Lightning protection measures for gas pipelines

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This post was last edited by liuquan1100 on 2017-12-14 11:24. I. Overview Lightning is a terrifying yet spectacular natural phenomenon. It has a profound impact on human life and industrial activities. Lightning facilitates the synthesis of organic substances, and it may have played a role in the origin of life on Earth. Forest fires caused by lightning might have inspired ancient humans to discover and utilize fire ； However, in modern life, lightning poses a threat to human lives, often causing damage to many sectors and facilities such as aviation, communications, computer systems, electricity supply, and construction, which has sustained public attention on lightning activities and their prevention. 　　Lightning strikes can severely damage electrical and electronic equipment. Lightning impulse currents of several dozen to hundreds of kA possess enormous electromagnetic, thermal, and mechanical effects. When such currents flow through a struck object, they generate impulse voltage waves with very high amplitudes, which can cause the insulation of electrical equipment to fail ； The electrodynamic force of the impulse current causes the struck object to explode ； Inrush current causes the temperature of metal objects such as wires to rise suddenly, resulting in their melting and damage. Among them, the former scenario is the most destructive and represents the main issue we should focus on. Chongqing is one of the areas in China with frequent thunderstorms. The economic losses caused by lightning strikes each year exceed 200 million, severely affecting people’s lives and property safety. In the 1980s, an accident occurred near Hongqiuba in Shangqingsi, Yuzhong District, Chongqing, where underground gas supply pipelines were damaged by lightning strikes ； At the end of the last century, the commercial and residential building in the Tiaodeng River farmers’ market in Yongchuan District served as a de facto lightning receptor due to gas pipelines installed on its roof, which did not meet the requirements of lightning protection standards. This led to explosions and fires in the gas pressure regulation units as a result of lightning strikes, blocking passages and causing panic among the residents. As a result, 72 residents jointly filed a complaint with the People’s Government of Yongchuan City and the Yongchuan City People’s Congress ； Entering the new century, lightning strikes caused static induction near the voltage regulation box in the Lijiatuo dormitory building of the Municipal Telecommunications Bureau, resulting in sparks. All these indicate that we should pay sufficient attention to lightning protection for gas pipelines. Furthermore, in recent years, due to the increasing number of high-rise buildings, and constrained by fire safety and other factors, gas supply pipes are often installed along the exterior of the buildings up to the roof before entering the interior. The emergence of this installation method has made lightning protection for gas supply pipelines particularly important. To prevent outdoor gas pipelines from suffering damage, fires, explosions, etc. due to lightning strikes, it is necessary to take lightning protection measures for such pipelines. II. Ways in which lightning poses hazards to gas supply pipelines 1. Hazards caused by direct lightning strikes When overhead or buried gas pipelines are struck by direct lightning, the voltage in the lightning bolts can reach tens of thousands of volts or even billions of volts, which can easily cause the pipelines to be damaged. The mechanical or thermal effects resulting from this can lead to fires or explosions involving gas, thereby causing injuries to people. 2. Hazards of induced lightning: During the processes of charge generation, movement, and leader discharge in thunderclouds, electrostatic induction occurs on gas supply pipelines, resulting in opposite electrical potentials. Once the trapped charges in the pipelines are released as a result of the thundercloud’s discharge to the ground, they move symmetrically toward both ends of the pipelines in the form of shock waves. The current generated by this movement of charges is what constitutes the induced voltage from lightning. The so-called induced lightning can be mainly divided into two types: one is the electrostatic induction by thunderclouds, which creates an equal amount of charge on the power/signal cables; as the thunderclouds expand, a large amount of opposite charges are gradually induced on the conductors. When a thundercloud discharges suddenly, the induced charges accumulated in the cable generate instantaneous surges (overvoltages) along the lines, which then enter buildings through power lines, metal pipes, etc., causing damage. Another type involves the generation of a strong alternating magnetic field (electromagnetic pulse) during thundercloud discharges, which induces overvoltage in conductors in its vicinity. These surge voltages, lasting from microseconds to milliseconds, are characterized by their high speed and large amplitude. 3. Hazards caused by lightning wave intrusion: Lightning wave intrusion refers to lightning strike accidents that occur when direct lightning or induced lightning enters a building through metal conductors such as gas supply pipes, resulting in a flash discharge. The incidence of such accidents is high, and they are often severe. In summary, the main ways in which lightning poses a threat to gas supply pipelines are the three mentioned above; whether it is direct lightning strike or the intrusion of lightning waves, the damage caused by these is far greater than that resulting from lightning-induced electrostatic effects. Electrostatic induction from lightning can cause electric sparks to be generated in a 1mm gap, thereby igniting flammable gases; hence, the hazards posed by direct lightning strikes and lightning wave intrusions are evident. III. Lightning protection measures for outdoor gas pipelines Based on the analysis above, three main aspects are considered for the lightning protection of buildings: first, protection against direct lightning strikes; second, protection against lightning induction; third, protection against the intrusion of lightning waves. The main design principles for lightning protection of gas pipelines are as follows: ① Install the gas pipelines within the protection range of the existing lightning protection devices to prevent direct lightning strikes; ② Establish an equipotential connection between the gas pipelines and the lightning protection devices to prevent induced lightning and lightning waves from penetrating. (1) Lightning protection measures against direct lightning strikes: 1. Place the gas pipelines within the protection area of the lightning receptor; therefore, it is crucial to determine the protection area of the lightning receptor. The protection area of a lightning receptor is determined using the rolling sphere method. When the grounded metal objects and other lightning receptors are located within the outer perimeter line and at the edges of the area to be protected, the protection area of the required end faces shall be determined in accordance with the following method (see Figure 1). A – Lightning receptor; B – Grounded metal object or lightning receptor. Figure 1: Protection areas of any two lightning receptors on a building at the required end faces. ① Arcs with center points A and B and radius r intersect at point O. ②With point O as the center and radius r, draw the arc AB; this arc AB constitutes the upper boundary of the protection area. 2. For the gas supply pipes installed on the roof, in order to prevent lightning from striking these pipes directly, they should not cross the parapet of the building (since they are not within the protection range of the building’s lightning protection system). Instead, they should enter the building through the bottom of the parapet. Additionally, a shielded metal grid should be installed along the gas pipes on the roof to minimize the risks posed by direct and induced lightning strikes. 3. Gas metal supply pipes installed outdoors along the exterior walls should be connected to the building’s equalization ring for lightning protection every 20 meters; there should be at least two connections for the pipes on the roof. Connection is also required when the clear distance between the gas pipe and the lightning protection network is less than 100 mm. The gas pipes and vent pipes on the roof should be kept as far away as possible from the parts of the building with the highest risk of lightning strikes, such as roof corners, eaves corners, areas above parapets, and roof ridges; generally, this distance should be at least 2 meters. (II) Protection techniques against lightning wave intrusion: To prevent damage caused by lightning strikes on gas supply pipelines or by lightning-induced current waves, insulation measures should be implemented at the points where the gas supply pipelines enter indoor areas. In other words, insulating joints should be installed at these entry points to isolate them from the outdoor gas pipelines, or insulating sections should be inserted at the flanges. For users who opt for external installation methods, especially those using aluminum-plastic composite pipes, insulating joints as shown in Figure 4 can be used as a separation measure to save costs, and this approach proves to be very effective. The specific method is as follows: select an insulating joint with a thickness comparable to that of the wall, attach it to the exterior wall pipe using an elbow or tee, and then connect it to the pipes inside the building after passing through the wall. It should be noted that the PE part of the insulated joint must remain inside the wall and must not be exposed outside it. If the hole through the wall is relatively large, it should be filled with appropriate material to ensure that the PE part is protected from ultraviolet radiation. Figure 5 shows workers connecting this type of wall-penetrating insulating connector, while Figures 6 and 7 are renderings of the outdoor and indoor sections after the wall-penetrating insulating connector has been installed. Currently, this lightning protection installation method has been promoted in the Sichuan and Chongqing regions, receiving praise from **the relevant regulatory authorities. (III) Lightning and electrostatic induction protection technology: In the insulation treatment of gas supply pipelines, the gas pipeline at the front end of the insulated section should be equilibrated in potential with the gas pipeline at the rear end of the insulated section (or the common equipotential bonding strip) through a discharge gap. (Figure 8) Figure 8: Equipotential bonding. If the flanges of gas supply pipelines or the joints of valves become corroded or have poor contact, sparks can occur between the flanges even at a relatively low current level (10.7 kA); in the presence of flammable gases, this can lead to immediate combustion and explosion. Therefore, indoor gas equipment and gas appliances should be provided with grounding to protect against lightning induction ； For gas meters and similar devices, bonding should be carried out to implement proper equipotential treatment. To protect against the hazards caused by lightning-induced electrostatic effects, gas supply pipelines should be equipped with grounding systems for lightning protection every 20–25 meters, and the resistance of such grounding systems should not exceed 10Ω ； At the same time, anti-static grounding should be provided at the branch points of the pipeline, with the grounding resistance not exceeding 30Ω. (IV) Lightning protection design for different types of buildings 1. For high-rise buildings with a height exceeding 45 m, in accordance with the \"Code for Design of Lightning Protection of Buildings\" (GB 50057 —94), for buildings with reinforced concrete or steel structures that are over 45 m high, metal pipes installed vertically along the exterior walls must be protected against side strikes and ensured to be at the same electrical potential; that is, the top and bottom ends of these metal pipes must be connected to the lightning protection system. 2. For multi-story buildings (with a height not exceeding 45 m), when the building’s roof is equipped with a proper lightning protection system, the gas pipelines in the building should be arranged in an upward supply and downward distribution pattern. The gas pipes on the roof must be completely within the protection range of the building’s lightning receptors; they should also be equipotentially connected to the lightning protection mesh/strip on the roof, and it may not be necessary to use a separate grounding electrode. If the roof of the building does not have a lightning protection system, try to avoid using pipes on the roof; otherwise, a lightning protection device must be installed in accordance with the regulations. 3 　Grounding devices (1) Grounding devices are divided into self-made grounding elements and the existing grounding devices of the building. When the building’s original grounding system is in good condition, it can be used for both lightning protection and the grounding of electrical equipment. When a self-made grounding electrode is not shared or connected to other grounding systems, the distance between them in the ground must satisfy the following formula, with a minimum value of 2 m: d ≥ 0.3 kc Ri. Where: d is the distance in the ground, in meters; kc is the current sharing factor; Ri is the impulse grounding resistance, in ohms. (2) The distance between the self-made grounding electrode and the entrance or exit of the building, or the sidewalk, should be no less than 3 m. When this distance is less than 3 m, the following measures should be taken: ① The local burial depth of the horizontal grounding electrode should be no less than 1 m; ② The local area of the horizontal grounding electrode should be covered with insulating material. (3) For vertical grounding elements buried in the soil, angle steel, flat steel, or round steel should be used; for horizontal grounding elements, flat steel or round steel is appropriate. The cross-sectional area of the flat steel should be no less than 100 mm2 and its thickness no less than 4 mm, while the diameter of the round steel should not be less than 10 mm. Since it is prone to corrosion when buried in soil, anti-corrosion measures such as hot-dip galvanizing should be taken or the cross-section should be increased. (4) The length of the vertical grounding electrode should be 2.5 m. When the grounding system consists of multiple horizontal or vertical grounding electrodes, in order to reduce the shielding effect of these electrodes, the distance between them should be 5 m; their depth in the soil should not be less than 0.5 m. (5) The grounding electrode shall be fully welded, with a length of not less than 60 mm, and the welded areas shall be treated for corrosion protection.