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Electrostatic grounding and equipotential bonding in explosive environments are important contents in the safety inspection of petrochemical installations. There are many debates about the requirements and implementation plans for equipotential bonding at metal pipeline flange connections. A correct understanding of the requirements for static electricity, electrostatic grounding and equipotential bonding is the basis for designing electrostatic protection and lightning protection projects. This article starts from the basic concept of static electricity, analyzes the requirements for electrostatic grounding and equipotential bonding, combines engineering practice, discusses several implementation methods of equipotential bonding at metal pipe flange connections, analyzes the different requirements and characteristics of electrostatic protection and lightning protection, and puts forward engineering suggestions. original: m.yunrun.com.cn/tech/4300.html 1. The concept of static electricity. Static electricity is the charge that exists on the surface of an object and is relatively static to the observer. Static electricity can be generated by processes such as separation and contact of objects, attachment of charged particles, electrostatic induction, and dielectric polarization. It is generated and accumulated on materials, equipment, human bodies, pipelines, and structures during production, storage and transportation. Static electricity safety refers to the state and conditions that prevent personal injury, ignition of flammable substances, damage to electronic equipment and other economic losses due to electrostatic discharge in production and the environment. The amount of static electricity on an electrostatically charged object is a relatively stable value that is a dynamic balance between the amount of static electricity generated and dissipated. Generally, as long as the rate of static electricity generation is less than the rate of static electricity dissipation, there will be no accumulation of static electricity and it is an electrostatic safe state. ； Maintaining a conductive state between objects can balance charges and avoid mutual electrostatic discharge, which is an electrostatic safe state. 2. Electrostatic ground resistance The movement of charges on or between objects does not require very low resistance. In order to release the charge on the object, grounding is usually used to avoid electrostatic discharge with the help of the equilibrium potential of the earth. Experiments have shown that charges can be moved if the surface resistance of an object is less than 1MΩ, so an object with an electrostatic grounding resistance of less than 1MΩ can release static electricity to the ground. Relevant standards and specifications stipulate that the electrostatic grounding resistance of objects should not be greater than 100Ω. In areas with high soil resistivity such as mountainous areas, the grounding resistance value can be relaxed to 1kΩ, which is very generous for electrostatic discharge. Under normal circumstances, when the electrostatic discharge resistance is not greater than 1MΩ, the rate of static electricity dissipation is greater than the rate of static electricity charging, or the accumulated static electricity can be released to eliminate static electricity. 3. Equipotential connection at the flange connection of metal pipes The conductive connection at the flange connection of metal pipes realizes equipotentiality for the purpose of electrostatic protection and lightning protection. Conductive connection is different from electrostatic grounding. Electrostatic grounding is to release static electricity with the help of the earth, achieve electrostatic balance of objects, and avoid static electricity accumulation on objects. The equipotential connection at the flange connection of metal pipes is to prevent the charge imbalance between the flange surfaces of metal pipes from forming a potential difference, and to avoid electrostatic gap discharge or lightning gap discharge due to potential difference. The equipotential connection at the flange connection of metal pipes should pay attention to two aspects: connection resistance and charge conduction and current carrying capacity. 4. Regulations of relevant standards: Different standards have different requirements for the equipotential connection of metal pipe flange pairs. The following are some excerpts of relevant standard requirements.: ①Article 4.2.2 of GB50057-2010 "Code for Lightning Protection Design of Buildings": For the anti-lightning induction of the first type of lightning protection building, when the transition resistance value at the connections of valves, flanges, elbows, etc. of long metal objects is greater than 0.03Ω, metal wires should be used to bridge the connection. Flanges connected by no less than 5 bolts do not need to be bridged in non-corrosive environments. Category II and Category III lightning protection buildings do not have the above requirements for lightning protection induction. ②Article 5.3.4 of SH/T3097-2017 "Design Specifications for Electrostatic Grounding in Petrochemical Industry": When the metal flange is fastened with metal bolts, it is usually not necessary to set up an additional electrostatic jumper connection line, but it should be ensured that at least two metal bolts have good conductive contact surfaces. ③Article 2.7.5 of HG/T20675-1990 "Electrostatic Grounding Design Regulations for Chemical Enterprises": When metal flanges are fastened with metal bolts, no additional electrostatic jumpers are required. Under corrosive conditions, the contact surfaces of no less than 2 bolts should be cleaned of rust and oil before installation, and lock nuts should be installed. Judging from practical experience, there is sufficient electrostatic conductivity between metal flanges connected by metal bolts. The bolt connection alone already has sufficient electrostatic conductivity. Installation requirements under corrosive conditions to ensure continuity. ④GB/T20801.4-2020 "Pressure Piping Specification Industrial Piping Part 4: Provisions in Article 10.12.1 of "Production and Installation": For pipelines designed with electrostatic grounding requirements, when the resistance between each pair of flanges or other joints is greater than 0.03Ω, wire jumpers should be installed. ⑤Article 13.2.12 of GB501056-2021 "Technical Standard for Automobile Refueling and Hydrogen Refueling Stations": Metal wire jumpers should be used at the connections between the two ends of process pipeline flanges in explosive environments. When the number of flange connecting bolts is greater than or equal to 5, bridging is not required in non-corrosive environments. Metal wire jumpers should be installed at the connections between hoses and flanges on oil, liquefied natural gas (LNG), compressed natural gas (CNG) and liquefied petroleum gas (LPG) pipelines in explosive environments. The purpose is to prevent static electricity and lightning sparks from occurring at the connection between hoses and flanges due to poor contact (resistance greater than 0.03Ω), resulting in fire or explosion accidents. Flanges with no less than 5 connecting bolts do not need to be bridged in non-corrosive environments. ⑥Article 9.0.4 of GB50177-2005 "Code for Design of Hydrogen Stations": The pipe flanges and valves in the hydrogen station and hydrogen supply station should be connected with metal wires. In a normal rust-free environment, the contact resistance of pipe joints, valves, flanges, etc. is below 0.03Ω. If the pipe joints are rusted, the contact resistance will increase. Tests have shown that when there is rust and corrosion between bolted flanges, gap discharge can occur between the flanges when the lightning current amplitude is quite low (10.7kA), inducing fire or explosion accidents. If the hydrogen station does not regularly check and test the transition resistance of pipe joints and flanges, it will be very dangerous if the contact connections become rusty. Therefore, it is stipulated that all pipelines in hydrogen stations and hydrogen supply stations, including heating pipes and water pipe flanges, valve joints, etc., should be connected with metal wires. ⑦Article 10.3.3A of GB50516-2010 "Technical Specifications for Hydrogen Refueling Stations (2021 Edition)": The flange connections on combustible pipelines such as hydrogen and liquid hydrogen should be bridged with metal wires, and the jumper resistance should be less than 0.03Ω. In a normal rust-free environment, the contact resistance of pipe flanges and other places is below 0.03Ω. If there is corrosion and rust, static electricity or lightning sparks may occur due to the increase in contact resistance, and fire and explosion accidents may occur. In order to prevent the contact resistance from increasing due to poor connection or metal corrosion at both ends of the upper flange of the pipeline, this article shall be implemented. During actual project design and implementation, an objective and scientific engineering plan for equipotential bonding should be determined through comprehensive analysis and the following factors should be taken into consideration:: ①How the connection between flange pairs takes into account both lightning induction and electrostatic protection. ②Are there different solutions for different medium properties (hydrogen, oxygen, etc.)? Can plans be unified for different explosive atmosphere areas? ③Is it a corrosive environment? Will anti-corrosion measures affect conductivity and to what extent? ④Are pipes, bolts, gaskets and flanges made of metal with good electrical conductivity? ⑤What connection method is most appropriate? ⑥Are the provisions of the standard specification appropriate? 5. Common problems on site 5.1 Corrosion and flange sealing For problems arising from corrosion and flange sealing on site: ①Since the lugs are made of copper and the bolts and nuts are made of alloy steel or stainless steel, galvanic corrosion and crevice corrosion will occur in rainy or severely humid environments. ②The sealing of the flange relies on the elastic compression of bolts. Among the multiple bolts that fasten the flange, the bolt that compresses the electrostatic jumper lug is likely to reduce the elastic compression force of the bolt due to the plastic deformation of the copper lug, resulting in uneven flange tightening stress and possible leakage of the flange sealing surface. 5.2 Redundant Jumpers There are different opinions on redundant jumpers, as follows:: ①Some people believe that connecting metal flanges with metal bolts also requires conductive jumpers. In fact, the bolts, nuts and washers of metal pipes are all made of metal and have good electrical conductivity. If there is no corrosion layer affecting conductivity, there is no need to jump over conductive wires. If the contact between the bolt nut and the flange does not have good electrical conductivity, then the contact with the lug also has the same problem. In fact, a good metal contact without corrosion layer has good electrical conductivity. The conductive cross-sectional area of multiple bolts on the flange is much larger than that of the jumper wire. ②Some opinions believe that symmetrical grounding is needed, that is, repeated grounding is needed to improve the reliability of grounding. In fact, there is no potential difference in static electricity on metal equipment, steel pipes, etc., and the release of static electricity does not require repeated grounding, let alone symmetrical grounding. ③An anti-static jumper was made on a fire water pipeline in an oil field. In fact, impure water is a conductive medium and does not require anti-static jumpers. ④An electrostatic jumper was made on both sides of the weld of a stainless steel pipe at a certain site. This was unnecessary because the weld was conductive. 6. Several methods of conductive bridging of flanges 6.1 Conductive bridging through bolts or gaskets The metal bolts and gaskets matching the flange are good conductors and can achieve conductive bridging. This method needs to consider the corrosion effect of the environment on the flange and bolt materials. Anti-corrosion measures are taken for many on-site environments, e.g.: Paint anti-rust paint, galvanize, surface treatment, etc. The effect of anti-corrosion measures on electrical conductivity should be evaluated. It is not recommended to remove anti-corrosion measures to improve conductivity, because if there are no good anti-corrosion measures in a corrosive environment, when bolts and nuts corrode, it will affect the pre-tightening force of the flange, and gasket corrosion will affect the sealing surface, causing safety hazards. The situations where conductive bridging can be achieved through bolts include the following aspects:: ①Non-corrosive environment. For example: Arid inland areas. ②Flanges and bolts of corrosion-resistant materials. For example: Stainless steel resistant to corrosion from process media. ③Conductive anti-corrosion measures. For example: Conductive anti-corrosion coating or plating. This anti-corrosion measure should be fully considered when selecting equipment materials, and should not be used simply for conductive bridging. 6.2 Equipotential connection is achieved by crimping the grounding piece with the nut. The metal connecting piece is crimped with the bolt nut matching the flange, and the connecting piece is crimped with the jumper wire to achieve conductive bridging. This method requires attention to the corrosion caused by improper configuration of the material of the connecting piece and the material of the flange bolt and nut, so as not to affect the connection performance of the flange bolt. Corrosion-proof metal configuration or connecting pieces made of the same material as the flange bolts should be used, otherwise corrosion may occur. 6.3 Set "grounding ears" to achieve conductive crossover. Set special "grounding ears" or transition connectors on pipes or equipment (valves), crimp the connecting piece, and crimp the connecting piece to the jumper metal wire to achieve conductive connection. This connection method is recommended when the flange bolt connection with anti-corrosion measures cannot achieve conductive bridging in a corrosive environment. It should be noted that the "grounding ear" on the pipeline is recommended to be designed by a pipeline professional. In some cases, heat treatment needs to be considered. ； A "grounding lug" on the valve is recommended that comes with the valve. “The type of "grounding lug" should facilitate connection construction. 6.4 Use bolt anti-corrosion glue There is a corrosive environment in the petrochemical plant. In order to ensure that the bolts will not affect the conductivity due to corrosion, bolt anti-corrosion glue can be used. This glue has a rust conversion function, can eliminate rust, and can isolate the penetration of external corrosive media, so that the bolts will not be corroded, and have a service life of more than 10 years. This method does not hinder the disassembly and assembly of bolts, and can maintain the conductive properties of the bolt connection. 7. The basis for relevant standards and specifications is based on the provisions in the literature and with reference to foreign data. Taking the test records of a company in Tianjin as an example, the actual measured values of the transition resistance at the flange connection are listed below. Actual measured values of transition resistance at flange connections in different parts of the device ① The upper flange of the residual liquid pipeline, with 4 bolts complete, and the transition resistance is 0.0075Ω without jumpers; ② The flange of the residual liquid pipeline, with 4 bolts complete, and the transition resistance with jumpers is 0.0120Ω; ③ The valve under the storage tank, with 8 bolts complete, and the transition resistance without jumpers is 0.0090Ω ④The valve under the storage tank has all 8 bolts, and the transition resistance is 0.0120Ω when there is a jumper. ⑤The φ89mm liquid phase pipe flange has 8 bolts, and the transition resistance is 0.0110Ω when there is a jumper. ⑥The newly installed flange of the φ89mm pipe has 8 bolts, and the transition resistance is 0.0070Ω when there is no jumper. ⑦φ57mm liquid phase flange, 4 bolts are complete, the transition resistance is 0.0050Ω when there is a jumper ⑧φ89mm liquid phase flange, 8 bolts are complete, the transition resistance is 0.0060Ω when the jumper is removed ⑨φ89mm liquid phase flange, 8 bolts are complete, the transition resistance is 0.0100Ω when there is a jumper It can be seen from the above measured values that the presence or absence of jumpers has little effect on the transition resistance of the connection. There is not much difference in the transition resistance values between 4 bolts and 8 bolts without jumpers. Even in some cases the transition resistance of 4 bolts is smaller than the transition resistance of 8 bolts. In summary, it can be seen that the factor that affects the transition resistance of the connection is mainly whether the conductive contact is good, not the number of bolts. 8. Questioning the connection resistance specified in the standard specification. As mentioned above, the movement of charges on or between objects does not require a very low resistance. In order to release the charge on the object, the grounding resistance is less than 1MΩ. The grounding resistance specified in the specification is no more than 100Ω, which is already very generous. Why does the electrostatic jumper need to be less than 0.03Ω? Metal bodies such as pipelines in explosive environments are electrostatically grounded to avoid gap discharges that may cause sparks. Since a 100Ω conductive connection is enough to balance the charge and potential between flanges, a connection resistance less than 0.03Ω is not required. The "anti-lightning induction" in the specification is actually to avoid sparks in the flange gap due to surges caused by lightning induction. It also does not require a connection resistance less than 0.03Ω. In addition, electric sparks will not occur in non-gas media. The conductivity of a metal contact surface without a corrosion layer must be good enough to discharge static electricity or lightning-induced surges. There is no need to use contact resistance to measure or judge. It is redundant to use the connection resistance less than 0.03Ω as a basis for judging good conductivity. 9. Conclusion This article introduces the concepts of static electricity, electrostatic protection and lightning protection. Based on the methods and requirements for equipotential connection of metal pipe flanges, it compares different standards and based on engineering reality, when flange bolt connections have good conductivity, bolt connections are preferred to achieve equipotential connections. For conductive bridging of flanges, attention should be paid to the conductivity of the flange bolts themselves. Not all flanges require additional conductive bridging. In some cases, the grounding lug method can be used for conductive bridging. Corrosion issues should also be paid attention to in flange connections and conductive bridging, and corresponding anti-corrosion measures need to be taken. Proper engineering design and implementation not only meet the requirements for electrostatic protection and lightning protection, but are also the main guarantee for the production safety of production equipment. author: Fan Yongfeng, Tang Bin, Lin Xiaohua