Method of controlling supply of feed water into a boiler, control system and boiler having the control system

Abstract

The invention relates to a system and method of controlling supply of feed water into a boiler (10). The boiler (10) comprises a feed water inlet (20) for supplying feed water into the boiler (10), a steam outlet (22) for taking steam from the boiler (10), and a liquid level sensor (14) for measuring a liquid level in the boiler (10). The steam outlet (22) comprises a steam valve (26) defining a steam valve (26) opening degree, the method comprising measuring the liquid level in the boiler (10) using the liquid level sensor (14), determining a rate of change of the steam valve (26) opening degree, and controlling the supply of feed water into the boiler (10) via the feed water inlet (20) based on the liquid level in the boiler (10), a reference liquid level for the boiler (10), and the rate of change of the steam valve (26) opening degree.

Description

TECHNICAL FIELD

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

[0002] The present invention relates to a strategy for controlling the water level in a boiler that experiences phenomena of shrinkage and expansion. These phenomena can occur when there is a change in the steam load on the boiler (a major 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 suspended in the boiler water. This results in a so-called inverse response of the water level (non-minimum phase behavior), in which the initial response of the liquid level is opposite to the long-term behavior.

[0003] For example: if the steam load increases, the boiler pressure will drop, which results in expansion of the steam bubbles suspended in the water. This expansion first results in an increase of the water level, and subsequently in a redistribution of steam and water in the boiler. The redistribution results in a long-term behavior of the water level dropping. The water level control strategy is designed to accommodate the adverse effect of the inverse response of the water level to the disturbance.

[0004] This adverse effect can be shown as follows: due to the initial increase of the water level as a result of the pressure drop during the load increase, the control will decrease the feed water flow when it actually needs to be increased. This initial decrease of the feed water flow exacerbates the subsequent drop of the water level. Therefore, it is desirable to reduce the fluctuations of the boiler water level by avoiding such initial control response.

[0005] Boiler drum level control systems are known in the art. US 9476584 describes a boiler drum level control system that uses the position of a bypass valve configured to control the steam flow. A controller controls the feed water flow and the heat input to the evaporator based on sensor signals. The sensor signals are generated by sensors that measure the fluid flow, the steam flow, the drum pressure, the drum temperature and the bypass position. The drum pressure can be measured directly by detecting the position of the bypass valve, which provides a leading indicator of the pressure changes in the boiler that can result in an adjustment of the water level in the drum.

[0006] JP 2504939 describes a boiler level control device that controls the opening of a bypass valve for diverting excess steam to a condenser, thereby bypassing a steam turbine. A pressure sensor is used to determine the steam flow.

[0007] US 10185332 describes a boiler drum level control method that uses the difference between the steam flow signal and the feed water flow signal. A transient controller calculates a gain parameter based on the absolute flow difference. Additional parameters include the bypass valve position.

[0008] CN 109028023 describes how to control changes in the steam load of a marine boiler by sending signals to a PLC that regulates the feed water valve. The water level and the steam flow are measured. The steam flow rate is introduced as a feed forward control signal. When the steam load changes suddenly, the steam flow rate signal causes the feed water regulating valve to initially move in the correct direction, i.e. when the steam flow increases, the feed water regulating valve opens. A vortex flow meter is used to monitor the load.

[0009] US 7931041 describes a method for controlling the liquid level in a boiler using filtered output signals representing the liquid level, gas / steam flow rate, feed liquid flow rate, vessel pressure, and vessel temperature.

[0010] US 10323547 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] JP 2018080672 describes a steam turbine control device with a steam control valve opening degree detector. It does not involve the supply of feed water into the boiler.

[0012] K. J. Åström and R. D. Bell, “Dynamics of a drum boiler”, Automatica, vol. 36, pp. 363-378, 2000, describe a nonlinear dynamic model for a natural circulation drum boiler.

[0013] S. W. Smith, The Scientist and Engineer’s Guide to Digital Signal Processing, 2nd Edition, (California Technical Publishing, 1999), describes edge detection in signal processing.

[0014] G. F. Gilman, Boiler Control System Engineering, 1st Edition, (ISA - Instrumentation, Systems, and Automation Society, 2005), describes a feed forward level control scheme using steam flow.

[0015] D. Lindsley, Power Plant Control and Instrumentation - Control of Boiler and Heat Recovery Steam Generator Systems, 1st Edition, (The Institute of Electrical Engineers, 2000), also describes a level control scheme using steam flow.

[0016] It is an object of the present invention to reduce fluctuations in the boiler water level by avoiding an undesirable initial control response while avoiding the dependency on a steam flow sensor. Such a sensor would increase the cost of the system and reduce the reliability. SUMMARY

[0017] In a first aspect of the application, the above object is achieved by a method of controlling the supply of feed water into a boiler, the boiler comprising a feed water inlet for supplying feed water into the boiler, a steam outlet for taking steam from the boiler, and a liquid level sensor for measuring the liquid level in the boiler, the steam outlet comprising a steam valve defining a steam valve opening degree, the method comprising:

[0018] measuring the liquid level in the boiler using the liquid level sensor,

[0019] determining a rate of change of the steam valve opening degree, and

[0020] controlling the supply of feed water into 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.

[0021] The boiler comprises a vessel for containing water and steam. Water is supplied into the boiler via a feed water inlet. Steam is generated by heat input into the vessel. Steam is taken from the boiler via a 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 type of steam consumer.

[0022] The boiler further comprises a liquid level sensor. The liquid level sensor measures the liquid level in the boiler. Below the liquid level, the vessel comprises a two-phase flow comprising liquid water and steam bubbles, and above the liquid level, the vessel mainly comprises steam.

[0023] The liquid level in the boiler is generally controlled using a control scheme by adjusting the supply of feed water into the boiler by means of a feed pump and / or a valve in response to the measured liquid level in the boiler. However, when the steam load on the boiler increases, the boiler pressure will drop. This causes the steam bubbles suspended in the liquid water to release, whereby the liquid level will initially expand. Thus, the liquid level in the boiler initially rises, and then drops as more steam is taken from the boiler. Using a naive controller, e.g. a simple PID not taking the expansion phenomenon into account, the response to the initial liquid level rise in the boiler will be to reduce the water fed into the boiler, which subsequently will cause the liquid level of the water in the boiler to be too low as more steam is taken from the boiler. This can potentially lead to damage of the boiler due to low liquid level. Instead, the initial response should be to increase the feed water to counteract the increase of steam taken from the boiler.

[0024] By the present solution, a signal representative of the opening of the steam valve and a signal representative of the level in the boiler are used for determining the supply of feed water into the boiler. Both signals are readily available and thus no additional sensors such as a boiler steam flow sensor are needed. The reference level for the boiler is a constant, which represents the normal level in steady state conditions and is determined by the design of the boiler. The rate of change of the steam valve opening is determined from the signal representative of the opening of the steam valve and is used to compensate for the inverse response to disturbances, i.e. the drop in the level when the steam load increases. When the steam valve opening is constant, i.e. when the steam valve is at rest and the steam load is constant, the rate of change of the steam valve opening is zero. When the steam valve opening changes due to load changes, i.e. when the steam valve is moving, the rate of change of the steam valve opening is non-zero.

[0025] By basing the supply of feed water into the boiler on the measured level in the boiler, the reference level for the boiler and the rate of change of the steam valve opening, the inverse response to disturbances can be avoided or reduced.

[0026] According to another embodiment, the feed water inlet comprises a feed water valve defining a feed water valve opening, wherein the method comprises determining the supply of feed water into the boiler by the feed water valve opening.

[0027] For controlling the supply of feed water to the boiler, a feed water valve can be used. The opening of the feed water valve can be used as an actuation signal for the boiler water level controller.

[0028] According to another embodiment, the feed water inlet is connected to a pump defining a flow rate by which the supply of feed water into the boiler is determined.

[0029] By using a flow control pump, a feed water valve in the feed water line can be omitted.

[0030] According to another embodiment, the flow rate is based on a rotational speed of the pump.

[0031] For varying the supply of feed water, the rotational speed of the pump can be varied.

[0032] According to another embodiment, the control of the supply of feed water into the boiler is performed using a PI control scheme.

[0033] The control action is designed to minimize the adverse effects of changes in disturbances. A PI (proportional-integral) controller performs calculations for a proportional-integral algorithm based on an integral parameter and a proportional or gain parameter, and the generated output is a flow control signal for controlling the supply of feed water to the boiler.

[0034] According to another embodiment, controlling the supply of feed water into the boiler comprises generating a feed water control action based on a difference between the level in the boiler and a reference level for the boiler, and determining the supply of feed water into the boiler based on a sum of the feed water control action and a rate of change of the steam valve opening.

[0035] The rate of change of the steam valve opening signal can be added to the control action in a feedforward manner (i.e., after the controller, the opening of the feed valve (or alternatively, the flow rate / velocity of the feed pump) is controlled based on the controller). In this way, when a disturbance occurs, a correction term for the feedwater control action is immediately introduced, without waiting for the water level to drop before the controller takes corrective action. The controller typically uses a linear SISO control scheme to generate the control action. The controller is preferably a PI controller, and the control action is preferably a PI control action.

[0036] According to another embodiment, controlling the supply of feedwater to the boiler includes: generating a feedwater control action 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 feedwater to the boiler through the feedwater control action.

[0037] Alternatively, the rate of change of the steam valve opening signal can be added via feedback, i.e., added before the controller to the measured liquid level, with the feed valve opening (or alternatively, the feed pump speed) controlled based on the controller. In this way, a correction term for the feedwater control input is introduced into the controller that takes the corrective action. The controller typically uses a linear SISO control scheme to generate the control action. The controller is preferably a PI controller, and the control action is preferably a PI control action.

[0038] According to another embodiment, the method includes determining the rate of change of the steam valve opening by using a phase lead filter on the steam valve opening.

[0039] The water feed opening is controlled by disturbance compensation in the water level control provided by the lead filter. The lead filter provides zero output when the input to the filter is constant, but provides non-zero output when the input changes (i.e., when the steam load changes suddenly). The rate of change of the filtered steam valve opening replaces the rate of change of the steam valve opening in the above feedforward and feedback schemes.

[0040] According to another embodiment, the method further includes the following steps:

[0041] Use a pressure sensor to measure the pressure in the boiler, and

[0042] The heat input to the boiler is controlled based on the pressure in the boiler and a reference pressure for the boiler.

[0043] The heat input to the boiler is controlled by a second controller. The heat input is based on pressure measurements and a reference pressure for the boiler. Pressure measurements are taken by a pressure sensor within the vessel. Therefore, the pressure 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 boiler's design.

[0044] According to another embodiment, controlling the heat input to the boiler includes: generating a heat input control action based on the difference between the pressure in the boiler and a reference pressure for the boiler, and determining the heat input to the boiler through the heat input control action.

[0045] The heat input into the liquid in the container is based on control actions. Higher heat input will cause more vapor to be generated, resulting in higher pressure, and vice versa.

[0046] According to another embodiment, a PI control scheme is used to perform control of the heat input to the boiler.

[0047] The control action is designed to minimize the adverse effects of changes in disturbances. The PI (Proportional-Integral) controller performs calculations for the proportional-integral algorithm based on integral parameters and proportional or gain parameters, and the resulting output is a flow control signal.

[0048] In a second aspect of the invention, the above objective is achieved by a boiler having a control system for controlling the supply of feedwater to the boiler, the boiler including a feedwater inlet for supplying feedwater to the boiler, a steam outlet for obtaining steam from the boiler, and a level sensor for measuring the liquid level in the boiler, the steam outlet including a steam valve defining the opening of a steam valve, the control system controlling the supply of feedwater to the boiler via the feedwater 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.

[0049] According to another embodiment, the boiler described above according to the second aspect may include any of the features of the method according to the first aspect.

[0050] In a third aspect of the invention, the above objective is achieved by a computer program including instructions which, when executed by a computer, cause the computer to perform the method according to the first aspect.

[0051] The control solution including feedback disturbance compensation according to the foregoing aspects can preferably be implemented as software in a programmable logic controller (PLC).

[0052] In a fourth aspect of the invention, the above objective is achieved by a computer-readable medium having a computer program of the third aspect stored thereon. Attached Figure Description

[0053] Figure 1 A schematic diagram of the boiler is shown.

[0054] Figure 2 A block diagram illustrating the feedwater level control strategy is shown.

[0055] Figure 3 The step response of the phase-lead filter is shown.

[0056] Figure 4 The simulation results are shown using the water supply flow rate without disturbance compensation.

[0057] Figure 5 The simulation results using feedwater flow rate and feedforward compensation are shown.

[0058] Figure 6 The simulation results using water flow rate and feedback compensation are shown.

[0059] Figure 7 The simulation results are shown with the valve open and no disturbance compensation.

[0060] Figure 8 The simulation results are shown with the valve open and feedforward compensation applied.

[0061] Figure 9 The simulation results are shown with the valve open and feedback compensation applied.

[0062] Figure 10 This demonstrates the use of PLC simulation software. Figure 9 The simulation results. Detailed Implementation

[0063] Figure 1 This is a simplified schematic diagram of boiler 10. Boiler 10 includes a container 12. The container defines a liquid level, indicated by corrugated lines. The liquid level is measured by a liquid level sensor 14. Below the liquid level is a two-phase flow 16 of liquid water and steam bubbles, while above the liquid level is primarily steam 18. Boiler 10 further includes a feedwater inlet 20 for supplying feedwater into container 12 and a steam outlet 22 for removing steam from container 12. Feedwater inlet 20 is supplied via pump 24' and controlled by feedwater inlet valve 24, and steam outlet is controlled by steam outlet valve 26.

[0064] Boiler 10 further includes a riser 28 and a downcomer 30 forming a closed loop together with vessel 12. Heat supply to the riser causes boiling. Steam rises into the vessel and circulates in the riser 28-vessel 12-downcomer 30 loop. In this schematic diagram, only one riser and one downcomer are shown; however, in practice, multiple risers and downcomers are used. Boiler 10 further includes a pressure sensor 32 for measuring the pressure in vessel 12.

[0065] Figure 2 This is a block diagram illustrating the feedwater level control strategy according to the present invention. The block with reference numeral 34 represents the equipment (i.e., the boiler). Therefore, the input to block 34 is feedwater. q f supply and heat Q The supply, while the output from box 34 is the measured liquid level in the container.l and the pressure measured p The disturbance is steam. q s The outflow, however, according to the present invention, steam q s The outflow is not directly measured. Steam q s The outflow from the boiler is controlled by a steam valve with reference number 36, and the measurable disturbance is the opening degree of steam valve 36. OD st Water supply q f The water supply is controlled by the water supply valve / pump 38, and the water supply valve / pump 38 is adjusted according to the opening degree of the water supply valve 38. OD fw As input, and optionally as the pressure in the boiler p The pump speed ω is used as input. Boiler pressure. p It can affect the feedwater flow rate in the boiler because it is the pressure on the secondary side of the feedwater valve / pump.

[0066] The frames marked "PI" with reference numbers 40 and 42 respectively implement measures targeting water levels. l and boiler pressure p The proportional-integral (PI) control action should be designed to minimize the adverse effects of changes in disturbances (i.e., changes in steam load) on the water level in the boiler. As a baseline, the control system is equipped with two independent PI controllers: one controls the heat input based on pressure measurements, and the other controls the feedwater input based on water level measurements. The following standard PI control expression is used for both system inputs. q f and Q :

[0067] [Equation 1]

[0068] [Equation 2]

[0069] [Equation 3]

[0070] [Equation 4]

[0071] in Indicates the corresponding controller status; >0 integral gain; >0 proportional gain; (·) ref The corresponding output reference value. The response of the closed-loop system using this reference control is exactly the opposite of what is desired, and it exacerbates the subsequent drop in boiler water level, see... Figure 4The initial response of a PI controller to an increase in steam load is to reduce the feedwater flow rate. This can be particularly problematic if the boiler volume is relatively small.

[0072] The main strategy for improving the response of a closed-loop system to changes in steam load is to use a filter to detect the (rising / falling) edges of disturbances. This filter has zero output for a constant input and provides a non-zero output whenever the input signal to the filter changes.

[0073] Steam value opening degree OD st The feed is sent to the output y of the generator filter. ll The lead filter 44. These sections are marked with dashed boxes and lines. The path to the box with reference number 46 is the feedforward strategy L. ff The path to the box with reference number 48 is the feedback strategy L. fb .

[0074] Figure 3 The step response of the phase-lead filter is shown. This is for detecting disturbances. k v The edges of the changes in the curve are addressed using a phase-lead filter with zero DC gain, described by the following continuous-time differential equation:

[0075] [Equation 5]

[0076] in p ll >0 represents a pole of the filter. Now, this term... No consideration was given to piecewise constants. k v It is well-defined. However, in order to illustrate the intuitive knowledge behind the filter, it can be assumed that it can be defined as follows: k v The time instance of the changing value is similar to a pulse (Dirac delta). Since the above non-forced equation describes the integrator, the filter's output response to this pulse initially rises to one at the pulse's time and then decays to zero. The rate of this decay is determined by the poles. p ll Sure.

[0077] Although the continuous-time implementation of [Equation 5] is for non-differentiable... k v There are some issues, but ultimately the filter needs to be implemented on a computer in discrete time, as long as there are differences between samples. k v The variation is limited, so there is no problem. In discrete time, [Equation 5] can be implemented at the k-th sample as:

[0078] [Equation 6]

[0079] in a 0 ,a 1 ,b 1>0 is a parameter of the filter. The parameters of [Equation 6] can be calculated based on the recommendations given in SW Smith's publication, "A Scientist's and Engineer's Guide to Digital Signal Processing," second edition (California Technology Press, 1999): and , of which 1- b 1 represents the expected attenuation between samples. That is, within x samples, the filter has attenuated to its initial value. Therefore, parameters b 1 and the poles of a continuous-time filter p II Relevant. In this case, the attenuation parameter. b 1 is selected in the following way: it may be desirable to design the filter to... T It decays to 10% of its initial value within seconds. During the sampling time... T s1 In this case, this corresponds to T / T s 1 sample, until the filter decays to 0.1 times its initial value. Then, b 1 is given by the following formula: The following section describes two strategies for using the filter output to compensate for disturbances.

[0080] The first strategy to improve the response of the level control is to add the output of the filter in [Equation 5] to the control signal from the PI controller in a feedforward manner. The aim is to immediately add a correction term to the feedwater control input when 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 a feedforward, any tampering with the PI loop is avoided. Therefore, the feedwater control input takes the following form:

[0081] [Equation 7]

[0082] [Equation 8]

[0083] in L ff >0 represents the feedforward gain. Gain L ff It should be located in it L ff y II It is in the same order of magnitude as the output of the PI controller.

[0084] A second strategy to improve the response of the level control is to add the output of the filter in [Equation 5] as feedback to the level measurement. The aim is to directly compensate for the immediate rise (fall) in the water level caused by the increase (decrease) in the steam load during the level measurement.

[0085] [Equation 9]

[0086] [Equation 10]

[0087] in L fb >0 should be located in l and L fb y II Regions of the same order of magnitude.

[0088] Simulations were performed to validate the control scheme. The simulation model used has been described in a publication by KJ Åström and RDBell. The boiler used in the simulation model is primarily located in... Figure 1 As shown in the image.

[0089] Two water level control strategies designed to adapt to the adverse effects of the inverse response of water level to disturbances have been used in simulations. This adverse effect is manifested by a baseline control strategy included for comparison and can be shown as follows: Due to the initial rise in water level caused by a pressure drop during load increases, the control will reduce the feedwater flow rate when an actual increase is needed. This initial reduction in flow rate exacerbates the subsequent drop in water level, see... Figure 4 Therefore, it is desirable to avoid such initial control responses.

[0090] In addition to these two control strategies, two different scenarios with available sensors and actuators were simulated. In the first scenario, an idealized situation was simulated where the feedwater flow is directly actuated and the steam valve resistance is a measurable disturbance. In the second scenario, a more realistic scenario was simulated where the feedwater flow is controlled by the valve opening, and the steam valve opening is a measurable disturbance.

[0091] The system model derived by KJ Åström and RD Bell in their publication is a nonlinear state-space model with four state variables. These state variables are: steam drum pressure. p Total water volume in the steam drum, riser pipe, and downcomer pipe V wt The steam mass at the riser outlet (i.e., the mass fraction of steam). a r The amount of steam below the water level in the steam drum.V sd The external input to the system is the feedwater mass flow rate into the boiler. q f Heat input to the riser pipe Q Steam mass flow rate exiting the boiler q s ,See Figure 1 .

[0092] Figures 4-6 The results of simulations under more ideal conditions are shown, where the feedwater 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 performed at time t = 10000 s and corresponds to an increase of approximately 30% in the desired energy / steam production. It should be noted that the results obtained using each individual control action could potentially be improved, as very little time is spent tuning them.

[0093] Figure 4 Simulation results are shown when the boiler water level is controlled using the standard / reference proportional-integral control action in [Equation 1]. Four plots of different time series are presented. The top left plot shows the boiler water level relative to some desired reference values. l The upper right figure shows the control signals. q f Or feedwater input. The lower left figure shows the boiler pressure. p The lower right figure shows the control signals used for heat input. Q The settings here are that the feedwater control should maintain the relative boiler water level at a reference value of 0 meters, while the heat control should maintain the boiler pressure at a reference value of 8 MPa.

[0094] The initial response of the feedwater control to the disturbance was to reduce the feedwater input, which was attributed to the significant expansion on the level chart, with the level reaching a maximum of 1.8 cm at approximately time t = 10100 s. This control response exacerbated the subsequent drop in level, with the level reaching a minimum of -8.5 cm below the desired level at approximately time t = 12900 s.

[0095] Note that the effort required to tune the gain of these reference controllers is minimal. However, they are used for this purpose to illustrate.

[0096] Figure 5 The simulation results are shown when the feedforward action described by [Equation 7] is added to the boiler water level control. The layout in the figure is similar to... Figure 4The same applies to the previous one. The initial response of the feedwater control to disturbances has now changed, so that despite the initial inverse response of the water level, an increase in steam demand now also leads to an increase in feedwater. Furthermore, at approximately time t = 15600 s, the subsequent drop in water level now decreases by approximately 37% to a minimum of -5.3 cm. However, at approximately time t = 10700 s, the rise in boiler water level before the drop now increases by 370% to a maximum of 8.4 cm.

[0097] Figure 6 Simulation results are shown when the disturbance feedback action described by [Equation 9] is added to the boiler water level control. Again, the layout and... Figure 4 and Figure 5 The same applies as in the baseline control. The initial control response is to increase the feedwater input, although this is attributed to the initial increase in water level due to expansion. The subsequent boiler water level drop is reduced by approximately 90% to -0.8 cm compared to the baseline control. Furthermore, at time t=10100s, the previous rise in boiler water level is increased by 172% to 4.9 cm compared to the baseline control, which is less than half the increase obtained using feedforward correction.

[0098] Figures 7-10 The simulation results for the more realistic second scenario mentioned above are shown. Figures 4-6 The simulation results presented assume that the PI controller used to control the boiler water level has access to the measured value of the steam flow valve resistance, and that the controller can directly actuate the feedwater flow. This actuation could potentially be achieved using an inner-loop flow controller. However, such an inner-loop controller would depend on the measurement of the feedwater flow rate.

[0099] This setting is Figure 1 The setup shown here illustrates the equipment for supplying feed water to the boiler and the valves that determine the steam flow rate. Assumptions will be made regarding: 1) the opening degree of the steam valves. OD st 1) It is known and can be used as an input to the boiler water level controller; 2) The feedwater pump operates at a constant speed; and 3) The opening degree of the feedwater valve. OD fw It can be used as an actuation signal for boiler water level controllers.

[0100] Furthermore, three scenarios for boiler setup were simulated: 1) The first simulation scenario utilizes a baseline PI boiler level control based on [Equations 1] and [Equations 2], which now controls the opening of the feedwater valve based on the measured value of the boiler water level, i.e.:

[0101] [Equation 11]

[0102] [Equation 12]

[0103] inOD fw This indicates the opening degree of the water supply valve.

[0104] 2) The second simulation scenario utilizes the steam valve opening signal. OD st The feedforward disturbance compensation of the advanced filtering version, namely

[0105] [Equation 13]

[0106] Furthermore, the feedforward disturbance compensation control can be described as follows:

[0107] [Equation 14]

[0108] [Equation 15]

[0109] 3) The third simulation scenario utilizes filtering again. OD st Feedback disturbance compensation of the signal makes disturbance compensation control describable by the following equation:

[0110] [Equation 16]

[0111] [Equation 17]

[0112] Figure 7 Simulation results obtained using a reference boiler water level controller as defined in controller equation [Equation 11] are shown. Again, the disturbance (here) OD st The step change in the simulation is introduced at time t = 10000 s. As previously simulated, the immediate response of the boiler water level to the disturbance is a rise to 1.7 cm at approximately time t = 10060 s. As can be seen, the subsequent response of the boiler water level is a drop to -3.25 cm at approximately time t = 10200 s. After this drop, there is a recovery, and then the level subsequently drops to -1.55 cm at approximately time t = 12000 s.

[0113] Note again that the time spent tuning the gain of the reference controller is not long. However, overall, the reference controller here appears to be tuned to be more responsive than in the first round of simulation.

[0114] Figure 8Simulation results obtained using feedforward disturbance compensation as defined in the controller equation [Equation 14] are shown. It can be seen that the initial rise in boiler water level is amplified by approximately 130% compared to the baseline controller, reaching a maximum value of 3.88 cm at approximately time t = 10060 s. This is immediately followed by a decrease in boiler water level that is reduced by 40% compared to the maximum decrease under the baseline condition, reaching a minimum value of -1.92 cm at approximately time t = 12050 s.

[0115] Figure 9 Simulation results obtained using feedback disturbance compensation as defined in the controller equation [Equation 16] are shown. It can be seen again that the rise in water level is aggravated compared to the baseline control. Here, the maximum (second) rise increases by 96%, reaching a maximum value of 3.34 cm at approximately time t = 10600 s. The maximum drop in boiler water level occurs at approximately time t = 10180 s, and the lowest water level is -0.72 cm, a reduction of 78% compared to the baseline.

[0116] Figure 10 The simulation results of the above control solution, including feedback disturbance compensation implemented in the PLC (Programmable Logic Controller) software, are shown. This implementation has been tested in a closed loop for PLC simulation using Siemens' "PLCSim" software. Figure 10 The results shown are consistent with Figure 9 The results shown are compared to verify the correctness of the PLC implementation. Overall, the results are the same, except for some quantization noise attributable to rounding errors caused by the limited representation of the control output in the PLC. This quantization in Figure 10 The zoom area is indicated in the text.

Claims

1. A method for controlling the supply of feedwater to a boiler (10), the boiler (10) including a feedwater inlet (20) for supplying feedwater to the boiler (10), a steam outlet (22) for obtaining steam from the boiler (10), and a level sensor (14) for measuring the liquid level in the boiler (10), the steam outlet (22) including a steam valve defining the opening of a steam valve, the method comprising: The liquid level in the boiler (10) is measured using the liquid level sensor (14). Determine the rate of change of the steam valve opening. Based on the liquid level in the boiler (10), a reference liquid level for the boiler (10), and the rate of change of the steam valve opening, the supply of feedwater to the boiler (10) via the feedwater inlet (20) is controlled. The pressure in the boiler (10) is measured using a pressure sensor (32), and The heat input to the boiler (10) is controlled based on the pressure in the boiler (10) and a reference pressure for the boiler (10).

2. The method according to claim 1, wherein, The water inlet (20) includes a water valve that defines the opening of the water valve, wherein the method includes determining the supply of water to the boiler (10) by means of the opening of the water valve.

3. The method according to claim 1, wherein, The water inlet (20) is connected to a pump (24') that defines a flow rate, wherein the method includes determining the supply of water to the boiler (10) by means of the flow rate.

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

5. The method according to any one of claims 1-4, wherein, A PI control scheme is used to control the supply of feedwater to the boiler (10).

6. The method according to any one of claims 1-4, wherein, Controlling the supply of feedwater to the boiler (10) includes: generating a feedwater control action based on the difference between the liquid level in the boiler (10) and a reference liquid level for the boiler (10), and determining the supply of feedwater to the boiler (10) based on the sum of the feedwater control action and the rate of change of the steam valve opening.

7. The method according to any one of claims 1 to 4, wherein, Controlling the supply of feedwater to the boiler (10) includes: generating a feedwater control action based on the sum of the rate of change of the steam valve opening and the difference between the liquid level in the boiler (10) and a reference liquid level for the boiler (10), and determining the supply of feedwater to the boiler (10) by means of the feedwater control action.

8. The method according to any one of claims 1-4, wherein, The method includes determining the rate of change of the steam valve opening by using a phase lead filter on the steam valve opening.

9. The method according to any one of claims 1-4, wherein, Controlling the heat input to the boiler (10) includes: generating a heat input control action based on the difference between the pressure in the boiler (10) and a reference pressure for the boiler (10), and determining the heat input to the boiler (10) by means of the heat input control action.

10. The method according to any one of claims 1-4, wherein, A PI control scheme is used to control the heat input to the boiler (10).

11. A boiler (10) having a control system for controlling the supply of feedwater to a boiler (10), the boiler (10) including a feedwater inlet (20) for supplying feedwater to the boiler (10), a steam outlet (22) for obtaining steam from the boiler (10), and a level sensor (14) for measuring the liquid level in the boiler (10), the steam outlet (22) including a steam valve defining the opening of a steam valve, the control system controlling the supply of feedwater to the boiler (10) via the feedwater inlet (20) based on the liquid level in the boiler (10), a reference liquid level for the boiler (10), and the rate of change of the opening of the steam valve, wherein, The control system uses a pressure sensor (32) to measure the pressure in the boiler (10) and controls the heat input to the boiler (10) based on the pressure in the boiler (10) and a reference pressure for the boiler (10).

12. A control system for controlling the supply of feedwater to a boiler (10), the control system being configured to perform the method according to any one of claims 1 to 10.

13. A computer program product comprising a computer program, which, when executed by a computer, causes the computer to perform the method of any one of claims 1 to 10.

14. A computer-readable medium having thereon a computer program having the computer program product of claim 13.

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