Summary of knowledge on the operation of condensate water system

Summary of knowledge on the operation of condensate water system

Summary of knowledge on the operation of condensate water system

Summary of knowledge on the operation of condensate water system

Summary of knowledge on the operation of condensate water system

1、 The function and composition of the main condensate system

The main function of the main condensate system is to transport condensate from the condenser hot well to the deaerator. To ensure the reliable operation of the entire system and improve efficiency, during the transportation process, it is necessary to perform desalination purification, heating, and necessary control adjustments on the condensed water. At the same time, relevant equipment cooling water, sealing water, cooling water, and control water should be provided during operation. In addition, steam and water losses during the thermal cycle should be supplemented.

The main condensate system is generally composed ofcondensate pump、Shaft seal heater、Low pressure heatingThe main equipment such as instruments and their connecting pipelines are composed. Subcritical and supercritical parameter units are equipped with desalination devices after the condensate pump due to the high quality requirements of the boiler for the feedwater, especially for the once through boiler. Due to the limitations of the pressure resistance conditions of the desalination device, domestic units adopt a two-stage boosting method for the condensate water. Therefore, a condensate water boosting pump is also installed after the desalination device. For large units, the main condensate system also includes a supplementary water system consisting of a supplementary water tank and a supplementary water pump.

The main condensate system of a typical unit has the following commonalities:

(1) Set up two condensate pumps or condensate booster pumps with a capacity of 100%, one in normal operation and the other as backup. In the event of a malfunction of the operating pump, the backup pump will be interlocked and started.

(2) The low-pressure heater is equipped with a main condensate bypass. The function of the bypass is to allow condensed water to enter the deaerator through the bypass when a heater fails or shuts down, without affecting the normal operation of the entire unit due to heater accidents. Each heater is equipped with a bypass, called a small bypass; Two or more heaters are equipped with a bypass, which is called a large bypass. The large bypass has the advantages of simple system, few valves, and investment savings. However, when one heater fails, the other heaters in the bypass will also be disconnected and shut down, resulting in a significant decrease in the condensate temperature. This not only reduces the thermal economy of the unit operation, but also lowers the inlet water temperature of the deaerator, making the operation unstable and the deaeration effect worse. Small bypass and large bypass are exactly the opposite. Therefore, the main condensate system of low-pressure heaters often adopts a combination of large and small bypass applications.

(3) Set the minimum flow rate for condensate recirculation. In order to prevent cavitation of the condensate pump during start-up or low load, and to ensure sufficient condensate flow through the shaft seal heater to prevent steam leakage from the shaft sealCondensation occurs to maintain a slight negative pressure state in the shaft seal heater, and a minimum flow recirculation pipe for condensate returning to the condenser is installed on the main condensate pipeline after 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 device. Because these waters require pure pressure water.

(5) At the bottom of the condenser hot well, the outlet condensate pipe of the last low-pressure heater (along the direction of condensate flow), and the bottom of the deaerator water tank, there are branch pipes connected to the drainage ditch, so as to discharge unqualified condensate into the ditch when flushing the condensate pipe before the unit is put into operation.

(6) Chemical makeup water enters the condenser through the makeup water regulating valve to supplement the steam and water loss during the thermal cycle process.

2、 Example of main condensate system

1. Condensed water pump and its pipelines

The condensed water is led out from the hot well water tank of the condenser through a pipeline, and is connected to the inlets of two condensate pumps (one in normal operation and one standby) through T-shaped tees. Electric gate valves and a flanged conical filter screen are installed on the inlet pipes of each pump. Gate valves are used for pump maintenance and isolation. The filter screen can prevent residue that may accumulate in the hot well from entering the pump. The filter screen is equipped with a differential pressure switch, which sends an alarm signal to the control room when the filter screen is blocked and the pressure drop reaches the limit value.

If it is confirmed that the interior of the hot well is clean, the filter can also be removed to reduce 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 for easy control and inspection of the valve's opening and closing status, while check valves prevent condensate from flowing back. The outlet pipes of two condensate pumps converge into a main pipeline connected to the chemical desalination device, and the recirculation pipe of the condensate pump is connected to this pipeline. There is an electric gate valve installed on the pipeline, which can be operated in the control room.

The function of the recirculation pipe is to:

① When debugging and starting the condensate pump, maintain the water level in the hot well to prevent cavitation of the pump;

② When each filter is put into operation, the amount of condensed water should be appropriately controlled to protect the filter;

③ When cleaning the condenser and condensate pump, circulation cleaning can be carried out through the recirculation pipe.

2. Chemical treatment of condensate water

In order to ensure the quality of boiler feedwater and prevent the presence of salt solids in the condensate due to leakage of condenser copper pipes or other reasons, a set of condensate desalination device is installed after the condensate pump to control the concentration of dissolved substances in the condensate.

One electric isolation valve and one electric bypass valve are installed on the inlet and outlet pipes of the desalination device. When the desalination device malfunctions and needs to be shut off during water filling or operation, the bypass valve opens, the inlet and outlet valves close, and the main condensate flows through the bypass. When the desalination device is put into operation, the inlet and outlet valves are opened and the bypass valve is closed.

3. Condensed water booster pump and its pipelines

Two condensate booster pumps, one in normal operation and one as backup. Between the inlet of the condensate pump and the inlet of the condensate booster pumpThere is a connecting pipe called a 'condensate tee'. The function of this tube is to:

① Used for separately debugging the condensate booster pump. At this time, water can be taken from the condenser and recycled back to the condenser through the condensate booster pump at the minimum flow rate;

② When the unit is temporarily shut down, the condensate pump can be used to maintain the circulating flow of condensate in the desalination device to avoid covering the filter and removing the membrane;

③ During unit start-up and low load operation, maintain a constant amount of condensate in the desalination device to ensure that the effectiveness of the filter is not affected. This is because when the outlet pressure of the covered filter is greater than its design pressure or when the inlet pressure changes significantly, the covered filter is unsafe.

Connect a pipeline from the supplementary water tank to the inlet manifold of the condensate booster pump. The function of this tube is to:

① For some reason, if the inlet of the condensate booster pump suddenly loses flow or the condensate booster pump exceeds the flow of the condensate pump, causing the inlet pressure of the condensate booster pump to be lower than 0.04 MPa, the supplementary water tank can immediately supply water to the condensate booster pump to temporarily balance the water volume and prevent the accident from expanding;

② When the condensate booster pump needs to be tested separately after installation or maintenance, it can be supplied with water from the supplementary water tank, and returned to the supplementary water tank through the high water level discharge pipe set before the shaft seal heater to the supplementary water tank;

③ When it is necessary to quickly fill the cold furnace and water supply tank (usually using a supplementary water pump), the condensate booster pump can also be started through this pipeline to complete the water supply work;

④ Before starting the condensate system, the condensate pipeline between the condensate pump and the inlet of the condensate booster pump can be filled and deflated from the supplementary water tank.

4. Minimum flow rate of condensate for recirculation

Install a minimum flow recirculation pipe for condensate to the condenser after the shaft seal heater and before the low-pressure heater H8. The pipe is equipped with a minimum flow recirculation device for condensed water, which consists of a regulating valve, two isolation valves, and a bypass valve. The signal of the regulating valve is taken from the condensate flow device before the shaft seal heater. When the flow rate during operation is less than the minimum flow required by the condensate booster pump and the shaft seal heater, the recirculation pipeline is automatically opened to maintain the minimum flow rate in the condensate booster pump and the shaft seal cooler.

5. Water level control of deaerator feedwater tank

The water level control console of the feedwater tank is located on the inlet side of the low-pressure heater H8 after the shaft seal heater, and it consists of two main and auxiliary regulating valves. During normal operation, the auxiliary valve is fully open, and the main regulating valve uses the three impulse control of water level in the feedwater tank, boiler feedwater flow rate, and condensate flow rate to automatically adjust and maintain the normal water level in the feedwater tank.

When the load of the unit is less than 30% MCR, or when the main control valve fails during maintenance, maintain the water level in the feedwater tank by manually controlling the auxiliary valve in the control room. The water supply tank is equipped with water level protection. When the water level rises to the high water level warning line, the high water level switch will activate, triggering an alarm in the control room to alert the operating personnel; When the water level continues to rise to the high one high water level warning line, the high one high water level switch will activate, triggering an alarm and simultaneously opening the drain valve on the water tank. All water level regulating valves on the main condensate pipeline and the normal drain valve of the high-pressure heater H3 will be closed, and the minimum flow recirculation of condensate will automatically start;

If the water level rises to the emergency water level warning line, the emergency water level switch will activate, and at the same time, the extraction check valve and electric isolation valve will be closed, and the front and rear drain valves will be opened to prevent condensed water from the deaerator from entering the turbine through the extraction pipeline. When the water level in the feedwater tank drops to the low water level warning line, the low water level switch will activate, triggering an alarm in the control room. If necessary, increase the water intake of the deaerator; If the water level continues to drop to the low one low water level warning line, the water supply pump should be interlocked and disconnected for shutdown treatment. Generally, the low low water level warning line should be designed as low as possible to avoid unnecessary unit shutdown accidents caused by easy disconnection.

6. Low pressure heater and its pipelines

There are two low-pressure heaters H8, each with a capacity of 50%, installed in parallel at the neck of the condenser, sharing a bypass pipeline. The inlet and outlet valves, as well as the bypass valve, are linked to the high water level of the heater. When the heater reaches a high water level, the bypass valve opens and the inlet and outlet valves close. The condensed water flows through the bypass to prevent water and steam from entering the turbine through the extraction pipeline without any protective valves due to leakage of the U-shaped pipe in the heater. Low pressure heaters H5, H6, and H7 are each equipped with electric small bypasses. Each heater is equipped with a pressure relief valve on the water side. A check valve is installed on the main condensate pipeline before entering the deaerator to prevent steam from flowing back into the condensate system during unit accident load shedding.

7. Supplementary water system and condenser water level control

The supplementary water for the unit comes from chemically treated water, which is automatically controlled by a pneumatic regulating valve to enter the supplementary water tank. When the unit is started, the water pump is supplemented through the flow orifice plate to complete the work of replenishing water to the condenser hot well, boiler water, sealing water for the water pump during the start-up period, and filling water in the pipeline. During normal operation, open the bypass valve of the supplementary water pump, stop the supplementary water pump, and rely on the pressure difference between the supplementary water tank and the condenser for self flowing water replenishment.

The water level of the condenser hot well is controlled by a regulating device installed on the supplementary water pipeline before the condenser, which is equipped with two main and auxiliary regulating valves. During normal operation, the hot well water level signal automatically controls the supplementary water volume through the main regulating valve to maintain the condenser water level. When the water level is low, turn up the main regulating valve; When the water level continues to drop and the low water level signal alarms, quickly open the bypass auxiliary valve in the control room to increase the amount of water replenishment. When the hot well reaches a high water level, turn down the regulating valve; When the water level continues to rise, open the high water level discharge control valve before the shaft seal heater, and the condensed water returns to the supplementary water tank through the condensed water pump, desalination device, and condensed water booster pump.

8. Various types of cooling water and miscellaneous water use

The water supply pipes connected to the outlet main of the condensate pump include: the steam turbine low-pressure cylinder exhaust cooling water pipe, the vacuum valve water seal water pipe, the steam turbine main stop valve, the intermediate pressure combined steam valve and the cat claw cooling water pipe, the boiler steam water sampling cooler cooling water pipe, and the high and intermediate pressure drain expansion tank cooling water pipe.

The water branch pipe connected to the main condensate pipeline after the condensate booster pump and before the shaft seal heater isSealed water pipes for feed pumps, condensate pumps, and condensate booster pumps, low-pressure bypass cooling water pipes, low-pressure bypass condenser three-stage cooling water pipes, condenser water curtain spray pipes, steam seal cooling water pipes, generator water internal cooling system supplementary water pipes, auxiliary steam cooling water pipes, etc.

9. Clean the branch pipe

A drainage pipe is connected to the bottom of the water tank at the bottom of the condenser hot well to the trench for cleaning and draining the condenser after installation and maintenance. A frequently closed water seal valve is installed on the pipe to prevent air from leaking into the condenser and damaging the vacuum. A condensate drainage ditch pipeline is installed on the outlet pipeline of the condensate pump, in order to start the condensate pump and its recirculation pipe to clean the condenser and condensate pump, and discharge the unqualified water into the ditch. Two water pipes are led out in front of the electric outlet gate valve on the outlet pipeline of the low-pressure heater H5. One route is an open cleaning pipe, leading to the trench. There is an electric gate valve installed on the pipe, which is locked in the closed position during normal operation of the unit. The drainage pipe is put into use during the initial start-up of the unit, and the low-pressure water pipeline is cleaned in an open manner to remove impurities from the equipment and piping system. The other route is the low-pressure circulating cleaning water pipe, which leads to the condenser and is also equipped with an electric gate valve. Through this pipe, relatively clean desalinated water is used to circulate and clean the low-pressure pipeline system, in order to meet the requirements of subcritical parameter boilers for condensate quality. After the water quality is qualified, close the electric gate valve and open the outlet gate valve of the low-pressure heater H5, and the condensed water enters the deaerator.

Main condensate system of imported 300MW unit. Its characteristic is that the chemical desalination device adopts a high-speed mixed bed, which can withstand high pressure and has no special requirements for changes in condensed water pressure and flow rate. Therefore, the main condensate system no longer requires a condensate booster pump, simplifying the system and reducing investment and footprint. H7 and H8 low-pressure heaters are both built-in and share a bypass pipeline. Desalination water and miscellaneous water for various purposes are discharged from the desalination device.

The water level control device of the deaerator feedwater tank consists of main and auxiliary regulating valves and an electric bypass valve. When the unit load is less than 30% MCR, the main regulating valve is fully closed, and the auxiliary regulating valve controls the deaerator water level with a single impulse. When the load of the unit exceeds 30% MCR, the auxiliary regulating valve is fully opened, and the main regulating valve controls the water level of the deaerator with three impulses. When both the main and auxiliary regulating valves fail, the operator manually adjusts the bypass valve control in the control room. Other main condensate systems are similar to those of domestically produced 300MW units. The main condensate system of the 600MW unit is basically the same as the above situation.

3、 Main condensate system operation

Taking the main condensate system of a domestically produced 300MW unit as an example, the operation of the main condensate system will be explained.

（1） Start up

1. Preparation before startup

Before starting the condensate system, the following conditions must be met:

① Fill the water tank to the normal water level, and fill the supplementary water pipeline to the condenser hot well. The supplementary water pump has been filled with water and is ready for operation;

② The condensate system and feedwater tank have been flushed, the condensate system has been filled with water and vented, and the condenser hot well and deaerator feedwater tank have been filled with waterHigher water level;

③ The electric valve for the recirculation of the condensate pump is opened, and the automatic control devices such as the minimum flow rate recirculation of condensate, the water level of the deaerator tank, and the water level of the condenser hot well are in a operable state. All control systems are in a complete state.

2. Start the condensate pump and condensate booster pump

When the above start-up conditions are met, first start the supplementary water pump to supply sealing water to the condensate pump and condensate booster pump, and then manually control the start-up of the condensate pump, using the condensate pump for recirculation operation. When the inlet pressure of the condensate booster pump reaches a certain value, manually start the condensate booster pump, close the electric valve of the condensate pump recirculation pipe, and put the shaft seal heater into operation for minimum condensate flow recirculation. When the vacuum of the condenser can meet the requirements of self flowing water replenishment, stop the supplementary water pump and switch the sealing water of various water pumps to be supplied by condensed water.

3. Startup of low-pressure heater

The start-up of low-pressure heaters is generally divided into start-up during unit operation and random start-up.

(1) Start up during unit operation.When starting, first slowly open the start-up vent valve from the low-pressure heater to the condenser to exhaust the large amount of air accumulated in the steam side space when the low-pressure heater is shut down. At this time, pay attention to the change in condenser vacuum to prevent the heater start-up vent valve from opening too quickly, which may cause a decrease in condenser vacuum.

Then, slowly open the water inlet valve of the low-pressure heater, inject water into the low-pressure heater, and monitor the drain water level of the heater, paying attention to whether there is any leakage in the heater. At the same time, open the vent valve of the water chamber and close it after water flows out. After the water injection is normal, gradually open the outlet and inlet valves on the water side, and let the heater supply water. When using the heater outlet valve to adjust the condensate water level, attention should be paid to determining the opening of the outlet valve based on the water level situation. Afterwards, put the check valve control system on the extraction steam pipeline into operation. Slowly open the electric isolation valve on the extraction steam pipeline to supply steam to the heater, preheat the heater according to the regulations for a period of time, fully open the inlet valve, and according to the drainage situation on the extraction steam pipeline, close the drain valves before and after the check valve until they are fully closed. For a reheating heater equipped with a drain pump, when the water level of the heater reaches 3/4 of the water level gauge, turn on the drain pump, open the outlet valve of the drain pump, and maintain the water level normal. If the drainage pump malfunctions, switch the drainage through the water seal pipe to the condenser.

(2) Randomly start.When the low-pressure heater is started together with the turbine, all the inlet valves, outlet valves, inlet valves, drain valves, and air valves of the heater should be opened, and the check valve control system should be put into operation. The steam side drain valve should be closed, and the operation is relatively simple.

（2） Normal operation

During normal operation of the unit, the condensate system operates under various variable load conditions according to the requirements of the steam turbine generator unit. The water levels of the deaerator feedwater tank, condenser hot well, and supplementary water tank are automatically maintained at normal values. The recirculation of the condensate system is automatically activated at low loads.

（3） Abnormal operation

1. Disassemble the low-pressure heater

If the water level on the steam side of the heater is too high due to leakage or poor drainage of the heater tube, the heater will be disconnected and the condensate will bypass. According to the number of heaters that have been shut down, reduce the output of the steam turbine as required.

2. The bypass operation of the desalination device

When the desalination device malfunctions or requires backwashing, and one or more desalination tanks are shut down, depending on the specific situation, some or all of the condensate water will be bypassed. When some of the condensate water passes through the bypass, the bypass valve should be manually adjusted to balance the condensate water volume.

3. Load rejection of steam turbine

When the main steam valve of the turbine trips due to load shedding, the water level regulating valve of the deaerator feedwater tank automatically closes, temporarily interrupting the flow of condensed water into the deaerator. This can slow down the sharp drop in deaerator pressure and prevent cavitation of the feedwater pump. At this point, the condensed water is circulated through the minimum flow rate.

（4） Shutdown

The unit begins to reduce the load, close the water level regulating valve of the condensate replenishment tank, lower the water level in the tank, and gradually reduce the load until the condensate cannot meet the flow requirements of the shaft seal heater. Then, the minimum flow rate of condensate is put into operation for recirculation. After the turbine is disconnected, close the water level control valves of the deaerator feedwater tank and condenser hot well. The condensate pump and condensate booster pump can only be shut down when all drainage connected to the hot well is interrupted and the vacuum of the condenser is destroyed. After the condensate pump is shut down, if the feedwater pump still needs to continue running, the supplementary water pump should be started to provide sealing water. If the unit needs to be shut down according to normal parameters, the low-pressure heater can only be shut down when the unit load drops below the value specified in the regulations.

When shutting down, first close the electric isolation valve and check valve on the extraction steam pipeline of the heater, and fully open the drain valves before and after the extraction check valve. Then fully open the water side bypass valve of the heater, and fully close the water side inlet and outlet valves. If there is a protective device, it should also be removed. At this point, attention should be paid to the drain water level of the heater. When it drops to one-third of the water level gauge, turn off the drain device of the heater. The heater with a drainage pump should be shut down and the outlet valve of the pump should be closed. When the unit is shut down due to sliding parameters, the heater adopts a random sliding stop method. After the turbine is shut down, the inlet and outlet valves on the water side of the low-pressure heater may not be closed, and there is no other operation except for opening the drain valve on the extraction steam pipeline and the steam side drain valve.