Selection of high-quality hardware
PLC controller: Choose a well-known brand PLC controller with high reliability, such as Siemens, Rockwell, etc. These brands of PLC have been tested in a large number of practical applications, and their internal processors and storage chips and other key components are of high quality, which can effectively reduce the probability of hardware failure. For example, Siemens S7-1500 series PLC uses advanced chip manufacturing technology and has strong anti-interference ability.
I/O module: For input and output modules, choose the appropriate type according to the actual needs of the chemical and petroleum site. For analog input modules, ensure that they have high-precision signal acquisition capabilities, such as 16-bit or higher resolution, to accurately obtain sensor signals such as temperature, pressure, and flow. Digital I/O modules should have high noise resistance and good electrical isolation performance to prevent external interference signals from affecting the normal operation of the system.
Power supply module: The use of redundant power supply configuration can improve the stability of the system. For example, a dual power supply module is selected. When one power supply fails, the other power supply can seamlessly take over the power supply task to ensure the continuous operation of the PLC control cabinet. And the power module should have good voltage stabilization and filtering functions to cope with voltage fluctuations and electromagnetic interference in the chemical and petroleum field grid.
Reasonable wiring and grounding
Wiring principles: Inside the control cabinet, classify and wire according to the signal type. Lay the power line and signal line separately. For example, lay the power line on one side of the control cabinet and the signal line on the other side, and keep a certain distance. It is generally recommended that the distance between the power line and the signal line is not less than 300mm. For analog signal lines, use shielded cables, and the shielding layer should be grounded at one end to reduce external electromagnetic interference.
Grounding system: Establishing a good grounding system is the key to ensuring stability. The PLC control cabinet should use a separate grounding electrode, and the grounding resistance should be less than 4Ω. At the same time, the grounding terminal of the PLC, the shielding layer of the shielded cable, and other equipment that needs to be grounded should be reliably connected to the grounding electrode to eliminate the potential difference caused by static electricity and electromagnetic interference, and avoid interference signals from entering the PLC system through the grounding loop.
Strict environmental control
Temperature control: The ambient temperature at the chemical and petroleum site may be high, and the components in the PLC control cabinet are more sensitive to temperature. An air conditioner or heat exchanger can be installed in the control cabinet to keep the temperature in the control cabinet within the normal operating temperature range of the PLC, generally 0 – 55℃. For example, in a high-temperature oil refinery workshop, the temperature in the control cabinet can be reduced by installing industrial air conditioners to ensure the stable performance of the PLC components.
Humidity control: High humidity environments may cause moisture damage to electronic components. Place desiccant or install dehumidification equipment in the control cabinet to control the relative humidity within a suitable range, usually 10% – 90%. At the same time, pay attention to the sealing of the control cabinet to prevent external humid air from entering.
Dust and corrosion prevention: There is a lot of dust and corrosive gases in the chemical production environment. Install air filters at the air inlet and vents of the control cabinet to filter out dust particles. For environments with corrosive gases, the control cabinet should adopt a sealed structure and use anti-corrosion materials to make the cabinet body, or install a gas purification device in the control cabinet to remove corrosive gases such as hydrogen sulfide and sulfur dioxide.
Software design optimization
Program structure: A modular programming structure is used to divide the control program into multiple functional modules, such as data acquisition module, control algorithm module, alarm processing module, etc. This not only facilitates program writing and debugging, but also does not affect the normal operation of other modules when a module has a problem. At the same time, the program should be annotated in detail to facilitate subsequent maintenance and upgrades.
Fault handling mechanism: A complete fault handling procedure is set up in the software. For example, when a certain I/O point fails, the program can automatically switch to the backup channel or take corresponding safety measures, such as stopping the operation of related equipment and issuing an alarm signal. And the program should be fully tested, including normal situation tests and various fault simulation tests to ensure the reliability of the program.
Data backup and recovery: PLC programs and data should be backed up regularly, and can be backed up through external storage devices (such as USB flash drives, mobile hard drives, etc.) or network storage. At the same time, a data recovery mechanism should be designed so that data can be quickly restored when the PLC fails or the program is damaged, reducing downtime.
Regular maintenance and testing
Hardware inspection: Make a regular maintenance plan to check the hardware in the PLC control cabinet. This includes checking whether the connection of the components is loose, the working status of the I/O module, the output voltage of the power module, etc. For vulnerable parts, such as relays and fuses, they should be replaced regularly to prevent failures.
Software maintenance: Regularly check the operation of the PLC program to see if there are program errors or abnormal operating conditions. At the same time, according to changes in production processes and equipment updates, the program should be upgraded and optimized in a timely manner. For example, when a chemical production line adds new equipment or changes the control process, the PLC program should be modified and debugged accordingly.
Cleaning and maintenance: Clean the control cabinet regularly to remove dust and debris inside. For components such as the operation panel and indicator lights outside the control cabinet, they should also be cleaned and inspected to ensure their normal operation. And the equipment in the control cabinet should be maintained, such as cleaning or replacing components such as fans and filters.
