PLC-based automatic control system for boiler dosing

by:Sure     2020-06-22
1 Introduction At present, my country has very strict requirements on the quality of feed water and steam of large boilers, so it is necessary to continuously monitor the quality of boiler water. The traditional PID control algorithm is mostly used to measure the pH value. However, in the reaction process, it is difficult to adjust the parameters of the traditional PID controller due to the high gain near the neutral point. Therefore, only a small proportional gain can be used, otherwise the system is unstable, and the proportional gain is too small, which will deteriorate the dynamic characteristics of the system. For the boiler feed water dosing measurement and control device, the dosing system has been automated, but there is no automatic dispensing equipment, but still need to manually dispense according to the results of the soda laboratory. This not only has a high work intensity, but also the added ammonia and hydrazine. It is highly toxic and volatile, which will cause serious harm to the operator and cause environmental pollution. To this end, two new pH control methods of variable gain three-section nonlinear PID and integral fuzzy control (IFC) algorithm are proposed. Through digital simulation of the pH neutralization process with time lag, the results show that these two control algorithms have the characteristics of strong robustness, fast response speed and high control accuracy, especially the IFC algorithm can overcome pH neutralization Large time lag in the process. Through practical application in a power plant, fully automatic control of the boiler feed water dispensing and dosing system has been achieved. 2. Research on pH control method 2.1 Conventional PID control PID control is a linear combination control method based on the ratio of deviation (P-Proportional), integral (I-Integral) and differential (D-Derivative). Figure 1 shows the conventional PID control system. Among them, r is the reference input signal; PID is the controller; P is the model of the controlled object; d is the amount of interference; e (k) is the system error; u (k) is the control amount; pH (k) is the output of the controlled process the amount. It can be seen from the figure that the proportional function in the conventional PID control is actually a linear enlargement or reduction effect, and it is difficult to adapt to the non-linear characteristics of the controlled object during the acid-base neutralization process. 2.2 Three-section nonlinear PID control with variable gain The change of pH value is divided into inflection points: a high gain area and two low gain areas with different gain coefficients. The high gain zone controller uses a lower gain; the low gain zone controller uses a different high gain to meet the desired performance index of the system. In addition, in order to prevent integral saturation, a PID control algorithm with dead band and output limiting is used. 2. 3 Fuzzy control The fuzzy control algorithm is summarized as: according to the system output value obtained by this sampling, the input variable is calculated; the precise amount of the input variable is changed to the fuzzy amount; according to the input variable (fuzzy amount) and the fuzzy control rules, the control is calculated according to the fuzzy inference synthesis rules Amount (blur amount); calculate the precise control amount from the control amount (blur amount) obtained above. 3. Power plant boiler feed water dosing control system There are 4 300 MW generating units in a power plant, divided into two units, one unit is 1#, 2# unit, and the second unit is 3# and 4# unit. Each unit dosing metering pump includes boiler make-up water (raw water is purified by various water treatment methods. It is used to supplement the steam water loss of thermal power plants) and boiler water. Taking the two units as an example, the dosing system uses dual-use and one-preparation, a total of 3 dosing metering pumps, that is, 3# and 4# units each use one dosing metering pump, and when one of them fails, it switches to the standby pump . In this system, the amount of phosphate added in the boiler water is controlled by detecting the pH value. The pH value needs to be controlled at 914 to 9.78. When the pH value of one of the units is lower than 9.4, the dosing pump of the corresponding unit is started. At this time, the phosphate solution in the phosphate dosing tank is pumped into the dosing point of the deaerator outlet pipe of the corresponding unit through the pipeline (the valves on the pipeline are manual valves, which are normally open). If the dosing metering pump of unit 3# fails, open the valve on the standby pump and the pipeline connected to it. The standby pump replaces the dosing metering pump of unit 3#, which is used to add medicine to the boiler water of unit 3#. . Since proper phosphate and sodium hydroxide are added to the boiler water, the buffering performance of the boiler water can be improved, and the stability of the pH value of the boiler water can be maintained, thereby preventing the scaling and corrosion of the boiler water wall. The system introduces the boiler water sample into the phosphoric acid meter and pH meter probe through the temperature and pressure reduction device for measurement. After the analog conversion, the PID output of the control system is used to control the output of the inverter and the speed of the dosing pump to control the furnace in real time. The dosing amount of water keeps the phosphate concentration and pH of the boiler water within the acceptable range. Figure 2 shows the control flow chart. The control is divided into 4 parts: regulator, actuator, controlled object and transmitter. Among them, the regulator is composed of S7-200 PLC and corresponding control software; the actuator is composed of frequency converter, motor and metering pump; the controlled object is boiler water; the transmitter uses analytical instruments, namely pH meter. 3.1 Control flow Figure 3 shows the boiler water dosing control system of unit 3#. The system extracts 4~20mA signals from online analytical instruments (phosphate meter, pH meter), performs window-type PID compound operation according to the operating process parameters and the determined mathematical model, the intermediate result is sent to the inverter, and the dosage of the dosing pump is controlled In order to achieve automatic closed-loop adjustment of dosing. 3.2 Control system composition The control system selects the combination program of the host computer software WinCC+Siemens PLC. The PLC system is connected to the host computer WinCC via the PorfiBus bus. As shown in Figure 4. The upper-level monitoring part is completed by the industrial computer (WinCC). The monitoring staff can monitor the operating status of the system in real time through CRT. Set or modify the operating parameters of the system, and at the same time control the system operation through the CRT remote software. The high-level industrial computer performs data processing and management, and is networked with the MIS system. The host computer can configure the controller. The configuration range includes the network address and time of the controller, selecting the control algorithm, setting the algorithm parameters, setting the set point of the control quantity, and selecting the input and output channels in the algorithm. Wait. The lower control part is completed by a set of programmable controllers (PLC) installed on site. It is the core of the automatic dosing control system and is used to collect corresponding water quality data. Because the chemical dosing system has a pure hysteresis property, it will cause the control function to be untimely, causing the system to produce overshoot or oscillation, and the use of a computer can facilitate hysteresis compensation. The improved digital PID control algorithm and fuzzy control algorithm are used to make the controller use the output control signal to adjust the on-site AC frequency converter, and then control the speed of the motor to adjust the dosing pump. The control mode of the electrical part is designed to be remote and local, to realize three functions of manual/semi-automatic/automatic, and the latter two functions are switched by the host computer. 4. Filtering application of IFC algorithm In the control system, the basic principle of the filter program is to continuously sample 5 values ??in the cycle, and find the average value to collect the current value, and find the difference between the collected value and the average value △=Xi-X;if|△| >0.2, then discard Xi, and take X=0.2 as the value of the signal fluctuation range set according to the actual situation; if |△|≤0.2, then Xl pops the stack, X2 replaces X1, X3 replaces X2, X4 replaces X3, and then recursively . Replace X5 with the currently sampled X6, and then use these 5 new values ??to find X and compare them. In this way, the program can be executed cyclically to realize the filtering function. Figure 5 shows the trend of the enlarged pH value after using the filtering procedure, which shows that the filtering effect is good. Figure 6 shows the control operation interface. 5 Conclusion Practice has proved that the PLC-based automatic chemical dosing control system can flexibly meet the online monitoring of various chemical dosing systems. Since the system was put into operation, the operation has been stable and reliable, and the boiler and auxiliary equipment can fully realize automatic adjustment, which has achieved the expected effect. It solves the problem of manual control that is difficult to ensure the stability of the water quality index in the past, and reduces the work intensity of the operating personnel, and is consistent. Praise.
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