Complete cylinder switching operation and cooling system for low-pressure cylinder of steam extraction and heat supply steam turbine

Abstract

The invention belongs to the technical field of improvement of power plant steam turbine devices and matched systems, and particularly relates to a complete cylinder switching operation and cooling system for a low-pressure cylinder of a steam extraction and heat supply steam turbine. In order to meet the requirement of maximum heat supply on the premise of safety and flexibility required by a user in a unit steam extraction heat supply stage, low-pressure cylinder through-flow steam inlet is completely cut off except for low-pressure cylinder shaft end sealing steam and low-pressure cylinder steam inlet hot standby steam, and a low-pressure cylinder and a low-pressure cylinder steam exhaust port are cooled by a small amount of steam subjected to temperature reduction, pressure reduction and steam-water separation; at the moment, the low-pressure cylinder is in a high-vacuum and hot standby state of complete cylinder switching operation; the system is composed of a middle-low pressure cylinder communicating pipe of a steam turbine, a valve on the communicating pipe, a low pressure cylinder, a low pressure cylinder steam exhaust port steam cooling system, a pipeline system for exhausting steam from the middle pressure cylinder to the low pressure cylinder minimum cooling flow, a low pressure cylinder shaft end sealing system, a system measuring point and a monitoring device.

Description

Technical Field

[0001] The invention belongs to the technical field of improvement of steam turbine devices and supporting systems of power plants, and in particular relates to a system for complete cylinder cutting operation and cooling of low-pressure cylinders of extraction steam heating steam turbines. Background Art

[0002] Steam turbines generally have a minimum load value. Below this value, if the steam turbine runs continuously, it is bound to overheat at the low-pressure exhaust cylinder of the steam turbine. This heat is the result of the blast loss generated by the last stage of the steam turbine. Steam turbine manufacturers generally require that when the temperature of the steam turbine exhaust cylinder is not higher than 80°C, the unit can continue to operate at low load, but the steam turbine cannot quickly increase the load at this time. Because it involves many factors, the load value at this temperature is inaccurate, but it is generally zero load. When the exhaust cylinder temperature does not exceed 52°C, the unit can quickly increase the load. In order to reduce or control the temperature of the steam turbine exhaust cylinder, a water spray device is installed at the outlet of the last stage blade of the steam turbine. The water spray of the low-pressure cylinder will cause water brushing at the root of the last stage blade.

[0003] For this reason, when the extraction steam heating unit is running with the low-pressure cylinder cut-off, a minimum cooling steam flow is required to flow through the low-pressure cylinder to make the low-pressure cylinder run at zero output and achieve the purpose of cooling the exhaust port of the low-pressure cylinder. The relevant manufacturers recommend leaving 20-40t / h of steam according to the blade length of different units. The steam inlet flow of the low-pressure cylinder can meet the cooling requirements of each level of the low-pressure cylinder, but the cooling flow is still relatively large, which does not fully meet the user's requirements for maximum heating under the premise of safety and flexibility. Summary of the invention

[0004] In view of this, in order to solve the above-mentioned problems existing in the prior art, the purpose of the present invention is to provide a system for complete cylinder cutting operation and cooling of the low-pressure cylinder of an extraction steam heating steam turbine, which consists of a connecting pipe between the medium and low pressure cylinders of the steam turbine and valves on the connecting pipe, a steam cooling system for the low-pressure cylinder and the exhaust port of the low-pressure cylinder, a minimum cooling flow piping system from the exhaust of the medium-pressure cylinder to the low-pressure cylinder, a low-pressure cylinder shaft end sealing system, system measuring points and monitoring; during the extraction steam heating stage of the unit, except for the steam for the low-pressure cylinder shaft end sealing and the hot standby steam for the low-pressure cylinder steam inlet, the steam inlet to the low-pressure cylinder is completely cut off, and the low-pressure cylinder is in a complete cylinder cutting operation state. A small amount of steam from the low-pressure cylinder and the low-pressure cylinder exhaust port to the medium-pressure cylinder exhaust port is cooled after temperature reduction, pressure reduction, and steam-water separation, which meets the manufacturer's requirements for the low-pressure cylinder exhaust port temperature, so as to use less or even no low-pressure cylinder water spraying, and prevent the occurrence of water erosion and water brushing on the low-pressure blades; at the same time, the cylinder temperature of the low-pressure cylinder steam inlet and exhaust port is monitored to be within the manufacturer's requirements. If necessary, the low-pressure cylinder shaft end sealing system is adjusted, and the medium-pressure cylinder exhaust to the low-pressure cylinder minimum cooling flow pipeline system is opened and adjusted, so that the low-pressure cylinder is in hot standby status at any time after complete cylinder cutting operation, meeting the flexible operation requirements of the unit; the low-pressure cylinder of the turbine is in a high vacuum state at this time, which greatly reduces the heating of the blade blast.

[0005] The medium and low pressure cylinder connecting pipes and valves on the connecting pipes of the S1 steam turbine are fully sealed valves. The valves are used to adjust the heating extraction steam pressure to meet the requirements during the heating stage. When the low pressure cylinder is cut off, the valve is completely closed to cut off the steam inlet from the medium pressure cylinder to the low pressure cylinder. Generally, a butterfly valve is used, and it has the function of fast opening and closing to prevent the low pressure cylinder exhaust port from overheating during the opening and closing process. The butterfly valve is generally installed on the steam inlet side of the low pressure cylinder to prevent the steam flow excitation phenomenon of the connecting pipe from occurring.

[0006] The steam cooling system for the S2 low-pressure cylinder and the exhaust port of the low-pressure cylinder consists of a control valve group, a temperature and pressure reducing device arranged at the throat of the condenser, an external temperature and pressure reducing water system, and a low-pressure cylinder steam supply pipe in the condenser; when the low-pressure cylinder is fully cut off, the above-mentioned control valve group is opened and controlled, and the temperature and pressure reducing device is put into operation; the exhaust steam from the intermediate-pressure cylinder of the steam turbine is reduced in temperature and pressure through the temperature and pressure reducing device, and then supplied to the low-pressure cylinder, enters the low-pressure cylinder to cool the low-pressure cylinder and the exhaust port of the low-pressure cylinder, and is finally discharged to the condenser; the unatomized water of the temperature and pressure reducing device and the water after steam-water separation are discharged to the condenser hot well through the U-shaped water seal to prevent steam from escaping from the drain port during normal operation of the unit.

[0007] A1 The desuperheater and pressure reducer arranged at the throat of the condenser adopts a two-stage pressure reduction and one-time water spraying desuperheating structure. The steam from the exhaust steam of the intermediate pressure cylinder enters the desuperheater and pressure reducer through the steam pipe, and is sprayed into the shell of the desuperheater and pressure reducer through the first-level opening area at the end of the pipe for the first expansion and pressure reduction. A stainless steel anti-erosion baffle is provided on the inner wall of the shell, and uniformly distributed atomizing nozzles are provided along the circumferential direction of the inner wall of the shell behind the first-level pipeline opening area. The desuperheating water from the condensate system is atomized and fully mixed with the steam in the shell of the desuperheater and pressure reducer for temperature reduction. The steam after the first-stage desuperheating and pressure reduction passes through several small holes on the conical spray net in the shell, and is expanded and pressure reduced for the second time and diffused to the rear area of ​​the desuperheater and pressure reducer, so that the steam is further expanded and pressure reduced. The steam temperature after the first desuperheating and second desuperheating does not exceed 50℃. The water that is not atomized in the desuperheater and pressure reducer and the water after steam-water separation are discharged to the hot well of the condenser through the drain port at the tail of the shell. In addition to the above-mentioned two-stage pressure reduction structure, a multi-stage pressure reduction structure with more than two-stage pressure reduction can also be adopted.

[0008] The water source of the A2 external cooling water system is taken from the condensate pump, filtered and connected to the cooling and pressure reducing device through the water spray regulating valve. The cooling steam parameters are controlled by controlling the water spray regulating valve.

[0009] The number of A3 desuperheater and pressure reducer arranged at the throat of the condenser is determined according to the low-pressure cylinder and the condenser structure. A single-shell condenser is arranged with one at its throat; a double-shell condenser turbine is arranged with two in total, and so on.

[0010] The low-pressure cylinder steam supply pipe in the A4 condenser supplies cooling steam to the low-pressure cylinder through the low-pressure cylinder steam extraction pipe connected to the condenser, and a check valve is installed on the pipe connected to the low-pressure cylinder steam extraction pipe to prevent steam leakage during normal operation of the unit.

[0011] The minimum cooling flow piping system from the exhaust steam of the S3 medium-pressure cylinder to the low-pressure cylinder consists of a control valve group, a flow meter before the valve group, and connecting pipelines; during the low-pressure cylinder complete cylinder cutting operation stage, hot standby steam is supplied to the low-pressure cylinder steam inlet through this system, so that the temperature of the low-pressure cylinder steam inlet can be kept at the switch-back requirement at any time; further monitoring of the low-pressure cylinder inlet temperature and the low-pressure cylinder exhaust temperature requires that the low-pressure steam inlet temperature is not lower than the medium-pressure exhaust temperature by more than 150°C. When the low-pressure cylinder steam inlet temperature is lower than 100°C, and the steam inlet temperature is lower than the medium-pressure exhaust temperature by more than 150°C, the electric door of the control valve group can be opened and the regulating valve in the system valve group can be controlled according to the given steam flow, but the control valve group should be closed at any time when the low-pressure cylinder exhaust temperature is higher than 52°C.

[0012] When the S4 medium-pressure cylinder exhaust to low-pressure cylinder minimum cooling flow piping system operates according to the low-pressure cylinder minimum cooling flow, the low-pressure cylinder and low-pressure cylinder exhaust port steam cooling system can also be put into operation, and cooling steam is supplied to the low-pressure cylinder exhaust port through the steam extraction pipeline in the condenser to prevent heating of the last and second-last stage blast.

[0013] The S5 system measurement points and monitoring include medium-pressure exhaust port temperature and pressure; low-pressure cylinder inlet temperature; low-pressure cylinder exhaust port cylinder temperature; temperature and pressure reducer outlet pressure and temperature; and flow meter of the medium-pressure cylinder exhaust to low-pressure cylinder minimum cooling flow pipeline system.

[0014] Beneficial effects: The low-pressure cylinder of the steam turbine for heating by extraction steam is completely cut off to run, which meets the manufacturer's requirements for the temperature of the low-pressure cylinder exhaust port, and achieves less or even no use of low-pressure cylinder water spraying, and prevents the occurrence of water erosion and water brushing of low-pressure blades. The low-pressure cylinder of the steam turbine is in a highly vacuum state, and the steam consumed by the hot standby of the low-pressure cylinder steam inlet and the cooling of the low-pressure cylinder exhaust port is much less than the original technology. The flexible operation of the steam turbine achieves maximum heating at the same time. The temperature and pressure reducer is arranged at the throat of the condenser, which greatly reduces the size of the pipeline of the low-pressure cylinder and the steam cooling system of the low-pressure cylinder exhaust port outside the condenser. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 System diagram of complete cylinder cutting operation and cooling of low-pressure cylinder of extraction steam heating turbine; Figure 1 Middle: Number 1 is the connecting pipe of the medium and low pressure cylinders of the steam turbine, number 2 is the electric valve of the pipeline system of the minimum cooling flow from the medium pressure cylinder exhaust to the low pressure cylinder, number 3 is the electric regulating valve of the pipeline system of the minimum cooling flow from the medium pressure cylinder exhaust to the low pressure cylinder, number 4 is the valve of the connecting pipe of the medium and low pressure cylinders, number 5 is the electric valve of the steam cooling system of the low pressure cylinder and the low pressure cylinder exhaust port, number 6 is the electric regulating valve of the steam cooling system of the low pressure cylinder and the low pressure cylinder exhaust port, number 7 is the manual valve of the steam cooling system of the low pressure cylinder and the low pressure cylinder exhaust port, number 8 is the bypass electric valve of the steam cooling system of the low pressure cylinder and the low pressure cylinder exhaust port, number 9 is the desuperheater and pressure reducer of the steam cooling system of the low pressure cylinder and the low pressure cylinder exhaust port, number 10 is the low pressure cylinder supplementary steam pipe in the condenser of the steam cooling system of the low pressure cylinder and the low pressure cylinder exhaust port, number 11 is the U-shaped drain pipe of the desuperheater and pressure reducer. The temperature measuring point is represented by T plus a box, the pressure measuring point is represented by P plus a box, and the flow measuring point is represented by F plus a box.

[0016] Figure 2 Structure diagram of temperature and pressure reducer; Figure 2 In the middle: Number 1 is the steam pipe inlet, number 2 is the primary opening area at the end of the pipe, number 3 is the stainless steel anti-impact plate, number 4 is the atomizing nozzle, number 5 is the conical spray net, number 6 is the cooling steam outlet, number 7 is the drain outlet, number 8 is the shell, number 9 is the condenser end plate, number 10 is the cooling water inlet. DETAILED DESCRIPTION

[0017] S1 As attached Figure 1The valve (number 4) on the connecting pipe (number 1) of the medium and low pressure cylinders of the steam turbine shown is a fully sealed valve. The valve is used to adjust the heating extraction steam pressure to meet the requirements during the heating stage. When the low pressure cylinder is cut off, the valve is completely closed to cut off the steam inlet from the medium pressure cylinder to the low pressure cylinder. Generally, a butterfly valve is used, and it has the function of fast opening and closing to prevent the low pressure cylinder exhaust port from overheating during the opening and closing process. The butterfly valve is generally installed on the steam inlet side of the low pressure cylinder to prevent the steam flow excitation phenomenon of the connecting pipe from occurring.

[0018] S2 as attached Figure 1 The steam cooling system of the low-pressure cylinder and the exhaust port of the low-pressure cylinder shown in the figure consists of an electric valve (number 5), an electric regulating valve (number 6), a manual valve (number 7), a bypass electric valve (number 8), a temperature and pressure reducing device (number 9) arranged at the throat of the condenser, an external temperature and pressure reducing water system, a low-pressure cylinder supplementary steam pipeline in the condenser (number 10), and a U-shaped drain pipe of the temperature and pressure reducing device (number 11). When the low-pressure cylinder is fully cut off, the valve (number 4) on the connecting pipe (number 1) of the turbine's intermediate- and low-pressure cylinders is closed, the valves (number 5, 6, and 7) mentioned above are opened and controlled, and the temperature and pressure reducer (number 9) is put into operation; the exhaust steam from the turbine's intermediate-pressure cylinder is reduced in temperature and pressure through the temperature and pressure reducer (number 9) and then replenished with steam to the low-pressure cylinder to enter the low-pressure cylinder to cool the low-pressure cylinder and the exhaust port of the low-pressure cylinder, and finally discharged to the condenser; the unatomized water in the temperature and pressure reducer and the water after steam-water separation are discharged to the condenser hot well through the U-shaped drain pipe (number 11) to prevent steam from escaping from the drain port during normal operation of the unit.

[0019] A1 As attached Figure 2 The desuperheater and pressure reducer shown in the figure adopts a two-stage pressure reduction and one-stage water spray desuperheating structure. The steam from the exhaust of the medium-pressure cylinder enters the desuperheater and pressure reducer through the steam pipe inlet (number 1), and is sprayed into the desuperheater and pressure reducer shell (number 8) through the first-stage opening area (number 2) at the end of the pipe for the first expansion and pressure reduction. A stainless steel anti-erosion baffle (number 3) is provided on the inner wall of the shell (number 8). Uniformly distributed atomizing nozzles (number 4) are provided along the circumferential direction of the inner wall of the shell after the opening area (number 2) of the first-stage pressure reducing pipeline. The desuperheating water from the condensate system is sprayed into the nozzle and fully mixed with the steam in the desuperheater and pressure reducer shell for desuperheating. After the first stage of temperature and pressure reduction, the steam passes through several small holes on the conical spray net (number 5) in the shell, and undergoes a second expansion and pressure reduction to diffuse to the rear area of ​​the temperature and pressure reducer, so that the steam further expands and reduces the pressure. The temperature of the steam after the first temperature reduction and the second pressure reduction does not exceed 50°C; the unatomized water of the temperature and pressure reducer and the water after steam-water separation are discharged to the condenser hot well through the drain outlet (number 7) at the tail of the shell; in addition to the above-mentioned two-stage pressure reduction structure, a multi-stage pressure reduction structure with more than two stages of pressure reduction can also be adopted.

[0020] A2 as attached Figure 1 , Attachment Figure 2The cooling water inlet shown needs to be connected to the external cooling water system. The water source is taken from the condensate pump, filtered and then connected to the cooling and pressure reducing device through the water spray regulating valve. The cooling steam parameters are controlled by controlling the water spray regulating valve.

[0021] The number of A3 desuperheaters and pressure reducers arranged at the throat of the condenser is determined according to the low-pressure cylinder and the condenser structure. A single-shell condenser is arranged with one at its throat; a double-shell condenser turbine is arranged with two in total, and so on. Figure 1 The single-shell condenser shown has a temperature and pressure reducer (numeral 9) arranged at its throat.

[0022] A4 as attached Figure 1 The low-pressure cylinder steam supply pipe (number 10) in the condenser shown in the figure supplies cooling steam to the low-pressure cylinder through the eighth-stage steam extraction pipe of the low-pressure cylinder in the condenser, and a check valve is set on the pipe connected to the eighth-stage steam extraction pipe of the low-pressure cylinder to prevent steam leakage during normal operation of the unit.

[0023] S3 as attached Figure 1 The pipeline system for the minimum cooling flow from the exhaust steam of the intermediate pressure cylinder to the low pressure cylinder shown in the figure is composed of an electric valve (number 2), an electric regulating valve (number 3), a flow meter and connecting pipelines; during the operation stage of the low pressure cylinder completely cutting off the cylinder, hot standby steam is supplied to the steam inlet of the low pressure cylinder through this system, so that the steam inlet temperature of the low pressure cylinder can be kept at any time to switch back to the normal operation requirement; further monitoring of the low pressure cylinder inlet temperature and the low pressure cylinder exhaust port temperature requires that the low pressure steam inlet temperature is not lower than the medium pressure exhaust port temperature by more than 150℃. When the steam inlet temperature of the low pressure cylinder is lower than 100℃, and the steam inlet temperature is lower than the medium pressure exhaust port temperature by more than 150℃, the electric valve (number 2) can be opened and the electric regulating valve (number 3) can be controlled according to the given steam flow, but the electric valve (number 2) should be closed at any time when the exhaust cylinder temperature of the low pressure cylinder is higher than 52℃.

[0024] S4 as attached Figure 1 When the shown intermediate pressure cylinder exhaust to low pressure cylinder minimum cooling flow piping system operates according to the low pressure cylinder minimum cooling flow, the low pressure cylinder and low pressure cylinder exhaust port steam cooling system can also be put into operation, and cooling steam is supplied to the low pressure cylinder exhaust port through the steam extraction pipeline in the condenser to prevent heating of the last stage and the second last stage blast.

[0025] S5 as attached Figure 1 The system measurement points and monitoring shown include the medium-pressure exhaust port temperature and pressure; the low-pressure cylinder steam inlet temperature; the low-pressure cylinder exhaust port cylinder temperature; the temperature and temperature of the temperature reducer and pressure reducer outlet; and the flow meter of the medium-pressure cylinder exhaust to the low-pressure cylinder minimum cooling flow pipeline system.

Claims

1. A system for complete cylinder cutting and cooling of low-pressure cylinders of steam turbines for steam extraction heating is composed of connecting pipes between medium-pressure and low-pressure cylinders of the steam turbine and valves on the connecting pipes, steam cooling system for low-pressure cylinders and low-pressure cylinder exhaust ports, piping system for minimum cooling flow from medium-pressure cylinder exhaust to low-pressure cylinders, low-pressure cylinder shaft end sealing system, system measuring points and monitoring; its characteristics are: During the steam extraction and heating stage of the unit, except for the steam used for the low-pressure cylinder shaft end seal and the hot standby steam supplement at the low-pressure cylinder steam inlet, the steam flow to the low-pressure cylinder is completely cut off. At this time, the low-pressure cylinder is in a completely cut-off operation state; a small amount of steam from the low-pressure cylinder and the low-pressure cylinder exhaust port to the medium-pressure cylinder exhaust port is cooled after temperature reduction, pressure reduction, and steam-water separation; the cylinder temperature of the low-pressure cylinder steam inlet and exhaust port is monitored, and the low-pressure cylinder shaft end sealing system is adjusted when necessary, and the pipeline system from the medium-pressure cylinder exhaust to the low-pressure cylinder minimum cooling flow is opened and adjusted to use a small amount of steam to heat the low-pressure cylinder steam inlet, so that the low-pressure cylinder is in a hot standby state at any time after the low-pressure cylinder is completely cut off. At this time, the low-pressure cylinder of the steam turbine is in a high vacuum state.

2. A system for complete cylinder cutting and cooling of low-pressure cylinders of an extraction steam heating turbine as claimed in claim 1, characterized in that The connecting pipes of the medium and low pressure cylinders of the steam turbine and the valves on the connecting pipes are fully sealed valves. The valves are used to adjust the heating extraction steam pressure to meet the requirements during the heating stage. When the low pressure cylinder is cut off, the valve is completely closed to cut off the steam inlet from the medium pressure cylinder to the low pressure cylinder.

3. A system for complete cylinder cutting and cooling of low-pressure cylinders of an extraction steam heating turbine as claimed in claim 1, characterized in that The steam cooling system for the low-pressure cylinder and the exhaust port of the low-pressure cylinder consists of a control valve group, a temperature and pressure reducing device arranged at the throat of the condenser, an external temperature and pressure reducing water system, and a low-pressure cylinder steam supply pipe in the condenser; when the low-pressure cylinder is fully cut off, the above-mentioned control valve group is opened and controlled, and the temperature and pressure reducing device is put into operation; the exhaust steam from the intermediate-pressure cylinder of the steam turbine is reduced in temperature and pressure through the temperature and pressure reducing device, and then supplied to the low-pressure cylinder to enter the low-pressure cylinder to cool the low-pressure cylinder and the exhaust port of the low-pressure cylinder.

4. The steam cooling system for the low-pressure cylinder and the low-pressure cylinder exhaust port as claimed in claim 3 is arranged in the condenser throat temperature and pressure reducer, characterized in that It is arranged at the throat of the condenser and adopts a two-stage pressure reduction and one-time water spray cooling structure; the steam from the exhaust of the intermediate pressure cylinder enters the cooling and pressure reducing device through the steam pipe, and is sprayed into the cooling and pressure reducing device shell through the first-level opening area at the end of the pipe for the first expansion and pressure reduction; a stainless steel anti-erosion baffle is provided on the inner wall of the shell, and uniformly distributed atomizing nozzles are provided along the circumferential direction of the inner wall of the shell behind the first-level pipe opening area, and the cooling water from the condensate system is atomized and fully mixed with the steam in the cooling and pressure reducing device shell for cooling; the steam after the first-stage cooling and pressure reduction passes through several small holes on the conical spray net in the shell, and undergoes the second expansion and pressure reduction to diffuse to the rear area of ​​the cooling and pressure reducing device, so that the steam further expands and reduces the pressure, and the steam temperature after the first cooling and second pressure reduction does not exceed 50°C; in addition to the above-mentioned two-stage pressure reduction structure, a multi-stage pressure reduction structure with more than two-stage pressure reduction can also be adopted.

5. The low-pressure cylinder supplementary steam pipeline in the condenser of the low-pressure cylinder and the low-pressure cylinder exhaust steam cooling system as claimed in claim 3, characterized in that Cooling steam is supplied to the low-pressure cylinder through a low-pressure cylinder extraction pipeline connected to the condenser, and a check valve is arranged on the pipeline connected to the low-pressure cylinder extraction pipeline.

6. The minimum cooling flow piping system from the exhaust steam from the intermediate pressure cylinder to the low pressure cylinder of the extraction steam heating steam turbine low pressure cylinder complete cylinder cutting operation and cooling system as claimed in claim 1 is composed of a control valve group, a flow meter before the valve group and a connecting pipeline, characterized in that During the low-pressure cylinder complete cylinder cutting operation stage, hot standby steam is added to the low-pressure cylinder steam inlet; the low-pressure cylinder inlet temperature and low-pressure cylinder exhaust temperature are further monitored, and the low-pressure steam inlet temperature is required to be no less than 150°C lower than the medium-pressure exhaust temperature. When the low-pressure cylinder steam inlet temperature is lower than 100°C, and the steam inlet temperature is lower than the medium-pressure exhaust temperature by more than 150°C, the electric door of the control valve group can be opened and the regulating valve in the system valve group can be controlled according to the given steam flow, but the control valve group should be closed at any time when the low-pressure cylinder exhaust temperature is higher than 52°C.

7. A system for complete cylinder cutting and cooling of low-pressure cylinders of an extraction steam heating turbine as claimed in claim 1, characterized in that When the piping system from the exhaust of the intermediate pressure cylinder to the minimum cooling flow of the low pressure cylinder operates according to the minimum cooling flow of the low pressure cylinder, the steam cooling system of the low pressure cylinder and the exhaust port of the low pressure cylinder can be put into operation, and cooling steam can be supplied to the exhaust port of the low pressure cylinder through the extraction pipeline in the condenser.

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