1 Introduction
Industrial production and urban life generate a large amount of smoke and dust, such as from thermal power generation and heating, which pollute the environment and harm health.
With the development of baghouse dust collection technology and increasingly stringent environmental protection requirements, the application range of baghouse dust collectors is becoming increasingly wide. Currently, baghouse dust collectors can be used to treat high-temperature, high-humidity, sticky, explosive, and abrasive flue gases, and even filter air containing ultrafine dust. In the field of baghouse dust collector control, PLCs occupy a dominant technological position. With the continuous development of control technology, the application of PLCs and touchscreens in industrial control is becoming increasingly widespread. Touchscreens replace traditional control panels and keyboards for intelligent operation, allowing for parameter setting, data display and storage, and depicting automated control processes in the form of curves and animations. The combined use of PLCs and touchscreens expands the functionality of PLCs, giving them a graphical, interactive working interface, making them independent systems. It also significantly reduces the number of switches, buttons, and instruments on the control cabinet, making operation simpler. Currently, in applications with high control requirements, frequent parameter changes, and complex hardware wiring, the combination of touchscreens and PLCs has become the dominant application.
However, the traditional point-to-point direct control method of dust collector PLCs is not only complex in design and on-site wiring, but also suffers from unstable system operation due to external signal interference, and is expensive. This paper combines a Siemens S7-300 PLC with a Delta A-series touch screen, using a matrix control method to realize the action of the dust collector's pulse valve. Data exchange between the master station and distributed I/O devices is realized through the PROFIBUS DP industrial bus, achieving optimized control results.
2. System Composition and Main Functions
2.1 System Composition
The control system of the bag filter is shown in Figure 1.
Figure 1 System Requirements
This baghouse dust collection system mainly consists of the dust collector body, the ash discharge system, and its pipeline temperature, pressure, and fault alarm functions. The dust collector body primarily handles online differential pressure dust removal control. The dust collector has a total of 6 chambers, each with 13 pulse solenoid valves, totaling 78 valves; and each chamber also has one outlet control valve, totaling 6 valves.
2.2 Functional Design
(1) Dust removal control
Control methods: online differential pressure/timed, offline differential pressure/timed, manual control (for debugging or testing); Pulse interval: 1~60s continuously adjustable; Pulse width: 0.02 ~ 0.2s continuously adjustable; Timed cleaning cycle: 0~99 minutes continuously adjustable; Differential pressure cleaning setting range: 0~3000pa continuously adjustable; Temperature setting range: 0~300℃ continuously adjustable; When the dust collector reaches the high resistance (set high differential pressure) value, the pulse valve of dust collector chamber #1 is activated to start jet cleaning, followed by dust collector chambers #4, #2, #5, #3, and #6, with the pulse valves working in groups; Cleaning ends when the last group of pulse valves in dust collector chamber #6 finishes working; Cleaning is then performed in the same way when the next high resistance is reached.
(2) Ash removal control
High and low material levels: Each dust collector has only one high and one low material level indicator on one ash hopper for alarm purposes (high material level indicates that ash needs to be discharged, low material level indicates that ash discharge can be stopped). When the material level is high or low, an audible and visual alarm will be triggered. If no action is taken after a certain period of time, the dust collection system will automatically stop. Ash discharge is carried out in both the control room and on-site. Signals are transmitted to the HMI to display the system's operating status.
(3) Temperature and pressure detection
The system needs to detect the inlet and outlet differential pressure, inlet temperature, outlet temperature, and leak detection pressure of each filter bag in the chamber.
(4) Control relationship (automatic mode)
When the flue gas temperature (referring to the inlet flue gas temperature) is normal (100~165℃), all inlet and outlet valves are open, and the bypass valve is closed; when the temperature is lower than (100℃) or higher than (165℃) the set temperature, the control system alarms; when the temperature is higher than (170℃) the set temperature, the bypass valve is opened first and then the outlet valves 1 to 6 are closed in sequence; the inlet valve is manually closed on site.
3 Hardware Configuration and Software Implementation
The system uses a matrix configuration of Siemens S7-300 series PLCs and intermediate relays to detect and control input points. A Delta touchscreen is used to set the pulse valve actuation width, interval, and timing cycle, and to view the operation of pulse valves in each chamber. By utilizing the PLC outputs and intermediate relays to form a row-column structure, with the output control points placed at the intersections of the row-column structure, the system's hardware cost is significantly reduced compared to point-to-point output control. For example, a point-to-point PLC automatic control system for a 6-chamber, 13-group online/offline pulse jet bag filter in Guangxi Mingyang requires at least 78 output points, while a matrix PLC control system using PLC outputs and intermediate relays only needs 19 output points to accomplish the same task.
This system features a user-friendly interface that clearly displays which chamber and pulse valve is being purged. It also includes differential pressure indication and pressure upper and lower limit alarm outputs. The control system can be interlocked with a local control box, facilitating on-site equipment maintenance, as shown in Figure 2. The system operates stably and reliably, is simple to operate, and easy to maintain, significantly reducing the workload of operators.
Figure 2 HMI main screen
The outputs of the 6×13 matrix control system are shown in Table 1.
Based on schematic diagram 3 and Table 1 above, the pulse valves in each chamber only require any combination of q0.0 ~ q1.4 and q1.5 ~ q2.2 to activate. Specifically, q0.0 ~ q1.4, connected to terminal m, energizes the intermediate relay, causing it to close and control the connection between the solenoid pulse valve and 24V+. Similarly , q1.5~ q2.2 , connected to terminal m, also energize the intermediate relay, controlling the connection between the solenoid pulse valve and terminal m. For example, when the 9# solenoid pulse valve in chamber e needs to activate, simply set q1.0 and q2.2 to 1. At this time , q1.0 outputs 24V, closing the normally open contact of the intermediate relay, connecting the 9# solenoid pulse valve in chamber e to 24V+. Likewise, q2.2 outputs 24V, closing the normally open contact of the intermediate relay, connecting the 9# solenoid pulse valve in chamber e to terminal m. At this point, the 9# solenoid pulse valve in chamber E is simultaneously connected to both the 24V+ and M terminals, thus activating. During this process, although all six solenoid pulse valves numbered 9# controlled by Q1.0 are connected to the L1 terminal, only the pulse valve in chamber E, controlled by Q2.2 , is connected to the M terminal. That is, the solenoid pulse valve at the intersection of the control points of Q1.0 and Q2.2 is activated; the solenoid pulse valves at other points are either connected only to the 24V+ terminal or only to the M terminal. In this principle, the solenoid valves could be activated using the 24V voltage output from the PLC, but to ensure stable operation of the solenoid pulse valves and reduce the load on the PLC, an external stable DC power supply is used, thus employing 19 intermediate relays.
On Delta's touchscreen HMI, the status display of pulse valve operation can be achieved by performing an AND operation between the individual pulse operation signals of each chamber. For example, to display the operation of the #5 solenoid pulse valve in chamber d, the following process can be used:
aq1.4
aq2 . 0
=m0 . 0
The touchscreen can determine whether the solenoid pulse valve is operating by reading the contents of m0.0 . The touchscreen's status indicator only shows that the solenoid pulse valve is operating when both q0.4 and q2.0 are simultaneously 1; otherwise, it is not operating. When manually controlling the #5 solenoid pulse valve in chamber D via the touchscreen, simply set m0.1 to 1. The PLC can then manually control the solenoid pulse valve using the following program segment:
am0 . 1
= q1.4
=q2 . 0
As can be seen from the above, by adding 19 relays to the original system, the number of output points was reduced by 59, achieving a very satisfactory result.
Figure 3 Valve matrix control principle
Figure 4 PROFIBUS DP configuration
To reduce the laying of control lines between the field and the control room and the transmission of external interference signals, the system uses PROFIBUS DP. Only one communication line connects the distributed I/O devices to the controller CPU. The DP master station exchanges data with the distributed I/O devices and monitors PROFIBUS DP data via PROFIBUS DP, as shown in Figure 4. The distributed I/O devices (=DP slave stations) collect data from field sensors and actuators so that it can be transmitted to the controller CPU via PROFIBUS DP.
4. Conclusion
In this pulse jet baghouse dust collector, a touchscreen was used to replace conventional digital display instruments and manual control panels. This enabled the setting and display of system operating parameters and the display of fault alarm information in Chinese, greatly improving the interactivity of system operation and reducing the burden on on-site operators, thus enhancing the human-machine interface of the system. The matrix control circuit was improved, enabling the control of a large number of pulse valves in the baghouse dust collector (reduced from n×m to n+m), saving a significant number of PLC input/output points, simplifying the control circuit, and reducing costs.
