Talking about lightning protection of natural gas station

Lightning is a common phenomenon of atmospheric discharge. Because the energy released by lightning is quite large, the physical effects such as strong current, hot high temperature, violent shock wave, dramatic electrostatic field and strong electromagnetic radiation have brought many kinds of harm to people. At present, the long-distance natural gas pipelines that have been built across the country traverse from east to west and north and south. The stations and valve rooms along the line are densely covered. The geographical location of the pipelines varies greatly. Some places have frequent thunderstorms and are easily affected by lightning. In the event of a lightning strike, the damage is difficult to estimate and the consequences are unimaginable. Therefore, it is imperative to do a good job of lightning protection at the natural gas station, the valve room along the line, and the Yinbao room.

1 Impact of lightning on natural gas stations

1. 1 Influence of lightning on instrument control system

Lightning can be divided into direct lightning strikes and inductive lightning strikes. The hazards that may be caused to the instrument control system are classified into the following types.

1 ) Direct lightning strikes. Lightning directly hitting the field instrumentation or piping connected to it can often damage the instrument's sensor module and can damage the transmitter's control board. The lightning current generates a powerful induced magnetic field in the process of flowing into the earth along the instrument bracket, and can be coupled to the control room equipment through the signal transmission line to damage the control device.

2 ) Inductive lightning strikes. One is electrostatic induction. When the thundercloud comes, the ground objects, especially the conductors, accumulate a large amount of electric charge to generate discharge. If the discharge current enters the field instrument and the electric equipment, the equipment will be damaged. The other is electromagnetic pulse radiation. The lightning current generates an electromagnetic field in the space around its channel, radiates electromagnetic waves outward, is coupled to the control equipment of the control room and various metal conductors, and generates induced electromotive force or induced current, which will cause equipment failure or damage, and even cause the control system to malfunction. .

3 ) Lightning overvoltage intrusion. Direct lightning strikes or lightning induction may cause overvoltages in wires or metal pipes. Overvoltages may cause high potentials to be introduced into the instrument control system along various metal pipes, cable troughs, and cable lines, causing interference or even damage.

4 ) Counterattack. When the lightning protection device is connected to the flash, a powerful instantaneous lightning current flows into the grounding device through the down conductor. Due to the presence of the earth resistance, the lightning charge cannot be quickly discharged to the ground, thus causing the local ground potential to rise (possibly hundreds of thousands of volts). If the grounding body of the instrument control system does not have a sufficient safety distance from this point, a discharge will occur between them, causing a counter-current, which can directly penetrate the insulating part of the electrical appliance, causing interference or damage to the control system.

1. 2 the impact of lightning on the pipeline

The ground portion of the process piping of the station and the spanning pipeline are excellent lightning receptors for the entire buried pipeline. When lightning is about to be generated in the vicinity of the pipeline, an electrostatic field is formed on the large area below the buried channel. The buried pipeline also induces the opposite charge on the same surface as the ground. When the charge accumulates to a certain extent and has a discharge condition, it will appear. A strong discharge process. At this point, the cloud charge quickly disappears and the ground charge becomes zero. However, due to the excellent insulation properties of the three layers of PE , the discharge rate of the induced charge of the pipeline is very slow. Once the partial discharge of the pipeline occurs, the induced charge of other parts will also dissipate to the ground, thus forming a strong inside the pipeline. Current. For pipes with low insulation resistance, the current will be dissipated through the leakage point of the insulation layer, and no large destructive force will be generated. For pipes with good insulation performance, when the surge cannot pass through the leakage of the insulation layer itself. When the point is quickly discharged into the ground, high voltage is generated in the insulated or poorly contacted parts of the pipeline, causing secondary discharge, which is the main reason for the lightning damage of the gas pipeline equipment and facilities. The metal pipe itself is a good conductor, and it is easy to become a large direct lightning strike channel for lightning strikes.

In addition, pipeline cathodic protection equipment and anti-corrosion power supply are also often attacked by lightning. The damaged components are mainly discharge tubes, capacitors, integrated circuits, fuses and so on. The most serious can burn the motherboard of the device. The cathodic protection equipment is directly connected to the pipeline through the cathode cable and the zero-connected cathode cable. It is the most convenient and direct bleed passage for the lightning current generated by the direct lightning strike and the induced lightning on the pipeline. If the lightning strikes the equipment, it will be easy to pipe. Causes harm.

2 Natural gas field station lightning protection measures

A perfect lightning protection project should include: 1 External lightning protection device (lighter, down conductor and grounding device) to receive more than 50% of the lightning current and discharge it into the earth; 2 use equipotential bonding, shielding, anti-flash Network technology and devices, blocking the way lightning waves invade along metal wires and space electromagnetic fields; 3 surge protection, using the nonlinear characteristics of some components to form a surge protector ( SPD ) and connect it to power distribution and signal lines In the process, the accumulated overvoltage and overcurrent are discharged into the earth through the SPD . These three systems are indispensable. The following describes the instrument control system as an example.

Measures to control the lightning hazard of intrusion into the instrument control system include lightning, shunting, voltage equalization, grounding and shielding. These measures must be combined to truly achieve lightning protection of the instrument system.

2. 1 flashing

The protection of direct lightning strikes is mainly realized by the lightning protection device of the building. The lightning protection of the field instrument system should be designed together with the lightning protection measures of the surrounding gas transmission equipment.

2. 2 pressure equalization

When a lightning strike occurs, a transient potential rise will occur on the path through which the lightning transient current passes, causing a transient potential difference between the path and the surrounding metal object, if the transient potential difference exceeds The insulation withstand strength between the two will lead to the breakdown discharge of the medium. This breakdown discharge can directly damage the instrumentation and can also generate electromagnetic pulses, which interfere with the normal operation of the instrument system. In order to eliminate the breakdown discharge between the lightning transient current path and the metal object, the metal casing of the field instrument, the frame, the metal equipment of the production device, the equipment, the metal casing of the components and components of the instrument control room, and the metal can be used. The facilities are connected together and connected to the lightning protection grounding system of the instrument control room to form a complete equipotential bonding.

2. 3 grounding

At present, there are two main measures for grounding of domestic instrumentation systems: floating ground and multi-point grounding.

1 ) Floating. Floating means that the working place of the instrument is insulated from the grounding system of the building, so that the electromagnetic interference in the grounding system of the building will not be transmitted to the instrument system, and the change of the ground potential has no effect on the instrument system. However, since the outer casing of the instrument is to be protected and grounded, when the lightning is strong, a high voltage may appear between the outer casing of the instrument and its internal electronic circuit, and the insulation gap between the two may be broken, causing damage to the electronic circuit.

2 ) Grounding. Grounding means that the working ground of the instrument, PLC, etc. is separated from the protective ground. The outstanding advantage of this type of grounding is that it can be grounded nearby, and the parasitic inductance of the grounding wire is small. However, if a strong lightning wave enters the system through the protective ground, the electronic circuit will also be damaged by the high voltage.

Since the above two grounding methods cannot meet the requirements of lightning protection, it is possible to consider connecting the protection ground to the working ground and accessing the lightning protection grounding system, and the problem can be solved.

2. 4 shielding

Since the instrument system uses a large number of semiconductor devices, integrated circuits and transmission signal cables, transient electromagnetic pulses generated by lightning strikes can be directly radiated to these components, and transient overvoltage waves can be induced on the power supply or signal lines along the line. Intrusion into electronic equipment, causing malfunction or damage to the electronic equipment. The use of a shield to block or attenuate the energy propagation of electromagnetic pulses is an effective measure of protection.

1 ) Control room shielding. The control system in the control room is the heart of the instrument system. It is very sensitive to the electromagnetic pulse generated by lightning, and special attention should be paid to its shielding problem. The instrument control room should be a closed structure, electrically connect the structural reinforcement points in the wall of the house, make a protective grounding ring (connected to the lightning protection ground) around the wall in the room, and make an effective electrical connection.

2 ) Field instrument shielding. The field instrument can be shielded by lightning with a metal instrument box. The instrument box should be equipotentially connected with other metal facilities on site and connected to the lightning protection grounding system.

3 ) Signal line and power line shield. In order to prevent lightning electromagnetic pulses from inducing transient overvoltage waves on the signal or power line, all signal lines and low voltage power lines should use a metal shielded cable. In the case of transient overvoltage protection, the shield of the signal or power line is required to be grounded at multiple points along the line or at least at the first and last ends of the line. When multi-point grounding is used, the shielding layer between the grounding points forms a loop along the line, and the electromagnetic field of the low-frequency interference current may partially pass through the shielding layer, which generates low-frequency interference in the sheath loop of the cable, which requires shielding. The layer can only be grounded at a single point along the line. In order to prevent low-frequency interference caused by multi-point grounding, the cable can be penetrated into the metal pipe or the double-shielded cable can be used to ground the outer shield of the metal pipe or double-shielded cable at multiple points, and the inner shield of the metal pipe or the double-shielded cable. The layer can be grounded at one end. This not only ensures safety, but also helps to suppress low frequency interference.

2. 5 shunt

Diversion is an effective measure for lightning protection. The method is to use a surge protector SPD in the signal or communication loop of the instrument control system and the power supply part of the system to limit transient overvoltage and surge current. An SPD can only provide protection for a part of the loop. Because there are too many equipment loops, it is impossible to use the SPD in each equipment loop. Instead, the SPD or the arrester must be selectively installed in the critical loop and the system power loop.

In order to achieve the lightning protection effect of the instrument system, the insured system and the valve room equipment, the entire production equipment should be designed according to the principle of equipotential bonding, from the control room, the insured system, the field instrument, the instrument signal and the power line. Comprehensive consideration, the use of lightning, shunt, voltage equalization, grounding, shielding and other measures, the need for electrical, construction, automation and other professional collaboration to achieve, in addition to considering system security, but also consider the cost of investment and operation Economical.

2. 6 Strengthen the management of lightning protection

First, set lightning protection and electrostatic devices according to regulations, and carry out regular inspection and maintenance to ensure that the grounding is in good condition and the grounding resistance meets the requirements; ensure that the valves and flanges are connected by wires and reliably grounded; Less than 0.03 Ω , the resistance to ground should not be greater than 100 Ω ; the jumper removed for inspection must be restored.

The second is to improve the intrinsic safety of the equipment and ensure that the equipment is in good condition. It is necessary to strictly control the procurement, installation and acceptance of equipment to ensure the safety of equipment. Process equipment should be frequently maintained, checked, and eliminated, run, run, drip, and leak; electrical equipment should be selected with appropriate safety level and explosion-proof grade. It is strictly prohibited to modify, dismantle or reduce the grade during operation and maintenance; leakage protection, Short-circuit protection, insulation, screen protection, etc. should be completed and effective at any time; the detection and alarm device should be regularly verified.