as specified in the procedures, fully open the steam inlet valve, and depending on the condensate situation in the steam extraction pipeline, gradually close the condensate valves located before and after the check valve until they are fully closed. For a regenerative heater equipped with a drain pump, when the water level in the heater rises to 3/4 of the gauge level, start the drain pump and open its outlet valve to maintain the water level at an appropriate level. If the drain pump fails to function properly, the drain water is routed to the condenser through the water seal tube. (2) Random start. When the low-pressure heater is started together with the turbine, all the inlet valve, outlet valve, steam inlet valve, drain valve, and air valve of the heater are opened, and a check valve control system is activated; meanwhile, the steam-side drain valve is closed, making the operation relatively simple. (II) During normal operation of the unit, the condensate water system operates under various load variation conditions in accordance with the requirements of the steam turbine generator set. The water levels in the deaerator feed tank, condenser hot well, make-up water tank, etc., are automatically maintained at normal values. Recycling of the condensate water system is activated automatically at low load. (III) Abnormal operation 1. Disconnection of the low-pressure heater: If there is a leak in the heater tubes or poor drainage, it causes the steam side water level in the heater to rise excessively, resulting in the disconnection of the heater and the condensate flowing through a bypass. Based on the number of heaters that are shut down, the turbine output is reduced as specified. 2. Bypass operation of the desalination unit: When a fault occurs in the desalination unit or backwashing is required, and one or more desalination tanks need to be shut down, depending on the specific circumstances, part or all of the condensate water may be routed through a bypass. When part of the condensate water is sent through the bypass, the bypass valve must be adjusted manually to balance the amount of condensate water. 3. When the turbine loses load and this causes the main steam valve of the turbine to trip, the water level control valve of the deaerator feed water tank closes automatically, temporarily stopping the flow of condensate water into the deaerator. This helps to slow down the rapid drop in pressure inside the deaerator and prevents cavitation in the feed water pump. At this time, the condensate water operates through minimum flow recirculation. (IV) The shut-down unit begins to reduce its load; the water level control valve of the condensate make-up tank is closed to lower the water level in the tank. As the load continues to decrease until the condensate can no longer meet the flow requirements of the shaft seal heaters, a minimum condensate flow recirculation mode is activated. After the turbine is disconnected, close the water level control valves for the deaerator feed tank and the condenser hot well. The condensate pump and the condensate booster pump can be shut down only after all drains connected to the hot well are disconnected and the condenser vacuum is lost. After the condensate pump is shut down, if the feed water pump still needs to operate, a make-up water pump should be started to supply seal water. If the unit is to be shut down under normal operating conditions, the low-pressure heater can be shut down only when the unit’s load drops below the value specified in the regulations. During shutdown, first close the electric isolation valve and check valve on the heater steam extraction pipeline; simultaneously, open fully the drain valves located before and after the steam extraction check valve. Then open fully the bypass valve on the water side of the heater, and close fully the inlet and outlet valves on the water side. If there is a protective device, it should also be removed. At this time, attention should be paid to the drain level of the heater; once it drops to 1/3 of the level gauge reading, the heater’s drain device should be turned off. For heaters equipped with a drain pump, the drain pump should be shut down and the outlet valve of the pump should be closed. During the gradual shutdown of the unit, the heater adopts a random gradual shutdown method. After the turbine is shut down, the inlet and outlet valves on the water side of the low-pressure heater need not be closed; no other actions are required apart from opening the drain valve on the steam extraction pipeline and the drain valve on the steam side.