Boiler system cleaning method and boiler system

Abstract

To provide a cleaning method of a boiler system which can shorten startup time, and removes powder scale by suppressing the amount of makeup water to be used and the amount of drainage, and to provide the boiler system.SOLUTION: In a cleaning method of a boiler system 1 which includes a main system where a boiler 2 and a condenser 5 are connected, based on the relation between a temperature of boiler water at a boiler outlet and a water supply flow rate to the boiler, a water supply condition in which water hammer does not occur is acquired beforehand; after the boiler is stopped, the temperature of the boiler water at the boiler outlet is measured; and the boiler water is circulated in the main system to cool it forcibly. At the same time, according to the measured temperature of the boiler water, within the range that satisfies the water supply condition, the water supply flow rate to the boiler is increased, and at the flow rate after the increase, the boiler water is circulated in the main system for predetermined time.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to a method for cleaning a boiler system and a boiler system. [Background technology]

[0002] In boiler plants where oxygen treatment is used as an anti-corrosion treatment, the phenomenon of hematite scale adhering and accumulating on the inner surface of the boiler's heat transfer tubes is observed. Hematite (Fe2O3) scale is called powder scale because it has a porous structure. The adhesion and accumulation of powder scale can impede heat transfer in the heat transfer tubes and cause the temperature of the heat transfer tube metal to rise.

[0003] Methods for removing powder scale that has adhered to and accumulated on the inner surface of a heat transfer tube include a cleaning method using chemicals (Patent Document 1) and a cleaning method using water.

[0004] Cleaning methods using chemicals can sufficiently remove powder scale, but cleaning using chemicals requires not only the duration of the cleaning work, but also construction costs and time for the preparation and removal of the chemicals and cleaning equipment.

[0005] Conventionally, water washing is performed when a plant is started up. For example, a boiler plant is shut down for repairs or regular inspection of the boiler body. After the plant is shut down, it goes through regular inspection, water filling blowing, water pressure testing, and cleanup of the water and steam system before it is started up. During water washing, water is repeatedly fed into the plant system and drained out of the system until the powder scale contained in the water in the plant system falls below a specified value during water filling blowing and cleanup.

[0006] Water-filled blowing is a process carried out before the water pressure test. The target areas of water-filled blowing are the economizer, furnace wall tube (evaporator tube), superheater and reheater.

[0007] Cleaning up is the process of cleaning the condensate and feedwater systems, feedwater heaters and condensate demineralizers before the boiler starts up, and removing impurities such as powder scale. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2016-160457 A Summary of the Invention [Problem to be solved by the invention]

[0009] It is difficult to thoroughly remove powder scale by water-filled blowing and cleanup at the start-up of a plant.

[0010] If there is a lot of scale when the plant is started up, the amount of makeup water used and the amount of wastewater increased during water filling blow and cleanup, and cleanup took longer.

[0011] The scale discharged from the boiler passes through a condensate demineralizer before being discharged outside the system. The condensate demineralizer contains ion exchange resin. When scale adheres to the ion exchange resin, the load increases, leading to an increase in differential pressure, and the time required for resin regeneration during cleanup increases.

[0012] Increasing the frequency of water blowing can expedite the removal of scale. However, doing so also increases the time required, the amount of make-up water used, and the amount of drainage.

[0013] The present disclosure has been made in consideration of the above circumstances, and aims to provide a boiler system cleaning method and a boiler system that can shorten the start-up time and remove powder scale while reducing the amounts of make-up water and drainage used. [Means for solving the problem]

[0014] In order to solve the above problems, the boiler system cleaning method and boiler system disclosed herein employ the following measures.

[0015] The present disclosure provides a cleaning method for a boiler system having a main system to which a boiler and a condenser are connected, which includes obtaining in advance water supply conditions that do not cause water hammer based on the relationship between the temperature of the boiler water at the boiler outlet and the water supply flow rate to the boiler, measuring the temperature of the boiler water at the boiler outlet after stopping the boiler, circulating the boiler water within the main system to forcibly cool it, and increasing the water supply flow rate to the boiler within a range that satisfies the water supply conditions according to the measured boiler water temperature, and circulating the boiler water within the main system at the increased flow rate for a predetermined period of time.

[0016] The present disclosure provides a system including a main system to which a boiler and a condenser are connected, a flow rate adjustment unit provided in the main system and adjusting a feedwater flow rate to the boiler, a temperature measuring device connected to an outlet of the boiler, and a control unit electrically connected to the temperature measuring device and the flow rate adjustment unit and controlling the feedwater flow rate at the flow rate adjustment unit based on a measurement result of the temperature measuring device, the control unit including a storage unit storing feedwater conditions that do not cause water hammer based on a relationship between a boiler water temperature at the boiler outlet and a feedwater flow rate to the boiler, a receiving unit receiving the measurement result of the temperature measuring device, a flow rate determination unit that compares the received measurement result with the feedwater conditions stored in the storage unit to determine a feedwater flow rate that satisfies the feedwater conditions, and a transmission unit that transmits information of the determined feedwater flow rate to the flow rate adjustment unit and controls the adjustment of the feedwater flow rate at the flow rate adjustment unit. After the boiler is stopped, the boiler water is forcedly cooled by circulating it in the main system, and the feed water flow rate of the boiler water is increased to the feed water flow rate determined by the flow rate determination unit. Provide a boiler system. Effect of the Invention

[0017] According to the boiler system cleaning method and boiler system of the present disclosure, the start-up time at the time of restart can be shortened.

[0018] According to the boiler system cleaning method and boiler system disclosed herein, powder scale can be removed while reducing the amount of makeup water and drainage water used. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a diagram illustrating a protective coating formed on a surface to be protected from corrosion. [Diagram 2] 1 is a schematic configuration diagram of a boiler system according to a first embodiment. [Diagram 3] FIG. 2 is a schematic diagram of a control unit. [Figure 4] 4 is an image diagram of a drop in boiler water temperature and a change in feed water flow rate during forced cooling in the first embodiment. FIG. [Diagram 5] FIG. 13 is a diagram illustrating the results of a preliminary test. [Figure 6] FIG. 1 is a diagram showing the procedure from shutting down to restarting a boiler system. [Figure 7] 13 is an image diagram of a drop in boiler water temperature and a change in feed water flow rate during forced cooling in the second embodiment. FIG. [Figure 8] FIG. 11 is a schematic configuration diagram of a boiler system according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of a boiler system cleaning method and a boiler system according to the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to this embodiment. When there are multiple embodiments, the present disclosure also includes a configuration in which each embodiment is combined. In the following description, up and above refer to the upper side in the vertical direction, and down and below refer to the lower side in the vertical direction, and the vertical direction is not precise and may include an error.

[0021] The boiler system according to the present disclosure can be applied to a thermal power plant, etc. The thermal power plant includes a boiler that generates high-temperature and high-pressure steam, a steam turbine that is rotationally driven by the steam generated in the boiler, and a generator that generates electricity by utilizing the force of rotation of the steam turbine (not shown).

[0022] In a boiler system, measures are taken to prevent corrosion of equipment that comes into contact with feed water for generating steam. In this embodiment, a case in which oxygen treatment is applied as the corrosion prevention measure will be described. Surfaces to be protected from corrosion are, for example, the inner surfaces of heat transfer tubes and heat transfer tube headers that are arranged in large numbers in a boiler. The objects to be protected from corrosion, such as heat transfer tubes, are members made of metal.

[0023] Oxygen treatment requires high water purity for the feed water, so boiler systems that use oxygen treatment generally include a condensate demineralizer.

[0024] In oxygen treatment, a small amount of oxygen is injected into feed water whose pH has been adjusted with ammonia, forming a protective film of magnetite (Fe3O4) on the surface of the object to be protected from corrosion. Figure 1 shows a cross-sectional view of the surface of an object to be protected from corrosion on which a protective film has been formed. On the surface of the object to be protected from corrosion M that has been oxygen-treated, a dense protective film (magnetite scale) S, which has been generated by the self-oxidation of the object to be protected from corrosion, and a porous powder scale (hematite scale) P have been formed.

[0025] Foreign matter that adheres to and accumulates on boiler heat transfer tubes can increase tube pressure loss and impede heat transfer, so it must be periodically cleaned and discharged outside the system. In boiler systems that use oxygen treatment, powder scale P adheres and accumulates on top of the protective coating during operation. Hematite powder scale P has an extremely high thermal resistance and has a significant impact on plant performance, so it must be periodically cleaned and discharged outside the system.

[0026] [First embodiment] (Boiler System) First, the boiler system according to this embodiment will be described. Fig. 2 shows a schematic configuration diagram of the boiler system.

[0027] The boiler system 1 includes a boiler (evaporator tube) 2, a steam separator 3, a steam separator drain tank 4, a condenser 5, a condensate demineralizer 6, a low-pressure feedwater heater 7A, a deaerator 8, a high-pressure feedwater heater 7B, a coal economizer 9, a temperature measuring instrument 10 and a flow measuring instrument 11.

[0028] The components of the boiler system 1 are connected by piping, and are arranged to circulate fluids such as steam, condensate C (C1 to C3), and feedwater F (F1, F2, F3) through the piping.

[0029] In this embodiment, the circuit in which the boiler 2, steam separator 3, steam separator drain tank 4, condenser 5, condensate demineralizer 6, low-pressure feedwater heater 7A, deaerator 8, high-pressure feedwater heater 7B and economizer 9 are connected in sequence is referred to as the main system.

[0030] In this embodiment, the section from the economizer 9 to the boiler 2 is referred to as a boiler inlet.

[0031] In this embodiment, the area between the boiler 2 and the steam separator 3 is called the boiler outlet. The boiler outlet is the location where the temperature of the boiler water circulating during forced cooling is the highest. The boiler outlet may be called the steam separator inlet or the furnace rear wall outlet, etc., depending on the system configuration of the furnace wall tube.

[0032] The boiler 2 evaporates the feed water F1 by exchanging heat between the heat generated by burning fuel and the feed water F1 flowing inside an evaporation tube (not shown) constituting the furnace wall of the boiler 2. As a result, steam B is generated inside the evaporation tube. The fuel may be coal, biomass fuel, petroleum coke, petroleum residue, heavy oil, natural gas, by-product gas, or a combination thereof.

[0033] The generated steam B is supplied to the steam separator 3 via the boiler outlet piping 21. The steam separator 3 separates water (drain) D from the steam B. The steam from which the drain has been separated is discharged from the steam separator 3 and guided to a steam turbine (not shown) that drives a generator.

[0034] A temperature measuring instrument 10 is connected to the boiler outlet pipe 21. The temperature measuring instrument 10 can measure the temperature of the fluid flowing through the boiler outlet pipe 21.

[0035] The water D separated in the steam separator 3 is guided to the steam separator drain tank 4 via a first drain pipe 22. The steam separator drain tank 4 stores the water D discharged from the steam separator 3.

[0036] The upstream end of the boiler circulation pipe 23 is connected to the steam separator drain tank 4. The downstream end of the boiler circulation pipe 23 is connected to a midway point of the second feedwater pipe 24. The boiler circulation pipe 23 is provided with a boiler circulation pump (BCP) 25 and a boiler recirculation flow control (BR) valve 26. The water D discharged from the steam separator drain tank 4 is guided to the second feedwater pipe 24 via the boiler circulation pipe 23. The water D guided to the second feedwater pipe 24 can be supplied to the economizer 9 as feedwater. The boiler circulation pump 25 may not be installed. Also, instead of the boiler recirculation flow control valve 26, the discharge flow rate may be changed by adjusting the rotation speed of the boiler circulation pump 25.

[0037] A second drain pipe 27 branches off from a midway position of the boiler circulation pipe 23. The second drain pipe 27 branches off from the boiler circulation pipe 23 upstream of the boiler circulation pump 25. The downstream end of the second drain pipe 27 is connected to the condenser 5. The second drain pipe 27 guides the water D discharged from the steam separator drain tank 4 to the condenser 5. A steam separator drain control (WDC) valve 28 is provided on the second drain pipe 27.

[0038] In the condenser 5, the steam after doing work in the steam turbine is cooled and condensed by cooling water (e.g., seawater). Water produced by the condensation and water D guided from the second drain pipe 27 are discharged from the condenser 5 as condensate C1.

[0039] The upstream end of a first condensate pipe 29 is connected to the condenser 5. The downstream end of the first condensate pipe 29 is connected to the condensate demineralizer 6. A condensate pump (CP) 30 is provided in the first condensate pipe 29. The condensate C1 discharged from the condenser 5 is guided to the condensate demineralizer 6 via the first condensate pipe 29.

[0040] In the condensate demineralizer 6, the condensate C1 passes through a resin layer such as an ion exchange resin. As a result, impurities are removed from the condensate C1 to produce condensate C2. The impurities include sodium ions. Sodium ions can cause scale formation and corrosion in the main system of the boiler system 1.

[0041] The upstream end of a second condensate piping 31 is connected to the condensate demineralizer 6. The downstream end of the second condensate piping 31 is connected to a low pressure feedwater heater 7A. A condensate booster pump (CBP) 32 is provided on the second condensate piping 31. The condensate C2 discharged from the condensate demineralizer 6 is guided via the second condensate piping 31 to the low pressure feedwater heater 7A.

[0042] A low-pressure extraction pipe (not shown) that extracts steam from the steam turbine is connected to the low-pressure feed water heater 7A. The low-pressure feed water heater 7A heats the condensate C2 with the steam extracted from the steam turbine.

[0043] The upstream end of a third condensate pipe 33 is connected to the low pressure feed water heater 7A. The downstream end of the third condensate pipe 33 is connected to the deaerator 8. The condensate C3 discharged from the low pressure feed water heater 7A is guided to the deaerator 8 via the third condensate pipe 33.

[0044] The deaerator 8 heats and deaerates the condensate C3. The upstream end of the first feedwater piping 34 is connected to the deaerator 8. The downstream end of the first feedwater piping 34 is connected to the high-pressure feedwater heater 7B. The first feedwater piping 34 is provided with a boiler feedwater pump (BFP) 35 and a feedwater flow rate control valve 36. The condensate C3 heated and deaerated by the deaerator 8 is guided as feedwater F3 to the high-pressure feedwater heater 7B via the first feedwater piping 34. Instead of the feedwater flow rate control valve 36, the discharge flow rate may be changed by adjusting the rotation speed of the boiler feedwater pump 35.

[0045] The high-pressure feedwater heater 7B heats the feedwater F3 with steam extracted from a high-pressure steam turbine (not shown) to produce feedwater F2. The upstream end of a second feedwater piping 24 is connected to the high-pressure feedwater heater 7B. The downstream end of the second feedwater piping 24 is connected to the economizer 9. The feedwater F2 discharged from the high-pressure feedwater heater 7B is guided to the economizer 9 via the second feedwater piping 24.

[0046] The second supply water pipe 24 is provided with a flow rate meter 11. The flow rate meter 11 can measure the flow rate of the supply water F2 flowing through the second supply water pipe 24.

[0047] In the economizer 9, the feedwater F2 is heated by heat exchange with the fuel combustion gas in the boiler 2 to produce feedwater F1. The upstream end of a third feedwater pipe 37 is connected to the economizer 9. The downstream end of the third feedwater pipe 37 is connected to the boiler 2. The feedwater F1 heated in the economizer 9 is introduced into the evaporator tube of the boiler 2 via the third feedwater pipe 37.

[0048] The boiler system 1 may include a control unit 12.

[0049] The control unit 12 is electrically connected to the temperature measuring instrument 10, the flow rate measuring instrument 11, the boiler circulation pump 25, the boiler recirculation flow rate control valve 26, the steam separator drain control valve 28, the condensate pump 30, the condensate booster pump 32, the boiler feedwater pump 35, and the feedwater flow rate control valve 36. The flow rate measuring instrument 11, the boiler recirculation flow rate control valve 26, the steam separator drain control valve 28, and the feedwater flow rate control valve 36 function as a flow rate adjustment unit. The control unit 12 controls the feedwater flow rate in the flow rate adjustment unit based on the measurement result of the temperature measuring instrument 10.

[0050] The control unit 12 is composed of, for example, a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and a computer-readable storage medium. A series of processes for realizing various functions is stored in a storage medium or the like in the form of a program, for example, and the CPU reads the program into the RAM or the like and executes information processing and arithmetic processing to realize various functions. The program may be installed in a ROM or other storage medium in advance, may be provided in a state stored in a computer-readable storage medium, or may be distributed via a wired or wireless communication means. The computer-readable storage medium may be a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, a semiconductor memory, or the like.

[0051] 3 shows an outline of the control unit 12. The control unit 12 has a storage unit 12-1, a receiving unit 12-2, a flow rate determining unit 12-3, and a transmitting unit 12-4.

[0052] Storage section 12-1 stores water supply conditions under which no water hammer occurs, which are based on the relationship between the temperature of the boiler water at the boiler outlet and the flow rate of the water supply at the boiler inlet.

[0053] The receiving unit 12-2 receives the measurement result of the temperature measuring device 10.

[0054] The flow rate determination unit 12-3 checks the received measurement results against the water supply conditions stored in the storage unit 12-1 and determines a water supply flow rate that satisfies the water supply conditions that do not cause water hammer.

[0055] The transmitting unit 12-4 transmits information about the determined feedwater flow rate to the flow rate adjustment unit and controls the adjustment of the feedwater flow rate in the flow rate adjustment unit. Specifically, the flow rate adjustment unit is composed of a flow meter 11 and a feedwater flow rate adjustment valve 36, and the opening of the feedwater flow rate adjustment valve 36 is adjusted so that the flow rate measured by the flow meter 11 becomes the determined feedwater flow rate. The flow rate adjustment unit can adjust the opening of the boiler recirculation flow rate adjustment valve 26 and the steam separator drain adjustment valve 28 so that the flow rate measured by the flow meter 11 becomes the determined feedwater flow rate.

[0056] The control unit 12 may be configured to control the opening and closing of each valve other than the temperature measuring instrument 10, the flow measuring instrument 11, and the water supply flow rate adjustment valve 36, and the starting and stopping of each pump.

[0057] Ammonia and oxygen for oxygen treatment are injected from an oxygen injection unit (not shown) into the boiler system 1. The ammonia and oxygen can be injected, for example, into the second condensate piping 31 between the condensate demineralizer 6 and the low-pressure feedwater heater 7A.

[0058] (How to clean a boiler system) Next, a method for cleaning a boiler system according to this embodiment will be described.

[0059] The method for cleaning the boiler system 1 includes a step of forcibly cooling the boiler 2 by circulating boiler water in the main system after the boiler is stopped.

[0060] Forced cooling can be performed by continuing to operate the condensate pump 30, the condensate booster pump 32, and the boiler feedwater pump 35 while stopping the combustion of fuel in the boiler 2. In forced cooling, the boiler recirculation flow control valve 26 is closed, and the steam separator drain control valve 28 and the feedwater flow control valve 36 are opened. By adjusting the opening of the feedwater flow control valve 36, the flow rate of the feedwater F2 supplied to the boiler 2 can be controlled.

[0061] Feedwater F sent by the boiler feedwater pump 35 enters the evaporator tubes of the boiler 2 via the economizer 9, and cools the evaporator tubes of the boiler 2 by receiving the heat retained in the evaporator tubes of the boiler 2. The boiler water discharged from the boiler 2 passes through the steam separator 3, steam separator drain tank 4, condenser 5, condensate demineralizer 6, low-pressure feedwater heater 7A, deaerator 8, and high-pressure feedwater heater 7B, and is sent again to the economizer 9 as feedwater. During forced cooling, heating is not performed in the low-pressure feedwater heater 7A, deaerator 8, and high-pressure feedwater heater 7B. The boiler water releases heat while flowing through the main system and is cooled. By circulating the water in the main system, the temperature of the evaporator tubes of the boiler 2 gradually decreases.

[0062] Fig. 4 is an image diagram of the drop in boiler water temperature at the boiler outlet and the change in feed water flow rate during forced cooling. In the figure, the horizontal axis is time, the vertical axis of the upper graph is the feed water flow rate, and the vertical axis of the lower graph is the boiler water temperature (°C) at the boiler outlet. In the figure, Example 1 (dashed line) relates to forced cooling of this embodiment, and Comparative Example 1 (solid line) relates to forced cooling based on the conventional method. In forced cooling based on the conventional method, the feed water flow rate is not increased during forced cooling.

[0063] In the method for cleaning the boiler system 1 according to this embodiment, after the boiler 2 is stopped, boiler water is circulated in the main system to forcibly cool the boiler 2, and the feedwater flow rate to the boiler 2 is gradually increased within a range that satisfies the feedwater condition that does not cause water hammer (see (1) in FIG. 4). After the feedwater flow rate reaches a predetermined flow rate, the feedwater flow rate is maintained and circulation is continued for a predetermined time (see (2) in FIG. 4).

[0064] The predetermined time is the time for the boiler water to make at least one circuit, preferably two circuits, in the main system. The time for the boiler water to make at least one circuit in the main system can be calculated from the amount of water held in the main system / the circulating flow rate.

[0065] After the specified time of circulation has ended, the feedwater flow rate is reduced to the initial value at the start of the increase (see (3) in Figure 4). Thereafter, circulation is continued while maintaining the feedwater flow rate, and cooling is continued at least until the temperature at which forced cooling is completed is reached (see (4) in Figure 4). Thereafter, the feedwater flow rate is reduced and circulation is stopped. The temperature at which forced cooling is completed is, for example, 100°C.

[0066] The water supply flow rate is increased within a range that satisfies the water supply conditions that do not cause water hammer. The specified flow rate after the increase should be set to as high a value as possible while still satisfying the water supply conditions. This makes it possible to promote the discharge of powder scale without increasing the forced cooling time.

[0067] The water supply conditions under which no water hammer occurs are obtained in advance through preliminary tests, etc. For example, in the preliminary test, boiler water is circulated in the main system, and the boiler water temperature at the boiler outlet and the feed water flow rate to boiler 2 are measured to confirm whether or not water hammer occurs.

[0068] Figure 5 shows the results of the preliminary test. The feedwater flow rate decreases in the order of Q1>Q2>Q3. The boiler water temperature decreases in the order of T1>T2>T3. According to Figure 5, at the highest feedwater flow rate Q1, water hammer occurs even if the boiler water temperature is reduced to the lowest T3. According to Figure 5, at boiler water temperatures T1 and T2, water hammer occurs even if the feedwater flow rate is reduced to Q3. According to Figure 5, if the boiler water temperature is below T3 and the feedwater flow rate is below Q2, water hammer does not occur.

[0069] In the above preliminary test, the water supply conditions under which no water hammer occurs can be obtained based on the relationship between the boiler water temperature at the boiler outlet and the water supply flow rate to the boiler 2.

[0070] According to the results of the above preliminary test, if the temperature of the boiler water at the boiler outlet is lowered to T3, the feedwater flow rate can be increased up to a maximum of Q2.

[0071] After boiler 2 is shut down, the boiler water temperature at the boiler outlet is measured, and the feedwater flow rate is increased according to the measured value within the range that satisfies the feedwater conditions, making it possible to supply water at a high flow rate close to the upper limit that does not cause water hammer.

[0072] The increase in the feedwater flow rate should be set so that the boiler water temperature drop rate is 220°C / hr or less. This makes it possible to prevent the boiler's lifespan from being shortened due to repeated thermal stress caused by sudden temperature changes, and also more reliably prevents the occurrence of water hammer caused by an increase in flow rate.

[0073] In the conventional method, the feedwater flow rate at the start of forced cooling is usually set within a range where water hammer does not occur. In the present embodiment, when the boiler water temperature at the boiler outlet is higher than T3, the feedwater flow rate at the start of forced cooling is the feedwater flow rate at the time of forced cooling that has been conventionally set in the boiler system actually used, and then, when the boiler water temperature reaches T3, the feedwater flow rate starts to increase.

[0074] If the boiler water temperature at the boiler outlet when forced cooling starts is T3 and the feed water flow rate previously set is smaller than Q3, the feed water flow rate at the start of forced cooling may be increased within the range of Q2 or less.

[0075] In circulating the boiler water, the circulating flow rate may be varied to promote the discharge of powder scale.

[0076] 6 shows the procedure from shutdown to restart of the boiler system 1. In the figure, "BEFORE" indicates the conventional procedure, and "AFTER" indicates the procedure including the cleaning method according to the above embodiment.

[0077] Both the "BEFORE" and "AFTER" procedures include boiler shutdown, forced cooling, water drainage, regular inspection (routine inspection), water filling and blowing, water pressure testing (water pressure), boiler cold cleanup (BCCU), boiler hot cleanup (BHCU), and boiler startup (operation to increase the pressure and flow rate of the generated steam to the specified values).

[0078] The conventional procedure does not include an operation of increasing the feedwater flow rate during forced cooling. In the conventional procedure, powder scale removal is performed during water filling blow-off after periodic inspection, boiler cold clean-up, and boiler hot clean-up.

[0079] In the procedure including the cleaning method according to the above embodiment, the powder scale removal process, which has conventionally been performed during water filling and blowing after regular inspection and during cleanup at startup, is performed during forced cooling after the boiler is stopped. Powder scale removal can be achieved by increasing the flow rate of water supplied to the boiler 2 during forced cooling. According to the boiler system cleaning method according to this embodiment, the time required for water filling and blowing after regular inspection and cleanup at startup can be shortened.

[0080] In the above embodiment, the increase in the feedwater flow rate is set according to the boiler water temperature at the boiler outlet within the range of feedwater conditions where the water hammer does not occur. Therefore, the feedwater flow rate may be increased up to the limit where the water hammer does not occur.

[0081] Conventional procedures include forced cooling, but it is difficult to sufficiently remove powder scale with forced cooling in conventional methods that do not include a step of increasing the feed water flow rate according to the boiler water temperature.

[0082] Second Embodiment This embodiment differs from the first embodiment in that it includes a step of circulating the boiler water in a recirculation system before circulating the boiler water in the main system, and supplying the water from the recirculation system to the boiler.

[0083] The recirculation system is a circuit that sequentially connects the boiler 2, the steam separator 3, the steam separator drain tank 4, and the coal economizer 9. The recirculation system does not include the condenser 5, the condensate demineralizer 6, the low-pressure feedwater heater 7A, the deaerator 8, and the high-pressure feedwater heater 7B.

[0084] The boiler water circulating through the recirculation system is circulated directly to the boiler 2 without passing through the condenser 5 and the condensate demineralizer 6 and the like.

[0085] In this embodiment, first, the boiler 2 is stopped, and then the condensate pump 30, the condensate booster pump 32, and the boiler feedwater pump 35 are stopped, and the boiler circulation pump (BCP) 25 is operated. The boiler recirculation flow control valve 26 is opened, the feedwater flow control valve 36 and the steam separator drain control valve 28 are closed, and the drain D of the steam separator 3 is supplied to the boiler 2.

[0086] The boiler water temperature at the boiler outlet of the recirculation system is measured, and after it is confirmed that the measured value has reached a predetermined temperature at which water can be fed to the boiler 2, the boiler recirculation flow control (BR) valve 26 is opened and the boiler circulation pump (BCP) 25 is operated to gradually increase the feedwater flow rate. The feedwater flow rate from the recirculation system can be controlled by adjusting the opening of the boiler recirculation flow control valve 26. The feedwater sent by the boiler circulation pump 25 enters the evaporator tubes of the boiler 2 via the coal economizer 9.

[0087] While boiler water is circulating to the recirculation system, the feedwater flow rate from the main system is gradually increased by opening the feedwater flow control valve 36 and the steam separator drain control valve 28 and starting the condensate pump 30, condensate booster pump 32, and boiler feedwater pump 35. The feedwater flow rate from the main system can be controlled by adjusting the opening of the feedwater flow control valve 36.

[0088] In the initial stage of forced cooling, the boiler water circulating in the recirculation system is at a higher temperature than the boiler water circulating in the main system. Therefore, by using water circulating in the recirculation system, the feed water flow rate can be increased compared to conventional methods without causing water hammer in the initial stage of forced cooling.

[0089] Fig. 7 is an image diagram of the drop in boiler water temperature at the boiler outlet and the change in feed water flow rate during forced cooling according to this embodiment. In this figure, the horizontal axis is time, the vertical axis of the upper graph is the feed water flow rate, and the vertical axis of the lower graph is the boiler water temperature (°C) at the boiler outlet. In this figure, Example 2 (dashed line, one-dot chain line, two-dot chain line) relates to forced cooling according to this embodiment, and Comparative Example 2 (solid line) relates to forced cooling based on the conventional method. In forced cooling based on the conventional method, the feed water flow rate is not increased during forced cooling.

[0090] In the boiler system cleaning method according to this embodiment, after the boiler 2 is stopped, the boiler water temperature at the boiler outlet is measured and it is confirmed that the measured value has reached a predetermined temperature. Then, the boiler water is circulated in the recirculation system, and the feedwater flow rate from the recirculation system is increased within a range that satisfies the feedwater condition that does not cause water hammer (see (1) in FIG. 7). The feedwater condition that does not cause water hammer is obtained in advance by a preliminary test or the like.

[0091] The increase in the feedwater flow rate should be set so that the boiler water temperature drop rate is 220°C / hr or less. This makes it possible to prevent the boiler's lifespan from being shortened due to repeated thermal stress caused by sudden temperature changes, and also more reliably prevents the occurrence of water hammer caused by an increase in flow rate.

[0092] After the boiler water temperature at the boiler outlet reaches a predetermined value, water supply from the main system is started and the feedwater flow rate is gradually increased, as in the first embodiment (see (2) in FIG. 7). By measuring the boiler water temperature at the boiler outlet and increasing the feedwater flow rate according to the measured value, the feedwater flow rate can be set to a value higher than that previously set.

[0093] At this time, the water supply from the recirculation system continues with the flow rate maintained at the time when the water supply from the main system started. This also increases the total flow rate of water passing through the boiler 2. The duration should be set by calculating backwards from the time required to reach the cooling completion temperature. In FIG. 7, when the water supply flow rate from the main system is reduced to the previous value, the water supply flow rate from the recirculation system starts to be reduced, and finally circulation in the recirculation system is stopped.

[0094] After the feedwater flow rate from the main system reaches a predetermined flow rate, that feedwater flow rate is maintained and circulation is continued for a predetermined time (see (3) in Figure 7).

[0095] The predetermined time is the time for the boiler water to make at least one circuit, preferably two circuits, in the main system. The time for the boiler water to make at least one circuit in the main system can be calculated from the amount of water held in the main system / the circulating flow rate.

[0096] After the specified time of circulation is completed, the feedwater flow rate from the main system is reduced to the previous value (see (4) in Figure 7). The previous value is the feedwater flow rate that was previously set for that boiler system.

[0097] After that, the water supply from the main system continues with the same flow rate as before, and is cooled until the forced cooling completion temperature is reached (see (5) and (6) in Figure 7). After the forced cooling completion temperature is reached, the flow rate of the water supply from the main system is reduced, and finally circulation in the main system is stopped.

[0098] The boiler water circulating through the recirculation system has a relatively higher temperature than the boiler water circulating through the main system. According to the above embodiment, by supplying water from the recirculation system, the supply water flow rate can be set higher than in the past at the initial stage of forced cooling. As a result, the occurrence of water hammer can be avoided while the powder scale removal efficiency can be increased.

[0099] While water is being supplied from the recirculation system, the boiler 2 is gradually cooled. After the boiler water temperature at the boiler outlet reaches a specified temperature, water supply from the main system is started, which further increases the water supply flow rate and improves the efficiency of powder scale removal. Also, increasing the water supply flow rate from the main system, which has a relatively low temperature, improves the cooling efficiency of the boiler 2.

[0100] The circulation time using the recirculation system can be set appropriately taking into consideration the time required to reach the forced cooling completion temperature, thereby increasing the efficiency of powder scale removal without excessively extending the forced cooling time.

[0101] Third Embodiment This embodiment differs from the first and second embodiments in that it includes a step of injecting gas into the water supplied to the boiler 2 between the inlet of the boiler 2 and the economizer 9.

[0102] A schematic configuration diagram of a boiler system according to this embodiment is shown in Fig. 8. The boiler system 100 includes a gas injection unit 101 connected to a third feedwater pipe 37 that connects the economizer 9 and the boiler 2. The other configurations are the same as those of the first embodiment.

[0103] The gas injection unit 101 injects gas into the feed water F1 supplied from the economizer 9 to the boiler 2. The method of injecting the gas is not particularly limited, but examples thereof include a pressure injection method and an ejector method.

[0104] The gas to be injected may be any gas that is unlikely to cause corrosion of the components (pipes, evaporation tubes, etc.) that constitute the boiler system 100. The gas to be injected may be, for example, an inert gas. Examples of the inert gas include nitrogen gas and carbon dioxide gas. When an inert gas is used, corrosion of the pipes, evaporation tubes, etc. during forced cooling can be suppressed.

[0105] The amount of gas injected is set according to the water supply flow rate, the size and shape of the evaporation tube, etc., so that a water film is not formed on the inner surface of the evaporation tube where the powder scale is attached, and the cleaning effect is not reduced. The amount of gas injected should be set so that the gas is 20 volume % or less in the water supply after mixing with the gas.

[0106] Tests have confirmed that the powder scale removal rate can be increased by approximately four times when 10 volume percent nitrogen gas is injected, and by more than seven times when 20 volume percent nitrogen gas is injected, compared to when no gas is injected.

[0107] Injecting gas increases the fluid force of the water supply, making it easier to peel off and remove powder scale.

[0108] <Additional Notes> The boiler system cleaning method and the boiler system described in the above-described embodiment can be understood, for example, as follows.

[0109] The present disclosure provides a cleaning method for a boiler system having a main system to which a boiler (2) and a condenser (5) are connected, the method comprising: obtaining in advance water supply conditions that do not cause water hammer based on the relationship between the temperature of boiler water at a boiler outlet and the flow rate of water supply to the boiler; after stopping the boiler, measuring the temperature of the boiler water at the boiler outlet; circulating the boiler water within the main system to forcibly cool it; and increasing the flow rate of water supply to the boiler within a range that satisfies the water supply conditions according to the measured boiler water temperature; and circulating the boiler water within the main system for a predetermined period of time at the increased flow rate.

[0110] Increasing the flow rate of water fed to the boiler promotes the removal of foreign matter that has adhered and accumulated on the inner surface of the heat transfer tubes. In particular, the powder scale that occurs on the heat transfer tubes of boilers to which oxygen treatment has been applied is made up of particles of about 1 μm in size that have a relatively low adhesion and can be easily removed by liquid flow. Increasing the flow rate of water fed to the boiler promotes the removal of powder scale.

[0111] Normally, there is hard scale underneath the powder scale. Hard scale is a dense scale (magnetite scale) that is produced by the self-oxidation of the base material and has high thermal conductivity. Hard scale has the property of growing when heated. Powder scale has a lower thermal conductivity than hard scale. If even a small amount of powder scale, which has low thermal conductivity, adheres to hard scale, the temperature of the base material rises, causing the hard scale to grow. For this reason, removing the powder scale can slow down the growth of hard scale. This makes it possible to extend the interval between cleanings using chemicals.

[0112] The temperature of the boiler water at the boiler outlet is measured, and the feed water flow rate is changed according to the measurement result. The temperature inside the boiler immediately after the boiler is stopped is high. On the other hand, the temperature of the feed water supplied to the boiler is lower than the temperature inside the boiler. If the feed water flow rate is increased when the temperature difference between the boiler water at the boiler outlet and the feed water at the boiler inlet is large, there is a high risk of water hammer occurring. If the temperature of the boiler water at the boiler outlet decreases, the temperature difference decreases, and the upper limit of the feed water flow rate at which water hammer does not occur also increases. In the above disclosure, the boiler water temperature at the boiler outlet is grasped by measurement, and the flow rate of the main feed water is increased according to the boiler water temperature, so that the feed water flow rate can be increased to a value that was not anticipated in the conventional method. By increasing the feed water flow rate and circulating it as boiler water for a predetermined time, the removal of powder scale can be promoted and forced cooling of the boiler can also be promoted.

[0113] By removing powder scale along with forced cooling after the boiler is shut down, the cleanup time can be shortened when the boiler system is restarted.

[0114] In a boiler system equipped with a condensate demineralizer, an operation to regenerate the resin in the condensate demineralizer is usually performed after the boiler is shut down and before it is restarted. Forced cooling is a process that precedes the regeneration operation. According to the above disclosure, since powder scale is removed during forced cooling after the boiler is shut down, even if powder scale discharged from the boiler adheres to the resin in the condensate demineralizer as it circulates in the main system, it can be removed in a later regeneration operation. Therefore, there is no concern that the cleanup at startup will take a long time.

[0115] As a shutdown procedure for a boiler system, forced cooling after the boiler is shut down has been conventionally performed. In the above disclosure, the powder scale is removed by utilizing the circulation of boiler water during forced cooling, so that the amount of make-up water and the amount of wastewater during cleanup can be reduced compared to the conventional method.

[0116] In one aspect of the above disclosure, the boiler system is provided with a recirculation system that circulates boiler water discharged from the boiler outlet directly to the boiler without passing through the condenser, and the boiler water is circulated within the recirculation system, and the water supply flow rate to the boiler is increased within a range that satisfies the water supply conditions depending on the measured temperature of the boiler water, and then circulation to the main system is started.

[0117] In a recirculation system, boiler water discharged from the boiler outlet returns directly to the boiler inlet without passing through a condenser or other device. Therefore, the feedwater supplied to the boiler through the recirculation system is at a higher temperature than the feedwater supplied from the main system. By supplying relatively hotter feedwater to the boiler first, it is possible to increase the feedwater flow rate at an early stage while avoiding the occurrence of water hammer. This promotes the removal of powder scale from the early stages of forced cooling.

[0118] In one embodiment of the disclosure, feed water that has passed through a recirculation system is supplied to the boiler, and then feed water that has passed through a main system, which has a relatively low temperature, is supplied to the boiler, thereby enabling forced cooling of the boiler while minimizing the decrease in cooling efficiency caused by using water at a relatively high temperature.

[0119] In one aspect of the disclosure above, it is preferable to continue supplying water from the recirculation system while also supplying water from the main system.

[0120] By continuing to feed water from the recirculation system, the flow rate of fluid passing through the boiler becomes greater than the flow rate of water fed from the main system, which increases the fluid force and improves the efficiency of powder scale removal.

[0121] In one embodiment of the disclosure above, the boiler system may have an economizer (9) connected to an inlet side of the boiler, and gas may be injected into the feedwater between the economizer and the inlet of the boiler in the circulation of the boiler water.

[0122] The gas injected into the feedwater between the economizer and the boiler inlet is supplied into the boiler together with the feedwater. As a result, the gas-liquid two-phase gas-mixed water flows inside the boiler. When the gas-mixed water flows, a pressure increase occurs at the front side of the gas layer (the front side in the flow direction). This can promote the peeling effect of powder scale at the front side of the gas layer. At the rear side of the gas layer, the pressure decreases. This can promote the removal effect of powder scale because the peeled powder scale can be rolled up at the rear side of the gas layer. Therefore, the removal efficiency of powder scale can be improved compared to the case where only water is flowed. Therefore, the removal rate of powder scale can be improved.

[0123] In one aspect of the disclosure above, the circulation flow rate of the boiler water can be varied.

[0124] By varying the circulation flow rate, the fluid force varies, which can promote the discharge of powder scale.

[0125] In one aspect of the disclosure above, the predetermined time may be a time required for the boiler water to make at least one circuit in the main system.

[0126] The time it takes for the boiler water to make one lap in the main system can be calculated from the formula: water volume held in the main system / circulating flow rate. Increasing the flow rate of the main feed water also increases the circulating flow rate. After increasing the flow rate to a specified level, the boiler water is allowed to circulate in the main system at least once, and preferably twice, allowing forced cooling of the boiler and removal of powder scale to be performed with high cooling efficiency and powder scale removal efficiency. This shortens the time required for cooling and powder scale removal compared to when the flow rate is not increased.

[0127] In one aspect of the disclosure, during the forced cooling, the temperature dropping rate of the boiler water may be 220° C. / hr or less.

[0128] By cooling the boiler water so that the temperature drop rate is 220°C / hr or less, it is possible to prevent the boiler's lifespan from being shortened due to repeated thermal stress caused by sudden temperature changes, and it is also possible to more reliably prevent the occurrence of water hammer caused by increased flow rate.

[0129] The present disclosure provides a boiler system comprising: a main system to which a boiler and a condenser are connected; a flow rate adjustment unit (11, 36, 26, 28) provided in the main system and adjusting the feedwater flow rate to the boiler; a temperature measuring device (10) connected to an outlet of the boiler; and a control unit (12) electrically connected to the temperature measuring device and the flow rate adjustment unit and controlling the feedwater flow rate at the flow rate adjustment unit based on the measurement result of the temperature measuring device, wherein the control unit has a storage unit (12-1) in which feedwater conditions that do not cause water hammer are stored based on the relationship between the temperature of the boiler water at the boiler outlet and the feedwater flow rate to the boiler; a receiving unit (12-2) that receives the measurement result of the temperature measuring device; a flow rate determination unit (12-3) that compares the received measurement result with the feedwater conditions stored in the storage unit to determine a feedwater flow rate that satisfies the feedwater conditions; and a transmitting unit (12-4) that transmits information of the determined feedwater flow rate to the flow rate adjustment unit and controls the adjustment of the feedwater flow rate in the flow rate adjustment unit.

[0130] In one aspect of the above disclosure, the main system of the boiler system (101) may include a coal economizer and a gas injection unit (101) connected between the coal economizer and an inlet of the boiler for injecting gas into the water supply to the boiler. [Explanation of symbols]

[0131] 1,100 Boiler system 2. Boiler 3 Brackish water separator 4. Brackish water separator drain tank 5 Condenser 6 Condensate demineralizer 7A Low Pressure Feedwater Heater 7B High pressure feed water heater 8 Deaerator 9 Economizer 10 Temperature measuring instruments 11 Flow Measuring Instruments 12 Control section 12-1 Storage area 12-2 Receiving section 12-3 Flow rate determination section 12-4 Transmitter 21 Boiler outlet piping 22 First drain pipe 23 Boiler circulation piping 24 Second water supply pipe 25 Boiler Circulation Pump (BCP) 26 Boiler recirculation flow control (BR) valve 27 Second drain pipe 28 Steam Separator Drain Control (WDC) Valve 29 No. 1 condensate pipe 30 Condensate pump (CP) 31 Second condensate pipe 32 Condensate booster pump (CBP) 33 Third condensate pipe 34 First water supply pipe 35 Boiler Feed Pump (BFP) 36 Water supply flow control valve 37 3rd water supply pipe 101 Gas injection section

Claims

1. A method for cleaning a boiler system having a main system to which a boiler and a condenser are connected, comprising the steps of: A water supply condition that does not cause a water hammer is obtained in advance based on a relationship between a temperature of the boiler water at a boiler outlet and a flow rate of the water supply to the boiler; After the boiler is stopped, the temperature of the boiler water at the boiler outlet is measured; The boiler water is circulated in the main system to be forcibly cooled, and a feedwater flow rate to the boiler is increased in accordance with the measured temperature of the boiler water within a range that satisfies the feedwater condition; A method for cleaning a boiler system, comprising circulating the boiler water in the main system at the increased flow rate for a predetermined period of time.

2. The boiler system includes a recirculation system that circulates the boiler water discharged from the boiler outlet directly to the boiler without passing through the condenser, 2. A boiler system cleaning method as described in claim 1, wherein the boiler water is circulated within the recirculation system, and the flow rate of the water supply to the boiler is increased within a range that satisfies the water supply conditions according to the measured temperature of the boiler water, and then circulation to the main system is started.

3. 3. The method for cleaning a boiler system according to claim 2, wherein water is also supplied from the main system while continuing to be supplied from the recirculation system.

4. The boiler system includes an economizer connected to an inlet side of the boiler, 4. The method for cleaning a boiler system according to claim 1, wherein gas is injected into the feed water between the economizer and the inlet of the boiler in the circulation of the boiler water.

5. 5. The method for cleaning a boiler system according to claim 1, wherein a circulation flow rate is varied in the circulation of the boiler water.

6. The boiler system cleaning method according to any one of claims 1 to 5, wherein the predetermined time is the time it takes for the boiler water to make at least one circuit through the main system.

7. 7. The boiler system cleaning method according to claim 1, wherein in the forced cooling, the temperature lowering rate of the boiler water is 220° C. / hr or less.

8. a main system to which a boiler and a condenser are connected; a flow rate adjusting unit provided in the main system and configured to adjust a flow rate of water supplied to the boiler; A temperature measuring instrument connected to an outlet of the boiler; a control unit electrically connected to the temperature measuring device and the flow rate adjusting unit, and configured to control the water supply flow rate at the flow rate adjusting unit based on a measurement result of the temperature measuring device; The control unit includes: a storage unit storing a water supply condition that does not cause a water hammer based on a relationship between a temperature of boiler water at a boiler outlet and a flow rate of water supplied to the boiler; A receiving unit that receives a measurement result of the temperature measuring device; a flow rate determination unit that compares the received measurement result with the water supply conditions stored in the storage unit and determines a water supply flow rate that satisfies the water supply conditions; a transmission unit that transmits information about the determined water supply flow rate to the flow rate adjustment unit and controls adjustment of the water supply flow rate in the flow rate adjustment unit; and after the boiler is stopped, the boiler water is circulated within the main system to be forcibly cooled, and a feedwater flow rate of the boiler water is increased to the feedwater flow rate determined by the flow rate determination unit.

9. The main system includes an economizer, 9. The boiler system according to claim 8, further comprising a gas injection section connected between the economizer and an inlet of the boiler for injecting gas into the feed water to the boiler.

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