Method for controlling the supply of feed water to a boiler

Abstract

The present invention relates to a system and method for controlling the supply of feedwater to a boiler 10. The boiler 10 includes a feedwater inlet 20 for supplying feedwater to the boiler 10, a steam outlet 22 for removing steam from the boiler 10, and a liquid level sensor 14 for measuring a liquid level within the boiler 10. The steam outlet 22 includes a steam valve 26 that defines a steam valve 26 opening, and the method includes measuring the liquid level within the boiler 10 using the liquid level sensor 14, determining a rate of change of the steam valve 26 opening, and controlling the supply of feedwater to the boiler 10 via the feedwater inlet 20 based on the liquid level within the boiler 10, a reference liquid level for the boiler 10, and the rate of change of the steam valve 26 opening.

Description

Technical Field

[0001] The present invention relates to a method for controlling the supply of feed water to a boiler, a system for controlling the supply of feed water to a boiler, a computer program, and a computer-readable medium.

Background Art

[0002] The present invention relates to a strategy for controlling the water level in a boiler that undergoes shrinkage and expansion phenomena. These phenomena can occur when there is a change in the steam load on the boiler (the main disturbance in the system). Such a change in the steam load will affect the steam pressure in the boiler, which in turn affects the amount of steam floating in the boiler water. This leads to a so-called reverse response of the water level (non-minimum phase behavior), where the initial response of the water level is opposite to the long-term behavior.

[0003] For example, if the steam load increases, the boiler pressure will decrease, which leads to the expansion of the steam bubbles floating in the water. This expansion first leads to an increase in the water level, followed by the redistribution of steam and water in the boiler. The redistribution leads to a long-term behavior of a decrease in the water level. A water level control strategy has been designed to counter the adverse effects of the reverse response of the water level to disturbances.

[0004] The adverse effects can be explained as follows. Due to the initial rise in the water level caused by the pressure drop during load increase, the feed water flow will decrease when an increase is actually required by the control. This initial decrease in the feed water flow worsens the subsequent decrease in the water level. Therefore, it is necessary to reduce the fluctuations in the boiler water level by avoiding such an initial response of the control.

[0005] Boiler drum level control systems are known in the prior art. Patent Document 1 describes a boiler drum level control system that uses the position of a bypass valve configured to control the steam flow. The controller commands the feed water flow and the heat input to the evaporator based on the sensor signals. The sensor signals are generated by sensors that measure fluid flow, steam flow, drum pressure, drum temperature, and bypass position. The drum pressure can be directly measured by detecting the position of the bypass valve, which provides an indication of the pressure change in the boiler and may lead to the adjustment of the water level in the drum.

[0006] Patent Document 2 describes a boiler water level control device that branches excess steam to a condenser and controls the opening degree of a bypass valve to bypass the steam turbine. A pressure sensor is used to determine the steam flow rate.

[0007] Patent Document 3 describes a boiler drum level control method that uses the difference between a steam flow signal and a feed water flow signal. A transient controller calculates a gain parameter based on the absolute flow difference. Additional parameters include the position of the bypass valve.

[0008] Patent Document 4 describes how the change in the steam load of a marine boiler is controlled by sending a signal to a PLC that adjusts the feed water valve. The water level and steam flow rate are measured. The steam flow rate is introduced as a feed forward control signal. When the steam load suddenly changes, the feed water control valve first moves in the correct direction by the steam flow rate signal, that is, when the steam flow increases, the feed water control valve opens. A vortex flow meter is used to monitor the load.

[0009] Patent Document 5 describes a method for controlling the liquid level in a boiler using filtered output signals representing liquid level, gas / steam flow rate, supply liquid flow rate, vessel pressure, and vessel temperature.

[0010] Patent Document 6 describes a steam level control system for a boiler that measures the position of a bypass valve that bypasses a heat recovery steam generator.

[0011] Patent Document 7 describes a steam turbine control device having a steam control valve opening detector. This is not related to the supply of feed water to the boiler.

[0012] Non-Patent Document 1 and describe a non-linear dynamic model for a natural circulation drum boiler.

[0013] Non-Patent Document 2 describes edge detection in signal processing.

[0014] Non-Patent Document 3 describes a feed-forward water level control scheme using steam flow.

[0015] Non-Patent Document 4 also describes a water level control scheme using steam flow.

Prior Art Documents

Patent Documents

[0016]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Non-Patent Documents

[0017]

Non-Patent Document 1

[0018] It is an object of the present invention to reduce fluctuations in boiler water level by avoiding negative initial responses of control while avoiding dependence on a steam flow sensor. Such a sensor would increase the cost of the system and reduce its reliability. [Means for Solving the Problems]

[0019] The above object is achieved, in a first aspect of the present invention, by a method for controlling the supply of feed water to a boiler, the boiler comprising a feed water inlet for supplying feed water to the boiler, a steam outlet for taking out steam from the boiler, and a liquid level sensor for measuring the liquid level in the boiler, the steam outlet including a steam valve defining a steam valve opening degree, the method comprising: measuring, using the liquid level sensor, the liquid level in the boiler; determining a rate of change of the steam valve opening degree; controlling the supply of feed water to the boiler via the feed water inlet based on the liquid level in the boiler, a reference liquid level for the boiler, and the rate of change of the steam valve opening degree; and including.

[0020] The boiler includes a container for containing water and steam. Water is supplied to the boiler via the feed water inlet. Heat input to the container generates a flow. Steam is taken out of the boiler via the steam outlet. The steam outlet is controlled by a steam valve defining a steam valve opening degree. The generated steam can be used to operate a steam turbine or other types of steam-consuming equipment.

[0021] The boiler also includes a liquid level sensor. The liquid level sensor measures the liquid level in the boiler. Below the liquid level, the container includes a two-phase flow containing liquid water and steam bubbles, and above the liquid level, the container essentially includes steam.

[0022] The liquid level in the boiler is typically controlled by adjusting the supply of feed water to the boiler by a supply pump and / or a valve using a control scheme in response to the measured liquid level in the boiler. However, when the steam load on the boiler increases, the boiler pressure will decrease. This leads to the release of steam bubbles floating in the liquid water, which causes the liquid level to initially expand. Thus, the liquid level in the boiler first rises and then decreases as more steam is taken out of the boiler. Using a simple controller, such as a simple PID that does not consider the expansion phenomenon, in response to the initial rise in the liquid level in the boiler, the feed water to the boiler is decreased, which will subsequently lead to the liquid level of the water in the boiler becoming too low as more steam is taken out of the boiler. This may lead to damage to the boiler due to the low liquid level. Instead, the initial response should be to increase the feed water to counter the increase in the extraction of steam from the boiler.

[0023] The present solution determines the supply of feed water to the boiler using a signal representing the opening degree of the steam valve and a signal representing the liquid level in the boiler. These two signals are readily available, and thus there is no need for an additional sensor such as a boiler steam flow sensor. The reference liquid level for the boiler is a constant representing the normal liquid level under steady state conditions and is determined by the design of the boiler. The rate of change of the steam valve opening degree is determined from the signal representing the opening degree of the steam valve and is used to compensate for the reverse response to disturbances, i.e., the decrease in the liquid level when the steam load increases. When the steam valve opening degree is constant, i.e., when the steam valve is stationary and the steam load is constant, the rate of change of the steam valve opening degree is zero. When the steam valve opening degree is changing due to a change in the load, i.e., when the steam valve is moving, the rate of change of the steam valve opening degree is non-zero.

[0024] By basing the supply of feed water to the boiler on the measured liquid level in the boiler, the reference liquid level for the boiler, and the rate of change of the steam valve opening degree, the reverse response to disturbances can be avoided or reduced.

[0025] According to a further embodiment, the water inlet includes a water supply valve that defines the water supply valve opening degree, and the method includes the step of determining the supply of water to the boiler based on the water supply valve opening degree.

[0026] To control the supply of water to the boiler, a water supply valve can be used. The opening degree of the water supply valve can be utilized as an operating signal for the boiler water level controller.

[0027] According to a further embodiment, the water inlet is connected to a pump, the pump defines a flow rate, and the supply of water to the boiler is determined by the flow rate.

[0028] By using a flow control pump, the water supply valve in the water supply line can be omitted.

[0029] According to a further embodiment, the flow rate is based on the rotational speed of the pump.

[0030] To change the supply of water, the rotational speed of the pump can be changed.

[0031] According to a further embodiment, the step of controlling the supply of water to the boiler is performed using a PI control scheme.

[0032] The control operation is designed to minimize the adverse effects of changes in disturbances. A PI (Proportional Integral) controller performs calculations for the proportional integral algorithm based on the integral parameter and the proportional or gain parameter, and the generated output is a flow control signal used to control the supply of water to the boiler.

[0033] According to a further embodiment, the step of controlling the supply of water to the boiler includes generating a water supply control operation based on the difference between the liquid level in the boiler and a reference liquid level for the boiler, and determining the supply of water to the boiler based on the sum of the water supply control operation and the rate of change of the steam valve opening degree.

[0034] The rate of change of the steam valve opening signal can be added in a feed-forward manner, i.e., after the controller and added to the control operation. Based on this, the opening of the supply valve (or alternatively the flow rate / speed of the supply pump) is controlled. In this way, when a disturbance occurs, a correction term for the feed water control operation is introduced immediately without having to wait for the water level to drop before the controller takes corrective measures. The controller typically uses a linear SISO control scheme to generate the control operation. The controller is preferably a PI controller, and the control operation is preferably a PI control operation.

[0035] According to a further embodiment, the step of controlling the supply of feed water to the boiler includes generating a feed water control operation based on the sum of the rate of change of the steam valve opening and the difference between the liquid level in the boiler and a reference liquid level for the boiler, and determining the supply of feed water to the boiler according to the feed water control operation.

[0036] The rate of change of the steam valve opening signal can alternatively be added in a feedback manner, i.e., added to the liquid level measured before the controller. Based on this, the opening of the supply valve (or alternatively the speed of the supply pump) is controlled. In this way, a correction term for the feed water control input is introduced to the controller taking corrective measures. The controller typically uses a linear SISO control scheme to generate the control operation. The controller is preferably a PI controller, and the control operation is preferably a PI control operation.

[0037] According to a further embodiment, the method includes determining the rate of change of the steam valve opening by using a phase advance filter for the steam valve opening.

[0038] The opening of the feed water is controlled by the disturbance compensation in the water level control provided by the advance filter. The advance filter provides a zero output when the input to the filter is constant, but provides a non-zero output when the input changes, i.e., when the steam load suddenly changes. The filtered rate of change of the steam valve opening replaces the rate of change of the steam valve opening in the above feed-forward and feedback schemes.

[0039] According to a further embodiment, the method comprises the steps of: measuring a pressure in the boiler using a pressure sensor; controlling heat input to the boiler based on the pressure in the boiler and a reference pressure for the boiler; Further includes:

[0040] The heat input to the boiler can be controlled by a second controller. The heat input is based on pressure measurements and a reference pressure for the boiler. The pressure measurements are made by a pressure sensor in the vessel. The pressure therefore represents the total energy in the boiler. The reference pressure for the boiler is a constant representing the normal operating pressure under steady state conditions and is determined by the design of the boiler.

[0041] According to a further embodiment, controlling the heat input to the boiler includes generating a heat input control action based on a difference between a pressure in the boiler and a reference pressure for the boiler, and determining the heat input to the boiler by the heat input control action.

[0042] The heat input to the liquid in the vessel is based on a control action: higher heat input will generate more steam, which results in higher pressure, and vice versa.

[0043] According to a further embodiment, the step of controlling the heat input to the boiler is performed using a PI control scheme.

[0044] The control action is designed to minimize the adverse effects of changes in disturbances. A PI (Proportional Integral) controller performs the calculations for a proportional-integral algorithm based on an integral parameter and a proportional or gain parameter, and the generated output is the flow control signal.

[0045] The above object is achieved, in a second aspect of the present invention, by a boiler having a control system for controlling the supply of feed water to the boiler. The boiler includes a feed water inlet for supplying feed water to the boiler, a steam outlet for taking out steam from the boiler, and a liquid level sensor for measuring the liquid level in the boiler. The steam outlet includes a steam valve for defining the steam valve opening degree. The control system controls the supply of feed water to the boiler through the feed water inlet based on the liquid level in the boiler, a reference liquid level for the boiler, and the change rate of the steam valve opening degree.

[0046] According to a further embodiment, the above boiler according to the second aspect can include any of the features of the method according to the first aspect.

[0047] The above object is achieved, in a third aspect of the present invention, by a computer program including instructions which, when the program is executed by a computer, cause the computer to execute the method according to the first aspect.

[0048] A control solution including feedback disturbance compensation according to the foregoing aspect can preferably be implemented as software in a programmable logic controller (PLC).

[0049] The above object is achieved, in a fourth aspect of the present invention, by a computer-readable medium storing the computer program according to the third aspect.

Brief Description of the Drawings

[0050]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Mode for Carrying Out the Invention

[0051] FIG. 1 is a simplified schematic diagram of the boiler 10. The boiler 10 includes a container 12. The container defines a liquid level indicated by a wavy line. The liquid level is measured by a liquid level sensor 14. Below the liquid level, there is a two-phase flow 16 of liquid water and steam bubbles, while above the liquid level, there is mainly steam 18. The boiler 10 further includes a water supply inlet 20 for supplying water to the container 12 and a steam outlet 22 for taking out steam from the container 12. The water supply inlet 20 is supplied via a pump 24' and is controlled by a water supply inlet valve 24, and the steam outlet is controlled by a steam outlet valve 26.

[0052] The boiler 10 further includes a riser 28 and a downcomer 30 that form a closed loop together with the container 12. Heat is supplied to the riser, causing boiling. Steam rises to the container, causing circulation in the loop of riser 28 - container 12 - downcomer 30. Only one riser and one downcomer are presented in this schematic diagram, but in reality, many risers and downcomers are used. The boiler 10 further includes a pressure sensor 32 for measuring the pressure inside the container 12.

[0053] Figure 2 is a block diagram showing the feedwater level control strategy according to the present invention. The block with reference number 34 represents the plant, i.e., the boiler. Therefore, the inputs to block 34 are the feedwater supply q f and the heat supply Q, while the outputs from block 34 are the measured liquid level l and the measured pressure p in the vessel. The disturbance is the steam outflow q s , but according to the present invention, the steam outflow q s is not directly measured. The steam outflow q s from the boiler is controlled by the steam valve with reference number 36, and the measurable disturbance at this time is the opening degree OD st of the steam valve 36. The feedwater supply q f has the opening degree OD fw of the feedwater valve 38 as an input, and is controlled by the feedwater valve / pump 38 and optionally by the pressure p in the boiler and the rotational speed ω of the pump. Since the boiler pressure p is the pressure on the secondary side of the feedwater valve / pump, it may affect the feedwater flow to the boiler.

[0054] The blocks marked "PI" with reference numbers 40 and 42 implement proportional-integral control operations for the water level l and the boiler pressure p, respectively. The control operations should be designed to minimize the adverse effects of changes in the disturbance, i.e., changes in the steam load, on the water level in the boiler. As a baseline, the control system is equipped with two independent PI controllers, one controlling the heat input based on the pressure measurement and the other controlling the feedwater input based on the water level measurement. The following standard PI control equations are used for the two system inputs q f and Q.

[0055]

Equation

[0056] where ξ (·) represents the state of each controller, K (·) >0 is the integral gain, N (·) >0 is the proportional gain, (·) refindicates the reference value for each output. The response of the closed-loop system using this baseline control is the exact opposite of what is required, which worsens the subsequent decrease in the boiler water level. See Figure 4. The initial response of the PI controller to an increase in steam load is to decrease the feed water flow rate. This can be particularly problematic if the boiler volume is relatively small.

[0057] The main strategy for improving the closed-loop system response to changes in steam load is to use a filter that has zero output for a constant input and always gives a non-zero output when the input signal to the filter changes value, to detect the (rising / falling) edge of the disturbance.

[0058] The opening degree OD of the steam valve st is supplied to the lead filter 44 that generates the filter output y ll . These parts are marked with dashed blocks and wires. The path leading to the block with reference number 46 is the feedforward strategy L ff while the path leading to the block with reference number 48 is the feedback strategy L fb .

[0059] Figure 3 shows the step response of the phase lead filter. To detect the edge of the change in the disturbance k v , a phase lead filter with zero dc gain is used, which is described by the following continuous-time differential equation.

[0060]

Equation

[0061] where p ll > 0 is the pole of the filter. Here, the term

[0062]

Equation

[0063] is not well - defined for the partition constant k v However, to explain the intuition behind the filter, we can assume that k v can be defined as something like an impulse (Dirac delta) at the point when its value changes. Since the above non - mandatory equation describes an integrator, the output response of the filter to this impulse first rises to 1 at the time of the impulse and then decays to zero. The rate of this decay is determined by the pole p ll .

[0064] The continuous - time implementation of [Equation 5] has some problems with the non - differentiable k at the end v , but the filter needs to be implemented on a computer in discrete time, which is not a problem as long as the change in k v between samples is finite. In discrete time, [Equation 5] can be implemented as follows at the k - th sample.

Number

[0065] where a0, a1, b1 > 0 are filter parameters. The parameters of [Equation 6] can be calculated based on the proposal given in Non - Patent Document 2, i.e., a0 = a1=(1 + b2) / 2 and 0 < b1 = 1, where 1 - b1 is the desired decay between samples. That is, within the samples of x, the filter has decayed to (b1) x of the initial value. Thus, the parameter b1 is related to the pole p II of the continuous - time filter. The decay parameter b1 is chosen as follows in this case. It may be desirable to design the filter to decay to 10% of the initial value within T seconds. At the sample time T s1 , this corresponds to the sample of T / T s until the filter decays to 0.1 times the initial value. Then, b1 is given by b1=(0.1) T s / T . Next, two strategies for compensating for disturbances using the output of the filter are explained.

[0066] The first strategy for improving the response of level control is to add the output of the filter in [Equation 5] to the control signal from PI control in a feed-forward manner. The intention is to add a corrective term to the feedwater control input as soon as a disturbance occurs without having to wait for the water level to drop before the PI controller takes corrective action. By adding this term as feed-forward, any tampering with the PI loop can be avoided. The feedwater control input thus takes the following form.

[0067]

Number

[0068] Here, L ff > 0 is the feed-forward gain. The gain L ff should be in a region where L ff y II has the same number of digits as the output of the PI controller.

[0069] The second strategy for improving the response of level control is to add the output of the filter in [Equation 5] to the level measurement in a feedback manner. The intention is to directly compensate for the immediate increase (decrease) in water level resulting from an increase (decrease) in steam load in the level measurement.

[0070]

Number

[0071] Here, L fb > 0 should be in a region where l and L fb y II have the same number of digits.

[0072] Simulations are being performed to verify the control scheme. The simulation model used in the simulations is described in Non-Patent Document 1 and. The boiler used in the simulation model is mainly shown in FIG. 1.

[0073] Two water level control strategies designed to counter the adverse effects of the water level inverse response to external disturbances are used in the simulation. This adverse effect is shown by the baseline control strategy included for comparison and can be explained as follows. Due to the initial rise in water level caused by the pressure drop during load increase, the feed water flow will decrease when an increase is actually required by the control. This initial decrease in water flow worsens the subsequent water level decrease. See Figure 4. Therefore, it is desirable to avoid such an initial response of the control.

[0074] In addition to the two control strategies, two cases that differ in terms of the availability of sensors and actuators are simulated. In the first case, an ideal scenario is simulated where the feed water flow can be directly actuated and the steam valve resistance is a measurable disturbance. In the second case, a more realistic scenario is simulated where the feed water flow is controlled by the valve opening and the steam valve opening is a measurable disturbance.

[0075] The system model derived in Non-Patent Document 1 is a non-linear state space model with four state variables. These state variables are the drum pressure p, the total amount of water V in the drum, riser, and downcomer wt , the quality of the steam at the riser outlet (i.e., the mass fraction of steam) α r , the amount of steam below the water level in the drum V sd . The exogenous inputs to the system are the feed water mass flow rate q f to the boiler, the heat input Q to the riser, and the steam mass flow rate q s from the boiler. See Figure 1.

[0076] Figures 4 to 6 show the results of simulations for a more ideal case where the feed water flow is directly available for actuation and the steam valve resistance is a measurable disturbance. In each simulation, a step is added to the disturbance (here the valve resistance parameter) and the closed-loop system response is observed. In all three cases, the disturbance step is executed at time t = 10000 seconds and corresponds to an increase of approximately 30% in the desired energy / steam generation. It should be noted that the results obtained with the individual control actions may be improved since little time is spent on tuning.

[0077] Figure 4 shows the simulation results when the boiler water level is controlled using the standard / baseline proportional-integral control action in [Equation 1]. Four plots of different time series are given. The upper left plot shows the boiler water level l with respect to some desired reference. The upper right plot shows the control signal q f or the feed water input. The lower left plot shows the boiler pressure p. The lower right plot shows the control signal Q for the heat input. The settings here are that the feed water control should hold the relative boiler water level at a reference of 0 m, while the heat control should hold the boiler pressure at a reference value of 8 MPa.

[0078] The initial response of the feed water control to the disturbance is to decrease the feed water input due to the obvious expansion phenomenon from the plot of the water level that reaches a maximum of 1.8 [cm] around time t = 10100 seconds. This control response worsens the next decrease in the water level, which reaches a minimum of -8.5 cm below the desired level around time t = 12900 seconds.

[0079] Note that minimal effort has been spent on adjusting the gains of these baseline controllers. However, for the sake of explanation, they are fulfilling their purpose.

[0080] Figure 5 shows the simulation results when the feedforward operation described by [Equation 7] is added to the boiler water level control. The layout in the figure is the same as that in Figure 4. Despite the initial inverse response of the water level, the initial response of the feedwater control to the disturbance has changed so that the increase in steam demand leads to an increase in feedwater here. Furthermore, the subsequent decrease in the water level is reduced by about 37% here and reaches a minimum of -5.3 cm around time t = 15600 seconds. However, the increase in the boiler water level before the decrease is increased by 370% here and reaches a maximum of 8.4 cm around time t = 10700 seconds.

[0081] Figure 6 shows the simulation results when the disturbance feedback operation described by [Equation 9] is added to the boiler water level control. Again, the layout is the same as that in Figures 4 and 5. Despite the initial increase in the water level due to expansion, the initial response of the control is to increase the feedwater input. Compared with the baseline control, the subsequent decrease in the boiler water level is reduced by about 90% to -0.8 cm. Furthermore, compared with the baseline control, the preceding rise in the boiler water level increases by 172% to 4.9 cm at time t = 10100 seconds, which is less than half of the increase obtained using the feedforward correction operation.

[0082] Figures 7 to 10 show the simulation results of the more realistic second case described above. In the simulation results presented in Figures 4 to 6, it was assumed that the PI controller used to control the boiler water level had access to the measurement of the steam flow valve resistance and the controller could directly actuate the feedwater flow. Such an operation may be achievable using an internal loop flow controller. However, such an internal loop controller would depend on the measurement of the feedwater flow.

[0083] The setup is as shown in Figure 1. Here, a device for supplying water to the boiler and a valve for determining the steam flow are shown. 1) The opening degree OD of the steam valve stis known and can be used as an input to the boiler water level controller, 2) the feed water pump operates at a constant speed, and 3) the opening degree OD fw of the feed water valve is available as an actuating signal for the boiler water level controller. This is assumed to be the case.

[0084] Again, three scenarios for the boiler settings are simulated. 1) The first simulated scenario is based on the baseline PI boiler water level control by [Equation 1] and [Equation 2] that controls the opening degree of the feed water valve here based on the measurement of the boiler water level, that is

[0085]

Equation

[0086] where OD fw indicates the opening degree of the feed water valve.

[0087] 2) The second simulated scenario is by feedforward disturbance compensation using an advanced filter version of the steam valve opening signal OD st and can be expressed by the following equation.

[0088]

Equation

[0089] The feedforward disturbance compensation control

[0090]

Equation

[0091] can be described by

[0092] 3) The third simulated scenario is the filtered OD such that the disturbance compensation control can be described by the following equation stIt is based on feedback disturbance compensation that re-uses the signal.

[0093]

Number

[0094] Figure 7 shows the simulation results obtained using the baseline boiler water level controller defined in the controller equation [Equation 11]. Again, a step in the disturbance, here OD st is introduced at time t = 10000 seconds. As simulated previously, the immediate response of the boiler water level to the disturbance rises to 1.7 [cm] around time t = 10060 seconds. As can be seen from the figure, the subsequent response of the boiler water level drops to -3.25 cm around time t = 10200 seconds. Following this drop, there is a recovery, and then subsequently the water level drops to -1.55 cm around time t = 12000 seconds.

[0095] It should be noted again that not much time has been spent adjusting the gain of the baseline controller. However, overall, the baseline controller here seems to be adjusted to react more aggressively than in the first round of the simulation.

[0096] Figure 8 shows the simulation results obtained using feedforward disturbance compensation as defined in the controller equation [Equation 14]. Here, it can be seen that compared to the baseline controller, the initial rise of the boiler water level deteriorates by about 130% and reaches a maximum of 3.88 cm around time t = 10060 seconds. Subsequently, the boiler water level drops, which is a 40% decrease compared to the maximum drop in the baseline case and reaches a minimum of -1.92 cm around time t = 12050 seconds.

[0097] Figure 9 shows the simulation results obtained using feedback disturbance compensation as defined in the controller equation [Equation 16]. Comparing with the baseline control, it can be seen again that the rise of the water level has deteriorated. Here, the maximum (second) rise increases by 96% and reaches a maximum of 3.34 cm near time t = 10600 seconds. The maximum drop of the boiler water level occurs near time t = 10180 seconds, and the minimum water level is -0.72 cm, which is a 78% decrease compared to the baseline case.

[0098] Figure 10 shows the results of the simulation of the above control solution including feedback disturbance compensation implemented in the PLC (Programmable Logic Controller) software. This implementation has been tested in a closed loop that emulates the PLC using the "PLCSim" software by Siemens. Comparing the results shown in Figure 10 with the results shown in Figure 9 can verify the accuracy of the PLC implementation. Overall, the results are the same except for some quantization noise due to rounding errors resulting from the finite representation of the control output in the PLC. This quantization is shown in the zoomed area in Figure 10.

Explanation of Symbols

[0099] 10 Boiler 12 Container 14 Liquid level sensor 16 Two-phase flow 18 Steam 20 Feed water inlet 22 Steam outlet 24 Feed water inlet valve 24′ Pump 26 Steam outlet valve 28 Rising pipe 30 Downcomer 32 Pressure sensor 36 Steam valve 38 Feed water valve 44 Advance filter

Claims

1. A method for controlling the supply of feed water to a boiler (10), wherein the boiler (10) comprises a feed water inlet (20) for supplying the feed water to the boiler (10), a steam outlet (22) for taking out steam from the boiler (10), and a liquid level sensor (14) for measuring the liquid level inside the boiler (10), and the steam outlet (22) comprises a steam valve (26) that defines the opening degree of the steam valve (26). In the method, the method comprises measuring the liquid level inside the boiler (10) by the liquid level sensor (14); determining the rate of change of the opening degree of the steam valve (26); controlling the supply of feed water to the boiler (10) through the feed water inlet (20) based on the liquid level inside the boiler (10), a reference liquid level for the boiler (10), and the rate of change of the opening degree of the steam valve (26); and the method further comprises measuring the pressure inside the boiler (10) by a pressure sensor (32); controlling the heat input to the boiler (10) based on the pressure inside the boiler (10) and a reference pressure for the boiler (10). A method.

2. The feed water inlet (20) comprises a feed water valve (24) that defines the opening degree of the feed water valve (24), and the method according to claim 1 comprises determining the supply of feed water to the boiler (10) by the opening degree of the feed water valve (24).

3. The feed water inlet (20) is connected to a pump (24'), the pump (24') defines a flow rate, and the method according to claim 1 or 2 comprises determining the supply of feed water to the boiler (10) by the flow rate.

4. The flow rate is based on the rotational speed of the pump (24'), according to the method of claim 3.

5. The step of controlling the supply of feed water to the boiler (10) is executed by a PI control scheme (40), according to any one of claims 1 to 4.

6. The step of controlling the supply of feed water to the boiler (10) generates a feed water control operation based on the difference between the liquid level inside the boiler (10) and the reference liquid level for the boiler (10). Based on the sum of the water supply control operation and the rate of change of the opening degree of the steam valve (26), determining the supply of water to the boiler (10); The method according to any one of claims 1 to 5, comprising this.

7. The step of controlling the supply of water to the boiler (10) is Based on the rate of change of the opening degree of the steam valve (26) and the sum of the difference between the liquid level inside the boiler (10) and the reference liquid level for the boiler (10), generating a water supply control operation; Determining the supply of water to the boiler (10) by the water supply control operation; The method according to any one of claims 1 to 5, comprising this.

8. The method according to any one of claims 1 to 7, wherein the method comprises the step of determining the rate of change of the opening degree of the steam valve (26) by using a phase advance filter (44) for the opening degree of the steam valve (26).

9. The step of controlling the heat input to the boiler (10) is Based on the difference between the pressure inside the boiler (10) and the reference pressure for the boiler (10), generating a heat input control operation; Determining the heat input to the boiler (10) by the heat input control operation; The method according to claim 1, comprising this.

10. The step of controlling the heat input to the boiler (10) is executed by a PI control scheme (42), the method according to any one of claims 1 to 9.

11. The boiler (10) having a control system for controlling the supply of water to the boiler (10), a water supply inlet (20) for supplying water to the boiler (10), a steam outlet (22) for taking out steam from the boiler (10), a liquid level sensor (14) for measuring the liquid level inside the boiler (10), and a pressure sensor (32) configured to measure the pressure inside the boiler (10), in the boiler (10), The steam outlet (22) is provided with a steam valve (26) that defines the opening degree of the steam valve (26), The control system controls the supply of water to the boiler (10) through the water supply inlet (20) based on the liquid level inside the boiler (10), the reference liquid level for the boiler (10), and the rate of change of the opening degree of the steam valve (26). A boiler, wherein the control system is configured to control heat input to the boiler (10) based on the pressure inside the boiler (10) and a reference pressure for the boiler (10).

12. A computer program comprising instructions, wherein, when the computer program is executed by a computer, the instructions cause the computer to execute the method according to any one of claims 1 to 10.

13. A computer-readable medium storing the computer program according to claim 12.

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