A control method for the system in the "steam maintenance mode" after the MFT of the boiler of a secondary reheat unit

Abstract

A control method for a system in a "steam maintenance mode" after the MFT of a secondary reheat unit boiler comprises the following steps: (1) clarifying the control target of the "steam maintenance mode"; (2) setting the judgment conditions for automatic triggering and automatic exit of the steam maintenance mode; (3) clearly explaining the automatic control of the high, medium and low pressure three-stage bypass and its desuperheating water in the "steam maintenance mode"; (4) detailing the control logic of the feed water pump and the induced draft fan in the "steam maintenance mode"; (5) further explaining the pressure control of the auxiliary steam header and the action of the steam trap group in the steam maintenance mode; (6) giving priority to the supply of the shaft seal in the "steam maintenance mode", while taking into account the control of the condensate system and the auxiliary steam system. The entire control process is transformed from extensive manual control to precise automatic adjustment. It is of great significance to achieve energy saving and consumption reduction of thermal power units, rapid recovery from accident shutdowns, and improvement of the automation level of units.

Description

Technical Field

[0001] The present invention relates to the technical field of large thermal power generating units, and in particular to a control method for a system in a "steam maintenance mode" after a secondary reheat unit boiler MFT. Background Art

[0002] Ultra-supercritical 660-1000MW secondary reheat units (hereinafter referred to as secondary reheat units) usually adopt a three-stage bypass and four-stage desuperheater arrangement, which are three-stage large bypasses of high, medium and low pressure. The unit is conventionally equipped with 1 100% steam-driven feedwater pump (or 2 50% steam-driven feedwater pumps), 1 electric feedwater pump, and 2 50% steam-driven induced draft fans. Each small unit has five-stage extraction steam and auxiliary steam dual-path steam sources. The auxiliary steam main pipe is supplied by the five extractions of this unit under normal operating conditions, and the secondary reheat cold section is used as the backup steam source. When starting the machine, temporary machine or starting boiler can be used to supply steam. Under general accident conditions, the starting boiler cannot replenish the steam source in time.

[0003] For the above-mentioned secondary reheat units equipped with steam-driven feedwater pumps and steam-driven induced draft fans, when the boiler MFT (main fuel trip) occurs, the steam-driven feedwater pumps and steam-driven induced draft fans in the conventional design will trip due to the loss of steam source, which will cause four adverse effects. First, it takes more time to restart the unit; second, the furnace cannot be purged due to the trip of the induced draft fan after the boiler MFT, and the incompletely burned coal powder and combustible gas accumulated in the furnace, flue, air preheater, etc. may explode when ignited next time; third, a large amount of high-quality hot steam is stored in the system pipelines of large secondary reheat units, which is not reasonably used, resulting in waste; fourth, for single-row or non-temporary heating units, there is no hot standby auxiliary steam after the boiler MFT, which brings a higher risk to the reliable supply of the unit shaft seal.

[0004] In the existing transformation and optimization, after the unit has MFT, most of them use the secondary reheating cold section to supply auxiliary steam, and then use the auxiliary steam to supply the shaft seal steam source, while the steam-driven induced draft fan and steam-driven feed water pump are directly tripped without speed reduction to maintain operation. The steam-driven induced draft fan and steam-driven feed water pump are restarted by using the boiler start-up or temporary steam source, which increases the time for hot start-up. Summary of the invention

[0005] In order to solve the above-mentioned adverse effects and meet the requirements of energy saving and consumption reduction, the present invention proposes a control method for the system in the "steam maintenance mode" after the MFT of the boiler of a secondary reheat unit. The method can reasonably utilize the system heat storage and provide sufficient auxiliary steam source to ensure the operation of the small feedwater pump and the small induced draft fan under the premise of giving priority to the steam supply to the shaft seal after the MFT occurs in the boiler. At the same time, by optimizing the control logic, the degree of automation of the unit is improved, the operation workload of the operating personnel under accident conditions is reduced, and the safe, stable and rapid restart of the unit is achieved.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A control method for a system in a "steam maintenance mode" after MFT of a secondary reheat unit boiler, specifically comprising:

[0008] When the unit is operating normally: the steam at the outlet of the boiler superheater A passes through the ultra-high pressure main steam valve 3 and the ultra-high pressure regulating valve 4 and then enters the ultra-high pressure cylinder VI to do work. The steam after doing work passes through the ultra-high discharge check valve 10 and enters the cold section D of the boiler primary reheater for reheating. The heated steam enters the high pressure cylinder II from the outlet of the primary reheater B through the high pressure cylinder main steam valve 18 and the high pressure cylinder regulating valve 19 for supply. The exhaust steam passes through the high discharge check valve 20 and enters the cold section E of the secondary reheater for further heating. The steam after secondary heating enters the medium pressure cylinder III from the outlet of the boiler secondary reheater C through the medium pressure cylinder main steam valve 25 and the medium pressure cylinder regulating valve 26 to do work. The exhaust steam of the medium pressure cylinder III enters the high back pressure condenser N respectively. A and low back pressure condenser N B The cooling water of the high-pressure bypass valve 5 comes from the outlet main pipe of the high-pressure feed water pump, the cooling water of the medium-pressure bypass valve 14 comes from the middle tap of the high-pressure feed water pump and the electric pump, and the cooling water of the low-pressure bypass valve 27 comes from the condensate main pipe;

[0009] After the occurrence of MFT in the boiler, the "steam maintenance mode" is operated. The steam from the boiler superheater A directly enters the primary reheater cold section D for heating through the high-pressure bypass 5. The heated steam enters the secondary reheater cold section E from the outlet of the boiler first-stage reheater B through the medium-pressure bypass valve 14. On the one hand, the steam of the secondary reheat cold section E enters the auxiliary steam header F through the secondary reheat cold section to auxiliary steam header regulating valve 42, and then is preferentially supplied to the main engine shaft seal K by using the auxiliary steam to shaft seal manual gate valve 31 and the auxiliary steam to shaft seal electric valve 32; on the other hand, the steam supplied to the auxiliary steam header F by the secondary reheat cold section E passes through the auxiliary steam to induced draft fan small unit electric valve 33 to maintain the steam-driven induced draft fan small unit I running at 3000r / min without tripping, and enters the steam-driven feed water pump small unit H through the auxiliary steam to feed water pump small unit electric valve 34 to maintain the steam-driven feed water pump small unit H running at 3000r / min without tripping. When this process occurs, the low-pressure bypass valve 27 remains fully closed.

[0010] A control method for a "steam maintenance mode" of a secondary reheat unit boiler after MFT, comprising the following steps:

[0011] (1) When the unit is operating normally in the load range above 35%Pe, and the "steam protection mode" interlock button is put into normal operation, after the boiler MFT is actuated, the steam turbine-boiler-electrical interlock protection is actuated, the steam turbine trips, the generator trips, and the boiler is extinguished. At this time, the "steam protection mode" is triggered, and the speed of the steam-driven feedwater pump small unit H is over-relaxed from the current speed value command to 3000r / min, and the steam-driven feedwater pump recirculation regulating valve 44 is fully opened. After 90 seconds, it is automatically put into automatic control, and the main feedwater to economizer inlet bypass electric door and the main feedwater to economizer inlet bypass electric door are interlocked and closed, cutting off the boiler water supply channel. The steam-driven feedwater pump small unit H maintains the minimum flow rate and recirculation operation. After the "steam protection mode" is triggered, the steam-driven induced draft fan small unit I also quickly reduces the current speed command to 3000r / min, maintaining the operation of the steam-driven induced draft fan small unit I, and performing normal furnace purge;

[0012] (2) After the "steam maintenance mode" is activated, check whether the three-stage bypass of the high-pressure bypass valve 5, the medium-pressure bypass valve 14, and the low-pressure bypass valve 27 are automatically switched to the "steam maintenance mode" control, that is, the high-pressure bypass valve 5 automatically controls the pressure of the primary reheat cold section, the medium-pressure bypass valve 14 automatically controls the pressure of the secondary reheat cold section, and the low-pressure bypass valve 27 is automatically fully closed;

[0013] (3) After checking that the electric feedwater pump J is in normal condition, start the electric feedwater pump J first. The electric feedwater pump J should be kept running in recirculation mode. If the unit is equipped with 2×50% steam-driven feedwater pumps H, manually shut down one steam-driven feedwater pump H and keep the other running. If there is only one steam-driven feedwater pump H, keep the speed unchanged at 3000r / min and run normally. The running electric feedwater pump J and steam-driven feedwater pump H provide cooling water to the high and medium pressure bypass. At the same time, in order to start the unit quickly, the boiler can be filled with water as soon as possible. After the "steam maintenance mode" runs for a period of time, the residual steam in the system gradually decreases. At this time, if the conditions for starting the unit are not met, once the electric feedwater pump J is running normally, consider manually shutting down one steam-driven feedwater pump H;

[0014] (4) After the "steam protection mode" is triggered, the medium-pressure bypass valve 14 controls the pressure of the secondary reheat cold section. At this time, the auxiliary steam header pressure p3 of the auxiliary steam header F is supplied by the secondary reheat cold section. The operating personnel manually set the pressure value of the auxiliary steam header F. The setting range is 0.6-1.0 MPa. The pressure setting value of the auxiliary steam header F is required to be less than the pressure value before the medium-pressure bypass valve 14. At this time, the auxiliary steam gives priority to providing steam source to the main engine shaft seal K to ensure that the main engine shaft seal K uses steam normally. At the same time, according to the requirements of the "steam protection mode", the auxiliary steam provides steam source to the steam-driven feedwater pump small unit H and the steam-driven induced draft fan small unit I to maintain the normal operation of the two small units at 3000r / min;

[0015] (5) 120 seconds after the "steam protection mode" is triggered, check whether the unit's steam traps are correctly linked. According to the requirements of the control logic, the steam trap pneumatic valve 39 and steam trap pneumatic valve 721 connected to the ultra-high pressure cylinder VI, high pressure cylinder II and medium pressure cylinder III are required to be opened to prevent the accumulation of cooled water vapor in the cylinder, which will increase the temperature difference between the upper and lower parts of the cylinder; at the same time, the steam traps in the pipelines before each main steam valve and the steam traps on the high, medium and low pressure bypass pipelines are interlocked and closed to minimize the waste of system steam. Through the automatic control of the three-level bypass, the system's heat storage is finally used as the steam source for the small machine and the main engine shaft seal K after the machine trip, achieving the effect of the "steam protection mode" design;

[0016] (6) After the "steam maintenance mode" is activated, it is confirmed that the control of the above subsystems meets the design requirements and the unit has the conditions for hot start, and it must be started as soon as possible. When the MFT signal is reset and the ignition signal is in place, the "steam maintenance mode" automatically switches to the normal start mode, that is, bypass follow-up.

[0017] In the "steam maintenance mode", the low-pressure bypass valve 27 is automatically fully closed, the medium-pressure bypass valve 14 automatically controls the pressure of the secondary reheat cold section E, and the high-pressure bypass valve 5 automatically controls the pressure of the primary reheat cold section D. This control method can achieve that when the boiler has MFT, the steam-driven feed water pump small unit H and the steam-driven induced draft fan small unit I quickly drop to a lower speed to maintain operation without tripping;

[0018] The interlock button in the "steam protection mode" is specifically arranged just below the original bypass control normal mode. Only after the button is put into operation can the automatic control of the "steam protection mode" be realized.

[0019] The automatic triggering condition of the "steam protection mode": the "steam protection mode" is automatically activated when and only when the following conditions a, b, and c are met at the same time;

[0020] a. The boiler has MFT action;

[0021] b. The unit load is greater than 35% Pe (Pe is the rated load of the unit) before MFT action;

[0022] c. Any water supply pump is running (electric water supply pump or any steam-driven water supply pump).

[0023] The automatic exit conditions of the "steam protection mode" are:

[0024] When the MFT signal is reset and the ignition signal is raised, it automatically switches from the "steam maintenance mode" to normal startup, that is, the high bypass valve 5 controls the main steam pressure, the medium pressure bypass valve 14 controls the primary reheater pressure, and the low pressure bypass valve 27 controls the boiler secondary reheater C pressure. The low pressure bypass valve 27 controls the current boiler secondary reheater C pressure, and the set value of the low pressure bypass valve 27 is given by the operator.

[0025] Control of the low-pressure bypass valve 27 in the "steam protection mode":

[0026] After MFT occurs, the low-pressure bypass valve 27 is overridden (skipping the protection logic and forcing the action) to close and switch to the manual state, in order to keep the low-pressure bypass valve 27 fully closed and save the steam storage in the system pipeline;

[0027] Control of the medium-pressure bypass valve 14 and the desuperheating water in the "steam protection mode":

[0028] a. After the low-pressure bypass valve 27 is quickly closed, it will no longer interlock the medium-pressure bypass valve 14 quickly closed;

[0029] b. Automatically start the pressure automatic control of the medium-pressure bypass valve 14 and switch to control the pressure after the medium-pressure bypass valve 14, that is, the pressure of the secondary reheat cold section. The given value is the pressure point controlled by the original low-pressure bypass valve 27 before the unit trips (the upper limit is set according to 75% Pe load pressure);

[0030] c. The middle bypass valve temperature reducing water regulating valve 16 is automatically put into control, and its temperature is initially set to 380℃, and the offset value is set to -100℃~+50℃;

[0031] d. The control logic of the middle bypass valve warm water stop valve 17 remains unchanged, that is, when the instruction of the middle bypass valve temperature reducing water regulating valve 16 is greater than 2%, the middle bypass valve warm water stop valve 17 is opened, and when it is less than 2%, it is closed after a delay of 300 seconds;

[0032] Control of the high-pressure bypass valve 5 and the desuperheating water in the "steam protection mode":

[0033] a. Automatically start the high bypass pressure and switch to control the pressure behind the high-pressure bypass valve 5, that is, the pressure of the primary reheat cold section. Its set value is set to the pressure point controlled by the original medium-pressure bypass valve 14 before the unit trips (the upper limit is set according to 75% Pe load pressure);

[0034] b. The high bypass desuperheating water regulating valve 7 is automatically put into operation, and its temperature is initially set to 400°C, with a bias value of -100°C to +50°C;

[0035] c. The control logic of the high bypass cooling water stop valve 8 remains unchanged: that is, when the high bypass cooling water regulating valve 7 command is greater than 2%, the high bypass cooling water stop valve 8 is opened; when it is less than 2%, it is closed after a delay of 300 seconds;

[0036] Control of the steam-driven feedwater pump small unit H in the "steam protection mode":

[0037] a. After MFT occurs, cancel the MFT interlocking steam-driven feedwater pump small machine H logic, and interlock the auxiliary steam to the feedwater pump small machine electric valve 34 to use the steam electric door;

[0038] b. Override the speed command of the small unit H of the steam-driven water supply pump to 3000r / min, but do not exit the remote control. The operator is allowed to operate only after the command is in place, otherwise the operator is locked out of operation;

[0039] c. Force the steam-driven feedwater pump recirculation regulating valve (44) to be fully opened, and then automatically put into automatic control after a delay of 90 seconds;

[0040] d. Close the main feed water door from the economizer inlet to the economizer inlet bypass electric door and the bypass electric door, and unlock the operator's operating authority after the feedback signal is in place.

[0041] Control of the steam-driven induced draft fan small unit I in the "steam protection mode":

[0042] a. After MFT occurs, cancel the MFT interlocking and tripping logic of the steam-driven induced draft fan small machine I;

[0043] b. Interlock and open the electric valve 33 from auxiliary steam to the small machine of the induced draft fan;

[0044] c. Override the speed command of the steam-driven induced draft fan small unit I to 3000r / min, exit the remote control, and allow the operator to operate after feedback is in place, otherwise the operator is locked out of operation;

[0045] Auxiliary steam header F pressure control in the "steam protection mode":

[0046] There are two regulating states for the automatic control of the regulating valve 42 from the secondary reheat cold section to the auxiliary steam manifold, one is the constant pressure control state, and the other is the sliding pressure control state. During normal operation, it can be selected as the sliding pressure state, and its setting value should refer to the five-stage extraction steam pressure of this unit. During normal startup and shutdown, the constant pressure operation state can be selected, and its setting value is given by the operating personnel. After the "steam maintenance mode" is triggered, the pressure p3 from the secondary reheat cold section to the auxiliary steam manifold selects constant pressure operation, and its set value is manually set by the operating personnel, with a setting range of 0.6 to 1.0 MPa. The desuperheating water regulating valve is automatically put into automatic control, and its setting value is 340°C, with a bias of -30 to +30°C.

[0047] Control of the steam trap in the "steam protection mode":

[0048] After the MFT is actuated, the logic of the interlocking opening of the high, medium and low pressure steam traps in the original logic remains unchanged. Considering that the "steam protection mode" has been triggered, in order to reduce the unnecessary waste of residual steam in the system, the following automatic control steam trap logic is added. After the steam protection mode is triggered, a pulse signal is sent to open or close the following steam traps after a delay of 120 seconds:

[0049] a. Close the pneumatic drain valve 1 on the pipeline in front of the main steam valve 3 of the ultra-high pressure cylinder, close the pneumatic drain valve 12 in front of the high pressure main steam valve 18 and the pneumatic drain valve 13 in the pipeline in front of the main steam valve 25 of the medium pressure cylinder;

[0050] b. Close the pipeline drain pneumatic valve 2 in front of the high-pressure bypass valve 5, close the pipeline drain pneumatic valve 13 in front of the medium-pressure bypass valve 14 and the pipeline drain pneumatic valve 24 in front of the low-pressure bypass valve 27;

[0051] c. Close the pneumatic drain valve 11 behind the ultra-high discharge check valve 10 and the pneumatic drain valve 8 22 behind the high discharge check valve 20;

[0052] d. Keep the steam pipe drain pneumatic valve 1139 for auxiliary steam to feedwater pump small unit debugging open;

[0053] e. Keep the steam pipe drain pneumatic valve 1240 open for auxiliary steam to induced draft fan small unit debugging;

[0054] g. Keep the drain pneumatic valve three 9 in front of the ultra-high discharge check valve 10 and the drain pneumatic valve seven 21 in front of the high discharge check valve 20 open.

[0055] A system for "steam maintenance mode" after MFT of a boiler in a double reheat unit;

[0056] It includes a boiler superheater A, the steam outlet of the boiler superheater A is connected to the ultra-high pressure cylinder VI through an ultra-high pressure main steam valve 3 and an ultra-high pressure regulating valve 4, and the output end of the ultra-high pressure cylinder VI is connected to the cold section D of the boiler primary reheater through an ultra-high discharge check valve 10;

[0057] The steam output end of the boiler superheater A is connected to the cold section D of the boiler primary reheater through a high-pressure bypass 5;

[0058] The steam output end of the cold section D of the boiler primary reheater is heated and connected to the first-stage reheater B of the boiler. The first-stage reheater B of the boiler is connected to the high-pressure cylinder II through the high-pressure cylinder main steam valve 18 and the high-pressure cylinder regulating valve 19. The exhaust steam of the high-pressure cylinder II is connected to the cold section E of the secondary reheater through the high-discharge check valve 20. The steam output end of the cold section E of the secondary reheater is connected to the secondary reheater C of the boiler. The outlet of the secondary reheater C of the boiler is connected to the medium-pressure cylinder III through the medium-pressure cylinder main steam valve 25 and the medium-pressure cylinder regulating valve 26. The exhaust end of the medium-pressure cylinder III is respectively connected to the high back pressure condenser N A and low back pressure condenser N B ;

[0059] The output end of the boiler primary reheater B is connected to the secondary reheater cold section E through a medium-pressure bypass valve 14.

[0060] The steam output end of the secondary reheater cold section E is connected to the auxiliary steam header F through the secondary reheater cold section to auxiliary steam header regulating valve 42, and the output end of the auxiliary steam header F is connected to the main engine shaft seal K through the auxiliary steam to shaft seal manual gate valve 31 and the auxiliary steam to shaft seal electric valve 32.

[0061] The steam output end of the auxiliary steam manifold F is connected to the steam-driven induced draft fan small unit I through the auxiliary steam to induced draft fan small unit electric valve 33.

[0062] The steam output end of the auxiliary steam manifold F is connected to the steam-driven feedwater pump H through the auxiliary steam to feedwater pump electric valve 34.

[0063] The exhaust end of the intermediate pressure cylinder III is connected to the high back pressure condenser N through the low pressure bypass valve 27 and the desuperheater 28. A and low back pressure condenser N B ;

[0064] The desuperheater 28 is provided with a low bypass desuperheating water regulating valve 29 and a low bypass desuperheating water stop valve 30 .

[0065] Beneficial effects of the present invention:

[0066] 1) The control objectives of the “steam maintenance mode” are clearly defined.

[0067] 2) The effect that needs to be achieved in the "steam maintenance mode" was proposed, and the speed reduction of the small feed water pump and induced draft fan after the boiler MFT was successfully achieved without stopping the machine. A complete set of control logic with a high degree of automation and small adjustment disturbance was proposed.

[0068] 3) Set the criteria for automatic triggering and automatic exit of the "steam protection mode".

[0069] 4) The entire control process is automatically adjusted and tracked, which reduces the amount of operation for operators, enables safe and rapid start-up of the unit after MFT, and greatly shortens the hot start-up time.

[0070] 5) Make full use of the characteristics of the secondary reheat unit to achieve flexible application of the three-stage large bypass, reduce the waste of system heat storage, and significantly improve the economic efficiency of the unit.

[0071] 6) Priority is given to the supply to the shaft seal in the "steam maintenance mode", while taking into account the control of the auxiliary steam system.

[0072] 7) Based on the overall safety of the unit, the switching action of the steam trap group in the "steam protection mode" was optimized.

[0073] 8) The entire control process has been transformed from extensive manual control to precise automatic adjustment. This is of great significance for achieving energy conservation and consumption reduction of thermal power units, rapid recovery from accident shutdowns, and improvement of the automation level of units. Practice has proved that this solution is feasible and can be promoted. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 This is a typical double reheat unit "steam maintenance mode" process and control measurement point layout diagram.

[0075] Among them, VI is an ultra-high pressure cylinder, II is a high pressure cylinder, III is a medium pressure cylinder (the high and medium pressure cylinders are combined cylinder structures), IV and V are low pressure cylinders, N A is the high back pressure condenser corresponding to low pressure cylinder IV, N B is the low back pressure condenser corresponding to the low pressure cylinder V; A is the boiler superheater, B is the first-stage boiler reheater, C is the second-stage boiler reheater, D is the first reheater cold section, E is the second reheater cold section, F is the auxiliary steam header, G is the shaft fan, H is the steam-driven feed water pump, I is the steam-driven induced draft fan, J is the electric feed water pump, and K is the main engine shaft seal system;

[0076] p1 is the pressure after the high bypass valve, t1 is the temperature after the high bypass valve, p2 is the pressure after the middle bypass valve, t2 is the temperature after the middle bypass valve, p3 is the pressure of the auxiliary steam header, t3 is the temperature of the auxiliary steam header, t4 is the temperature after the desuperheating water regulating valve from the secondary reheat cold section to the auxiliary steam header;

[0077] 1, 2, 9, 11, 12, 13, 21, 22, 23, 24, 39, 40 are respectively the steam-drain pneumatic valves, 5 is the high-pressure bypass valve, 14 is the medium-pressure bypass valve, 27 is the low-pressure bypass valve, 6, 15, 28, 43 are desuperheaters, 7 is the high-bypass cooling water regulating valve, 8 is the high-bypass cooling water stop valve, 16 is the medium-bypass cooling water regulating valve, 17 is the medium-bypass cooling water stop valve, 29 is the low-bypass cooling water regulating valve, 30 is the low-bypass cooling water stop valve, 44 is the steam-driven feedwater pump recirculation regulating valve, 3 is the ultra-high-pressure main steam valve, 4 is the ultra-high-pressure regulating valve, 18 is the high-pressure main steam valve, 19 is a high-pressure regulating valve, 25 is a medium-pressure main steam valve, 26 is a medium-pressure regulating valve, 35 is a small-unit speed closing valve for the feedwater pump, 36 is a small-unit speed closing valve for the induced draft fan, 10 is an ultra-high exhaust check valve, 20 is a high-exhaust check valve, 31 is a manual gate valve from auxiliary steam to shaft seal, 32 is an auxiliary steam to shaft seal electric valve, 33 is an auxiliary steam to a small-unit electric valve for the induced draft fan, 34 is an auxiliary steam to a small-unit electric valve for the feedwater pump, 37 is a five-pump to a small-unit electric valve for the feedwater pump, 38 is a five-pump to a small-unit electric valve for the induced draft fan, 41 is a five-pump to auxiliary steam manifold electric valve, and 42 is a regulating valve from the secondary reheat cold section to the auxiliary steam manifold. DETAILED DESCRIPTION

[0078] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0079] like Figure 1As shown, a typical secondary reheat unit is equipped with four cylinders, namely, an ultra-high pressure cylinder (VI), a high and medium pressure combined cylinder (II and III), and two low pressure cylinders (IV and V). The steam at the outlet of the boiler superheater A passes through the ultra-high pressure main steam valve 3 and the ultra-high pressure regulating valve 4 and then enters the ultra-high pressure cylinder VI to perform work. The steam after performing work passes through the ultra-high pressure exhaust check valve 10 and enters the cold section D of the boiler primary reheater for reheating. The heated steam enters the high pressure cylinder II from the outlet of the primary reheater B through the high pressure cylinder main steam valve 18 and the high pressure cylinder regulating valve 19 for supply, and the exhaust steam passes through the high pressure exhaust check valve 20 and enters the cold section E of the secondary reheater for further heating. The steam after secondary heating enters the medium pressure cylinder III from the outlet of the boiler secondary reheater C through the medium pressure cylinder main steam valve 25 and the medium pressure cylinder regulating valve 26 to perform work, and the exhaust steam of the medium pressure cylinder III enters the high back pressure condenser N respectively. A and low back pressure condenser N B , completing the normal cycle;

[0080] During unit startup and accident conditions, the steam from the boiler superheater directly enters the primary reheater cold section D for heating through the high-pressure bypass valve 5. The heated steam enters the secondary reheater cold section E from the outlet of the boiler's first-stage reheater B through the medium-pressure bypass valve 14. The steam after reheating enters the condenser temperature and pressure reduction device from the outlet of the boiler's second-stage reheater C through the low-pressure bypass 27 to complete the entire cycle. The cooling water of the high-pressure bypass valve 5 comes from the high-pressure feed water pump outlet main pipe, the cooling water of the medium-pressure bypass valve 14 comes from the middle tap of the high-pressure feed water pump and the electric pump, and the cooling water of the low-pressure bypass valve 27 comes from the condensate main pipe.

[0081] The steam source of the auxiliary steam header F includes the steam from the startup boiler or temporary machine, the steam from the fifth extraction zone of the machine, and the steam from the secondary reheat cold section; the main users of auxiliary steam include steam for the steam-driven feedwater pump small machine H, steam for the steam-driven induced draft fan small machine I, and steam for the main engine shaft seal. In addition, the steam source of the steam-driven feedwater pump small machine H and the steam-driven induced draft fan small machine I not only comes from the auxiliary steam header, but also includes steam from the fifth extraction zone of the machine. During the startup stage and below 30% Pe load, the steam source of the small machine is provided by auxiliary steam, and the steam source above 30% Pe load is cut to the fifth extraction zone of the machine. The auxiliary steam for the shaft seal is supplied back to the steam main pipe through the shaft seal, and finally discharged to the atmosphere through the shaft fan G.

[0082] like Figure 1 As shown: after the unit is normally connected to the grid with load, the high-pressure bypass valve 5, the medium-pressure bypass valve 14 and the low-pressure bypass valve 27 are all in the normal following mode. At this time, the set values ​​of the high, medium and low-pressure bypasses are the current actual pressure plus 0.5MPa, so the three-level bypasses are all in the automatic closing position.

[0083] When the unit load reaches 30%Pe or more, the steam traps 1, 2, 9, 11, 12, 13, 21, 22, 23, 24, 39, and 40 on the system pipeline are automatically closed. At this time, the auxiliary steam manifold is supplied by the five-stage extraction steam, and the electric valve 41 from the five-stage extraction to the auxiliary steam manifold is fully opened. The auxiliary steam regulating valve 42 for the secondary reheat cold section is adjusted as a standby steam source, and its pressure setting value is manually input by the operator, usually set to the current five-stage extraction steam pressure minus 0.15MPa. The boiler water supply is switched from the electric feedwater pump J to the steam-driven feedwater pump H, and the electric feedwater pump J is ready to start at any time as a hot standby. At the same time, the steam source of the steam-driven feedwater pump H is switched from the auxiliary steam to the five-stage extraction steam. It should be emphasized that the electric valve 34 from the auxiliary steam to the feedwater pump should be kept fully open at this time as a hot standby pipeline. The steam source of the steam-driven induced draft fan small unit I is also switched from auxiliary steam to the five-stage extraction steam supply, and the auxiliary steam to the induced draft fan small unit electric valve 33 should be kept fully open, also serving as a hot standby pipeline.

[0084] When the unit load reaches 35%Pe or above, the "steam maintenance mode" button is put into operation. At this time, once the boiler MFT is activated, any electric feedwater pump J or steam feedwater pump H is running, the "steam maintenance mode" is automatically triggered.

[0085] After the "steam protection mode" is activated, the steam-driven feedwater pump small machine H command is over-relaxed from the current value to 3000r / min, the five-stage steam extraction to the feedwater pump small machine electric valve 37 is interlocked and closed, the auxiliary steam to the feedwater pump small machine electric valve 34 triggers the opening command again, and the auxiliary steam to the feedwater pump small machine electric valve before the water drain pneumatic valve 11 39 is interlocked and opened. Similarly, the steam-driven induced draft fan small machine I command is over-relaxed from the current command value to 3000r / min, the five-stage steam extraction to the induced draft fan small machine electric valve 38 is interlocked and closed, the auxiliary steam to the induced draft fan small machine electric valve 33 is triggered again to open the command, and the auxiliary steam to the induced draft fan small machine electric valve before the water drain valve 40 is interlocked and opened.

[0086] After the "steam maintenance mode" is triggered, the high-pressure bypass valve 5 automatically tracks the pressure p1 after the high-bypass valve, that is, the pressure of the primary reheating cold section. The pressure p1 after the high-bypass valve is set according to the value of the medium-pressure bypass valve 14 before the MFT. The high-bypass cooling water regulating valve 7 automatically sets the temperature t1 after the high-bypass valve to 400℃, and adjusts it according to the bias value of -100℃~+50℃; the high-bypass cooling water stop valve 8 tracks the high-bypass cooling water regulating valve 7, and when the high-bypass cooling water regulating valve 7 instruction is greater than 2%, it is opened, and when it is less than 2%, it is closed with a delay of 300 seconds. After the "steam protection mode" is triggered, the medium-pressure bypass valve 14 automatically tracks the pressure p2 after the medium bypass valve, that is, the pressure of the secondary reheat cold section. The pressure value is set according to the value of the low-pressure bypass valve 27 before the MFT. The medium bypass cooling water regulating valve 16 automatically sets the temperature t2 after the medium bypass valve to 380℃, and adjusts it according to the offset value of -100℃~+50℃; the medium bypass cooling water stop valve 17 tracks the opening of the high bypass cooling water regulating valve 7. When the high bypass cooling water regulating valve 7 instruction is greater than 2%, it is connected to open, and when it is less than 2%, it is closed with a delay of 300 seconds. The low-pressure bypass valve 27 is automatically interlocked and closed after the "steam protection mode" is triggered. After closing in place, it exits the automatic state and switches to the manual state.

[0087] After the "steam protection mode" is triggered, the turbine trips and interlocks to close the five-stage steam extraction check valve and the five-stage steam extraction electric valve. At this time, the auxiliary steam header pressure p3 is supplied by the secondary reheat cold section. The operating personnel manually set the auxiliary steam header pressure p3 through the secondary reheat cold section auxiliary steam header regulating valve 42. The setting range is 0.6~1.0MPa, and the auxiliary steam header pressure setting auxiliary steam header pressure p3 value is required to be less than the pressure value p2 before the middle bypass valve. The secondary reheat cold section auxiliary steam header desuperheating water regulating valve is automatically put into automatic control, and its setting value is the temperature t4 after the secondary reheat cold section to the auxiliary steam header desuperheating water regulating valve, which is initially set to 340℃, and adjusted according to the -30~+30℃ offset.

[0088] After the "steam maintenance mode" is triggered, the water regulating valve on the deaerator is overrun and closed to 0, and then the automatic water level adjustment is restored after a delay of 20 seconds. The set value is still the original value given by the operator. The automatic minimum value of the inverter is set to 30Hz. After a delay of 15 seconds, the condensate main pipe pressure is tracked. The pressure value is required to be no less than 1.8MPa to ensure the minimum pressure required for the operation of condensate polishing and low bypass cooling water.

[0089] 120 seconds after the "steam protection mode" is triggered, the main body drain pneumatic valve connected to the ultra-high pressure cylinder VI, high pressure cylinder II and medium pressure cylinder III and the drain valves 9, 21, 39 and 40 before the check valve are required to be opened to prevent the accumulation of cooled water vapor in the cylinder, which will cause the temperature difference between the upper and lower parts of the cylinder to increase; at the same time, the drain valves 1, 2, 11, 12, 13, 22, 23 and 24 on the high, medium and low pressure bypass pipelines in each main steam valve are interlocked and closed to minimize the waste of system steam. Through the automatic control of the three-level bypass, the system's heat storage is finally used as the steam source for the small machine and the shaft seal after the machine tripping, so as to achieve the effect of the "steam protection mode" design.

Claims

1. A control method for a system in a "steam maintenance mode" after a secondary reheat unit boiler MFT, characterized in that: Specifically include: When the unit is operating normally: the steam at the outlet of the boiler superheater (A) passes through the ultra-high pressure main steam valve (3) and the ultra-high pressure regulating valve (4) and then enters the ultra-high pressure cylinder (VI) to perform work. The steam after performing work passes through the ultra-high pressure exhaust check valve (10) and enters the cold section (D) of the boiler primary reheater for reheating. The heated steam enters the high pressure cylinder (II) from the outlet of the primary reheater (B) through the high pressure cylinder main steam valve (18) and the high pressure cylinder regulating valve (19) for supply. The exhaust steam passes through the high pressure exhaust check valve (20) and enters the cold section (E) of the secondary reheater for further heating. The steam after secondary heating enters the medium pressure cylinder (III) from the outlet of the boiler secondary reheater (C) through the medium pressure cylinder main steam valve (25) and the medium pressure cylinder regulating valve (26) to perform work. The exhaust steam of the medium pressure cylinder (III) enters the high back pressure condenser (N A ) and low back pressure condenser (N B ), the cooling water of the high-pressure bypass valve (5) comes from the outlet main pipe of the high-pressure water supply pump, the cooling water of the medium-pressure bypass valve (14) comes from the middle tap of the high-pressure water supply pump and the electric pump, and the cooling water of the low-pressure bypass valve (27) comes from the condensate main pipe; After the boiler has MFT, it runs in the "steam maintenance mode". The steam from the boiler superheater (A) directly enters the primary reheater cold section (D) for heating through the high-pressure bypass valve (5). The heated steam enters the secondary reheater cold section (E) from the outlet of the boiler primary reheater (B) through the medium-pressure bypass valve (14). The steam from the secondary reheater cold section (E) passes through the secondary reheater cold section to the auxiliary steam header regulating valve (42) and enters the auxiliary steam header (F). Then, the auxiliary steam is used to pass through the shaft seal manual gate valve (31) and the auxiliary steam to the auxiliary steam header regulating valve (42). The shaft seal electric valve (32) supplies steam to the main engine shaft seal (K) first; on the other hand, the steam supplied to the auxiliary steam header F by the secondary reheater cold section (E) passes through the auxiliary steam to induced draft fan small unit electric valve (33) to maintain the steam-driven induced draft fan small unit (I) running at 3000r / min without tripping, and passes through the auxiliary steam to feed water pump small unit electric valve (34) to enter the steam-driven feed water pump small unit (H) to maintain the steam-driven feed water pump small unit (H) running at 3000r / min without tripping. When this process occurs, the low-pressure bypass valve (27) remains fully closed.

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