Most of the instrumentation automation systems used in chemical fiber plant utilities and water purification plant installations are EK series conventional instrumentation systems and DCS distributed control systems. Although the control room instrument uses a single EK series and DCS meter, there are many types of field instruments and the technical aspects involved are extensive. During the many years of operation of the device, there have been many instrumentation system failures and damage to the instrumentation system. Summarizing and analyzing these faults is the fault of the instrumentation system itself. There is also a fault that can not be ignored because of the wiring specifications of the respective signal lines and power lines between the meters and the cable laying and grounding lines between the control room and the field devices. Especially in some weak signal detection instrument systems such as electromagnetic flowmeter sensors, the instrumentation of field instruments and control room instruments, cable laying, signal lines and power line wiring conditions affect the accuracy of the instrument measurement and the safe operation of the instrument. Therefore, after summing up many years of work experience, with reference to the combination of instrument data and actual work, we have specified a number of precautions and precautions for instrumentation grounding, power supply, and cable laying. Forum of Chemical Technology - Gifts from Rose Hand 9 s/ T, S6 k/ X7 E7 D! l5 C, n
1. The signal interference factor of instrument automatic control system is also called noise. It is an electrical signal that is intruded or superimposed on the system power supply and signal cable and has nothing to do with the signal. Interference can cause measurement errors and severe interference can cause equipment damage. Common disturbances are the following: pressure vessels, chemical processes; d* j! t3 p2 R" g
(1) Conducted interference 1) When several kinds of signal cables are transmitted together, due to aging and leakage of insulating materials, other signals are affected, that is, interference is introduced into other signals. Bbs.hgbbs.net& j4 [" d# \* C% o2 T+ Q
2) In some field execution agencies, 220V power supply is used, sometimes equipment is burned out, or human factors cause short circuit between the power supply and the signal cable, and the strong electricity enters the weak current, resulting in greater interference and instrument damage. 8 o% n7 a" N) j ( Y
3) Common mode interference due to unreasonable grounding. If the two ends of the signal cable are grounded at the same time, the potential difference ΔE between the two points of the grounding system may occur, which may generate a large circulation current on the signal cable, which is superimposed on the signal current and causes the analog signal to fluctuate. Caused failure of the instrument or card. Pressure Vessel, Chemical Process 7 P: Q& j1 d$ l9 z# G
4) During operation, the acquisition signal sent by the card is grounded for a long time, the formed ground current exceeds the allowable value, and the channel is damaged.
Among these types of conducted interferences, since the instrumentation system itself is now protected, the interference caused by the aging of the insulating material or the burning of the device is less likely to occur, and the interference caused by grounding is more common. It is more elusive and should be given enough attention. "l" x4 H; P) U: ^! R( \( f
(2) Capacitive and inductive coupling disturbances bbs.hgbbs.net8 o% Q1 {6 ^ [+ Z! w" y- n$ Q
In the controlled field, there are often many signals passing through the cable ducts or cable ducts that are connected to the control system. These signal cables are routed together with distributed capacitances. These distributed capacitances will add interference to other signals. On the cable. In addition, alternating magnetic fluxes are generated around alternating signal cables, and these alternating magnetic fluxes generate electromotive force between parallel conductors, which also causes disturbance on the line. " Q* ~- d! D7 D8 b" `
(3) Disturbance caused by large-scale electrical equipment In places where large-scale electrical equipment is started up and where the switchgear operates frequently, sparks generated by starting the motor and closing the switch will generate a large alternating magnetic field around it. These alternating magnetic fields can either cause interference by coupling on the signal cable, or they can generate high-frequency interference by coupling on the power cable. If these interferences exceed the allowable range, the operation of the system will also be affected. . J. y6 |( H. r) U. @/ r
(4) Other factors) j-S6 p: ~$ m1 W7 K
Lightning strikes may generate significant electromagnetic interference around the system, and may also introduce interference through various ground lines. Static electricity is also often the killer of destroying system equipment. % `1 y% e; }5 W7 B6 M& \
2. General methods to prevent interference and equipment damage $ x4 \6 L* f9 r: e( [$ F" t
(1) The system power supply system power supply should be redundant. Each power distribution module should have an independent peak protection diode (overvoltage), automatic circuit breaker (overcurrent) protection. The power supply system is preferably an isolation transformer, so that the grounding point of the DCS system and the power system grounding point are separated, and the power low-pass filter is used to eliminate the higher harmonics on the power grid. In order to avoid fluctuations, DCS power supply should try to come from electricity grids with small load changes. To strictly prevent strong electricity from entering the DC24V power supply circuit through the terminal row circuit, and periodically check whether the cabinet power supply system is normal, whether the power supply voltage is within the specified range, whether the system grounding is reliable, good, and whether the line insulation is qualified, and stop and send power as required. Program execution. Through the above work, this risk can be effectively avoided. Pressure Vessel, Chemical Process 2 B6 Q/ _2 L9 d! [
(2) The cable laying strong and weak cables should be laid separately. The distance between the power supply voltage below 220V and the power supply cable below 10A and the signal cable should be greater than 150mm. The power supply voltage should be 220V or more and the current should be more than 10A. The distance between cables should be greater than 600mm. If you can only put in the same bridge, between the isolation board should be installed. Thermal cables must not be placed in high voltage electric fields. For the secondary cable of a capacitive device, such as a secondary cable of a capacitive voltage transformer, the construction should be close to the ground. In the vicinity of the generator there is a strong radiation, it should be noted that there should be copper or aluminum foil and other sealed boxes to play a role in shielding. The signal loop must have a unique reference ground. The shielded cable must be grounded only on the site or in the control room in the event of possible conductive interference, thus preventing the formation of a “ground loopâ€.
(3) Signal isolation For analog input/output (AI/AO) circuits, it is necessary to prevent the strong electric string from the field from entering the card, as well as the potential difference between the local equipment and the DCS system. The important circuit should adopt signal isolator. For digital input/output (DI/DO) loops, the common solution is to use relay isolation for DI/DO signals. For example, for a motor control off feedback input loop: when the normally open contact of the site is closed, the relay coil is energized, the output contact is closed, and the contact signal is introduced into the switch acquisition card. In this way, strong signals will not enter the signal circuit of the card, and in the event of a failure, the isolated external circuit will also be overhauled. The disadvantage of adopting relays for signal isolation is that power loops need to be added to the external loops; switching isolators using electromagnetic isolation and photoelectric isolation technology can reduce the workload of supplying power to the external loop. Bbs.hgbbs.net" S$ y `, h9 j! }* h
(4) Anti-static and lightning protection measures 6 y- }. x; xt# O
Enter the control room and electronic room, wear anti-static overalls, touch the module, you must bring a static release wrist cover. During inspection and maintenance, the clips removed from the rack should be placed on a well-grounded anti-static mat and cannot be tampered with. Take comprehensive lightning protection measures, especially the DCS system can not be connected to electrical and lightning protection grounding public grounding network, and the distance between them must meet the requirements.
% a& i2 _! Y" x2 W" d- |
3. Concluding remarks on the Chemical Technology Forum - giving roses to people with fragrance; D" d4 U (@* Q% w
From this point of view, in the application of automatic instrument control system, the grounding problem is very worthy of attention. Grounding design is actually a complex system engineering. In the design of grounding system, it should be carried out according to relevant national standards. In the infrastructure construction, construction must be strictly carried out according to the design drawings, and electrical grounding should be strictly tested to ensure that it meets the design requirements. In addition, for the instrument automation system interference and safety issues, we require that we familiar with the scene, take appropriate comprehensive measures, we can ensure the normal operation of the system to the maximum extent possible.
1. The signal interference factor of instrument automatic control system is also called noise. It is an electrical signal that is intruded or superimposed on the system power supply and signal cable and has nothing to do with the signal. Interference can cause measurement errors and severe interference can cause equipment damage. Common disturbances are the following: pressure vessels, chemical processes; d* j! t3 p2 R" g
(1) Conducted interference 1) When several kinds of signal cables are transmitted together, due to aging and leakage of insulating materials, other signals are affected, that is, interference is introduced into other signals. Bbs.hgbbs.net& j4 [" d# \* C% o2 T+ Q
2) In some field execution agencies, 220V power supply is used, sometimes equipment is burned out, or human factors cause short circuit between the power supply and the signal cable, and the strong electricity enters the weak current, resulting in greater interference and instrument damage. 8 o% n7 a" N) j ( Y
3) Common mode interference due to unreasonable grounding. If the two ends of the signal cable are grounded at the same time, the potential difference ΔE between the two points of the grounding system may occur, which may generate a large circulation current on the signal cable, which is superimposed on the signal current and causes the analog signal to fluctuate. Caused failure of the instrument or card. Pressure Vessel, Chemical Process 7 P: Q& j1 d$ l9 z# G
4) During operation, the acquisition signal sent by the card is grounded for a long time, the formed ground current exceeds the allowable value, and the channel is damaged.
Among these types of conducted interferences, since the instrumentation system itself is now protected, the interference caused by the aging of the insulating material or the burning of the device is less likely to occur, and the interference caused by grounding is more common. It is more elusive and should be given enough attention. "l" x4 H; P) U: ^! R( \( f
(2) Capacitive and inductive coupling disturbances bbs.hgbbs.net8 o% Q1 {6 ^ [+ Z! w" y- n$ Q
In the controlled field, there are often many signals passing through the cable ducts or cable ducts that are connected to the control system. These signal cables are routed together with distributed capacitances. These distributed capacitances will add interference to other signals. On the cable. In addition, alternating magnetic fluxes are generated around alternating signal cables, and these alternating magnetic fluxes generate electromotive force between parallel conductors, which also causes disturbance on the line. " Q* ~- d! D7 D8 b" `
(3) Disturbance caused by large-scale electrical equipment In places where large-scale electrical equipment is started up and where the switchgear operates frequently, sparks generated by starting the motor and closing the switch will generate a large alternating magnetic field around it. These alternating magnetic fields can either cause interference by coupling on the signal cable, or they can generate high-frequency interference by coupling on the power cable. If these interferences exceed the allowable range, the operation of the system will also be affected. . J. y6 |( H. r) U. @/ r
(4) Other factors) j-S6 p: ~$ m1 W7 K
Lightning strikes may generate significant electromagnetic interference around the system, and may also introduce interference through various ground lines. Static electricity is also often the killer of destroying system equipment. % `1 y% e; }5 W7 B6 M& \
2. General methods to prevent interference and equipment damage $ x4 \6 L* f9 r: e( [$ F" t
(1) The system power supply system power supply should be redundant. Each power distribution module should have an independent peak protection diode (overvoltage), automatic circuit breaker (overcurrent) protection. The power supply system is preferably an isolation transformer, so that the grounding point of the DCS system and the power system grounding point are separated, and the power low-pass filter is used to eliminate the higher harmonics on the power grid. In order to avoid fluctuations, DCS power supply should try to come from electricity grids with small load changes. To strictly prevent strong electricity from entering the DC24V power supply circuit through the terminal row circuit, and periodically check whether the cabinet power supply system is normal, whether the power supply voltage is within the specified range, whether the system grounding is reliable, good, and whether the line insulation is qualified, and stop and send power as required. Program execution. Through the above work, this risk can be effectively avoided. Pressure Vessel, Chemical Process 2 B6 Q/ _2 L9 d! [
(2) The cable laying strong and weak cables should be laid separately. The distance between the power supply voltage below 220V and the power supply cable below 10A and the signal cable should be greater than 150mm. The power supply voltage should be 220V or more and the current should be more than 10A. The distance between cables should be greater than 600mm. If you can only put in the same bridge, between the isolation board should be installed. Thermal cables must not be placed in high voltage electric fields. For the secondary cable of a capacitive device, such as a secondary cable of a capacitive voltage transformer, the construction should be close to the ground. In the vicinity of the generator there is a strong radiation, it should be noted that there should be copper or aluminum foil and other sealed boxes to play a role in shielding. The signal loop must have a unique reference ground. The shielded cable must be grounded only on the site or in the control room in the event of possible conductive interference, thus preventing the formation of a “ground loopâ€.
(3) Signal isolation For analog input/output (AI/AO) circuits, it is necessary to prevent the strong electric string from the field from entering the card, as well as the potential difference between the local equipment and the DCS system. The important circuit should adopt signal isolator. For digital input/output (DI/DO) loops, the common solution is to use relay isolation for DI/DO signals. For example, for a motor control off feedback input loop: when the normally open contact of the site is closed, the relay coil is energized, the output contact is closed, and the contact signal is introduced into the switch acquisition card. In this way, strong signals will not enter the signal circuit of the card, and in the event of a failure, the isolated external circuit will also be overhauled. The disadvantage of adopting relays for signal isolation is that power loops need to be added to the external loops; switching isolators using electromagnetic isolation and photoelectric isolation technology can reduce the workload of supplying power to the external loop. Bbs.hgbbs.net" S$ y `, h9 j! }* h
(4) Anti-static and lightning protection measures 6 y- }. x; xt# O
Enter the control room and electronic room, wear anti-static overalls, touch the module, you must bring a static release wrist cover. During inspection and maintenance, the clips removed from the rack should be placed on a well-grounded anti-static mat and cannot be tampered with. Take comprehensive lightning protection measures, especially the DCS system can not be connected to electrical and lightning protection grounding public grounding network, and the distance between them must meet the requirements.
% a& i2 _! Y" x2 W" d- |
3. Concluding remarks on the Chemical Technology Forum - giving roses to people with fragrance; D" d4 U (@* Q% w
From this point of view, in the application of automatic instrument control system, the grounding problem is very worthy of attention. Grounding design is actually a complex system engineering. In the design of grounding system, it should be carried out according to relevant national standards. In the infrastructure construction, construction must be strictly carried out according to the design drawings, and electrical grounding should be strictly tested to ensure that it meets the design requirements. In addition, for the instrument automation system interference and safety issues, we require that we familiar with the scene, take appropriate comprehensive measures, we can ensure the normal operation of the system to the maximum extent possible.