I. Introduction
In recent years, the scale and number of petrochemical companies have continuously expanded and increased, and the instrumentation system has been rapidly developing toward networking and intelligence. Instrumentation and equipment have generally suffered from low dielectric strength, poor tolerance to overvoltage and overcurrent, and sensitivity to electromagnetic interference. Such weaknesses: Once the instrumentation equipment is directly struck by lightning or a lightning strike occurs in a nearby area, lightning overvoltage, overcurrent, and pulsed electromagnetic fields will reach the instrumentation equipment through power supply lines, instrument signal lines, cable trunkings, and conduits, threatening the instrumentation equipment. The normal work and safe operation.
If it is improperly protected, it will cause the instrumentation equipment to fail to work, and will cause the instrumentation equipment to be permanently damaged. In severe cases, it may also result in casualties and production accidents.
Therefore, the design of modern petrochemical instrumentation systems must place great emphasis on the design of lightning protection.
II. Review of Lightning Protection of Petrochemical Instrumentation System
At present, when designing petrochemical instrumentation systems in China, the issue of lightning protection is basically not considered, but foreign countries have already had nearly 20 years of research and experience in this area. Some domestic petrochemical plants often suffer from lightning strikes, which paralyzes the control system and causes the device to stop, causing huge economic losses. Therefore, some remedial measures for lightning protection have been taken mainly by adopting the shunting method, which is to use a Surge Protective Device (SPD) SPD (Surge Protective Device) in the signal or communication circuit of the instrument system and the power supply of the system to limit the transient overvoltage. And take the inrush current. However, an SPD can only provide protection for part of the loop; for example, an SPD installed in the DCS control room can only provide protection for the DCS card channel; an SPD installed at the field transmitter output can only be used with the transmitter. Provide protection. If all the I/O channels are equipped with SPD, the cost will increase significantly; furthermore, if two SPDs are added for each loop, the SPD itself will fail (as proven in production practice), The failure rate of the instrumentation system will greatly increase.
Therefore, some domestic petrochemical plants can only partially use SPDs in some relatively important occasions. Protection is also limited to field instruments or control rooms DCS, PLC, etc., and the overall lightning protection of instrument systems is not truly realized.
Third, lightning damage to the instrument system
Lightning strikes can be divided into direct lightning strikes and induced lightning strikes. The possible forms of damage to the instrument system are divided into the following types:
1. Direct lightning strike
Lightning directly hits the field instrumentation or the pipe connected to it, often damaging the instrument's sensor module and possibly damaging the transmitter's electronic circuit board. Lightning currents flow into the ground along the instrument support and generate a strong magnetic field that can be coupled to electronic devices such as control rooms DCS through signal transmission lines, damaging DCS and other electronic devices.
2. Induced lightning strike
(1) Electrostatic induction. When the thundercloud arrives, ground objects, especially conductors, accumulate a large amount of charge and cause discharge. If the discharge current enters the on-site meter and power equipment, the equipment will be damaged.
(2) electromagnetic pulse radiation. Lightning currents generate electromagnetic fields in the space around their channels, radiate electromagnetic waves outward, couple to the control room's computers, meters, and on-site instrumentation, as well as all kinds of metal conductors, generate induced electromotive force or induced currents, causing equipment failure, damage and so on. The control system failed.
3. Lightning overvoltage intrusion
Direct lightning or lightning induction may cause overvoltage in the wire or metal pipeline. The lightning overvoltage may introduce high potential into the instrument system along various metal pipelines, cable trenches and cable lines, causing interference and destruction.
4. Counterattack
When the lightning protection device is connected to the flash, a powerful momentary lightning current flows into the grounding device through the down conductor. Due to the existence of the earth resistance, the lightning charge cannot be quickly discharged to the earth, and it will inevitably cause a rise in the local ground potential (possibly hundreds of kilovolts). If the grounding body of the instrument control system does not have a sufficient safety distance from this point, discharge will occur between them, resulting in a counter-attack current that can directly break through the insulation of the appliance and cause interference or even damage to the instrument control system.
Fourth, instrumentation system major measures for lightning protection
The measures for the intrusion of the instrument system on mine damage are various and mainly include lightning, diversion, pressure equalization, earthing and shielding. These measures must be applied comprehensively to truly achieve the lightning protection of the instrument system. Current lightning protection measures adopted by petrochemical instrumentation systems are as follows:
1. Flash
The direct lightning protection is mainly achieved by the lightning protection devices of the building. The lightning protection of the field instrument system should be designed together with the lightning protection measures of the surrounding oil storage tanks and other equipment.
2. Pressure balance
When a lightning strike occurs, a transient potential rise occurs in the path through which the lightning transient current passes, causing a transient potential difference between the path and the surrounding metal objects. If the transient potential difference exceeds Insulation resistance between the two will lead to dielectric breakdown discharge, this breakdown discharge can directly damage the instrumentation equipment, but also can generate electromagnetic pulses, interfere with the normal operation of the instrument system. In order to eliminate the breakdown discharge between lightning transient current paths and metal objects, all metal enclosures, frameworks, metal devices of production plants, facilities, instrumentation, components, and components of metal enclosures of all field instruments can be eliminated. The metal facilities are connected together and connected to the lightning protection grounding system of the instrument control room to form a perfect equipotential connection.
3. Grounding
At present, there are mainly two measures for grounding of domestic petrochemical instrumentation systems: floating ground and multi-point grounding.
(1) Floating ground means that the working place of the instrument is kept insulated from the grounding system of the building, so that electromagnetic interference in the building grounding system will not be transmitted to the instrument system, and the change of the ground potential will have no effect on the instrument system. However, since the outer shell of the meter is to be protected and grounded, when the lightning is strong, a very high voltage may appear between the meter housing and its internal electronic circuit, breaking down the insulation gap between the two and causing damage to the electronic circuit.
(2) Grounding refers to the separation of the working ground and protection ground of the instrument, DCS, PLC, etc. The outstanding advantage of this grounding method is that it can be grounded nearby and the parasitic inductance of the ground wire is small. However, if a strong lightning wave enters the system through protective grounds, the electronic circuit will also be damaged due to high voltage. Because the above two grounding methods cannot meet the need for lightning protection, the protection ground and the working ground can be connected and the lightning protection grounding system can be connected, and the problem can be solved.
4. shield
The petrochemical instrumentation system uses a large number of semiconductor devices, integrated circuits, and cables for transmitting signals. The transient electromagnetic pulses generated by lightning strikes can be directly radiated to these components, and transient overvoltage waves can also be induced on the power or signal lines. The intrusion of electronic devices along the line causes the electronic device to malfunction or become damaged. The use of shields to block or attenuate the propagation of electromagnetic pulse energy is an effective protective measure. The lightning protection of instrumentation system mainly includes three aspects: control room shielding, field instrument shielding, signal line and power line shielding.
(1) Control Room Screening
The control system in the control room is the heart of the instrumentation system and is very sensitive to the electromagnetic pulses generated by lightning. Special attention must be paid to its shielding. The instrument control room shall be a closed structure without windows. It shall be electrically connected at the intersections of the structural steel bars in the house walls and be welded with the metal door frame to form a shielding cage with a door opening. A protective ring shall be made in the room around the wall. (Integrated lightning protection), the grounding ring and the shielding cage for effective electrical connection.
(2) Field Instrument Masking
Field instruments can use metal instrument boxes (hoods) to achieve lightning protection. The instrument box (hood) must be equipotentially connected to other site metal facilities and be connected to a lightning protection grounding system.
(3) Signal and power line shields
In order to prevent lightning electromagnetic pulses from inducing transient overvoltage waves on the signal or power lines, all signal lines and low voltage power lines should use cables with a metal shield. In terms of transient overvoltage protection, it is necessary that the shielding layer of the signal line or the power line be grounded at multiple points along the line or at least at the first and the end of the line. When a multi-point grounding is adopted, the shielding layer between the grounding points forms a loop between the lines, and the electromagnetic field of the low-frequency interference current may partially pass through the shielding layer, which generates low-frequency interference in the core-sheath loop of the cable. It is required that the shield can only take a single point of grounding along the line. In order to prevent low-frequency interference caused by multi-point grounding, the cable can be threaded into a metal pipe or a double-shielded cable can be used, and the outer shield of the metal pipe or double-shielded cable can be grounded at multiple points, and the inner shield of the metal pipe or double-shielded cable can be shielded. The layer can be grounded at one end, which not only ensures safety, but also helps to suppress low-frequency interference.
5. Diversion
Diversion is an effective measure for lightning protection. Because there are too many instrument loops, it is not possible to use SPD in each instrument loop. SPDs or arresters must be selectively installed in important loops and system power loops.
V. Overview
In order to achieve the lightning protection of the petrochemical instrumentation system, the entire production equipment shall be designed according to the principle of equipotential bonding, comprehensive considerations shall be taken from the control room, on-site instrumentation, instrumentation signals and power cords, etc., with flashover, shunting and pressure equalization Measures such as grounding, shielding, shielding, etc., need to be realized by the cooperation of electric, construction, and automation. In addition to considering system safety, the cost of investment and the economics of operation must also be considered.
In recent years, the scale and number of petrochemical companies have continuously expanded and increased, and the instrumentation system has been rapidly developing toward networking and intelligence. Instrumentation and equipment have generally suffered from low dielectric strength, poor tolerance to overvoltage and overcurrent, and sensitivity to electromagnetic interference. Such weaknesses: Once the instrumentation equipment is directly struck by lightning or a lightning strike occurs in a nearby area, lightning overvoltage, overcurrent, and pulsed electromagnetic fields will reach the instrumentation equipment through power supply lines, instrument signal lines, cable trunkings, and conduits, threatening the instrumentation equipment. The normal work and safe operation.
If it is improperly protected, it will cause the instrumentation equipment to fail to work, and will cause the instrumentation equipment to be permanently damaged. In severe cases, it may also result in casualties and production accidents.
Therefore, the design of modern petrochemical instrumentation systems must place great emphasis on the design of lightning protection.
II. Review of Lightning Protection of Petrochemical Instrumentation System
At present, when designing petrochemical instrumentation systems in China, the issue of lightning protection is basically not considered, but foreign countries have already had nearly 20 years of research and experience in this area. Some domestic petrochemical plants often suffer from lightning strikes, which paralyzes the control system and causes the device to stop, causing huge economic losses. Therefore, some remedial measures for lightning protection have been taken mainly by adopting the shunting method, which is to use a Surge Protective Device (SPD) SPD (Surge Protective Device) in the signal or communication circuit of the instrument system and the power supply of the system to limit the transient overvoltage. And take the inrush current. However, an SPD can only provide protection for part of the loop; for example, an SPD installed in the DCS control room can only provide protection for the DCS card channel; an SPD installed at the field transmitter output can only be used with the transmitter. Provide protection. If all the I/O channels are equipped with SPD, the cost will increase significantly; furthermore, if two SPDs are added for each loop, the SPD itself will fail (as proven in production practice), The failure rate of the instrumentation system will greatly increase.
Therefore, some domestic petrochemical plants can only partially use SPDs in some relatively important occasions. Protection is also limited to field instruments or control rooms DCS, PLC, etc., and the overall lightning protection of instrument systems is not truly realized.
Third, lightning damage to the instrument system
Lightning strikes can be divided into direct lightning strikes and induced lightning strikes. The possible forms of damage to the instrument system are divided into the following types:
1. Direct lightning strike
Lightning directly hits the field instrumentation or the pipe connected to it, often damaging the instrument's sensor module and possibly damaging the transmitter's electronic circuit board. Lightning currents flow into the ground along the instrument support and generate a strong magnetic field that can be coupled to electronic devices such as control rooms DCS through signal transmission lines, damaging DCS and other electronic devices.
2. Induced lightning strike
(1) Electrostatic induction. When the thundercloud arrives, ground objects, especially conductors, accumulate a large amount of charge and cause discharge. If the discharge current enters the on-site meter and power equipment, the equipment will be damaged.
(2) electromagnetic pulse radiation. Lightning currents generate electromagnetic fields in the space around their channels, radiate electromagnetic waves outward, couple to the control room's computers, meters, and on-site instrumentation, as well as all kinds of metal conductors, generate induced electromotive force or induced currents, causing equipment failure, damage and so on. The control system failed.
3. Lightning overvoltage intrusion
Direct lightning or lightning induction may cause overvoltage in the wire or metal pipeline. The lightning overvoltage may introduce high potential into the instrument system along various metal pipelines, cable trenches and cable lines, causing interference and destruction.
4. Counterattack
When the lightning protection device is connected to the flash, a powerful momentary lightning current flows into the grounding device through the down conductor. Due to the existence of the earth resistance, the lightning charge cannot be quickly discharged to the earth, and it will inevitably cause a rise in the local ground potential (possibly hundreds of kilovolts). If the grounding body of the instrument control system does not have a sufficient safety distance from this point, discharge will occur between them, resulting in a counter-attack current that can directly break through the insulation of the appliance and cause interference or even damage to the instrument control system.
Fourth, instrumentation system major measures for lightning protection
The measures for the intrusion of the instrument system on mine damage are various and mainly include lightning, diversion, pressure equalization, earthing and shielding. These measures must be applied comprehensively to truly achieve the lightning protection of the instrument system. Current lightning protection measures adopted by petrochemical instrumentation systems are as follows:
1. Flash
The direct lightning protection is mainly achieved by the lightning protection devices of the building. The lightning protection of the field instrument system should be designed together with the lightning protection measures of the surrounding oil storage tanks and other equipment.
2. Pressure balance
When a lightning strike occurs, a transient potential rise occurs in the path through which the lightning transient current passes, causing a transient potential difference between the path and the surrounding metal objects. If the transient potential difference exceeds Insulation resistance between the two will lead to dielectric breakdown discharge, this breakdown discharge can directly damage the instrumentation equipment, but also can generate electromagnetic pulses, interfere with the normal operation of the instrument system. In order to eliminate the breakdown discharge between lightning transient current paths and metal objects, all metal enclosures, frameworks, metal devices of production plants, facilities, instrumentation, components, and components of metal enclosures of all field instruments can be eliminated. The metal facilities are connected together and connected to the lightning protection grounding system of the instrument control room to form a perfect equipotential connection.
3. Grounding
At present, there are mainly two measures for grounding of domestic petrochemical instrumentation systems: floating ground and multi-point grounding.
(1) Floating ground means that the working place of the instrument is kept insulated from the grounding system of the building, so that electromagnetic interference in the building grounding system will not be transmitted to the instrument system, and the change of the ground potential will have no effect on the instrument system. However, since the outer shell of the meter is to be protected and grounded, when the lightning is strong, a very high voltage may appear between the meter housing and its internal electronic circuit, breaking down the insulation gap between the two and causing damage to the electronic circuit.
(2) Grounding refers to the separation of the working ground and protection ground of the instrument, DCS, PLC, etc. The outstanding advantage of this grounding method is that it can be grounded nearby and the parasitic inductance of the ground wire is small. However, if a strong lightning wave enters the system through protective grounds, the electronic circuit will also be damaged due to high voltage. Because the above two grounding methods cannot meet the need for lightning protection, the protection ground and the working ground can be connected and the lightning protection grounding system can be connected, and the problem can be solved.
4. shield
The petrochemical instrumentation system uses a large number of semiconductor devices, integrated circuits, and cables for transmitting signals. The transient electromagnetic pulses generated by lightning strikes can be directly radiated to these components, and transient overvoltage waves can also be induced on the power or signal lines. The intrusion of electronic devices along the line causes the electronic device to malfunction or become damaged. The use of shields to block or attenuate the propagation of electromagnetic pulse energy is an effective protective measure. The lightning protection of instrumentation system mainly includes three aspects: control room shielding, field instrument shielding, signal line and power line shielding.
(1) Control Room Screening
The control system in the control room is the heart of the instrumentation system and is very sensitive to the electromagnetic pulses generated by lightning. Special attention must be paid to its shielding. The instrument control room shall be a closed structure without windows. It shall be electrically connected at the intersections of the structural steel bars in the house walls and be welded with the metal door frame to form a shielding cage with a door opening. A protective ring shall be made in the room around the wall. (Integrated lightning protection), the grounding ring and the shielding cage for effective electrical connection.
(2) Field Instrument Masking
Field instruments can use metal instrument boxes (hoods) to achieve lightning protection. The instrument box (hood) must be equipotentially connected to other site metal facilities and be connected to a lightning protection grounding system.
(3) Signal and power line shields
In order to prevent lightning electromagnetic pulses from inducing transient overvoltage waves on the signal or power lines, all signal lines and low voltage power lines should use cables with a metal shield. In terms of transient overvoltage protection, it is necessary that the shielding layer of the signal line or the power line be grounded at multiple points along the line or at least at the first and the end of the line. When a multi-point grounding is adopted, the shielding layer between the grounding points forms a loop between the lines, and the electromagnetic field of the low-frequency interference current may partially pass through the shielding layer, which generates low-frequency interference in the core-sheath loop of the cable. It is required that the shield can only take a single point of grounding along the line. In order to prevent low-frequency interference caused by multi-point grounding, the cable can be threaded into a metal pipe or a double-shielded cable can be used, and the outer shield of the metal pipe or double-shielded cable can be grounded at multiple points, and the inner shield of the metal pipe or double-shielded cable can be shielded. The layer can be grounded at one end, which not only ensures safety, but also helps to suppress low-frequency interference.
5. Diversion
Diversion is an effective measure for lightning protection. Because there are too many instrument loops, it is not possible to use SPD in each instrument loop. SPDs or arresters must be selectively installed in important loops and system power loops.
V. Overview
In order to achieve the lightning protection of the petrochemical instrumentation system, the entire production equipment shall be designed according to the principle of equipotential bonding, comprehensive considerations shall be taken from the control room, on-site instrumentation, instrumentation signals and power cords, etc., with flashover, shunting and pressure equalization Measures such as grounding, shielding, shielding, etc., need to be realized by the cooperation of electric, construction, and automation. In addition to considering system safety, the cost of investment and the economics of operation must also be considered.
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