JP2013088107A - Once-through boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a carry-over and steam quality from decreasing on change in combustion amount.SOLUTION: A once-through boiler, which has a water level set halfway in a heat transfer pipe, is configured to set a combustion amount of a boiler in steps and change a water level adjustment range according to the combustion amount. When the water level in the boiler becomes higher than a suitable range made to correspond to the combustion amount as a result of an increase in the combustion amount in a state of combustion, boiler water is discharged to a water level matched to the combustion amount. The combustion amount of the boiler is controlled under multi-position control of four stages or more including a stop of combustion. The boiler water is discharged when the combustion amount is further increased within a predetermined time with time in increase in the combustion amount as a starting point. The boiler water is not discharged when the combustion amount is not increased within the predetermined time.

Description

  The present invention relates to a once-through boiler in which a water level is set in the middle of a heat transfer tube and steam is generated by heating the heat transfer tube.

  Through-flow boilers that install multiple vertical heat transfer tubes, supply water from the bottom of the heat transfer tubes, heat the boiler water in the heat transfer tubes to generate steam, and extract the steam from the top of the heat transfer tubes are widely used. In recent years, large capacity once-through boilers are also increasing. Since the once-through boiler sets the water level in the middle of the heat transfer tube, the amount of boiler water is smaller than that of other types of boilers. Therefore, since the amount of heat held by the boiler is reduced, the damage is limited even if an accident occurs, and there is an advantage that the steaming time when starting from a cold can is shortened.

  However, in the once-through boiler that sets the water level in the middle of the heat transfer tube, severe water level control is required. Even if the water level is set in the middle of the heat transfer tube, the boiler water is boiling because it boils violently in the heat transfer tube. For this reason, the boiler water that is lifted even in the portion above the water level of the heat transfer tube takes heat, so that the heat transfer tube can be prevented from overheating if the water level is maintained at an appropriate level. However, when the water level in the heat transfer tube is low, or when the amount of boiling water in the boiler water decreases, if the boiler water is insufficiently cooled at the top of the heat transfer tube, the heat transfer tube may be overheated at the top. If the water level in the heat transfer tube is too high, the amount of boiler water lifted to the top of the heat transfer tube increases, and a large amount of boiler water is sent to the steam separator. In this case, since the separation performance in the steam / water separator deteriorates, a carry-over in which boiler water that could not be separated is taken out together with the steam may occur, resulting in a decrease in steam quality. Therefore, water level control is important for once-through boilers.

  The appropriate water level varies depending on the combustion amount of the boiler, and when the combustion amount is increased or decreased in stages, such as high combustion, medium combustion, and low combustion, the appropriate water level varies depending on the combustion amount. In the case of high combustion with a large amount of combustion, the boiling of boiler water in the heat transfer tube increases, and therefore the possibility of carryover and the decrease in steam dryness increases unless the water level is kept low. Conversely, in the case of low combustion with a small amount of combustion, the boiling of boiler water in the heat transfer tube is reduced, so that the heat transfer tube is likely to be overheated unless the water level is raised.

  Japanese Patent No. 294080 discloses that the water level is set corresponding to each of 100% combustion, 65% combustion, and 30% combustion, and the water level is changed in accordance with the amount of combustion. Here, the water level in the water level detection container is low between Hb and Hc during 100% combustion, and the water level in the water level detection container is intermediate between Hc and Hd during 65% combustion, At the time of 30% combustion, the water level in the water level detection container is set to a high level between Hd and He, so that the water level in the heat transfer tube is changed. If it does in this way, even if it changes the amount of combustion, it can be maintained at an appropriate water level, and it can control the occurrence of overheating and carryover of a heat exchanger tube.

  However, when the combustion amount is shifted to increase the combustion amount, the boiling of the can water is increased and a carryover may occur. In this case, the amount of combustion is strengthened when the water level at low load is high, but the increase in the amount of combustion is performed in a short time, while the fluctuation of the water level takes longer than that, Therefore, the balance between the water level and the combustion amount will be lost. Carryover and steam deterioration can be prevented by discharging boiler water when the amount of water in the steam separator increases. To prevent carryover, etc., a water level electrode is added to the air / water separator and the water level can be detected by the air / water separator. If the water level in the air / water separator rises, the boiler This can be done by discharging water, but in this case, the cost for adding the water level detection device increases. Further, since the flow of can water (such as swirling) is large in the steam separator, there is a possibility that air bubbles enter the electrode portion of the water level detection device, so that the detection accuracy is lowered and appropriate control cannot be performed.

Japanese Patent No. 294080

  The problem to be solved by the present invention is to provide a once-through boiler capable of suppressing carryover and deterioration of steam quality when the amount of combustion is changed.

  The invention according to claim 1 is a once-through boiler in which the water level is set in the middle of the heat transfer tube, the combustion amount of the boiler is set stepwise, and the water level adjustment range is switched according to the combustion amount. If the water level in the boiler becomes higher than the appropriate range corresponding to the combustion amount due to the increase in the combustion amount in the state of combustion, the boiler reaches the water level that matches the combustion amount. It is characterized by controlling the discharge of water.

According to a second aspect of the present invention, in the once-through boiler, the combustion amount of the boiler is multi-position control of four or more stages including combustion stop, and further combustion is performed within a predetermined time starting from when the combustion amount is increased. The boiler water is discharged when the amount is increased, and the boiler water is not discharged when the combustion amount is not increased within a predetermined time.

  According to a third aspect of the present invention, in the once-through boiler, a water level detection cylinder connected to the steam separator is connected to the boiler, and the water level in the boiler is detected based on the water level in the water level detection cylinder. The boiler water is discharged from the air / water separator or the return pipe from the air / water separator to the lower header.

  When the combustion amount is changed while combustion is being performed, the change of the combustion amount is performed in a short time because it is completed only by changing the fuel supply amount and the combustion amount air supply amount. However, since the water level changes gradually after the water level adjustment range is switched, a relatively long time is required until the water level is switched to the appropriate water level. By adding control that discharges boiler water from the boiler when the combustion amount increases, the water level does not remain high even though the combustion amount has increased, and it is possible to suppress the occurrence of carryover and deterioration of steam quality. it can. If the boiler water is discharged from the steam / water separator or the return pipe from the steam / water separator to the lower header, the water level in the steam / water separator can be quickly lowered to secure the steam / water separation space. The vapor quality can be kept high.

  In addition, when the combustion amount of the boiler is multi-position control of four or more stages including combustion stop, if only changing the combustion amount by one step, the appropriate water level determined for each combustion amount does not change much. If the amount is different by two stages, the appropriate water level may change greatly. In this case, if the change in the combustion amount is made slowly over a period of time, there is no problem because the difference between the appropriate water level and the actual water level does not increase, but the change in the combustion amount changes greatly in a short time. In this case, the difference between the appropriate water level and the actual water level becomes large, which causes carryover. When the combustion amount is further increased within a predetermined time starting from the time when the combustion amount is increased, the boiler water is discharged. When the combustion amount is not increased within the predetermined time, the boiler water is discharged. If the discharge is not performed, the boiler water is not discharged unnecessarily, and the occurrence of carryover or the like can be prevented.

Outline diagram of boiler implementing the present invention Explanatory drawing which showed the fluctuation state of the combustion state in this invention, and the water level in a heat exchanger tube Explanatory drawing showing the fluctuation of the combustion state and water level in the heat transfer tube when there is no need to drain

  An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a boiler for carrying out the present invention. At the upper part of the boiler, a combustion device 2 with a variable combustion amount for generating a downward flame is provided, and the flame is burned in the combustion chamber 3 in the center of the boiler. Combustion air used in the combustion device 2 is supplied by a blower 5 connected to the combustion device 2 through a blower passage. An air flow control device 8 for controlling the amount of air supplied from the blower 5 to the combustion device 2 is provided, and the air supply amount for combustion is adjusted by the air flow control device 8. A fuel supply control device 7 is also provided in the fuel supply pipe 4 for supplying fuel to the combustion device 2 so that the amount of fuel supplied to the combustion device 2 can also be adjusted by the fuel supply control device 7.

  Water supply to the boiler is performed by operating a water supply pump 10 provided in the middle of the water supply pipe connected to the lower part. Boiler water that has entered from the bottom of the boiler enters into a large number of installed heat transfer tubes, boils in the heat transfer tubes by heating from around the heat transfer tubes, and generates steam. The generated steam is collected from the heat transfer tube in the upper header and then taken out through the upper connecting tube 12. In the heat transfer tube during boiler combustion, the boiler water boils violently, and the steam taken out from the upper header through the connecting tube 12 contains a large amount of boiling boiler water. Therefore, the steam containing boiler water is separated into steam and boiler water by swirling in the steam separator 11, and only steam is taken out. A return water pipe for returning the boiler water separated by the steam / water separator to the lower part of the boiler is connected to the lower part of the steam / water separator, and the separated boiler water is returned to the lower part of the boiler through the return water pipe. The steam-water separator or return water pipe is provided with a drain valve 13 for discharging a part of the boiler water separated by the steam-water separator, so that the boiler water can be discharged by opening the drain valve 13. ing.

  The boiler water supply control is performed based on the water level in the water level detection device 9 whose upper part is connected to the steam separator. The water level detection device 9 detects the water level by detecting the presence or absence of water with a plurality of electrode rods installed at different height positions. In the water supply control, the operation of the water supply pump 10 is started when the water level in the water level detection device falls to the water supply start water level, and the operation of the water supply pump 10 is stopped when the water level rises to the water supply stop water level by water supply. Also, the operation of the water supply pump 10 is started when a predetermined time has elapsed from the time when the water level becomes lower than the water supply stop water level, and the operation of the water supply pump 10 is stopped when the water level rises to the water supply stop water level due to water supply. ing. In this embodiment, when the low combustion is performed, the water level lowering time from the water supply stop water level is detected to start the water supply, and during the middle combustion and the high combustion, the water level decrease to less than the water supply start water level is detected to start the water supply.

  The boiler is operated based on the steam pressure value detected by the pressure detection device 1 that detects the steam pressure value of the boiler. When the steam pressure value is low, the combustion amount is increased, and when the steam pressure value is high, the combustion amount is decreased to keep the steam pressure within a predetermined range. The water level information detected by the water level detection device 9 and the steam pressure value detected by the pressure detection device 1 are sent to the operation control device 6 that controls the operation of the boiler. The operation control device 6 is connected to each of the blower 5, the air flow control device 8, the fuel supply control device 7, the water supply pump 10, and the drain valve 13 to control the operation of each device, and the operation control device. 6 controls the operation of the boiler.

  The operation control device 6 controls the combustion amount at four positions of high combustion, medium combustion, low combustion, and combustion stop based on the steam pressure value. When the combustion amount for high combustion is 100% and the combustion amount for low combustion is 20%, intermediate combustion is an intermediate value, and about 50% to 70% is appropriate. The operation control device 6 controls the fuel amount supplied to the combustion device 2 and the combustion air amount by operating the fuel supply control device 7, the blower 5, and the air flow control device 8, and the combustion amount in the combustion device 2. Adjust.

  If the steam pressure value for increasing the combustion amount and the steam pressure value for decreasing the combustion amount are set to the same value, the combustion amount increase and the combustion amount decrease may be alternately repeated with a slight pressure fluctuation. For this reason, in this embodiment, the occurrence of hunting is prevented by providing a difference between the steam pressure value that increases the combustion amount and the steam pressure value that decreases the combustion amount. If the pressure value that decreases the combustion amount is set higher than the pressure value that increases the combustion amount, the combustion amount is not changed until the difference pressure changes, so the combustion amount is repeatedly increased and decreased in a very short time. This is no longer the case.

  The water level adjustment range in the water level detection device 9 is set in two stages for high combustion, medium combustion, and low combustion, and the water level adjustment range is switched according to the amount of combustion. In the case of low combustion with a small amount of combustion, the water level is increased because the amount of boiler water that rises decreases. Conversely, in the case of high combustion and medium combustion with a large amount of combustion, the amount of boiler water that rises increases, so the water level is lowered.

  2 and 3 are explanatory diagrams schematically showing the combustion state and the fluctuation state of the water level in the heat transfer tube in the present invention. Since the water level in the boiler rises when water supply is started and falls when water supply is stopped, the water level is repeatedly raised and lowered alternately. The water levels a to d described in FIG. 2 and FIG. 3 indicate the height positions in the water level detection device of FIG. The water level is cd. If it is high combustion and medium combustion, the operation of the feed water pump is started when the water level drops to the position a, and the operation of the feed water pump is stopped when the water level rises to the position b due to water supply. In the case of low combustion, when the water level falls below the d position and the water level drops to the c position, the operation of the feed water pump is started, and when the water level rises to the d position by supplying water, the operation of the feed water pump is stopped.

  In FIG. 2, the combustion amount determined from the steam pressure value starts from a low combustion state. The water level at this time is between c and d, and is maintained at an appropriate water level with low combustion. Thereafter, the steam pressure value decreases due to an increase in the amount of steam used, and the combustion amount determined from the steam pressure value changes to medium combustion. In this case, the operation control device 6 changes the combustion level to medium combustion and changes the water level. Water supply was carried out until the combustion amount was changed, but since there is no need for water supply from the set water level at the time of high combustion and medium combustion, the water supply is stopped. Furthermore, the elapsed time is counted starting from the time when the combustion amount is changed, and if the combustion amount becomes higher combustion within the predetermined time set in advance, the drainage Open the valve 13. In FIG. 2, since the time until high combustion is shorter than the predetermined time, the drain valve 13 is opened and the boiler water is drained, and the water level in the boiler quickly decreases. When the water level drops to position b, which is the water supply stop water level in high combustion and medium combustion, the drain valve 13 is closed, and thereafter the water level is maintained at high combustion and medium combustion.

  If high combustion is performed in a state where there is a water level between the c position and the d position, which is the water level adjustment range in low combustion, the amount of boiler water boiling in the heat transfer tube increases. If the feed water pump is not operated during boiler operation, the water level drops because the boiler water is removed from the boiler as steam, but it takes time to reach an appropriate water level. The amount of boiler water sent to the water separator increases. When the amount of boiler water sent to the steam separator increases and the boiler water accumulates in the steam separator, the free capacity of the steam separator decreases, so the separation performance in the steam separator Decreases. If the boiler water cannot be separated by the steam separator, the boiler water is taken out together with the steam, so that the steam quality is lowered and temporary carryover occurs. When the amount of boiler water sent to the steam separator increases due to the increase in the amount of combustion, the boiler water can be discharged quickly by opening the drain valve 13 to reduce the steam level to the appropriate level. It is possible to prevent quality degradation and carryover.

  In the present invention, the water level in the steam separator is not detected and the boiler water is not discharged, so it is not necessary to provide a water level detector in the steam separator, and the water level detector provided in the steam separator Accuracy does not matter. In addition, if the boiler water is discharged from the steam / water separator or in the middle of the return pipe from the steam / water separator to the lower header, the water level in the steam / water separator quickly decreases and the steam / water separation space is reduced. It can be ensured and the vapor quality can be kept high.

  Note that the operation of draining when changing the combustion amount is not always necessary. The explanatory diagram shown in FIG. 3 schematically shows the combustion state and the fluctuation state of the water level in the heat transfer pipe when the combustion amount is changed from low combustion to medium combustion and then to high combustion. However, the length of time during which medium combustion is maintained is different from that in FIG. If the change from low combustion to medium combustion and further change to high combustion is performed in a short time, the balance between the water level and the combustion amount will be lost as shown in FIG. However, the change to intermediate combustion, which is an intermediate combustion amount, has a small effect of lowering the vapor quality, and if the time of intermediate combustion becomes longer, the water level in the boiler becomes a level suitable for high combustion and intermediate combustion during that time. There is no problem even if drainage is not performed.

  The boiler water separated by the steam separator is discharged, and the concentrated portion is drained, so that it is also used as a concentration blow. For this reason, drainage is originally necessary, but if the drainage exceeds the amount of concentrated blow, heat will be released, so it becomes a problem that the drainage amount becomes too large. In this control, high combustion and medium combustion are set to the same water level, but medium combustion does not have to be the same water level as high combustion. Since the intermediate combustion is an intermediate combustion amount between the high combustion and the low combustion, there is no particular inconvenience even if the intermediate combustion is performed at the low combustion water level or the intermediate combustion is performed at the high combustion water level. Therefore, it is not necessary to drain water when changing from low combustion to medium combustion, and if the time of medium combustion is sufficiently long, it is not necessary to discharge boiler water even if high combustion occurs thereafter. Therefore, in that case, wasteful discharge can be eliminated by not discharging boiler water. In this embodiment, the elapsed time is counted starting from the time when the combustion amount is changed, and the combustion amount becomes another higher level of combustion within a predetermined time set in advance. In this case, the drain valve 13 is opened. If the amount of combustion does not increase within a predetermined time, the drain valve 13 is not opened. Therefore, wasteful drainage can be prevented.

  The present invention is not limited to the embodiments described above, and many modifications can be made by those having ordinary knowledge in the art within the technical idea of the present invention.

1 Pressure detector
2 Combustion device
3 Combustion chamber 4 Fuel supply piping 5 Blower 6 Operation control device
7 Fuel supply control device 8 Air flow control device 9 Water level detection device 10 Water supply pump 11 Air / water separator 12 Connection pipe 13 Drain valve

Claims (3)

  In a once-through boiler in which the water level is set in the middle of the heat transfer tube, the combustion amount of the boiler is set in stages, and the water level adjustment range is switched according to the combustion amount. If the water level in the boiler becomes higher than the appropriate range corresponding to the combustion amount due to the increase in the combustion amount in the state where the combustion is performed, the control is performed to discharge the boiler water to the water level that matches the combustion amount. A once-through boiler characterized by being.   The once-through boiler according to claim 1, wherein the combustion amount of the boiler is multi-position control of four or more stages including combustion stop, and the combustion amount is further increased within a predetermined time starting from the time when the combustion amount is increased. The once-through boiler is characterized in that boiler water is discharged when it is broken and boiler water is not discharged when the combustion amount is not increased within a predetermined time. 3. A once-through boiler according to claim 1 or 2, wherein a water level detection cylinder connected to the steam separator is connected to the boiler, and the water level in the boiler is detected based on the water level in the water level detection cylinder. The once-through boiler is characterized in that the discharge is taken out from the steam / water separator or the return pipe from the steam / water separator to the lower header.

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