Abstract: This paper details the software design of the blast furnace gas control system and elaborates on its characteristics and functions.
1. Introduction
With the completion of Xuan Steel's 2500 m³ blast furnace, there was an overproduction of blast furnace gas, leading to increased and drastic fluctuations in gas pipeline pressure, which affected the production of downstream gas users. Therefore, the company constructed a gas venting system for the 2500 m³ blast furnace area. To stabilize gas pipeline pressure, reduce gas venting volume, alleviate the workload of operators, and achieve automated control of gas venting, we designed and developed an automatic gas venting control program, and debugged it to meet the needs of process production.
2 Design Requirements
All field data from the venting system are displayed on the screen via PLC, perfectly matching the actual process, facilitating overall observation and operation by the operator. Automatic control of the gas pipeline pressure is achieved through PID regulation in the program, stabilizing the gas pipeline pressure, reducing venting volume, saving energy, and ensuring continued gas supply for users. The gas venting system hardware consists of a CPU, I/O modules, and a touchscreen. The system software uses S7-200 WIN32 software for the lower-level machine and Protools software for the upper-level machine.
3. Introduction to the S7200 Control System
The S7-200 series is a type of programmable logic controller (Micro PLC). This series of products can meet a wide variety of automation control needs. Figure 1 shows an S7-200 Micro PLC. Due to its compact design, good expandability, low price and powerful instructions, the S7-200 can almost perfectly meet small-scale control requirements. In addition, the rich variety of CPU types and low voltage levels make it highly adaptable to solving users' industrial automation problems.
The S7200 small PLC control system has advantages such as strong anti-interference, good operational stability, and good adjustment characteristics.
Figure 14 Gas venting system
4.1 To ensure safe operation, the system features a two-level access control system. The gas pressure setpoint is modified by operators on the touchscreen based on operational conditions. Maintenance personnel can enter the modification mode by entering a password, allowing them to make more appropriate adjustments to various parameters based on on-site conditions. After logging in, they can modify and maintain parameters such as the proportional, integral, derivative time, and control cycle of the control loop. The login screen is shown in Figure 2.
Figure 2. Maintenance personnel login screen
4.2 The system mainly controls the blast furnace gas venting from both the blast furnace and the gas pipeline. Each pipeline has a gas venting valve, and the venting volume is controlled by managing these two valves. The touchscreen displays the venting volume and valve position of each venting valve. The two venting valves have both automatic and manual control modes.
(1) In manual mode, the opening of the blast furnace gas pressure regulating valve is controlled by the operator to achieve gas pressure release and regulation.
(2) In automatic mode, the pressure point is manually set (currently 17 kPa). The PLC (S7200) calculates the opening degree of the regulating valve based on the actual pressure and the set pressure. The control program is shown in Figure 3.
Figure 3 Control diagram of gas regulating valve
4.3 Currently, the system is set to issue an audible and visual alarm when the blast furnace gas pressure exceeds 17 kPa, and to open the venting nitrogen valve to prevent backfire in the venting tower when the blast furnace gas venting flow rate is below 10,000 m³ /h. The venting nitrogen valve is closed when the blast furnace gas venting flow rate is above 20,000 m³ /h (because the flow rate is higher at this point, making it less likely for the flame to extinguish and backfire). The control program is shown in Figure 4.
Figure 4 Gas Emission Control Diagram
4.4 In the automatic control of the gas venting system, we made good use of PID control, which stands for proportional, integral, and derivative control. A larger proportional gain results in smaller control actions and a more stable system, achieving a larger damping rate during oscillations. However, if the proportional gain is too large, the controller's response to disturbances becomes less sensitive, and the dynamic deviation is large. A smaller proportional gain results in a smaller steady-state error. Integral action is used to achieve error-free control. As long as a deviation exists, the integral action continuously increases (or increases in the opposite direction) the controller's output, further reducing the deviation. Integral action speeds up the control process, which can easily lead to overshoot and oscillations. If the integral time is too long and the integral action is too weak, the unilateral deviation will persist for a long time, prolonging the control process. Derivative control cannot eliminate deviations; its role is to strengthen the initial control action, thereby reducing dynamic deviations. However, if the derivative action is too strong, it often leads to excessive overshoot by the controller, amplifying system oscillations and reducing the damping rate and stability. Generally speaking, derivative action is only beneficial for objects with slow response; it is usually unnecessary for gas venting control systems. In summary, proportional action ensures stability in the control process; integral action ensures accuracy; and derivative action promotes speed. This system employs both proportional and integral control, effectively controlling the venting amount. The control program is shown in Figure 5.
Figure 5 PID program control diagram
5. Evaporation ignition control system
The gas venting ignition control system has two control methods: local and remote. The control points are all on the control panel. In local control, it is manually controlled. For ignition: first open the coke oven gas valve, then open the nitrogen valve, and then press the ignition button (press and hold for 20-30 seconds). The ignition indicator light will illuminate, indicating successful ignition (each venting pipe has three thermocouples at the top; when the on-site temperature detector detects that any one thermocouple's temperature is greater than 70℃ and rises for 20 seconds, an ignition signal is output). For extinguishing: first open the nitrogen valve, then close the coke oven gas valve. When the ignition indicator light goes out, the extinguishing is successful (all three thermocouples are below 70℃ or have continuously cooled for 15 seconds). In remote control, clicking the programmable ignition button will automatically execute the above ignition process using the PLC (S7200) in the ignition control cabinet. Similarly, clicking the programmable extinguishing button will automatically execute the above extinguishing process using the PLC.
6. Conclusion
The 2500 m³ blast furnace gas venting control system displays parameters such as blast furnace gas pressure, pipeline gas pressure, and venting flow rate on the screen, enabling real-time monitoring of this data. PID control connects the gas pressure to the venting valve for automatic pressure control, and includes high-limit audible and visual alarms. The venting flow rate controls the nitrogen valve in the venting ignition system; when the venting flow rate falls below 20,000 m³, the nitrogen valve automatically opens to purge the venting pipeline, preventing backfire. This automated control system ensures stable gas pipeline pressure, reduces unnecessary gas venting, and guarantees production safety.
About the author: Wang Haiqin, female, born in April 1982, assistant engineer, Xuanhua Iron and Steel Group Co., Ltd.
Mailing Address : Unit 5, Building 3, Boyang Residential Area, Xuanhua District, Hebei Province, China Telephone: 13253133154
E-mail:[email protected]
