Abstract: The process flow of the gas pressurizing fan is analyzed. The gas pressurization fan is controlled by the variable frequency speed control technology. The Siemens S7-300 PLC is used with the Wincc industrial configuration control technology and the modular design scheme. The continuous and on-line measurement and processing of the operating status provide a powerful guarantee for the safe and economical operation of the gas pressurization fan.
Keywords: Gas pressurization fan; Variable frequency speed control; PLC; Configuration control; Control system, etc.
0 Preface
The gas pressurizing fan is the core equipment of the active lime sleeve kiln and belongs to the long-time operation equipment. Its role is to pressurize the gas from the gas cabinet to the active lime sleeve kiln as fuel. AC frequency conversion speed regulation technology is a major means to save energy and reduce consumption, improve process structure, improve product quality, improve the environment, and promote technological progress. The frequency conversion speed control system is used to directly control the speed of the gas pressurizing fan, and a PID closed-loop automatic control system consisting of a PLC and a pressure sensor to form a gas outlet mains is used. The motor can be operated at variable speed according to the load changes, and the gas flow and pressure are automatically adjusted. Satisfy the production needs, but also achieved the purpose of saving energy and reducing consumption and improving the level of control.
1 Analysis of Operation Characteristics and Control Process of Xining Special Steel Active Limestone Sleeve Kiln Gas Pressurizing Fan
1) Set up 2 sets of pressurized fan systems, 1 open and 1 ready.
Single fan interlock control
1.1 Bearing temperature is greater than 90 °C alarm, more than 80 °C shutdown.
1.2 The stator temperature is greater than 140 °C alarm, more than 145 °C shutdown.
(The above content must be confirmed by fan factory)
2) Electric butterfly valve control.
3) When the inlet pressure of the pressure fan is lower than 1KPA, the system alarms and the remote operator of the operation room
Move open the electric regulating butterfly valve on the return pipe to a certain degree (determined according to the on-site commissioning);
When the inlet pressure of the pressurized fan is lower than 0.7 KPA, the program will automatically
Adjust the butterfly valve fully open; when the inlet pressure of the pressurized fan is lower than 0.5KPA, the system stops automatically
machine.
4) According to the kiln's output and calorific value, the flow rate of the gas required by the tunnel kiln is calculated automatically.
The flow orifice on the orifice tube feeds back and automatically adjusts the opening of the electrically adjustable butterfly valve through the return tube.
Reach the required flow; and ensure that the pressure of the outlet of the pressurized fan is between 17~19KPa.
The system alarms when the pressure of a single pressurized air outlet pipe is lower than 17KPa.
5) When the outlet manifold pressure is greater than 19KPa, the return pipe regulator gradually opens; when the outlet
When the pressure is greater than 21KPa, the system will alarm; when the outlet pressure is greater than 23KPa, the system will
Active shutdown.
6) Start of the fan
The gas pipeline is set up;
Imported electric butterfly valve and return tube electric adjustment butterfly valve closed;
Inlet pressure not less than 1KPA;
The fan 5HZ is started, the electric butterfly valve of the inlet pipe is opened in turn after a delay of 10S, and the outlet pipe is powered
Butterfly valve and return tube electric regulation disc valve;
7) Fan stop
In addition to the automatic shutdown of the above-mentioned system, when all burners of the kiln are extinguished, they shall be immediately powered on.
Then notify the press control room and shut down the system.
Shutdown step: At the same time, all the electric valves of the return pipe are automatically opened, and the inlet and outlet pipe valves are closed.
8) The cooling water inlet manifold pressure is lower than 0.1MPa and the system alarms.
9) The pressure after nitrogen decompression is higher than 60KPa and the system alarms.
2 Xining Special Steel Limestone Kiln Gas Pressurized Fan System Design
1) Electrical design of pressurized fan system
System electrical design
Power conditions
— Power supply voltage: AC 380V 3P+N+PE 2-way 320KW power supply
— Control supply voltage: AC 220V
— Frequency: 49.0 to 50.5 Hz
— Three-phase five-wire system: (TN-S)
— Voltage fluctuation range: 380V+10% to -15%
The Xining Special Steel Active Limestone Sleeve Kiln Gas Pressurization Station has two MZ400-2800 gas-pressurized blowers, which are usually transported and prepared. The main circuit electrical structure diagram is designed according to actual working conditions. The dual power conversion cabinet uses Schneider WATSGM-800/4 dual power transfer switches to ensure that the gas pressurization station has normal power supply, and the actual main and standby power supplies are quickly switched. The frequency conversion cabinet adopts ABB ACS800-04P-0400-3+P901 frequency converter.
2) Automation Network Design
The control system consists of master PLC, touch screen, and upper computer. Among them, the main control system adopts Siemens S7-300 PLC, upper computer monitoring selects SIEMENS Wincc software, and the touch screen adopts SIENEMS MP277 10.4'inch true color screen. The communication network adopts the Profibus DP bus at the bottom, the master PLC, the touch screen and the host computer monitoring system adopt Ethernet communication. Siemens S7-300 PLC, as the control core of the entire system, handles various requests from the man-machine interface to the system, monitors the parameters of the entire system, realizes PID adjustment of the manifold pressure, and maintains a constant pressure in the pipe network. The upper computer system adopts the wincc monitoring software familiar to the user, and the connection with the PLC adopts the Ethernet method. The touch screen adopts a safe and stable SEMENS MP27710.4'inch true color screen, and the connection to the PLC is Ethernet. Configuration: S7-300 selects 315-2 DP and configures CP341-1T Ethernet module. Ethernet module CP343-1T is used for Ethernet connection with touch screen and upper computer.
The control network has the following characteristics: good stability, expansibility, openness of software and hardware, and friendly man-machine interface, easy to operate.
figure 1
3) Software Design
The upper computer system selects Wincc software produced by SIEMENS. The software has a friendly man-machine interface and supports Ethernet. It can be very convenient to achieve remote monitoring and other functions, reports, alarms powerful, support OPC, support for a variety of types of PLC communication. The touch screen system uses WinCC flexible 2008 software manufactured by SIEMENS, which can easily realize field-level human-computer interaction.
S7-300 PLC is programmed with Step 7 software. The software adopts the modular programming method, and compiles each work of the system into a functional block, which is called in an OB, which is easy to use and easy for the user to understand and modify. Supports ladder diagrams and statement tables. Some fault-tolerant programming is used to enhance the stability of the system.
4) System Design
4.1 Two blowers without disturbance switching control
When 1# blower runs with frequency conversion, when it is necessary to stop and check the inverter or fan, input 2# blower. Operation starts 2# blower first, and stop 1# inverter at the same time. Because the pressure before the machine achieves PID closed-loop control, 1# frequency conversion follows the set deceleration time, smoothes the parking, and the inlet valve flow decreases slowly. On the contrary, the 2# blower follows the set acceleration time, the rotation speed rises smoothly, and the inlet valve flow increases slowly. This ensures that the pressure of the main gas collection tube in front of the machine is constant, and the two blowers can be switched without disturbance, which solves the problem of severe fluctuations in the system pipe network pressure caused by the two blowers in the original control system. (See system flow chart for details)
figure 2
4.2 Implementation of PID Control
A. Determination of PID controlled volume
PID control is closed-loop control. The monitored quantity of the monitored signal (that is, the actual value detected by the pressure sensor) is fed back to the PLC or the inverter and compared with the controlled quantity of the target signal to determine whether the predetermined control objective is achieved. If not, adjust according to the difference between the two until the desired control is achieved.
The operation of the gas pressurizing fan must first ensure that the gas cabinet has sufficient gas, and the suction volume of the gas pressurizing fan depends on the gas volume of the gas cabinet and the production demand of the active lime sleeve kiln. The operation of the pressurized fan is based on the pressure of the fan outlet manifold. The fan outlet manifold pressure comes directly from the fan outlet pressure changes, so taking the fan outlet manifold pressure as the system control parameters, is conducive to the operation of the active lime kiln operation and system stability.
B. Basic Principle of Frequency Control System
AC asynchronous motor speed formula is
N1=60 f1/2P (1)
From equation (1), the power frequency f is proportional to the speed n. That is changing the frequency can change the speed of the motor.
There is a slip relationship between asynchronous motor speed n and synchronous speed n1
n=n1(1-S)=60f1/2P(1-S) (2)
From equation (2), we can know that any speed can be adjusted by changing any of the parameters f1, P, and S. The best method for asynchronous motors is to change the frequency f1 to achieve speed control.
Actually, most frequency conversion speed control applications are used below the rated frequency. The compensation used at low frequencies is to solve the low frequency torque drop. The methods used are various, including vector control and direct torque control. Its role is mainly to dynamically change the inverter output voltage, output phase or output frequency at low frequency, using circuit and computer technology to change the output characteristics of the asynchronous motor in real time.
The basic principle of the gas pressure blower variable frequency speed control system is shown in Fig. 3. The field signal is fed back to the PLC via a pressure sensor. The setpoint is determined according to the actual conditions of the work. The PLC's function block, PID regulator, and PID regulator are used for routine operation. After being output to the inverter, the inverter regulates the flow and pressure of the gas by changing the rotation speed of the gas pressurizing fan motor, thereby achieving a single closed-loop PID control of the gas outlet manifold, which contributes to the stabilization of the gas pipeline pressure.
image 3
In the gas pressurizing fan system, the variable frequency speed control operation mode is adopted, which can automatically adjust the fan speed according to the change of load, solve the problem of “big horse-drawn carâ€, reduce production costs, extend equipment life, save energy, and reduce consumption. Reducing labor intensity and improving the working environment has created new ways.
C. PID control process
The basic requirement of the gas pressure blower variable frequency speed control system is to keep the outlet pressure of the outlet fan constant. The working process of the system: Let X Ñ‚ be the target signal setting value, its size corresponds to the required gas manifold pressure, and XF is the feedback signal of the pressure sensor. Then the output frequency fx of the inverter is determined by X Ñ‚ -XF.
If the gas main pressure P exceeds the set value, then XF>XÑ‚, and X Ñ‚-XF <0, the output frequency fx of the inverter decreases, the motor rotation speed decreases, and the gas main pressure P decreases until the required setting The settings are equal in size.
Conversely, if the gas main pressure P is lower than the set value, then XF<XÑ‚, X Ñ‚-XF>0, the inverter output frequency fx increases, the motor rotation speed increases, the gas manifold pressure P increases until it meets the requirements. The settings are the same size.
D. Implementation of PID control
Through the PLC's PID function block, the single closed-loop control of the pre-cooling air suction is completed, and the variable table is established at the same time for the parameter setting and modification of the PID. The programming language adopts a ladder diagram to realize various logic sequence control and closed-loop control of the suction force before the initial air-cooling. The software flow is shown in Figure 4.
Figure 4
The key to the control of the entire system is to maintain the gas manifold pressure setpoint in the process requirements. Under normal circumstances, the gas main pressure does not generally need to be changed. However, when the lime kiln fuel is insufficient and the corresponding butterfly valve adjustment mechanism is closed to the end under system control, the computer control will automatically modify the gas manifold pressure parameter setting value, change the output frequency of the inverter, and accordingly change the speed of the gas blower. , And then change the pressure of the gas manifold to ensure the normal production needs of the lime kiln.
For the adjusted parameters such as pressure and flow rate, the time constant T0 of the object adjustment channel is small, and the load changes rapidly. At this time, both the differential action and the integral action must cause oscillation, which has a great influence on the adjustment quality, so no differential is used. Regulation law. Therefore, PI regulation is used for the automatic adjustment of the gas main pressure and the automatic adjustment of the small circulation valve. The larger the value of P, the stronger the effect of proportional adjustment, and the smaller the value of I, the stronger the integral effect.
Automatic control of air volume (pressure) in the system, reducing labor intensity and failure rate; intuitive observation of operating parameters, simultaneous display of operating parameters such as pressure, frequency, rotational speed, voltage, current, torque, etc.; all pipeline valves open, greatly reducing the loss of regulating doors .
E. Dropout control of pressure sensor
For some highly reliable control systems, the controlled object proposes the theory of multi-point acquisition. Therefore, two pressure collection points are taken on the front mother tube. Compare these two pressure values ​​into the PLC S7-300. If the difference is greater than a certain value, it is considered that the sensor with a small pressure value fails. This ensures the accuracy of the collected pressure values, thus ensuring system reliability.
F. Drop resistor control
For some highly reliable control systems, temperature is an important parameter of the system. According to the working principle of the Pt100 thermal resistance, if the Pt100 thermal resistance is disconnected for some reason then this is the limit value of the infinite resistance temperature. For those devices whose temperature cannot exceed the set value, it may cause the device to trip, causing the system to be unnecessary. parking. The temperature signal of the Pt100 RTD can be processed in the PLC S7-300. If the temperature value is greater than a certain value, the Pt100 RTD sensor is considered to be faulty. Avoid unnecessary parking to ensure system reliability.
4.3 Emergency Breakdown Measures
The pressure fan's variable frequency control system has an on-site operation box designed on site. In the case of a control system, operate the transfer switch and stop the inverter through a hard-wired cable to prevent the accident from expanding. For example, if the Profibus bus cable is faulty, the pressurized fan is in an out-of-control state, and the pressure in front of the fan is slowly reduced by the inertia of the fan. At this time, the control system will send out sound and light alarms and report communication failures. It needs manual intervention to quickly handle the fault.
4 Conclusion
The use of a frequency converter to control the pressurizing fan enables a constant control of the pressure at the outlet of the pressurized blower, which greatly improves the lime kiln production and on-site environment and fully meets the production process requirements. The perfect combination of PLC control technology, PROFIBUS bus technology and frequency conversion technology makes the integration highly automated, stable in operation, simple in operation, and energy-saving and highly efficient. Solve the problem that the two pressurized blower parallel operation by adjusting the return valve can not achieve a constant pressure and mutual non-perturbative switching, this lime kiln production problems.
The successful application of the frequency converter control system of the lime kiln pressurizing fan has high economic value for improving the environment, improving the gas utilization rate, and ensuring that the gas cabinet is not evacuated into a vacuum, and is worthy of promotion.
references
[1] Du Zhenhui.Application of frequency converter in gas blower system[J].Glass,2003(3):20-21,51.
[2] Wang Zhenmin, Yan Yu, Jiang Fulu, et al. Preliminary research on frequency conversion speed regulation of coke oven gas blower[J]. City Gas, 1995(11):24-26,11.
[3] Li Sheng, Xu Jianning. Reformation of blower variable frequency speed regulation and collector pressure control device[J]. Fuel and Chemical Industry, 2000(4); 188-191.
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