Turbine low-pressure cylinder system running towards zero load and running method

Abstract

The invention belongs to the technical field of steam turbines, and particularly relates to a steam turbine low-pressure cylinder system running towards zero load and a running method. According to the technical scheme, the steam turbine low-pressure cylinder system running towards zero load comprises a steam turbine low-pressure cylinder, a plurality of stages of partition plates are arranged on the steam turbine low-pressure cylinder, each partition plate comprises a partition plate inner ring and a partition plate outer ring, and a whole circle of guide vanes are connected between the partition plate inner rings and the partition plate outer rings; at least part of the guide vanes in the whole circle of guide vanes of the partition plate are hollow guide vanes, and the hollow guide vanes are provided with low-temperature steam introduction ports and a plurality of steam nozzles which are communicated through the hollow guide vanes. The invention provides a steam turbine low-pressure cylinder system running towards zero load and a running method.

Description

Technical Field

[0001] The invention belongs to the technical field of steam turbines, and in particular relates to a steam turbine low-pressure cylinder system operating at zero load and an operating method. Background Art

[0002] During the operation of the steam turbine, in winter heating period, in order to increase the heating capacity of the exhaust steam of the intermediate pressure cylinder, the connecting pipe butterfly valve is often used to adjust the heating flow and pressure. However, there is a limit on the cooling flow of the low-pressure cylinder. In order to ensure that the low-pressure cylinder does not produce blast overheating and ensure the safe operation of the low-pressure cylinder, there is a minimum cooling flow for the low-pressure cylinder according to different last-stage blades to ensure the safe operation of the last-stage blades, prevent flow reflux, blast overheating, etc., and these problems will affect the efficiency and life of the steam turbine.

[0003] In the prior art, in order to expand the heating capacity of the middle row, the low-pressure cylinder bare axis operation is often adopted. This operation mode can avoid the blast of the low-pressure cylinder and does not have the problem of blade safety. However, these methods are complicated and require huge investment. The low-pressure rotor needs to be replaced every heating season, and the installation and maintenance costs are huge. Summary of the invention

[0004] In order to solve the above problems existing in the prior art, an object of the present invention is to provide a steam turbine low-pressure cylinder system and an operating method that operate towards zero load.

[0005] The technical solution adopted by the present invention is:

[0006] A steam turbine low-pressure cylinder system that runs towards zero load comprises a steam turbine low-pressure cylinder, on which a plurality of stages of partitions are arranged, wherein the partitions comprise a partition inner ring and a partition outer ring, and a full circle of guide vanes are connected between the partition inner ring and the partition outer ring; at least part of the guide vanes in the full circle of the partition are hollow guide vanes, and the hollow guide vanes are provided with low-temperature steam introduction interfaces and a plurality of steam nozzles that are connected through the hollow guide vanes.

[0007] The system of the present invention replaces the guide vanes of each level of the low-pressure cylinder with specially developed hollow guide vanes, injects low-temperature low-pressure steam into the hollow guide vanes, and axially sprays low-temperature steam through the opening in the middle of the hollow guide vanes, injecting steam into the center of the impeller to prevent the moving blades from blasting. When the low-temperature steam enters the steam turbine, it can cool the blades, slow down the occurrence of flow reflux and blast overheating, and improve the efficiency and reliability of the steam turbine.

[0008] As a preferred embodiment of the present invention, the entire circle of guide vanes of the partition are hollow guide vanes.

[0009] As a preferred solution of the present invention, the hollow guide vanes and the solid guide vanes each account for half of the entire circle of guide vanes of the partition, and the hollow guide vanes and the solid guide vanes in the partition are cross-distributed.

[0010] As a preferred embodiment of the present invention, the guide vane includes an inner arc and an outer arc, a plurality of steam nozzles are arranged in the inner arc of the hollow guide vane, and a low-temperature steam introduction interface is arranged on the outer edge side of the hollow guide vane.

[0011] As a preferred embodiment of the present invention, a steam turbine low-pressure cylinder steam inlet pipe is connected to the middle of the steam turbine low-pressure cylinder, and a heating butterfly valve and a cooling steam bypass are connected in parallel to the steam turbine low-pressure cylinder steam inlet pipe.

[0012] As a preferred embodiment of the present invention, the partitions arranged from the middle to both sides of the low-pressure cylinder of the steam turbine are respectively the first-stage partition, the second-stage partition, the third-stage partition, and the fourth-stage partition, and the low-pressure cylinder of the steam turbine is connected with the first-stage low-temperature steam inlet pipeline, the second-stage low-temperature steam inlet pipeline, the third-stage low-temperature steam inlet pipeline, and the fourth-stage low-temperature steam inlet pipeline; the first-stage low-temperature steam inlet pipeline is connected to the low-temperature steam inlet interfaces of all hollow guide vanes on the first-stage partition, the second-stage low-temperature steam inlet pipeline is connected to the low-temperature steam inlet interfaces of all hollow guide vanes on the first-stage partition, the third-stage low-temperature steam inlet pipeline is connected to the low-temperature steam inlet interfaces of all hollow guide vanes on the third-stage partition, and the fourth-stage low-temperature steam inlet pipeline is connected to the low-temperature steam inlet interfaces of all hollow guide vanes on the fourth-stage partition.

[0013] As a preferred embodiment of the present invention, it also includes a shaft seal leakage steam main pipe, which is respectively connected to the first-stage low-temperature steam introduction pipeline, the second-stage low-temperature steam introduction pipeline, the third-stage low-temperature steam introduction pipeline, and the fourth-stage low-temperature steam introduction pipeline.

[0014] As a preferred embodiment of the present invention, the first-stage low-temperature steam inlet pipeline is connected to a first-stage low-temperature steam cooler, the second-stage low-temperature steam inlet pipeline is connected to a second-stage low-temperature steam cooler, the third-stage low-temperature steam inlet pipeline is connected to a third-stage low-temperature steam cooler, and the fourth-stage low-temperature steam inlet pipeline is connected to a fourth-stage low-temperature steam cooler.

[0015] As a preferred embodiment of the present invention, the exhaust port of the low-pressure cylinder of the steam turbine is connected to a condenser through a pipeline, the condenser is connected to a deaerator through a pipeline, a hydraulic pump is connected to the pipeline between the condenser and the deaerator, the outlet of the condenser is connected to the water inlet of the first-stage low-temperature steam cooler, the water inlet of the second-stage low-temperature steam cooler, the water inlet of the third-stage low-temperature steam cooler, and the water inlet of the fourth-stage low-temperature steam cooler through pipelines, and the deaerator is connected to the water outlet of the first-stage low-temperature steam cooler, the water outlet of the second-stage low-temperature steam cooler, the water outlet of the third-stage low-temperature steam cooler, and the water outlet of the fourth-stage low-temperature steam cooler through pipelines.

[0016] A method for operating a low-pressure cylinder system of a steam turbine running at zero load comprises the following steps:

[0017] S1: When the heating demand increases and the heating butterfly valve is completely closed, the low-pressure cylinder cooling steam enters the turbine low-pressure cylinder through the cooling steam bypass;

[0018] S2: When the heating flow rate continues to increase and reaches the fourth threshold, the valve on the fourth-stage low-temperature steam introduction pipeline is opened;

[0019] S3: When the heat supply flow rate continues to increase and reaches the third threshold, the valve on the third-stage low-temperature steam introduction pipeline is opened;

[0020] S4: When the heat supply flow rate continues to increase and reaches the second threshold, the valve on the second-stage low-temperature steam introduction pipeline is opened;

[0021] S5: When the heat supply flow rate continues to increase and reaches the first threshold value, the valve on the first-stage low-temperature steam introduction pipeline is opened.

[0022] The beneficial effects of the present invention are:

[0023] The present invention replaces the guide vanes of each level of the low-pressure cylinder with specially developed hollow guide vanes, injects low-temperature low-pressure steam into the hollow guide vanes, and axially sprays low-temperature steam through the opening in the middle of the hollow guide vanes, injecting steam into the center of the impeller to prevent the moving blades from blasting. When the low-temperature steam enters the steam turbine, it can cool the blades, slow down the occurrence of flow reflux and blast overheating, and improve the efficiency and reliability of the steam turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure of the present invention;

[0025] Figure 2 It is a schematic diagram of the structure of the hollow guide vane;

[0026] Figure 3 It is a diagram of the connection between the guide vane and the baffle and a schematic diagram of the steam flow path;

[0027] Figure 4 It is a flow chart of the method of the present invention.

[0028] In the figure: 1-first-stage partition; 2-second-stage partition; 3-third-stage partition; 4-fourth-stage partition; A-first-stage low-temperature steam introduction pipeline; B-second-stage low-temperature steam introduction pipeline; C-third-stage low-temperature steam introduction pipeline; D-fourth-stage low-temperature steam introduction pipeline; E-heating butterfly valve; F-cooling steam bypass; G-turbine low-pressure cylinder steam inlet pipe; P-guide vane inner arc; Q-guide vane outer arc; M-low-temperature steam introduction interface; K-steam nozzle; I-steam inlet side; O-steam outlet; a-first-stage low-temperature steam cooler; b-second-stage low-temperature steam cooler; c-third-stage low-temperature steam cooler; d-fourth-stage low-temperature steam cooler; R-partition outer ring; S-partition inner ring; W-outer ring steam inlet; T-annular steam channel; N-hollow guide vane chamber. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0030] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0031] like Figure 1 , Figure 2 and Figure 3 As shown, the steam turbine low-pressure cylinder system for zero-load operation of this embodiment includes a steam turbine low-pressure cylinder, on which a plurality of stages of partitions are arranged, the partitions including a partition inner ring S and a partition outer ring R, a full circle of guide vanes are connected between the partition inner ring S and the partition outer ring R; at least some of the guide vanes in the full circle of guide vanes of the partition are hollow guide vanes, and the hollow guide vanes are provided with a low-temperature steam introduction interface M and a plurality of steam nozzles K connected through the hollow guide vanes.

[0032] The system of the present invention replaces the guide vanes of each level of the low-pressure cylinder with specially developed hollow guide vanes, injects low-temperature low-pressure steam into the hollow guide vanes, and axially sprays low-temperature steam through the opening in the middle of the hollow guide vanes, injecting steam into the center of the impeller to prevent the moving blades from blasting. When the low-temperature steam enters the steam turbine, it can cool the blades, slow down the occurrence of flow reflux and blast overheating, and improve the efficiency and reliability of the steam turbine.

[0033] Among them, the hollow guide vane is an upgraded version of the traditional guide vane. The guide vanes of each level of the baffle of the low-pressure cylinder of the steam turbine can be set to all hollow guide vanes or to set solid guide vanes and hollow guide vanes to be cross-distributed according to demand and investment returns. The structure of the hollow guide vane includes an inner arc P of the guide vane and an outer arc Q of the guide vane. A low-temperature steam introduction interface M is set on the inner outer edge side of the guide vane. A steam nozzle K is set on the inner arc P of the guide vane. The steam inlet introduces low-temperature steam through the steam pipe processed on the outer ring of the baffle. One side of the guide vane is the steam inlet side I, and the other side is the steam outlet side O.

[0034] Specifically, a steam turbine low-pressure cylinder steam inlet pipe G is connected to the middle of the steam turbine low-pressure cylinder, and a heating butterfly valve E and a cooling steam bypass F are connected to the steam turbine low-pressure cylinder steam inlet pipe G in parallel.

[0035] The partitions arranged from the middle to the two sides of the low-pressure cylinder of the steam turbine are respectively the first-stage partition 1, the second-stage partition 2, the third-stage partition 3, and the fourth-stage partition 4. The low-pressure cylinder of the steam turbine is connected with the first-stage low-temperature steam introduction pipeline A, the second-stage low-temperature steam introduction pipeline B, the third-stage low-temperature steam introduction pipeline C, and the fourth-stage low-temperature steam introduction pipeline D; the first-stage low-temperature steam introduction pipeline A is connected to the low-temperature steam introduction interface M of all hollow guide vanes on the first-stage partition 1, the second-stage low-temperature steam introduction pipeline B is connected to the low-temperature steam introduction interface M of all hollow guide vanes on the first-stage partition 1, the third-stage low-temperature steam introduction pipeline C is connected to the low-temperature steam introduction interface M of all hollow guide vanes on the third-stage partition 3, and the fourth-stage low-temperature steam introduction pipeline D is connected to the low-temperature steam introduction interface M of all hollow guide vanes on the fourth-stage partition 4.

[0036] Specifically, Figure 3 As shown, the outer ring of the partition is provided with a connected outer ring steam inlet W and an annular steam channel T, the low-temperature steam introduction interface M on the hollow guide vane is provided near the annular steam channel T, a hollow guide vane chamber N is also provided in the hollow guide vane, and a plurality of steam nozzles K are connected to the hollow guide vane chamber N. The cooling steam is connected to the outer ring steam inlet W on the outer ring of the partition through a pipeline, and the steam passes through the annular steam channel T, the low-temperature steam introduction interface M and the hollow guide vane chamber N in sequence, and then is ejected from a plurality of steam nozzles K and enters the low-pressure cylinder flow channel to cool the blades.

[0037] The present invention also includes a shaft seal leakage steam main pipe (SSR), which is respectively connected to the first-stage low-temperature steam introduction pipeline A, the second-stage low-temperature steam introduction pipeline B, the third-stage low-temperature steam introduction pipeline C, and the fourth-stage low-temperature steam introduction pipeline D. The cooling steam is provided by the shaft seal leakage steam main pipe (SSR), introduced into the hollow guide vane through the pipeline embedded in the partition, and ejected from the steam nozzle K.

[0038] The steam nozzle K sprays low-temperature steam radially to fully mix the low-temperature steam in the center of the impeller to cool the base of the impeller; the low-temperature steam can also use residual pressure to do work to prevent blade blasting.

[0039] In order to improve the cooling effect of low-temperature steam, a cooler is usually installed for each low-pressure steam line. The first-stage low-temperature steam inlet pipeline A is connected to the first-stage low-temperature steam cooler a, the second-stage low-temperature steam inlet pipeline B is connected to the second-stage low-temperature steam cooler b, the third-stage low-temperature steam inlet pipeline C is connected to the third-stage low-temperature steam cooler c, and the fourth-stage low-temperature steam inlet pipeline D is connected to the fourth-stage low-temperature steam cooler d. The high-temperature and low-pressure outlet steam is cooled and introduced into the hollow guide vanes.

[0040] The exhaust port of the low-pressure cylinder of the steam turbine is connected to a condenser through a pipeline, and the condenser is connected to a deaerator through a pipeline. A hydraulic pump is connected to the pipeline between the condenser and the deaerator. The outlet of the condenser is connected to the water inlet of the first-stage low-temperature steam cooler a, the water inlet of the second-stage low-temperature steam cooler b, the water inlet of the third-stage low-temperature steam cooler c, and the water inlet of the fourth-stage low-temperature steam cooler d through pipelines, respectively. The deaerator is connected to the water outlet of the first-stage low-temperature steam cooler a, the water outlet of the second-stage low-temperature steam cooler b, the water outlet of the third-stage low-temperature steam cooler c, and the water outlet of the fourth-stage low-temperature steam cooler d through pipelines. The cooling water in the first-stage low-temperature steam cooler a, the second-stage low-temperature steam cooler b, the third-stage low-temperature steam cooler c, and the fourth-stage low-temperature steam cooler d comes from the condenser and returns to the deaerator, which can improve the circulation efficiency of the thermal system and reduce energy consumption.

[0041] The steam for the first-stage low-temperature steam cooler a, the second-stage low-temperature steam cooler b, the third-stage low-temperature steam cooler c, and the fourth-stage low-temperature steam cooler d usually comes from SSR. When the load is low, the cooling steam can also come from auxiliary steam or other steam sources.

[0042] like Figure 4 As shown, the operation method of the steam turbine low-pressure cylinder system in zero-load operation of this embodiment includes the following steps:

[0043] S1: When the heating demand increases to the set threshold (Xt / h), the heating butterfly valve E is completely closed, and the low-pressure cylinder cooling steam enters the turbine low-pressure cylinder through the cooling steam bypass F;

[0044] S2: When the heat supply flow rate continues to increase and reaches the fourth threshold value (X-10t / h), the valve on the fourth-stage low-temperature steam introduction pipeline D is opened;

[0045] S3: When the heat supply flow rate continues to increase and reaches the third threshold value (X-20t / h), the valve on the third-stage low-temperature steam introduction pipeline C is opened;

[0046] S4: When the heat supply flow rate continues to increase and reaches the second threshold value (X-30t / h), the valve on the second-stage low-temperature steam introduction pipeline B is opened;

[0047] S5: When the heat supply flow rate continues to increase and reaches the first threshold value (X-40t / h), the valve on the first-stage low-temperature steam introduction pipeline A is opened.

[0048] Through the cooling effect of low-temperature steam, the occurrence of flow reflux and blast overheating is slowed down. Taking a 300MW subcritical unit as an example, the wheel shaft work of the last blade of the low-pressure cylinder under the conventional low-pressure cylinder cutting condition is -780KW. When the low-pressure cylinder system of the present invention is used, the wheel circumference of the last blade is -378KW under the cylinder cutting state, and the last-stage blast power is reduced by 50%. When the total output of the low-pressure cylinder remains basically unchanged, the blast power of the other low-pressure stages of the moving blades is greatly reduced. By using the low-pressure cylinder system of the present invention, the heating supply can be increased by 26t / h, and the heating capacity is increased by about 5%. By selling at a reasonable heating price, a relatively ideal investment return can be obtained.

[0049] The present invention reduces the occurrence of flow backflow and blast overheating through the cooling effect of low-temperature steam. In addition, the present invention not only improves the heating capacity of the steam turbine, but also reduces the turbulence caused by the blast of the low-pressure moving blades, so that the vibration of the low-pressure cylinder is well controlled, and the reliability and life of the steam turbine are improved.

[0050] In the existing steam distribution system, the existing low-temperature and low-pressure steam source can be used without additional investment, reducing costs. The upgrade and transformation can be completed by simply modifying the low-pressure cylinder partition, modifying the SSR system, and supplementing the processing of the low-pressure inner cylinder.

[0051] The invention improves the thermal efficiency of the steam turbine by introducing low-temperature steam into the hollow guide vanes, reduces energy waste, and improves the operating efficiency of the steam turbine.

[0052] The present invention is not limited to the above-mentioned optional implementation modes. Anyone can derive other various forms of products under the inspiration of the present invention. However, no matter what changes are made in the shape or structure, all technical solutions that fall within the scope defined by the claims of the present invention fall within the protection scope of the present invention.

Claims

1. A steam turbine low-pressure cylinder system running towards zero load, comprising a steam turbine low-pressure cylinder, on which a plurality of partitions are arranged, wherein the partitions comprise a partition inner ring (S) and a partition outer ring (R), and a full circle of guide vanes are connected between the partition inner ring (S) and the partition outer ring (R); characterized in that: At least part of the guide vanes in the entire circle of the partition are hollow guide vanes, and the hollow guide vanes are provided with low-temperature steam introduction interfaces (M) and a plurality of steam nozzles (K) connected through the hollow guide vanes.

2. A steam turbine low-pressure cylinder system for zero-load operation according to claim 1, characterized in that: The entire circle of guide vanes of the partition are hollow guide vanes.

3. A steam turbine low-pressure cylinder system for zero-load operation according to claim 1, characterized in that: The hollow guide vanes and the solid guide vanes each account for half of the entire circle of guide vanes of the partition, and the hollow guide vanes and the solid guide vanes in the partition are cross-distributed.

4. A steam turbine low-pressure cylinder system for zero-load operation according to claim 1, characterized in that: The guide vane comprises an inner arc (P) and an outer arc (Q), a plurality of steam nozzles (K) are arranged in the inner arc (P) of the hollow guide vane, and a low-temperature steam introduction interface (M) is arranged on the outer edge side of the hollow guide vane.

5. A steam turbine low-pressure cylinder system for zero-load operation according to claim 1, characterized in that: The middle part of the steam turbine low-pressure cylinder is connected with a steam turbine low-pressure cylinder steam inlet pipe (G), and a heating butterfly valve (E) and a cooling steam bypass (F) are connected in parallel to the steam turbine low-pressure cylinder steam inlet pipe (G).

6. A steam turbine low-pressure cylinder system for zero-load operation according to claim 5, characterized in that: The baffles arranged from the middle to the two sides of the low-pressure cylinder of the steam turbine are sequentially a first-stage baffle (1), a second-stage baffle (2), a third-stage baffle (3), and a fourth-stage baffle (4); the low-pressure cylinder of the steam turbine is connected with a first-stage low-temperature steam introduction pipeline (A), a second-stage low-temperature steam introduction pipeline (B), a third-stage low-temperature steam introduction pipeline (C), and a fourth-stage low-temperature steam introduction pipeline (D); the first-stage low-temperature steam introduction pipeline (A) is in communication with the low-temperature steam introduction interfaces (M) of all hollow guide vanes on the first-stage baffle (1), the second-stage low-temperature steam introduction pipeline (B) is in communication with the low-temperature steam introduction interfaces (M) of all hollow guide vanes on the first-stage baffle (1), the third-stage low-temperature steam introduction pipeline (C) is in communication with the low-temperature steam introduction interfaces (M) of all hollow guide vanes on the third-stage baffle (3), and the fourth-stage low-temperature steam introduction pipeline (D) is in communication with the low-temperature steam introduction interfaces (M) of all hollow guide vanes on the fourth-stage baffle (4).

7. A steam turbine low-pressure cylinder system for zero-load operation according to claim 6, characterized in that: It also includes a shaft seal leakage steam mother pipe, which is respectively connected to the first-stage low-temperature steam introduction pipeline (A), the second-stage low-temperature steam introduction pipeline (B), the third-stage low-temperature steam introduction pipeline (C), and the fourth-stage low-temperature steam introduction pipeline (D).

8. A steam turbine low-pressure cylinder system for zero-load operation according to claim 6, characterized in that: The first-stage low-temperature steam introduction pipeline (A) is connected to a first-stage low-temperature steam cooler (a), the second-stage low-temperature steam introduction pipeline (B) is connected to a second-stage low-temperature steam cooler (b), the third-stage low-temperature steam introduction pipeline (C) is connected to a third-stage low-temperature steam cooler (c), and the fourth-stage low-temperature steam introduction pipeline (D) is connected to a fourth-stage low-temperature steam cooler (d).

9. A steam turbine low-pressure cylinder system for zero-load operation according to claim 8, characterized in that: The exhaust port of the low-pressure cylinder of the steam turbine is connected to a condenser through a pipeline, the condenser is connected to a deaerator through a pipeline, a hydraulic pump is connected to the pipeline between the condenser and the deaerator, the outlet of the condenser is connected to the water inlet of the first-stage low-temperature steam cooler (a), the water inlet of the second-stage low-temperature steam cooler (b), the water inlet of the third-stage low-temperature steam cooler (c), and the water inlet of the fourth-stage low-temperature steam cooler (d) through pipelines, and the deaerator is connected to the water outlet of the first-stage low-temperature steam cooler (a), the water outlet of the second-stage low-temperature steam cooler (b), the water outlet of the third-stage low-temperature steam cooler (c), and the water outlet of the fourth-stage low-temperature steam cooler (d) through pipelines.

10. A method for operating a steam turbine low-pressure cylinder system that runs toward zero load, using the steam turbine low-pressure cylinder system that runs toward zero load according to claim 6, characterized in that: The following steps are involved: S1: When the heating demand increases and the heating butterfly valve (E) is completely closed, the low-pressure cylinder cooling steam enters the turbine low-pressure cylinder through the cooling steam bypass (F); S2: When the heat supply flow rate continues to increase and reaches the fourth threshold, the valve on the fourth-stage low-temperature steam introduction pipeline (D) is opened; S3: When the heat supply flow rate continues to increase and reaches the third threshold, the valve on the third-stage low-temperature steam introduction pipeline (C) is opened; S4: When the heat supply flow rate continues to increase and reaches the second threshold, the valve on the second-stage low-temperature steam introduction pipeline (B) is opened; S5: When the heat supply flow rate continues to increase and reaches the first threshold value, the valve on the first-stage low-temperature steam introduction pipeline (A) is opened.

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