JP2008267742A - Drum level control method and device for boiler - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the economic efficiency of a boiler by minimizing drainage accompanied by drum level control. <P>SOLUTION: Drum level control for controlling the drum level of a drum in the boiler based on a predetermined control level is performed at the start of the boiler by obtaining a starting control level Ls by a starting control level computing element 11 based on the state quantity of the drum obtained by measurement on the drum before starting, and using the starting control level Ls as a control level. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to a drum level control technique for a boiler, and more particularly to a boiler with a relatively small capacity that is operated by a boiler, for example, an independent power generation company, which is desired to reduce waste water generated during operation of the boiler. The present invention relates to a drum level control technique suitable for a boiler or the like.

  In recent years, with the deregulation of electric power, the spread of distributed power sources has progressed, and many independent thermal power generation facilities have been installed by independent power generation companies. Accordingly, there is an increasing demand for more economical designs of boilers for small capacity thermal power generation facilities. In order to meet such demands, waste water from the boiler is one of the major factors in improving the economic efficiency of boilers for thermal power generation facilities.

  Generally, a boiler for a thermal power generation facility has a drum as a steam generator, and steam is generated by heating water stretched on the drum (usually pure water is used). Drum level control is performed to keep the water level, i.e., the drum level, appropriate. Drum level control is performed between the upper limit drum level for preventing moisture from entering the steam supplied to the load and the lower limit drum level (trip level) for preventing the drum from becoming empty. The control level is set by the control unit, and the drum level is maintained at the control level (for example, Patent Documents 1 and 2).

  In such drum level control, when the drum level becomes excessive, it is necessary to drain the excess amount, and accordingly drainage occurs. Drainage accompanying this drum level control affects the economics of the boiler by increasing the amount of pure water used, and it is also necessary to use other scale drainage facilities depending on the amount of wastewater to treat it. Affects economics. For this reason, if the drainage accompanying the drum level control can be reduced, the drainage facility can be reduced in size, the amount of pure water used can be reduced, and the economics of the boiler can be improved.

JP 2006-46874 A Japanese Patent Laid-Open No. 11-351503

  As described above, the reduction of waste water associated with drum level control is effective in improving the economics of the boiler. The present invention has been made from such a viewpoint, and the problem is to improve the economics of the boiler by minimizing the amount of drainage associated with drum level control.

  Drainage accompanying drum level control occurs mainly during the startup process of the boiler. That is, in the starting process, the drum water undergoes a significant temperature rise, and the drum level rises due to the volume expansion of the drum water due to the temperature rise. As a result, the drum level becomes excessive, and thus drainage is required. In order to reduce drainage during the startup process, it is effective to perform drum level control during the startup process using a startup control level that is set lower than the normal operation control level during normal operation. So it was like that in the past. However, conventionally, even if drum level control is performed by the start-up control level, for example, if the generated steam capacity is 100 tons, several tons of drainage is generated, and even on the order of several percent of the generated steam capacity. There were not a few cases where wastewater was generated.

  The inventors have conducted a detailed study on the occurrence of such waste water in the past. As a result, it became clear that there was a problem with the method of setting the startup control level. That is, conventionally, the startup control level is obtained and set by a test operation, and the startup control level is uniformly used. To that end, it sufficiently responds to drum level rise due to temperature rise in the startup process that will be affected by many factors such as boiler stop time, standby state of the stopped boiler, seasonal temperature conditions of the boiler installation environment Can not do it. As a result, a considerable amount of drainage was to be generated.

  From such knowledge, in the present invention, the drum state amount such as pressure and temperature is measured for the drum before start-up, and the drum level increase accompanying drum temperature rise is predicted for each start-up process from the measured drum state amount, The control level for starting is set based on the prediction, thereby enabling drum level control for the starting process in accordance with the actual drum state for each starting process, resulting in the temperature rise of the drum. To reduce drainage.

  That is, in the present invention, in the drum level control method in which the drum level of the drum in the boiler is controlled based on a predetermined control level, when the boiler is started, the drum obtained by measuring the drum before the start is measured. The starting control level set based on the state quantity is used as the control level.

  When the activation control level is set based on the actually measured drum state quantity in this way, the activation control level is a level close to the lower limit drum level for preventing the drum from being cooked empty. It is normal. Therefore, there is a possibility that the drum level control is unstable due to the drum level being lowered to the lower limit drum level without the water supply being able to follow the sudden drop of the drum level accompanying the start of steam supply in the starting process. Therefore, it is preferable that the drum level control method as described above can effectively prevent the drum level from reaching the lower limit drum level when the drum level suddenly decreases as the steam supply starts. Therefore, in the present invention, in the drum level control method as described above, a drum level decrease due to the start of steam supply in the startup process is estimated, and water supply control based on the estimation is performed in the startup process. It is supposed to be.

  It is particularly appropriate to estimate such a decrease in drum level based on the rate of change in drum pressure. Therefore, in the present invention, in the drum level control method as described above, the estimation of the decrease in the drum level is performed based on the rate of change in the drum pressure.

  In the present invention, the drum level control device used in the drum level control method as described above includes a start control level calculation means for calculating the start control level based on the drum state quantity before the start. Further, the water supply control means for performing the water supply control based on the estimation of the drum level drop is provided.

  According to the present invention as described above, drainage accompanying drum level control can be reduced as much as possible, and the economics of the boiler can be improved.

  Hereinafter, modes for carrying out the present invention will be described. FIG. 1 shows a functional configuration of a drum level control apparatus 1 according to an embodiment in relation to a drum 2 that is a steam generator to be controlled. A water supply pipe 3 is connected to the drum 2. The water supply pipe 3 is provided with a water supply control valve 4, and the drum level of the drum 2 is controlled by controlling the valve opening degree of the water supply control valve 4 by the drum level control device 1. A drain pipe 5 is connected to the drum 2. A drainage control valve 6 is provided in the drainage pipe 5, and the drainage control valve 6 is controlled by the drum level control device 1 to control the drainage from the drum 2. The drainage from the drum 2 by the drainage pipe 5 is performed when the drum level becomes excessive, and the drainage discharged thereby is sent to the wastewater treatment device 7 and subjected to a predetermined treatment. Further, the drum 2 is connected to an evaporator 8 that receives heat from a boiler heating unit (not shown).

  The drum level control device 1 includes a startup control level calculator 11, a drum temperature calculator 12, a control level switch 13, a pressure change rate calculator 14, a preceding valve opening command value calculator 15, and a valve opening command value calculation. A device 16 and an adder 17 are provided. In addition to these, the drum level control device 1 also includes a drainage control system for controlling the opening and closing of the drainage control valve 6, but these are not shown.

  The start control level calculator 11 is a start control level calculation means, and calculates the start control level Ls every time the boiler is started. The activation control level Ls in the activation control level calculator 11 is calculated based on the drum state quantity obtained by measuring the drum 2. In the example shown in the figure, the drum pressure before startup measured by the pressure gauge 18 (the internal pressure of the drum 2 before startup of the boiler) is used as a basic drum state quantity. That is, the startup control level calculator 11 calculates the pre-startup drum pressure measured by the pressure gauge 18 and the pre-startup drum pressure by the drum temperature calculator 12 (the saturation temperature of the drum 2 before the boiler start-up). ) To calculate the drum holding heat amount before starting (the holding heat amount of drum 2 before starting the boiler), and further increasing the drum level due to the volume expansion that occurs in the drum water due to the temperature rise of the drum 2 until the start of steam generation in the starting process Minutes are estimated from the drum holding heat amount before starting, and the starting control level Ls is calculated based on the predicted drum level increase.

  The startup control level Ls thus calculated is a value correlated with the pre-startup drum pressure, and as an example, as shown in FIG. 2, a value close to the trip level for preventing the drum from becoming empty. It becomes. A situation in which the drum level becomes excessive during the start-up process by using the start-up control level Ls that correlates with the pre-start-up drum pressure, that is, expects the drum level to rise due to the temperature rise predicted from the pre-start-up drum pressure. Can be effectively avoided, and therefore drainage associated with drum level control during the start-up process can be reduced. And by reducing the waste water accompanying the drum level control, the waste water treatment device 7 can be miniaturized, the amount of pure water used can be reduced, and the economic efficiency of the boiler can be improved.

  The control level switching unit 13 is a control level switching unit, and stably generates steam when the boiler is started (more specifically, when water is filled in the startup process) as a reference for drum level control. It functions to selectively switch during normal operation. That is, the startup control level Ls calculated by the startup control level calculator 11 is set as a control level during startup, and the normal operation control level Ln given in advance during normal operation is set as the control level.

  The pressure change rate calculator 14, the preceding valve opening command value calculator 15, the valve opening command value calculator 16, and the adder 17 control the valve opening of the water supply control valve 4 to supply water to the drum 2. The water supply control means 19 to control is comprised. In this water supply control means 19, the pressure change rate calculator 14 and the preceding valve opening command value calculator 15 function in feedforward control, and the valve opening command value calculator 16 functions in feedback control.

  In the feedforward control, when a sudden change occurs in the drum level due to a change in the drum pressure, the opening degree of the water supply control valve 4 is controlled to increase before the sudden change in the drum level. More specifically, the pressure change rate calculator 14 calculates the rate of change from the drum pressure provided by the pressure gauge 18 as pressure change rate calculation means, and the preceding valve opening command value calculator 15 calculates the pressure change. The rate calculator 14 predicts whether or not there is a sudden change in the drum level from the rate of change in the drum pressure calculated. If there is a sudden change, the amount of change in the drum level due to the sudden change is estimated, and based on the estimated change amount. The preceding valve opening command value is calculated and output to the adder 17. The adder 17 adds the preceding valve opening command value to a valve opening command value from a valve opening command value calculator 16 described later, and outputs the result to the water supply control valve 4 as an effective valve opening command value.

  On the other hand, in the feedback control, the deviation between the actual drum level measured by the drum level meter 20 and the control level is obtained, and the valve opening command value necessary for eliminating the deviation is calculated and output to the adder 17. To do.

  Below, the drum level control at the time of boiler starting performed with the drum level control apparatus 1 is demonstrated. The drum level control at the time of start-up includes the processing steps of steps 101 to 111 as shown in FIG.

  In step 101, the start control level Ls is set. Specifically, the startup control level calculator 11 calculates the startup control level Ls as described above, and the control level switch 13 sets it as the control level.

  In step 102, the drum level based on residual drum water is determined. Specifically, the actual drum level before water filling in the starting process is obtained from the measured value by the drum level meter 20, and it is determined whether the actual drum level is lower than the starting control level Ls. If the determination result in step 102 is affirmative, the process proceeds to step 103 to supply water to the drum 2 to complete water filling (step 105). On the other hand, if the determination result in step 102 is negative, step 102 is performed. Proceeding to 104, draining from the drum 2 is performed, and water filling is completed (step 105).

  When the water filling is completed, heating is started and the temperature of the drum 2 rises, and the drum level rises accordingly (step 106). When the drum level starts to rise, step 107 determines the actual drum level that is being heated. Specifically, the actual drum level during temperature increase is obtained from the measured value by the drum level meter 20, and it is determined whether the actual drum level during temperature increase is higher than the normal operation control level Ln. If the determination result in step 107 is affirmative, the process proceeds to step 108 to drain the drum 2. On the other hand, if the determination result in step 107 is negative, the actual drum level is maintained.

  When the drum 2 is heated to the saturation temperature, stable generation of steam starts in the drum 2 (step 109). When stable generation of steam is started, supply of steam to the load is started. When the supply of steam to the load is started, the drain valve that has been opened until then is closed, so that the drum pressure changes suddenly, and the drum level is suddenly lowered accordingly. This sudden decrease in the drum level may cause instability in the drum level control in the startup process using the startup control level Ls as described above. In other words, since the start control level Ls is close to the trip level, the water supply cannot follow the sudden drop in the drum level due to the start of steam supply, so that the drum level is lowered to the trip level and the drum level control becomes unstable. There is a possibility of inviting. Therefore, in order to cope with this, water supply corresponding to a sudden drop in the drum level is performed as step 110 by the water supply control to which the above feedforward control is added. The water supply in step 110 is performed using the normal operation control level Ln as the control level. Therefore, in step 111, the control level is switched to the normal operation control level Ln, and the starting process is completed.

  As mentioned above, although one of the forms for implementing this invention was demonstrated, this is only a representative example and this invention can be implemented with various forms in the range which does not deviate from the meaning. it can. For example, in the above embodiment, the drum pressure is used as the drum state quantity for obtaining the activation control level, but the drum temperature may be used as the drum state quantity instead.

It is a figure which shows the structure of the drum level control apparatus by one Embodiment. It is a figure which shows the example of the relationship between the drum pressure before starting, and the control level for starting. It is a figure which shows the flow of a process in the drum level control at the time of starting.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Drum level control apparatus 2 Drum 11 Start-up control level calculator (Start-up control level calculation means)
14 Pressure change rate calculator 19 Water supply control means

Claims (5)

In the drum level control method adapted to control the drum level of the drum in the boiler based on a predetermined control level,
2. A drum level control method according to claim 1, wherein when the boiler is started, a start control level set based on a drum state quantity obtained by measuring the drum before start is used as the control level.   2. The drum level control according to claim 1, wherein a drum level decrease due to the start of steam supply in the startup process of the boiler is estimated, and water supply control based on the estimation is performed in the startup process. Method.   The drum level control method according to claim 2, wherein the estimation of the drum level drop is performed based on a rate of change in drum pressure. A drum level control device used in the drum level control method according to claim 1,
A drum level control apparatus comprising: a start control level calculating means for calculating the start control level based on the drum state quantity before the start. A drum level control device used in the drum level control method according to claim 2 or claim 3,
A drum level control device comprising water supply control means for performing water supply control based on the estimation of the drum level drop.

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