JP5221971B2 - A once-through exhaust heat recovery boiler with water level control and superheat control. - Google Patents

Description

  The present invention relates to a control method and apparatus for superheat degree control in a once-through exhaust heat recovery boiler, and more particularly to continuously and stably transition from water level control to superheat degree control of a brackish water separator drain tank during a plant start-up process. In particular, the present invention relates to a method and apparatus for switching between water level control and superheat degree control of a preferred once-through exhaust heat recovery boiler.

  Conventionally, there are known a natural circulation type and a once-through type exhaust heat recovery boiler, and the outline of the natural circulation type exhaust heat recovery boiler will be described first with reference to FIGS. 5 and 6. FIG. 5 is a block diagram showing a configuration of a boiler water natural-circulation exhaust heat recovery boiler in the prior art as disclosed in Patent Document 1, for example. FIG. 6 is a control block diagram of drum water level control in a conventional natural circulation type exhaust heat recovery boiler.

  In FIG. 5, in the natural circulation type exhaust heat recovery boiler, the feed water supplied from the feed water pump 69 passes through the economizer 70 and is supplied to the steam drum 71 and the evaporator 72. Thereafter, the boiler water is naturally circulated by the steam drum 71 and the evaporator 72, the steam separated by the steam drum 71 is sent to the superheater 73, and the superheated steam is sent to the steam turbine 74. The Further, the exhaust gas from the gas turbine 75 is supplied to the superheater 73, the evaporator 72, and the economizer 70 to superheat the steam, evaporate the boiler water, and heat the feed water.

  Next, drum level control in the conventional natural circulation type exhaust heat recovery boiler will be described with reference to FIG. A difference between the drum water level measurement value detected by the drum water level transmitter 77 installed in the steam drum 71 and the drum specified water level 79 is obtained, and a water level control deviation is output from the subtractor 80. The water level control deviation output from the subtractor 80 is subjected to proportional / integral / differential processing by a proportional integral derivative controller 81, and the processed water level control deviation is detected by the steam flow meter 78 in the adder 82. The flow rate is added and output to the subtracter 83.

  The adder 82 outputs a feed water flow value necessary for eliminating the water level control deviation, and a subtractor 83 obtains a difference between the feed water flow value and a feed water flow measurement value detected by the feed water flow meter 76. Outputs control deviation. The output flow rate control deviation is proportionally integrated by the proportional integration controller 84 and then input to the feed water flow rate adjustment valve 85 as a control command.

Thus, in the natural circulation type exhaust heat recovery boiler, it is necessary to keep the water level of the drum constant in order to maintain a stable flow of the evaporator. The water level of the drum is the difference between the evaporation amount and the water supply amount, and the evaporation amount is determined by the heat input from the gas turbine 75. Therefore, the feed water flow rate is adjusted to keep the drum water level constant (that is, the steam determined by the heat input amount). This is a control for replenishing the amount of steam sent to the turbine by the amount of water supply). The water level control described above is performed in the startup process of the natural circulation type exhaust heat recovery boiler.
JP 2000-146108 A

  In the startup process of the conventional natural circulation type exhaust heat recovery boiler described above, only the water level control of the steam drum is performed. However, in the once-through type exhaust heat recovery boiler targeted by the present invention, the water level control of the brackish water separator drain tank is performed by the conventional water supply amount in the starting process (see FIG. 4), and then the once-through operation, that is, the evaporator The outlet superheat degree control is performed.

  In view of this, the present invention provides a method for switching from the conventional water level control to the superheat degree control at the evaporator outlet that does not exist in the conventional natural circulation exhaust heat recovery boiler control method in the startup process of the once-through exhaust heat recovery boiler. Another object is to provide a once-through exhaust heat recovery boiler that can smoothly and quickly switch from water level control to superheat control.

In order to solve the above problems, the present invention adopts the following configuration.
An evaporator that heats feed water supplied by a feed water pump with exhaust gas from a turbine, a brackish water separator tank that separates fluid from the evaporator into brackish water, and superheats steam from the brackish water separator tank with the exhaust gas A superheater, a water level control function unit for controlling the water level of the brackish water separator tank, and a superheat degree control function unit for controlling the superheat degree of the outlet steam of the brackish water separator tank. A recovery boiler,
Control switching that adjusts the feed water flow rate in the transition period for switching control from the water level control state by the water level control function unit to the superheat degree control state by the superheat degree control function unit during the start-up process of the once-through exhaust heat recovery boiler Provide an operation function unit,
Feed water flow adjustment command value by the control switching operation function section state, and are not given the feed water flow rate decrement is added to the feed water flow rate command value in switching level control by the level control function unit,
The permission condition for switching from the water level control state by the water level control function unit to the control switching operation state to which the predetermined water supply flow rate decrease by the control switching operation function unit is added is the steam from the brackish water separator tank or the superheater. It is a condition that the flow rate has reached a predetermined amount,
Further, when the outlet temperature of the brackish water separator tank exceeds (saturation temperature + specified value), the feed water flow rate adjustment command value by the control switching operation function unit is changed to the feed water flow rate command value by the superheat degree control function unit. switching,
The control switching operation function unit is provided with an analog memory and an analog signal generator for generating a negative value,
The analog memory is configured to hold a feed water flow rate command value when the water level control is switched by the water level control function unit, and the analog signal generator generates the predetermined feed water flow rate decrease.

  According to the present invention, in the start-up process of the once-through type exhaust heat recovery boiler, the control switching operation period is provided in the transition from the water level control to the superheat degree control of the brackish water separator drain tank, and the amount of water supply is controlled within this control switching operation period. It is possible to smoothly shift to superheat degree control by lowering the water level by an operation of temporarily reducing and forcibly drying the evaporator outlet.

  A switching device and method for water level control and superheat degree control of a once-through exhaust heat recovery boiler according to an embodiment of the present invention will be described below with reference to FIGS. 1, 2, 3, and 4. FIG. 1 is a diagram illustrating a method for switching between water level control and superheat degree control of a brackish water separator drain tank in a once-through exhaust heat recovery boiler according to an embodiment of the present invention. FIG. 2 is a diagram illustrating control blocks in the water level control function unit, the superheat degree control function unit, and the control switching operation function unit of the brackish water separator drain tank in the once-through exhaust heat recovery boiler according to the present embodiment. FIG. 3 is a diagram showing the operating conditions of the analog switch and change rate limiter used in the control block shown in FIG. FIG. 4 is a block diagram showing the configuration of the once-through exhaust heat recovery boiler according to the present embodiment.

  First, in FIG. 4, a schematic configuration of the once-through exhaust heat recovery boiler according to the present embodiment will be described. In the once-through exhaust heat recovery boiler according to the present embodiment, the feed water supplied from the feed water pump 61 passes through the economizer 62 and is supplied to the evaporator 63. A mixture of water and steam is generated in the evaporator 63 by the heat quantity of the exhaust gas from the gas turbine 68 and added to the brackish water separator drain tank 64. Steam is sent from the steam separator drain tank 64 to the superheater 65, and the superheated steam is supplied to the steam turbine 66. Here, when the once-through exhaust heat recovery boiler is not in a once-through operation, a recirculation line 67 of a dotted line path shown in FIG. 4 is formed, and saturated water separated by the brackish water separator drain tank 64 is supplied to the inlet of the evaporator 63. Circulate to.

  In FIG. 2, the once-through exhaust heat recovery boiler according to the present embodiment has, as a control system, a water level control function unit 23, a superheat degree control function part 38, and a control switching operation for performing control for switching from water level control to superheat degree control. A functional unit 48 and a water supply flow rate adjustment valve 58 are provided. The water level control function unit 23 is basically the same as in FIG. 6 showing the conventional water level control, and the addition value of the brackish water separator water level setting unit 12 and the water level setting bias 13 passes through the change rate limiter 16 and is detected. Compared to the water level 19 of the brackish water separator, the subtractor 20 subtracts the water level deviation. After the water level deviation has passed through the proportional integral derivative controller 21, the detected steam flow rate 18 is added by the adder 22 to obtain a feed water flow rate command value 49 for water level control, which is applied as a signal b to the analog switch T1 (52). Is done.

  The superheat degree control function unit 38 outputs a saturated steam temperature in the function generator 30 based on the pressure detected by the brackish water separator drain tank outlet pressure gauge 29, and adds the saturated steam temperature and the superheat degree set value 31 to the adder. Add at 32. This added value is compared with the steam temperature by the steam separator drain tank outlet steam thermometer 36 and the subtractor 35 through the analog switch 33 and the change rate limiter 34, and is output from the proportional integral derivative controller 28 as a superheat degree deviation. The On the other hand, based on a gas turbine (GT) load (combustion amount) 24 representing the heat quantity of the exhaust gas, a feed water flow rate leading value 25 at the time of superheat control is calculated and passed through a first-order lag element 26, and is added to one of the adders 37. Entered. The other input of the adder 37 is a superheat degree deviation. The output from the adder 37 becomes the feed water flow rate command value 51 at the time of superheat control, and is applied as a signal b to the analog switch T2 (47). The superheat degree control function part 38 mentioned above should just employ | adopt a conventionally well-known structure.

  In the present embodiment, when the transition from the water level control to the superheat degree control in the once-through exhaust heat recovery boiler provided with the brackish water separator drain tank, the control switching operation function unit 48 shown in FIG. To do. As will be described later, the control switching operation function unit 48 outputs the feed water flow rate command value 50 at the time of the control switching operation in the control switching operation 8 connecting the water level control 7 and the superheat control 9 shown in FIG. This is a configuration for applying the signal a to the analog switch T2 (47). Details of the function of the control switching operation function unit 48 will be described with reference to FIG.

  3, FIG. 3 (1) is provided in the analog switch T3 provided in the water level control function unit 23, the analog switch T4 provided in the superheat degree control function unit 38, and the control switching operation function unit 48. A display 59 indicates that T2, T5, T6 and the analog switch T1 provided in the path of the target water supply flow rate command value to be compared with the detected water supply flow rate 55 select the switching symbol under the illustrated switching condition. FIG. FIG. 3B is a diagram for displaying 60 when the change rate limiter V operates and the change rate limit is valid.

  Next, the operation | movement in the starting process of the once-through-type waste heat recovery boiler which concerns on embodiment of this invention is demonstrated below, referring FIG. In the present embodiment, as shown in FIG. 4, superheated steam is generated using the heat quantity of the exhaust gas from the gas turbine (GT). At this time, the GT load (exhaust gas heat amount) from the gas turbine has a gradually increasing characteristic in the boiler start-up process, and the characteristic that the output to the boiler rapidly increases in a step-like manner like other combustion devices other than the gas turbine. It is assumed that it does not have.

  In (1) and (4) of FIG. 1, in the starting process of the once-through exhaust heat recovery boiler according to the present embodiment, the water level control 7 is first performed as the control state 6, and then the control switching operation 8 is performed. Subsequently, the superheat control 9 is performed. The water level 3 of the brackish water separator drain tank is first subjected to water level control 7 and controlled as shown in FIG. Switching start permission condition 1 for switching from water level control to superheat degree control (plant conditions such as steam flow rate and water quality condition), that is, the condition that, for example, steam exceeding 20% of the rated steam flow rate is obtained from the drain tank or superheater, For example, if it is determined that the condition that the impure component concentration in the can water reaches a predetermined value or less is satisfied (this determination is determined by a configuration different from the configuration shown in FIG. 2), the analog switch T1 The control state automatically shifts from the water level control 7 to the control switching operation 8 by switching.

  In FIG. 1 (3), a mode in which the feed water flow rate 5 changes corresponding to each control state 7, 8, 9 is shown, and a substantially constant feed water flow rate is maintained during the period of the water level control 7. When the switching start permission condition 1 is satisfied, a certain amount of the decrease in the feed water flow rate is added to the feed water flow rate 5 at the time of the water level control 7, and the feed water flow rate is the decrease in the feed water flow rate as illustrated in the period of the control switching operation 8. The water level of the tank decreases (see (1) in FIG. 1). As a result, the outlet of the evaporator 63 starts to dry, and the superheat control condition is adjusted. Here, since the GT load due to the exhaust gas keeps a tendency to gradually increase as described above, the superheat degree control condition is established by the drying of the evaporator outlet by the decrement of the constant water supply flow rate shown in (3) of FIG. The control switching operation period, which is the period, can be shortened.

  In (2) of FIG. 1, when the outlet temperature 4 of the brackish water separator drain tank is seen, it is below the saturation temperature at the time of the water level control 7, and exceeds the saturation temperature 11 due to a drop in the tank water level at the end of the control switching operation 8. The outlet temperature is detected. When the outlet temperature exceeds (saturation temperature 11 + predetermined value α), it is determined that the superheat control start condition 2 (see (1) in FIG. 1) is satisfied, and the control switching operation 8 is performed by switching the analog switch T2. The process proceeds to the superheat degree control 9, and thereafter, the outlet temperature 4 changes as shown in the figure depending on the feed water flow rate corresponding to the GT load 24 so as to follow the superheat degree set value 10.

  Referring to FIGS. 1 (5) to (7), the control / operation function units 23, 38, and 48 shown in FIG. The integral differential controller 21, the control switching operation function unit 48 is the analog memory 45, and the superheat control function unit 38 is the proportional integral differential controller 28, and the control state 6 shown in FIG. The proportional integral derivative controller 21 performs water level control, the analog memory 45 performs control switching operation, and the proportional integral derivative controller 28 performs superheat control. In cases other than these controls / operations, tracking is performed according to the movements of the respective water supply flow rate command signals 49, 50, 51 (actually, input to the water supply flow rate adjustment valve 58 by switching the analog switches T1 and T2). Is never done).

  Next, the operation of the control switching operation function unit for the water level control and superheat degree control of the brackish water separator drain tank according to the present embodiment will be described with reference to FIG. In FIG. 2, the control switching operation function unit 48 performs a switching operation between water level control and superheat degree control. During a brackish water separator drain tank water level control operation, if it is determined in another configuration that the control switching start permission condition 1 such as the evaporation amount and water quality condition shown in FIG. 1 (1) is satisfied, the analog switch T1 (52) automatically The input control is switched from “b” to “a.” (The same applies to the analog switch T2).

  When the control switching operation 8 is started, the analog switch T6 (41) outputs a negative value of a signal output from the analog signal generator 39 (a constant ratio (eg, 10 to 30%) of the feed water flow rate 5 balanced at the switching start time). In the analog switch T5 (42), the signal a output from the analog signal generator 39 is similarly selected (the signal b from the analog signal generator 43 that generates 0 t / h is not selected). . The signal a output by the analog signal generator 39 is input to the change rate limiter 44, and the change rate is limited during the control switching operation period from the change rate limiter operating condition table 60 shown in (2) of FIG. . The function of the change rate limiter 44 is described in the top row of FIG. The positive value of the analog signal generator 40 of the control switching operation function unit 48 in FIG. 2 is that the once-through exhaust heat recovery boiler is changed from the once-through type to the circulation type, and the once-through type exhaust heat recovery boiler is stopped. Sometimes for use when increasing the feedwater flow rate.

  The output signal limited by the rate-of-change limiter 44 is stored in the adder 46 during the control switching operation in the analog memory 45 (in the analog memory 45, immediately before shifting from the water level control 7 to the control switching operation 8). Water level control-time feed water flow rate command value 49 during the switching operation) and superheat degree control feed water flow rate leading value 25 (a value including primary delay 26) obtained from GT load (combustion amount) 24 representing the heat quantity of exhaust gas. Are added, and the feed water flow rate command value 50 at the time of the control switching operation is output. The feed water flow value held in the analog memory 45 may be a feed water flow value when the switching start permission condition 1 is satisfied.

  According to the analog switch condition table 59 shown in FIG. 3A, the input signal a is selected in the analog switches 47 (T2) and 52 (T1) during the control switching operation period. The signal a selected by the analog switch 52 is limited by the upper / lower limiter 53, outputs a water supply control deviation from the water supply flow rate 55 detected by the subtractor 54, and the proportional-integral controller 56 performs proportional integration processing. After performing, the automatic / manual switching device 57 is passed to the feed water flow rate adjusting valve 58 as a control command.

  During the period of the control switching operation 8, the superheat degree control is started when the superheat degree control start condition 2 shown in FIG. At that time, during a period of the control switching operation 8, the flow rate 87 of a certain ratio (for example, 10 to 30%) of the feed water flow rate 5 balanced at the switching start time of the switching start permission condition 1 is forcibly and slowly decreased. (Refer to the characteristics of the feed water flow rate in FIG. 1 (3)), the water separator drain tank water level 3 shown in FIG. 1 (1) gradually decreases and the evaporator outlet begins to dry, and the superheat control start condition 2 is satisfied. When the condition of the steam separator drain tank outlet steam temperature> (saturation temperature + specified value α) is satisfied, the superheat degree control 9 is automatically switched. Here, the determination that the condition 2 is satisfied is not shown in the configuration for the determination, but the detected value of the outlet steam temperature, the saturated steam temperature calculated based on the pressure by the outlet pressure gauge, This can be done based on the set specified value α. As described above, the target value is set while receiving the change rate limit 86 (for example, about 2 to 5 ° C./min) by the change rate limiter 34 with the measured value when the superheat degree control start condition 2 is satisfied as a starting point. The value is increased to 10 (see (2) in FIG. 1).

  The measured value detected from the brackish water separator drain tank outlet pressure gauge 29 is input to the function generator 30 to calculate the saturated steam temperature, and the adder 32 adds the saturated steam temperature and the superheat setting value 31. In the analog switch 33 (T4), since the signal a is selected during the superheat degree control from the analog switch condition table 59 shown in FIG. 3 (1), the superheat degree from the change rate limiter operation condition table 60 shown in FIG. 3 (2). The rate of change is limited by the rate-of-change limiter 34 that is effective during control (see the third row of the table shown in FIG. 3 (2), see the rate-of-change limit 86 in FIG. 1 (2)), and the subtractor. 35.

  The subtractor 35 subtracts the measured value of the brackish water separator drain tank outlet thermometer 36 and the superheated steam temperature value based on the saturated steam temperature and the superheat degree setting value, and outputs a steam temperature control deviation. The output steam temperature control deviation is subjected to proportional / integral / derivative processing by the proportional integral derivative controller 28 using the control gain correction 27. In the adder 37, the superheat degree control feed water flow rate leading value 25 (including the primary delay 26) determined from the GT load (fuel amount) 24 representing the exhaust gas heat amount from the gas turbine, and the proportional integral derivative controller 28 The control deviation value is added and the feed water flow rate command value 51 at the time of superheat control is output.

  From the analog switch condition table 59 shown in FIG. 3A, the analog switch 47 (T2) selects the signal b during superheat control, and the analog switch 52 (T1) selects the signal a during superheat control. The selection of the analog switches 47 and 52 is automatically selected when it is determined that the superheat degree control start condition 2 shown in FIG. This automatic selection operates similarly when it is determined that the switching start permission condition 1 is satisfied.

  The superheat degree control water supply flow rate command value 51 is limited in upper and lower limits by a water supply flow rate upper / lower limit limiter 53 via analog switches 47 and 52, and a subtractor 54 outputs a feed water flow rate deviation from the water supply flow rate measurement value 55. Then, the proportional-integral controller 56 performs proportional-integral treatment, and the control command is input to the feed water flow rate control valve 58.

  As described above, the main feature of the once-through exhaust heat recovery boiler according to the embodiment of the present invention is that brackish water is used in order to continuously and smoothly transition from the brackish water separator drain tank water level control to the superheat degree control. At the time of transition from separator drain tank water level control operation to superheat control operation, a control switching operation period is provided, and during this period, the water level is lowered by the operation of temporarily reducing the water supply amount, and the evaporator outlet is forcibly dried. After the steam temperature separator drain tank outlet steam temperature is in the state of (saturation temperature + specified value α), an operation of switching to superheat degree control is performed. At this time, in order to make the transition to the control switching operation smoothly and in a short time, the command signal of the feed water flow rate by the brackish water separator drain tank water level control is tracked to the command signal operated by the control switching operation. Like that. Specifically, the water supply flow rate value immediately before the switching is held in the analog memory 45 provided in the control switching operation function unit 48, and the control switching operation is performed based on the held value and taking into account the decrease in the water supply flow rate. Here, the value of the specified value α is about 1 to 10 ° C. as an example, but for the measurement, if the steam water drain tank outlet steam temperature is clearly distinguishable from the saturated state and can be detected as an overheated state good.

  Thus, by the operation of temporarily reducing the water supply amount within the control switching operation period, the water level of the brackish water separator drain tank is lowered, the evaporator outlet is forcibly dried, and the steam temperature of the brackish water separator drain tank outlet After the temperature exceeds the state of the saturation temperature + the specified value α, the superheat degree control is started to smoothly switch between the water level control and the superheat degree control. Moreover, in this embodiment, since it is considered that the water level control by water supply of the conventional natural circulation type exhaust heat recovery boiler is replaced with the superheat degree control of the evaporator outlet, the control of the conventional natural circulation type exhaust heat recovery boiler is performed. The knowledge of the commissioner skilled in adjustment can be easily adapted to the once-through exhaust heat recovery boiler.

It is a figure explaining the switching method of the water level control of a brackish water separator drain tank, and superheat degree control in the once-through-type waste heat recovery boiler which concerns on embodiment of this invention. It is a figure which shows the control block in the water level control function part, superheat degree control function part, and control switching operation function part of a brackish water separator drain tank in the once-through-type exhaust heat recovery boiler which concerns on this embodiment. It is a figure showing the operating condition of the analog switch and change rate limiter used for the control block shown in FIG. It is a block diagram which shows the structure of the once-through-type waste heat recovery boiler which concerns on this embodiment. It is a block diagram which shows the structure of the natural circulation type waste heat recovery boiler of the boiler water in a prior art. It is a control block diagram of drum water level control in the natural circulation type exhaust heat recovery boiler of the prior art.

Explanation of symbols

1: water level control → superheat degree control switching start permission condition, 2: superheat degree control start condition, 3: brackish water separator drain tank water level, 4: brackish water separator drain tank temperature, 5: feed water flow rate, 6: control state, 7 : Water level control, 8: control switching period, 9: superheat degree control, 10: superheat degree target set value, 11: saturation temperature, 12: brackish water separator water level setter, 13: water level setting bias, 14: adder, 15 : Analog switch, 16: Change rate limiter, 17: Control gain correction, 18: Steam flow rate, 19: Steam separator water level,
20: Subtractor, 21: Proportional integral derivative controller, 22: Adder, 23: Water level control function, 24: GT load (fuel amount), 25: Advance value of feed water flow rate during superheat control, 26: Primary delay, 27 : Control gain correction, 28: proportional integral derivative regulator, 29: brackish water separator outlet pressure gauge, 30: function generator, 31: superheat setting value, 32: adder, 33: analog switch, 34: change rate limit 35: subtractor, 36: steam separator drain tank outlet steam thermometer, 37: adder, 38: superheat control function, 39: analog signal generator, 40: analog signal generator, 41: analog switch, 42: Analog switch, 43: Analog signal generator, 44: Change rate limiter, 45: Analog memory, 46: Adder, 47: Analog switch, 48: Switching operation function,
49: Feed water flow rate command value during water level control, 50: Feed water flow rate command value during switching operation, 51: Feed water flow rate command value during superheat control, 52: Analog switch, 53: Upper / lower limiter, 54: Subtractor, 55: Feed water flow rate, 56: Proportional integral regulator, 57: Automatic / manual switch, 58: Feed water flow rate control valve, 59: Analog switch switching condition table, 60: Change rate limiter effective condition table, 61: Feed water pump, 62: Economizer, 63: evaporator, 64: brackish water separator drain tank, 65: superheater, 66: steam turbine,
67: recirculation line, 68: gas turbine, 69: feed water pump, 70: economizer, 71: steam drum, 72: evaporator, 73: superheater, 74: steam turbine, 75: gas turbine, 76: feed water Flow meter, 77: Drum water level transmitter, 78: Steam flow meter, 79: Specified water level, 80: Subtractor, 81: Proportional integral derivative controller, 82: Adder, 83: Subtractor, 84: Proportional integrator, 85: Feedwater flow rate adjustment valve, 86: Change rate limit, 87: Reduced feedwater flow rate at a certain rate,

Claims (1)

An evaporator that heats feed water supplied by a feed water pump with exhaust gas from a turbine, a brackish water separator tank that separates fluid from the evaporator into brackish water, and superheats steam from the brackish water separator tank with the exhaust gas A superheater, a water level control function unit for controlling the water level of the brackish water separator tank, and a superheat degree control function unit for controlling the superheat degree of the outlet steam of the brackish water separator tank. A recovery boiler,
Control switching that adjusts the feed water flow rate in the transition period for switching control from the water level control state by the water level control function unit to the superheat degree control state by the superheat degree control function unit during the start-up process of the once-through exhaust heat recovery boiler Provide an operation function unit,
The feed water flow rate adjustment command value by the control switching operation function unit is a predetermined feed water flow rate decrease amount added to the feed water flow rate command value at the time of switching the water level control by the water level control function unit,
The permission condition for switching from the water level control state by the water level control function unit to the control switching operation state to which the predetermined water supply flow rate decrease by the control switching operation function unit is added is the steam from the brackish water separator tank or the superheater. It is a condition that the flow rate has reached a predetermined amount,
Further, when the outlet temperature of the brackish water separator tank exceeds (saturation temperature + specified value), the feed water flow rate adjustment command value by the control switching operation function unit is changed to the feed water flow rate command value by the superheat degree control function unit. switching,
The control switching operation function unit is provided with an analog memory and an analog signal generator for generating a negative value,
The analog memory holds a feed water flow rate command value when the water level control is switched by the water level control function unit, and the analog signal generator generates the predetermined reduced feed water flow rate. Heat recovery boiler.

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