Lightning protection and countermeasures

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This post was last edited by yinkuilin6868 on 2016-5-17 at 15:12. On May 23, 2007, 7 elementary school students in Xingye Village School, Yihe Town, Kaixian County, Chongqing, died after being struck by lightning during class, while more than 30 other students were injured; this tragic accident drew the attention of the whole society. With the arrival of summer, thunderstorm weather increases gradually, having an adverse effect on the working environment and conditions, and it easily leads to various safety accidents. In recent years, fires and explosions in petrochemical enterprises caused by lightning strikes have occurred frequently, resulting in huge losses to people’s lives and property. Therefore, proper lightning protection is also one of the important measures for ensuring safe production in enterprises, and it should be given sufficient attention. This special issue analyzes the causes and hazards of lightning, as well as the types and characteristics of lightning. Taking into account the characteristics of petrochemical enterprises, it proposes lightning protection measures for petrochemical production facilities, storage tanks, and human beings. Accident case ● At 9:55 a.m. on August 12, 1989, in the old tank farm of the Huangdao Oil Depot under Shengli Oil Pipeline Company, Pipeline Bureau of China National Petroleum and Natural Gas Corporation, Concrete Oil Tank No. 5, which had a storage capacity of 23,000 cubic meters of crude oil, exploded and caught fire. The fire burned for a total of 104 hours, consuming over 40,000 cubic meters of crude oil. All facilities in the 250-mu-sized old tank farm and production area were completely destroyed. This accident resulted in direct economic losses amounting to 35.4 million yuan. During the fire fighting and rescue operations, 10 fire trucks were destroyed, 19 people lost their lives, and over 100 people were injured. Among them, 14 public security fire fighters lost their lives and 85 were injured. The direct cause of the accident was a defect inherent in the non-metallic oil tank, which led to the ignition of the oil and gas when induced sparks from a landmine strike struck it. ● At 4:10 p.m. on July 13, 1998, the Hengdian Oil Storage Facility of the Huangpi County Petroleum Corporation in Wuhan City was struck directly by lightning. This resulted in a fire and explosion in Storage Tank No. 4 within the facility; 125 tons of No. 0 diesel fuel and a diesel storage tank with a capacity of 1,000 cubic meters were destroyed, causing significant economic losses. ● On April 4, 2006, Tank No. 1 for toluene at Nanjing Huajing Chemical Co., Ltd. experienced an explosive combustion caused by a ball lightning strike, resulting in the roof of Tank No. 1 being torn off and various attachments such as pipelines and spraying systems being damaged. Fortunately, there were no casualties, with direct economic losses of nearly 70,000 yuan. The cause of this accident was inadequate investment in safety by the organization, deficiencies in management, and certain potential hazards that could lead to accidents. This facility has failed to have its lightning and static protection systems inspected and maintained for over 4 months (the last inspection was in May 2005). The entire storage tank, especially the areas where the tank top meets the tank body, is severely corroded. Due to this severe corrosion, the connections between components such as the vent valves and light-transmitting holes on the tank top and the tank body are ineffective; they no longer provide an electrical connection, which makes flashovers highly likely and increases the risk of fire ； Of the original 3 lightning and anti-static grounding wires in the storage tank, 2 have rusted and broken, failing to meet the requirement for instant discharge of large currents. How petrochemical enterprises can carry out effective lightning protection 1. The causes and hazards of lightning 1. Lightning is essentially a massive electrostatic discharge phenomenon that occurs in charged clouds in nature. Lightning is primarily caused by the formation of charged clouds in the atmosphere. There are various reasons for the formation of such charged clouds, with the most common one being the freezing of cloud droplets as they rise into the air, which generates electricity; as a result, the bottom part of the cloud becomes negatively charged while the top part becomes positively charged. When thunderclouds carrying opposite charges accumulate and come into contact, or when thunderclouds with a large amount of charge approach the ground, discharge occurs between the clouds or between the clouds and the ground. With the instantaneous conversion of energy in the air, intense light and sound are generated – this is what people commonly refer to as lightning. A massive lightning discharge can rapidly heat the surrounding air to over 20,000°C. The heated air expands violently, generating shock waves that travel through the air at a speed of 5 meters per second. These shock waves cause intense mechanical vibrations and thermal effects in the equipment and buildings within the range of the lightning strike, resulting in their complete destruction. 2. When a lightning protection system is struck by lightning, the lightning current generates very high voltages; if the insulation distance between the lightning protection system and the equipment inside and outside the building is insufficient, a \"backflow\" phenomenon can occur. When lightning current flows into the surrounding soil through the lightning strike point on the ground or a grounding electrode, a voltage drop is created around it. If someone stands near the grounding electrode, they will be exposed to the hazard of step voltage caused by lightning current. When lightning current flows through the down conductor to the grounding device, a high voltage drop occurs due to the impedance of both the down conductor itself and the grounding device; this voltage drop can reach tens of thousands or even hundreds of thousands of volts. If a person comes into contact with such voltage, their body can suffer severe damage. All of these phenomena can generally cause disability, and in severe cases, they can lead to death immediately. 2 Types of lightning: Based on its different shapes, lightning can be roughly divided into sheet lightning, streak lightning, and ball lightning. From a hazard perspective, they can be classified into direct lightning strikes, induced lightning (including electrostatic induction and electromagnetic induction), and ball lightning. Based on their formation mechanisms, they can be classified into thermal thunderstorms, frontal thunderstorms, and low-pressure thunderstorms. Sheet lightning occurs within clouds and has little impact on humans ； Linear lightning is a relatively common phenomenon of lightning striking the ground ； Ball lightning is a special type of lightning phenomenon, also known as “ball thunder”; it is a luminous sphere of purple or red color, with a diameter ranging from a few millimeters to several dozen meters, and it typically exists for 3 to 5 seconds. Ball bombs usually roll along the ground or float through the air; they can also enter buildings through gaps and explode. 3 Lightning protection measures: Lightning protection devices are the primary means of preventing lightning strikes. Common lightning protection devices include lightning rods, lightning grids, lightning belts, lightning conductors, and surge protectors. A lightning protection system mainly consists of three components: a lightning receptor, downconductors, and grounding electrodes. Basically, it utilizes its elevated position above the objects to be protected to attract lightning onto itself; thereafter, through down conductors and grounding devices, the lightning current is discharged into the ground, thereby protecting people or buildings from damage caused by lightning strikes. 1. Lightning rods: In practical applications, lightning rods are divided into various types, such as single lightning rods, two lightning rods of equal height, two lightning rods of unequal height, three lightning rods of equal height, four or more lightning rods of equal height, and slope-mounted lightning rods. The protection area of a lightning rod is a frustum-shaped cone with the lightning rod as its axis, resembling the shape of a tent. At a certain height below the lightning rod, there is a safe zone, which is the protection area provided by the lightning rod; within this area, objects are hardly likely to be struck by lightning. This safe zone is known as the protection area of the lightning rod. The protection radius of a single lightning rod is generally 1.5 times its height. However, buildings within the protection area cannot be absolutely guaranteed to be free from lightning strikes. 2. Lightning arresters: Lightning arresters are protective devices that prevent lightning overvoltage from damaging distribution and other electrical equipment. There are valve-type arresters and tube-type arresters. The arrester is installed at the incoming end of the equipment to be protected, with one end connected to the circuit and the other end grounded. Under normal conditions, the gap in the arrester remains insulated, without affecting the operation of the system. When a lightning strike generates a high-pressure shock wave that travels along the line, the gap in the lightning rod breaks down and connects to ground, thereby forcibly interrupting the shock wave. At this point, the voltage that can reach the protected object is merely the \"residual voltage\" generated by the lightning current passing through the arrester, its leads, and the grounding system. After the lightning current passes, the gap in the arrester returns to its insulating state, allowing the system to continue operating normally. 3. Distance from lightning protection devices: Ionization-based lightning protection consists of an ionization device at the top, a ground current collection device underground, and connecting wires. Ionization lightning protection devices do not prevent lightning strikes by controlling the point of impact; instead, they take advantage of the induction effect of thunderclouds, or use radioactive elements to create a strong electric field in the vicinity of the ionization device, thereby ionizing the air and generating an ion flow that moves toward the thundercloud. This allows the charges carried by the thundercloud to be slowly neutralized and dissipated, so that the electric field strength in that area does not exceed the breakdown threshold of the air, thus eliminating the conditions necessary for lightning to strike and preventing lightning incidents. 4 Lightning protection measures for petrochemical production facilities and storage tanks. Since most petrochemical production facilities and storage tanks have metal enclosures, and some special facilities—such as distillation towers—are extremely tall in order to meet production requirements, they are particularly susceptible to lightning strikes. 1. Lightning protection for petrochemical production facilities: During production, petrochemical companies often use and store large quantities of flammable and explosive materials. Moreover, there are many tall chemical processing units in these production areas; especially in areas classified as Q-1 and Q-2, explosive gas mixtures may exist, and combustion and explosion can occur whenever struck by lightning. During a lightning strike, the intense thermal and mechanical effects it generates can cause severe damage to chemical processing units as well as to the flammable and explosive materials stored in the tank areas, resulting in catastrophic damage to petrochemical production facilities. It can very easily cause the combustion and explosion of flammable and explosive materials. The raw materials and products used by petrochemical enterprises are basically flammable gases, liquids, and solids; all equipment and pipelines at the production site must be grounded. At the inlets and outlets of metal pipes, as well as every 10–20 meters, at points where pipes are parallel or intersecting at a distance of less than 0.5 meters, and at all connection points of the pipes, wires should be used for bridging and proper grounding should be ensured. The metal roofs in petrochemical production facilities should be connected to the grounding wire via down conductors at intervals of 15 meters along their perimeters. For reinforced concrete roofs, during construction, the steel frame should be welded into a single unit, and down conductors should be used to connect it to the grounding wire every 15 meters. To prevent backflow, the flashover voltage of the insulating medium between the lightning protection device and the building’s metal conductors should be higher than the backflow voltage. For pipes transporting flammable liquids in parallel, when the distance between them is less than 10 centimeters, the pipes should be connected together with wires every 20 meters along their length. For chemical plants and their associated buildings, all power supply lines are supplied by cables buried underground. Or, the wires 50 to 100 meters before entering the building should be replaced with cables that are buried to supply power. At the connection point between the cable and the overhead line, a valve-type arrester is installed, and the arrester, the cable’s metal sheath, and the insulator feet are all grounded together; the grounding resistance is generally between 5 and 30 ohms. 2. Lightning protection for above-ground storage tanks: The gas cylinders in chemical enterprises and the tanks used for storing flammable liquids are mostly made of metal; some large tanks are prone to being struck by lightning during thunderstorms, and should be protected with separate lightning rods. When the wall thickness of above-ground horizontal oil tanks equipped with flame arresters, and the roof thickness of above-ground fixed-roof steel oil tanks is 4 millimeters or greater, lightning rods shall not be installed. Aluminum roof oil tanks and steel oil tanks with a roof thickness of less than 4 millimeters shall be equipped with lightning rods (nets). Lightning rods (nets) should protect the entire oil tank. Floating roof tanks or internal floating roof tanks should not be equipped with lightning rods, but the floating roof should be electrically connected to the tank body using two wires. For specific lightning protection requirements, refer to GB50074-2002 Code for Design of Oil Depots. 5 Human lightning protection measures: During lightning activities, direct discharge from thunderclouds to the human body, as well as ground voltage or secondary back-discharge, can all cause electric shock to humans. Therefore, attention should be paid to the necessary safety requirements. (1) During thunderstorms, unless it is necessary for work, one should minimize time spent outdoors or in open areas ； It is best to wear a raincoat made of plastic or similar material that does not get wet when outdoors or in the wild ； If possible, enter a building, car, or ship with a sturdy metal framework or lightning protection measures ； When taking shelter on streets shaded by buildings or tall trees, make sure to keep at least 8 meters away from walls and tree trunks. (2) During thunderstorm activity, one should stay as far away as possible from hills, mounds, or raised paths. One should also stay away from the seaside, lakesides, riverbanks, and pond edges. It is advisable to keep a distance from wire fences, metal clotheslines, as well as flagpoles, chimneys, tall towers, and solitary trees. In addition, one should avoid small buildings or other structures that lack lightning protection. (3) During thunderstorm activity, one should be aware of the danger of lightning intrusion waves indoors; it is necessary to stay away from lighting wires, power lines, telephone lines, broadcast lines, radio power cords, radio and television antennas, as well as various devices connected to them, in order to prevent secondary discharges from these wires or devices to the human body. Survey data show that over 70% of indoor human secondary discharge accidents occur within a distance of 1 meter; no fatal accidents have been reported when the distance is more than 1.5 meters. Thus, during thunderstorms, it is advisable for people to stay at least 1.5 meters away from wires and equipment that might be subjected to lightning intrusion waves. It should be noted that during thunderstorm activity, simply turning off the switches to prevent lightning strikes is not sufficient; it is also important to close doors and windows to stop ball lightning from entering the building and causing damage. (4) When a lightning protection system is struck by lightning, the lightning current typically generates very high voltages, which can cause injuries or deaths. To prevent backflashovers, the network voltage of the insulating medium between the lightning protection device and the metal conductors of the building must be greater than the backflashover voltage; furthermore, a certain hazardous zone should be designated where personnel are not allowed to approach. (5) When the lightning current flows into the surrounding soil through the grounding electrode at the point of ground strike, a very high potential is generated in its vicinity. If a person stands near the grounding electrode, they can be harmed by the step voltage caused by the lightning current. (6) When lightning current flows through the down conductor to the grounding device, a high voltage drop occurs due to the impedance of both the down conductor itself and the grounding device. If a person comes into contact with this current, they may suffer from contact voltage hazards, so this aspect requires attention. (7) To prevent injuries caused by step voltage, the distance between the lightning protection grounding device and the entrances/exits of buildings and structures as well as pedestrian paths shall be no less than 3 meters. When the distance is less than 3 meters, safety measures such as locally burying the grounding electrode deeper, insulating it with an asphalt layer, and laying underground equipotential bonding bars should be taken.