Discussion on Lightning Protection Measures of Pressure Control Instrument in Chemical Industry

In the development of China's chemical industry, the improvement of automation and intelligence of various control instruments is an important condition to ensure the continuity of chemical production. However, in the pressure control instruments used in domestic chemical enterprises at this stage, there are many defects, such as poor over-voltage or over-current tolerance, low insulation strength, sensitive to electromagnetic interference and so on, which are also the main factors causing lightning accidents of pressure control instruments. Due to the performance defects of the product, it is very likely that when the pressure control instrument is directly struck by lightning or nearby area, the lightning overvoltage or pulsed electromagnetic field is transmitted to the instrument equipment through the instrument signal line, power supply line, cable trunking, threading pipe, etc., seriously threatening the normal work and safe operation of the instrument and related ancillary equipment, It is of great significance to strengthen the research of lightning protection measures for pressure control instruments, which must be paid great attention by relevant technical and management personnel.

1. Analysis of possible hazards caused by lightning stroke

According to the introduction of China's lightning and anti-static department, lightning can be divided into three forms: direct lightning, inductive lightning and surge. However, the most common harm of pressure control instruments is not caused by direct lightning strike, but by the current surge induced in power supply and communication lines when lightning strike occurs. The main reasons are as follows: 1) due to the increase of signal source paths, the system is more vulnerable to lightning wave invasion than before, and the surge voltage can rush into computer equipment from power line or signal line; 2) Due to the high integration of the internal structure of electronic equipment, the level of withstand voltage and over-current of the equipment is reduced, and the ability to withstand lightning is reduced. 

2. Lightning protection measures for pressure control instruments 

At present, in the process of manufacturing and using pressure control instruments in China, the lightning protection problem has gradually attracted the attention of the industry, and constantly through technological innovation, process improvement, quality enhancement and other ways, in order to obtain the continuous improvement of the lightning protection performance of pressure control instruments. For the study of lightning protection measures of industrial pressure control instruments, we must adhere to the principle of comprehensive consideration and analysis, and strictly control the whole process of production, installation, use and maintenance, otherwise it is very likely to be struck by lightning. 

2.1 selection and installation of pressure control instrument

When selecting the pressure control instrument, it is necessary to select the instrument with strong anti-interference ability, and the instrument is equipped with special lightning protection and grounding measures. The transmitter is equipped with a lightning arrester to protect it from the abnormal voltage impulse induced by lightning on the transmission cable. The arresters are field mounted, built-in and disk mounted. When installing the pressure control instrument, the fasteners of the instrument should be installed correctly, and the meter box must be well grounded. At the same time, the position of the pressure control instrument should be considered. The instrument installed at the high altitude measurement position is easy to be struck by lightning, so it is necessary to install lightning rod according to the actual situation; If the pressure control instrument is far away from the production device and there is no lightning protection device around, the lightning rod should also be installed. The signal cable of pressure control instrument must be connected into the slot box according to the regulations. When the signal of primary components is connected on the device site, the shielding isolation of the cable must be grounded according to the regulations. The outside of the cable shall be laid through the pipe with galvanized metal pipe. When the lightning strikes the cable through the pipe, the protection circuit will be generated and discharge to the ground. All metal parts of the pressure control instrument insulated from the ground shall be jumper connected by metal wire. 

2.2 try to shield the harm caused by lightning electromagnetic pulse 

In the formulation of lightning protection measures for pressure control instruments, it is necessary to pay attention to the control of the same circuit. All links and components are connected by shielded wires, and the two ends of the shielded wire are grounded, which can consume the energy of lightning electromagnetic pulse radiation. In the process of consumption, although there is induced current in the shielding layer, the induced current also induces current to the signal line, and its direction is just opposite to that of lightning electromagnetic pulse to the signal line, which weakens the harm of lightning electromagnetic pulse to the signal line of pressure control instrument. When the shielding wire is short, it can be grounded at one end, and the redundant signal wiring should also be grounded at both ends. The metal skin of the controller and other equipment of the pressure control instrument should also be grounded, which can not only play a shielding role, but also release the accumulated electric charge. Therefore, there should be multi-level shielding grounding to minimize the harm of lightning electromagnetic pulse to the signal line of the pressure control instrument. The shielding grounding should be connected to the grounding network of the information system. 

2.3 improve the immunity of pressure control instrument system 

The new pressure control instrument system contains many electronic circuits, which are used in the harsh industrial environment such as chemical industry, metallurgy, oil refining and power generation. It is under the influence of electromagnetic disturbance produced by various industrial equipment. In order to ensure that the sensitive electronic circuit can work normally, the electromagnetic compatibility of pressure control instrument system must be considered. There are seven types of power interference in pressure control instrument system: drop, frequency offset, power loss, electrical noise, harmonic distortion, surge and transient; The signal interference types of pressure control instruments mainly include common mode interference, series mode interference, surge, etc. Among them, drop, surge and transient interference are mainly produced by lightning. 

2.4 pay attention to the setting of surge protective device

The three-level lightning protection for the power supply part of pressure control instrument involves the strong current part, which will not be discussed in detail here. Focus on the common surge protective device for pressure control instruments. The main functions of surge protective devices for pressure control instruments are as follows: under normal conditions, it does not affect the circuit; When the surge invades, the connected part will be short circuited to the ground, so that the surge current in the line will be released into the ground quickly, so that the equipment connected to the line will not be affected by the surge; After the surge stopped, the surge protector returned to normal.

3. Conclusion

The formulation and implementation of lightning protection measures for pressure control instruments is a complex and comprehensive project, which not only needs to increase the investment of technology, capital and manpower, but also needs the later management and maintenance. In the process of chemical production, different types of pressure control instruments also have their particularity. In the formulation of lightning protection measures, all kinds of external factors should be comprehensively analyzed, especially the actual use and working environment of the instruments should be considered. In the formulation of lightning protection measures for pressure control instruments, in order to ensure the overall lightning protection standards and requirements of chemical automation system, the whole production device should be designed according to the principle of equipotential bonding, considering from the control room, field instruments, analytical instruments, control instruments, instrument signals and power lines. At present, the lightning protection of pressure control instruments used in domestic chemical production adopts a variety of measures, such as lightning protection, shielding, equipotential bonding, grounding, surge protection, and so on. In addition to considering the system safety, it also needs to consider the cost of investment and the economy of operation.

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