IMPLEMENTATION OF CONTROL AND MONITORING SYSTEM ON PLC FOR INDUSTRIAL STEAM BOILER

Applying our earlier work, we present a practical application in industry, including the numerical method to identify technological processes based on the “cleft-over-step” algorithm [1, 2], and the robust-based control theory to tune the PID (proportional integral derivative) controller [3-5]. These studies are implemented into an industrial boiler control system using a PLC (programmable logic controller). To do this, we design and program in detail a control and monitoring system for a CFB (circulating fluid bed) industrial boiler, including control software and operating interface on the Siemens PLC S7-1200. When equipped with this control and monitoring system, industrial boilers can be operated fully automatically, allowing operators to control and monitor at the control room and intervene remotely when necessary


Introduction
In [1,2], the author proposed numerical methods for process identification in the open-loop. Data from the step response of processes (obtained when excited by step pulses) is used to model the objects by numerical method using the "cleft-over-step" algorithm. The identified models are first order plus dead time (FOPDT), second order plus dead time (SOPDT) and integrating plus first order plus dead time (IFOPDT) with high accuracy. With these models, in studies [3][4][5], the authors demonstrated a method of PID tuning according to the robust-based controller, which allows pre-selection of the robustness index of the control system. In this follow-up study, we design and program the CFB (circulating fluid bed) industrial boiler control system using a PLC (programmable logic controller), in which the entire theoretical part of process identifying and PID tuning will be installed into the control system. CFB industrial boilers are commonly used for steam generation and play a very important role in industry. This also is object of many studies of simulation [6][7][8] and development [9,10]. In fact, there are still many industrial boilers that are operated manually or semi-automatically, the control circuit is a simple power circuit ON/OFF. Boiler operators mainly operate by experience, sometimes not in accordance with the nature of the technology as well as the regulating process, giving poor working quality. These greatly affect economic efficiency as well as operational safety. From this fact, it is necessary to research and equip control and monitoring systems for automatic operation of industrial boiler, helping to improve the efficiency and safety of industrial boiler operation. Owing to the diversity and popularity of control devices, suitable for technological objects, the programmable logic controller (PLC) was selected to equip industrial boilers.
There are many studies that have done work designing boiler automation systems using PLC. In [11], Welekar et al. presented a solution using Aduino and LM35 to monitor the temperature of the boiler. In [12], Talware and Chaudhari designed and programmed the drum water level control system Implementation of Control and Monitoring System on PLC … 3 using PLC controller. In [13], Shankar used PLC to equip the control system for the boiler of the thermal power plant, the controlled parameters of which are level, pressure, flow and temperature. In [14], Sivasruthi et al. used PLC to equip the system to control temperature, pressure and level parameters.
These studies equipped one or several control and monitoring systems for boilers but were not complete for the entire boiler and especially did not mention the problem of identifying and tuning PID controllers for technological processes. This is the issue determining the level of stable, correct, and efficient operation of the boiler automation system. In this study, the author presents a total solution for industrial boiler operation automation system using PLC including operating interface, and control logic system. Especially, the method of process identifying and PID controller tuning installed into the control system.
The system is built on the S7-1200 controller, CPU 1217C of SIEMENS due to its popularity in the Vietnamese market. The program is written on TIA PORTAL v14 software, and the operating interface design software is implemented on WIN CC 7.4. The object of application is the CFB industrial boiler.

Process control loops
The CFB boiler control system is designed including nine modulating control loops as follows: -Boiler load control loop: due to the small scale of the furnace, the need to stop at a level to keep the output steam pressure stable, no need to act quickly, so the solution is to adjust the load by controlling amount of fuel supplied to the furnace. The system input reads the steam pressure value, then sends to the PID controller which will vary the coal feeding speed to regulate the fuel to the furnace.
-Fluidizing bed pressure control loop: the essence of this adjustment is to control the height of the boiling layer in the furnace, so that the Trung Do Cao 4 combustion process is stable, improving the furnace efficiency. In this system, the input uses a differential pressure transmitter, one end is located in the first level air box, and the other is located in the back of the furnace. The measured signal will be sent to the controller to calculate the opening angle of the pulse valve. This will affect the amount of material removed from the furnace causing the boiling layer to increase or decrease, increase if the amount of material introduced is greater than the amount of matter given, otherwise, it will decrease.
-Furnace pressure control loop (furnace vacuum): this control system ensures that the combustion process will not expand the fire if the furnace pressure is too positive as well as the furnace will be turned off if the vacuum is increased excessively. The system will measure the furnace pressure by the sensor placed at the middle of the furnace wall to control the speed of induced draft fan accordingly, thereby keeping the furnace pressure at a level of the range from 10 to 12 mmH 2 O.
-Limestone feeding control loop: this system has the effect of adding limestone to the furnace to reduce SOx gas. Due to the small scale of the furnace, it is quite expensive to equip as SOx content instrument. So, this case will control the amount of limestone supplied depending on the amount of coal supplied to the furnace. And this ratio is determined by the quality of coal burnt. This parameter will be set by the operator when the furnace is operating.
-Combustion air distribution control loop: using proportional control, the supplied air flow will be proportional to the coal flow into the furnace. Here, the system will take the signal of the coal supply speed to the furnace, and calculate the amount of coal supplied to the furnace. From the amount of coal supplied to the furnace, we will have the amount of air to supply. After calculating the quantity of air supply, the amount of air will be divided among primary air (PA) fans, secondary air (SA) fans and high pressure air fans. Primary air fans (for creating a fluidized layer) will always be guaranteed from 40% to 50% of the total air volume fed into the boiler and Implementation of Control and Monitoring System on PLC … 5 its lower limit value is always fixed to ensure the boiler's boiling mode. Secondary air (SA) fans (exhausted air supply) will be calculated by subtracting the total air volume from the primary air (PA) fans.
-Drum water level control loop: the level of water in the steam drum will affect the steam production capacity and the quality of outlet steam. If it is too high, it will cause the steam generation rate to decrease, increase the temperature of flue gas, and affect the boiler's performance. If it is too low, it will endanger the drum and water walls. The control loop uses a singlepulse diagram with the input signal of the water level. The level measuring signal will be sent to the regulator, which will adjust the valve opening angle to change the flow of feed water into the boiler. In addition, to ensure the safety of the device, this system also uses 2 pole pair level sensors to provide four levels of water level alarms including: high 1 (H-High), high 2 (HH-High High), low 1 (L-Low), low 2 (LL-Low Low). For alarm LL, the system will give a signal to stop the boiler and HH to stop the pump. The H and L alarms will give a chime alarm and a warning on the HMI (Human Machine Interface).
-Bed temperature control loop: this control loop ensures that the temperature in the furnace is always kept stable in the range of C 800 o to C 900 o in normal operation. That also forces the emission gas quantity of SOx and NOx to be in range of designed values for environment protection. The input of the control loop is the temperature signal transmitted by the thermocouple which be placed on the boiler water wall, about the middle position according to the furnace height. The controller will calculate the amount of primary air for the boiler.
-Flue gas oxygen (O 2 ) content control loop: to increase the efficiency of the combustion process, it is necessary to regulate the optimal flow of air supply to the furnace. This will be assessed by the oxygen (O 2 ) content in the flue gas outlet from the furnace. The regulator uses the parameter of flue gas oxygen content as a signal to control the speed of the secondary air (SA) fans.
Trung Do Cao 6 -Continuous blowdown control loop: this control loop is used to remove excess Na and SiO 2 salts accumulated during steam generation. The controller will automatically regulate the control valve based on the conductivity of water in the boiler drum.

Control programs
The control program is designed including sub-programs to make it convenient for management, modification and upgrading.

(a) Main program -main [OB1]
The main program names the sub-program to serve the control loops.

(b) PID cyclic interrupt [OB30]
In the control system, PID controllers need to be continuously scanned to quickly and accurately regulate the technological processes. Therefore, PID controllers are put into OB30 so that it is scanned continuously without being affected by the main program's scan cycle [OB1].

(d) Sub-programs
The master control program includes many sub-programs. For each sub-program, there are two modes of manual and automatic control. "Manual mode" allows direct change of parameters of motor speed, valve opening angle, while "automatic mode" activates the PID controllers to Trung Do Cao 8 operate according to the setpoint value, and also synchronizes control data from the operation screen (HMI).

Group 2: Data import and export programs:
 Read value from sensors  Output signal to actuator.
These programs include many function blocks AI and AO Converts. The main function is to process the measured signals from the sensors and put values into memory as well as get the control data of the actuator to convert the signal and send to the actuator.  AUTO [FC18]: switch the control system to automatic operating mode. After finishing the startup process, the control loops are switched to automatic operating mode (auto). When the AUTO sub-program is called, the control loop operating mode will be checked, switched to automatic mode, thereby automatically operating the whole system. In this mode, the control loops are not allowed to operate manually.

(e) Data block
In order to manage the system's data to make it easier to upgrade, repair and tune during actual work, DATA blocks will be built separately. Include:

Algorithm and programming
The control loops are designed with algorithm diagrams and programmed on Tia Portal v14 control software. For example, Figure 2 shows the flowcharts of the load control and the limestone feeder control, while Figure 3 shows the algorithm flowcharts of the furnace pressure control and bed pressure control.
The control loops are equipped with a PID controller which is set into operation when the loop control is switched to automatic mode. Trung Do Cao 10  The control software is programmed in the ladder language, shown, for example, in Figure 4

Process identification and PID controller tuning [2-5]
The control loops listed in Group 1 of sub-programs have structure as shown in Figure 5 with process of   In work [1], the thermal process   s O is clarified into five types ( Figure   6) basing on step response in which the main are numbers 2, 3 and 5. This study uses three typical types.
In the models: K -gain factor; 2 1 , , T T T -lag constants;  -dead time; and s -complex variable.
The process step response is obtained when the process is excited by a step pulse. In this program, the measured parameters will be read and written automatically. The collected data is used for process identification numerically [1] which is set up in the control system.

PID tuning [3-5]
The   s R controller is defined as: This is robust-based controller proposed in studies [2][3][4][5]. In (4), the lag constant  is calculated basing on the robustness index s m [3]. With the identified process   s O in (1) and (2), the robust-based controller in (4) will be PID and PI, respectively. Particularly, in case (3) (for integrating process), to obtain the PID controller, we need to add the integral component [2,5]. To tune robust-based PID controller  , s R firstly, choose the robustness index s m in the range of 0.461 to 2 and calculate lag constant  [2]; and secondly, define the robust-based controller   s R by (4). The whole procedure is installed in the control system for automatic tuning PID controller of modulating control loops.

Man-machine Interface
The boiler operating interface is designed on the basis of WIN CC 7.4. Including the overall system interface, from this master interface, the operator can access the interfaces showing detailed technology items of the system.

Technical diagram
The overall technical diagram of the boiler is presented in Figure 7. The technology diagram on the operator interface is designed to ensure that the operator visually monitors the cycle, thereby making adjustments to help the furnace operate safely and efficiently. Operators are allowed to change process parameters such as fan speed, coal feeder speed, valve opening angle, etc. according to specific operating requirements. The manual parameter setting can be done through the master interface or the interface of each control loop.

Modulating control system
At the end of the start-up process, the control loops can be switched to the automatic operation mode (AUTO) in turn with the setpoint set by operator in the interface. In case, whenever the AUTO subroutine is called, the modulating control loops will be also switched to automatic mode.

Conclusions
The paper presents the application of the author's research platform system to a practical application in industry, which is a control and The results show that the theoretical foundation system is installed very effectively for the CFB industrial boiler control system, allowing the identification, tuning and re-tuning to be performed completely automatically. Besides, the work also shows that the PLC S7-1200 fully meets the control and monitoring requirements for industrial boilers, regardless of the specific type of industrial boiler. In addition, the success also demonstrates that it is possible to equip industrial boiler automation with a universal PLC controller, which is user-friendly to design, program, use and properly selected configuration. With a relatively low initial cost (compared to the cost of industrial boilers) while the result is safety, convenience, improved efficiency when operating the boiler, the installation of a control system monitoring using a PLC controller for industrial boilers currently operating manually, is a technical solution with good prospects for application.