I. Introduction
The main function of the decentralized control system (DCS) is to control, monitor, manage, and make decisions on the production process. Therefore, it must have high reliability to ensure the safe and economic operation of the plant. With the continuous development of large-scale computer systems and computer communication networks, the reliability problem has become a very important problem, and its theory is constantly developing and improving under this situation. The research content of reliability technology is roughly divided into four aspects: reliability design, reliability analysis, reliability test, and reliability management.
2. Reliability measures of DCS system
In the DCS system, many technical measures to improve reliability are adopted. These technical measures are based on the following four basic ideas: â‘ Make the system itself not prone to failure, so-called failure prevention; â‘¡ When the system fails, minimize the impact of the failure, so-called failure security and failure Weakening; â‘¢ When the system fails, the system can continue to run, so-called fault tolerance; â‘£ When the system fails, maintenance can be performed without stopping the system, which is called online maintenance. Based on these four basic ideas, various reliability measures are adopted in the DCS system to reduce the occurrence of accidents and the losses caused.
1. Strict quality management and improve system hardware level
In order to realize the hardware reliability of the DCS system, the manufacturer of the DCS system has adopted a series of measures to improve the quality of the hardware, such as strict screening of components, derating of aging components, full consideration of the impact of parameter changes, and the use of low Power consumption components, using noise suppression technology, etc.
2. Reliability measures of DCS system site design
After the DCS has a problem, there will be situations where the device will be out of control due to inoperability and monitoring. If the equipment is out of service, the economic benefits will be affected, and the safety of people and equipment will be endangered. Therefore, the following reliability of the system must be seriously considered in the design according to the specific situation of the site:
(1) Power supply system
The power supply system of the DCS system is the basis of its operation. The power load should not exceed 60% to avoid high load operation. At the same time, it is necessary to consider the use of high-quality power supplies. In general, the author uses UPS power supply. It is best to use two UPSs as backups for each other. The backup machine can be switched without disturbance after a fault occurs, improving the reliability of the power supply. Do not use the same electrical bus for the UPS input power supply, and select different bus section power supplies with high reliability according to the electrical plant wiring, so as to ensure that the electrical section of the bus will not affect the power supply of the DCS system after a problem. The DCS power system (UPS, power cabinet, etc.) should be installed in the electronic equipment room that meets the requirements of dust-free, vibration-free, anti-corrosion, etc. at the time of installation, and the cable hole should be blocked before the power is installed to prevent leakage of steam from the cable. The hole enters the power system and causes a short circuit in the power supply. After the DCS power supply is powered up, the UPS, power cabinet and other equipment should be hung with the "equipment forbidden operation during power-on operation" sign. It is even more forbidden to connect the DCS power system with loads that are not related to the DCS system, such as maintenance power supply Lighting, charging equipment, air conditioning, etc.
(2) Redundant system
Considering the need for people to perform redundancy on important I / O modules on site, the N: 1 redundancy structure of the operator station is also considered to avoid equipment outage due to hardware failures in the card or operator station.
(3) Adopting backup measures
â‘ Manual backup
For important control loops, manual backup methods can be used to improve reliability. Once the automatic control fails, the manual operation station is used for direct operation. In this case, the manual operation station directly outputs 4 ~ 20mA or 1 ~ 5V analog signal to control the actuator or directly ground to perform the operation of the device, which can be controlled manually. production process.
â‘¡ Automatic backup
Automatic backup is to set up another set or sets of backup control devices in a redundant manner. When the automatic control device in the running state fails, the standby control device is automatically put in to maintain the automatic control of the system. Automatic backup is a form of redundant system. The redundancy includes main control unit redundancy, power supply system redundancy, network redundancy, I / O module redundancy, operator station redundancy, server redundancy, etc. When the operating equipment fails, the hot standby equipment will run automatically and without disturbance, and it will not affect the maintenance of the failed equipment.
3. Make the system in a safe state when it fails
The DCS system itself continuously performs online fault detection during work. Once a fault is found, the faulty device is isolated from the system so that it will not affect the normal operation of other devices. After the fault occurs, an audible and visual alarm will be issued at the same time, and the maintenance personnel will analyze the cause of the fault according to the alarm prompt and deal with the fault as soon as possible to avoid the expansion of the fault. On-site technical measures should be taken during the process, and the CPU should not be restarted easily to avoid disturbance to the equipment. If you need to restart the CPU, you need to cooperate with the operator to take precautions. Generally, the DCS resets the analog output signal when the CPU is reset, so that you can manually switch the control method between the actuator and the Inverter. In the electric mode or the way of pulling out the analog output card (the actuator has the function of holding off the signal), after the system is normal, add the signal to the normal value and then power on the actuator and insert the card. However, it must be carried out according to the characteristics of the DCS itself. Some designs have switching safety outputs (on, off, hold, etc.). If the safety output selected by the user during configuration is on, then the switching will be issued after restarting the CPU. Open instructions.
3. Site conditions
1. On-site environment
DCS systems are generally placed in the control room or the computer electronics room. The computer room should be insulated and dustproof. It is also necessary to avoid the interference of strong electromagnetic fields and away from places with strong vibration and strong noise. The clear height of the computer room should not be less than 3.2m, the area should not be less than 20m2 (a computer and its peripheral equipment), and there should be electrical sockets in the room. The ground should be a dust-free, static-free smooth floor. The floor should be a raised floor, and the height of the lower space should not be less than 150mm. The computer room should ensure that the indoor temperature is in the range of 18 to 25 ° C, the temperature change rate is less than 5 ° C / h, and the relative humidity is in the range of 45% to 70%. No condensation is allowed under any circumstances. When the air conditioning equipment fails, the room temperature should be maintained within 24 hours not exceeding the manufacturer's allowable value. The above conditions need to be strictly controlled before the DCS system is powered on. The working environment is the basis to ensure the reliable operation of the equipment. If the environmental requirements do not meet the requirements, the DCS system will be affected to varying degrees during the operation, such as high temperature and high humidity , Vibration, etc. will affect the normal operation of the DCS system.
2. Cable laying
(1) Copper wires and PVC insulated wires should be used for the connecting wires inside the control panel, table and relay cabinet. The connecting wire that needs to go through the plug-in (or the connecting wire of the FK type switch) should be a copper core PVC insulated flexible wire.
(2) The thermocouple wire to the cold junction compensator, constant temperature box or direct connection with the instrument should use a thermocouple wire extension wire or a compensation wire that matches the thermoelectric characteristics of the thermocouple wire.
(3) The analog input and output signals and low-level switching signals should be connected with shielded cables, and the cross section of the signal cable core should be greater than or equal to 1 mm2. The high-level (or high-current) switching input and output signals can be connected with a common twisted pair cable (control cable), but should be separated from the analog signal and low-level switching signal, and go separately in the cable slot.
(4) Weak signals and low-level signals, especially those requiring anti-interference signals (such as in front of computers and split-phase instrument converters), should not be combined with a strong electric circuit by a cable or laid in the same protective tube.
(5) Cables with different signals or different voltage levels should not be laid in the same layer of brackets; when laid in the same layer of brackets, they should be categorized and separated by partitions if necessary.
(6) It is strictly forbidden to lay the cable in parallel above the oil pipeline or directly above the gas pipeline and under the oil pipeline connection.
(7) The input and output signal cables of the computer should be laid in the cable channel with cover, and the cable channel and the cover should be well grounded. A single signal cable should be laid in a steel cable tube, and the cable tube should be well grounded. The cable shielding layer should use copper tape shielding or aluminum foil shielding.
3. Common notes on site grounding
The DCS system has strict requirements for grounding. When grounding on site, you must strictly follow the grounding specifications of the DCS manufacturer. Otherwise, the reliability of the DCS system will be affected. Pay attention to the following points when grounding the DCS system:
(1) The power ground of the I / O cabinet and the power ground of the UPS must be connected to the same ground to ensure equal potential.
(2) The grounding cable must meet the requirements of the DCS manufacturer and must be connected to the dedicated grounding screw in the cabinet.
(3) The system ground and shield ground cannot be merged in the cabinet, and must be connected to the grounding grid separately.
(4) The cabinet cannot be replaced indirectly with cabinet parallel bolts, and must be strictly connected with a ground wire.
(5) The grounding shall be firm and reliable, and the busbar shall be coated with anti-corrosion paint.
(6) The grounding resistance meets the requirements of the DCS system and must be strictly measured.
(7) Grounding is strictly grounded in accordance with the DCS manufacturer's grounding requirements. You cannot take shortcuts or save grounding according to the grounding wire diameter requirements.
The DCS system grounding does not meet the requirements. During the operation, unpredictable failures will occur, such as signal acquisition errors, abnormal communication, computer crashes, control failures, and protection malfunctions.
4. System maintenance
Summarize the following points for DCS system maintenance:
(1) Regularly check the power supply of the DCS system, and periodically carry out switching experiments for the redundant power supply system. At the same time, the UPS power supply shall be regularly switched and checked, and the Battery shall be regularly discharged and charged as required.
(2) Regularly check whether the network connector and each connecting wire are firm, and whether the wiring terminals in the control cabinet are firm and reliable.
(3) Always check whether the control unit, I / O modules, other modules, etc. are working properly.
(4) Regularly check whether the grounding is firm and test whether the grounding resistance meets the requirements.
(5) Regularly check the workload of the Controller, computer, etc., and pay attention to whether there is a rise.
(6) Regularly check the MMI hard disk and delete fragmented files. Historical files are often archived and backed up by peripherals.
(7) Use the DPU (Distributed Processing Unit) and MMI (Human-Machine Interface) of the DCS system to reset one by one during the overhaul of the unit to eliminate the accumulated errors of the computer during long-term operation.
(8) For the interface between the DCS system and other systems (such as MIS (computer management system), SIS (factory-level monitoring information system), etc.), it is recommended to install a virus firewall on the gateway station on the other system side and update the virus in time Library. At the same time, update the operating system patches in time to improve the security of the system.
(9) Regularly check whether the system fan is working properly and the air duct is not blocked to ensure that the system can run reliably for a long time.
(10) When the unit is overhauled and shut down, pay attention to check the power of the COMS battery of the DPU host card. When the COMS data is lost due to the lack of CMOS battery, the CMOS battery of the motherboard should be replaced in batches.
(11) Regular patrol inspection habits, regular inspections, and regular cleaning to ensure that the operating environment of the DCS system is clean.
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