Static grounding and equipotential bonding in explosive environments are important contents in the safety inspection of petrochemical plants. There are many debates on the requirements and implementation of equipotential bonding at the flange joints of metal pipes. Correct understanding of static electricity, static grounding and equipotential bonding The requirements are the basis for doing a good job in electrostatic protection and lightning protection engineering design. This article starts from the basic concept of static electricity, analyzes the requirements of static grounding and equipotential connection, combines engineering practice, discusses several implementation methods of equipotential connection at the flange connection of metal pipes, and analyzes the different requirements and characteristics of static electricity protection and lightning protection , and put forward engineering suggestions.

01 The concept of static electricity

Static electricity is the charge existing on the surface of an object, which is relatively static for the observer. Static electricity can be generated by processes such as separation and contact of objects, attachment of charged particles, electrostatic induction, and medium polarization. During production and storage, it is generated and accumulated on materials, equipment, human bodies, pipelines, and structures.

Electrostatic safety refers to the state and conditions that do not cause economic losses due to electrostatic discharge, such as personal injury, ignition of combustible substances, and damage to electronic equipment, in production and the environment. The amount of static electricity on an object with static electricity is a relatively stable value of the dynamic balance between the amount of static electricity generated and the amount dissipated. Usually, as long as the rate of static electricity electrification is lower than the rate of static electricity dissipation, there will be no static electricity accumulation, which is a safe state of static electricity; The conductive state can balance the charge and avoid mutual electrostatic discharge, which is an electrostatic safe state.

02 Static Grounding Resistance

The movement of charge on or between objects does not require very low resistance. In order to release the charge on the object, the way of grounding is usually adopted to avoid electrostatic discharge by means of the equalization potential of the earth. Experiments have proved that the surface resistance of the object is less than 1MΩ, and the charge can be moved, so the electrostatic grounding resistance of the object is less than 1MΩ, and the static electricity can be released to the ground. Relevant standards stipulate that the electrostatic grounding resistance of objects is not 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.

Normally, 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.

03Equipotential bonding at flange connections of metal pipes

The conductive connection at the flange connection of the metal pipe realizes equipotentiality, and the purpose is electrostatic protection and lightning protection.

Conductive connection is different from static grounding. Static grounding is to release static electricity by means of the earth, to achieve static balance of objects, and to avoid static accumulation of objects. The equipotential connection at the flange connection of the metal pipe is to avoid the charge imbalance between the flange surfaces of the metal pipe, form a potential difference, and avoid electrostatic gap discharge or lightning gap discharge due to the potential difference. The equipotential connection at the flange connection of metal pipes should pay attention to two aspects: connection resistance and charge conducting current carrying capacity.

04Relevant standards

For the equipotential connection of metal pipeline flange pairs, different standards have different requirements. The following are some excerpts of relevant standard requirements:

1) Article 4.2.2 of GB50057-2010 “Code for Lightning Protection Design of Buildings” [1] stipulates: For the lightning protection induction of the first type of lightning protection buildings, when long metal objects, valves, flanges, elbows, etc. When the transition resistance value at the connection is greater than 0.03Ω, metal wires should be used for jumper connection. Flanges connected by not less than 5 bolts may not be bridged in non-corrosive environment. There are no above requirements for the lightning protection induction of the second and third types of lightning protection buildings.

2) Article 5.3.4 of SH/T3097-2017 “Petrochemical Static Grounding Design Code” [2] stipulates that when the metal flange is fastened with metal bolts, it is usually not necessary to set up an additional static jumper connection line, but it should be ensured that at least There are 2 metal studs with a good conductive contact surface.

3) Article 2.7.5 of HG/T20675-1990 “Design Regulations for Electrostatic Grounding of Chemical Enterprises” [3] stipulates that when metal flanges are fastened with metal bolts, an additional electrostatic jumper is not required. Under corrosive conditions, there should be no less than two bolt contact surfaces to remove rust and oil before installation, and anti-loosening nuts should be installed. According to practical experience, between the metal flanges connected by metal bolts, there is enough electrostatic conductivity only if the bolts are connected. Installation requirements under corrosive conditions in order to ensure continuity.

4) Article 10.12.1 of GB/T20801.4-2020 “Pressure Piping Specifications for Industrial Piping Part 4: Fabrication and Installation” [4] stipulates that when designing a pipeline with electrostatic grounding requirements, when each pair of flanges or other joints When the resistance value between them is greater than 0.03Ω, a wire jumper should be set.

5) Article 13.2.12 of GB501056-2021 “Technical Standards for Automobile Refueling and Hydrogenation Stations” [5] stipulates that metal wire jumpers should be used at the joints at both ends of the flanges of process pipes in explosive environments.

When the flange connection bolts are more than or equal to 5, no bridging is required in a non-corrosive environment. Metal wire jumpers should be set at the joints of the hoses and flanges on oil, liquefied natural gas (LNG), compressed natural gas (CNG) and liquefied petroleum gas (LPG) pipelines in explosive environments, in order to prevent hoses and Static electricity and lightning sparks occur at the joints at both ends of the flange due to poor contact connection (resistance greater than 0.03Ω), resulting in fire or explosion accidents. For flanges with no less than 5 connecting bolts, no bridge connection is required in non-corrosive environments.

6) Article 9.0.4 of GB50177-2005 “Code for Design of Hydrogen Stations” [6] stipulates that metal wires should be used to bridge the connection between pipeline flanges and valves in hydrogen stations and hydrogen supply stations. In the case of no rust in the normal environment, the contact resistance of pipe joints, valves, flanges, etc. are all 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 the bolted flanges, the gap discharge between the flanges can also cause fire or explosion accidents when the lightning current amplitude is quite low (10.7kA). If the hydrogen station does not check and test the transition resistance of the pipe joints and flanges frequently, it will be very dangerous once the connection is rusted. Therefore, it is stipulated that all pipelines in the hydrogen station and hydrogen supply station, including heating pipes and water pipe flanges, valve joints, etc., should be bridged with metal wires.

7) Article 10.3.3A of GB50516-2010 “Technical Specifications for Hydrogen Refueling Stations (2021 Edition)” [7] stipulates that flange connections on pipelines for combustibles such as hydrogen and liquid hydrogen should be bridged by metal wires. The resistance should be less than 0.03Ω. In the case of no rust in the normal environment, the contact resistance of pipe flanges and other places is below 0.03Ω. If corrosion and rust occur, static electricity or lightning sparks may occur due to the increase in contact resistance, resulting in fire and explosion accidents. To prevent The contact resistance at both ends of the upper flange of the pipeline is increased due to poor connection or metal corrosion, and the provisions of this article shall be followed.

During actual engineering design and implementation, an objective and scientific equipotential connection engineering scheme should be determined through comprehensive analysis, taking into account the following factors:

1) How to take into account the lightning induction and electrostatic protection for the connection between flange pairs.

2) Are there different solutions for different medium properties (hydrogen, oxygen, etc.)? Is it possible to unify the scheme for different explosive atmosphere areas?

3) Whether it is a corrosive environment, and whether the anti-corrosion measures will affect the conductivity and the degree of influence?

4) Are the pipes, bolts, gaskets and flanges made of metallic materials with good electrical conductivity?

5) What is the most suitable connection method?

6) Are the provisions of the standard specification appropriate?

05On-site common problems

5.1 Corrosion and flange sealing

For the problems of corrosion and flange sealing in the field:

1) Since 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 form in rainy or heavily humid environments. 2) The seal of the flange is elastically compressed by the bolts. Among the multiple bolts fastening the flange, the bolts of the electrostatic jumper lugs are compressed, which is likely to cause the elastic compression of the bolts due to the plastic deformation of the copper lugs. The force is reduced, resulting in uneven fastening force of the flange, which may cause leakage of the flange sealing surface.

5.2 Redundant jumper wires There are different views on redundant jumper wires, as follows:

1) Part of the view is that the use of metal bolts to connect metal flanges also requires conductive jumpers. In fact, the bolts, nuts and gaskets of metal pipes are all made of metal, which has good electrical conductivity. If there is no corrosion layer to affect the conductivity, there is no need to jumper the conductive wires. If the contact between the bolt and nut and the flange does not have good electrical conductivity, then the same problem exists in the contact with the lug. In fact, a good metal contact without corrosion layer has good electrical conductivity. The conductive cross-sectional area of the multiple bolts on the flange is much larger than the jumper wires.

2) Some views believe that symmetrical grounding is required, that is, repeated grounding is required to improve the reliability of grounding. In fact, there will be no potential difference in the static electricity of metal equipment, steel pipes, etc., and the discharge of static electricity does not require repeated grounding, let alone symmetrical grounding.

3) The fire-fighting water pipeline in an oil field has been electrostatically bridged. In fact, impure water is a conductive medium, and anti-static bridges are not required.

4) Electrostatic bridging was done on both sides of the weld of stainless steel pipes on a certain site, which is also unnecessary, because the weld is conductive.

06Several ways of conductive bridging of flanges

6.1 Conductive bridging through bolts or gaskets The metal bolts and gaskets matched with the flange are good conductors and can realize conductive bridging. This method needs to consider the influence of the environment on the corrosion of flange and bolt materials.

For many on-site environments, anti-corrosion measures will be taken, such as: painting anti-rust paint, galvanizing, surface treatment, etc. The effect of corrosion protection measures on electrical conductivity should be evaluated. It is not recommended to use the method of removing anti-corrosion measures to improve the electrical conductivity, because if there are no good anti-corrosion measures in a corrosive environment, when the bolts and nuts are corroded, the pre-tightening force of the flange will be affected, and the gasket corrosion will affect the sealing surface. Create a safety hazard. The occasions where conductive bridging can be achieved through bolts are as follows:

1) Non-corrosive environment. Example: arid inland areas.

2) Flanges and bolts made of corrosion-resistant materials. Example: stainless steel resistant to corrosion by process media.

3) Conductive anti-corrosion measures. For example: conductive anti-corrosion coating or plating, this anti-corrosion measure should be considered in the overall consideration of equipment material selection, and should not be used simply for conductive bridging.

6.2 Realize equipotential connection by crimping the grounding piece with nuts. Crimping the metal connecting piece through the bolts and nuts matching the flange, and crimping the connecting piece with the jumper wire to realize conductive bridging. This method needs to pay attention to the corrosion caused by improper configuration of the material of the connecting piece and the material of the flange bolts and nuts, so as not to affect the connection performance of the flange bolts. Corrosion-resistant metal arrangements or lugs of the same material as the flange bolts should be used, otherwise corrosion may exist.

6.3 Set up “grounding lugs” to achieve conductive bridging. Set up special “grounding lugs” or transition connectors on pipes or equipment (valves), crimp the connecting piece, and connect the connecting piece to crimp the bridging metal wire to realize conductive connection. This connection method is recommended when the flange bolt connection with anti-corrosion measures in a corrosive environment cannot achieve conductive bridging. It should be noted that the “grounding lug” on the pipeline is recommended to be professionally designed by the pipeline, and heat treatment needs to be considered in some occasions; the “grounding lug” on the valve is recommended to be attached to the valve. The type of “earth lug” should be convenient for connection construction.

6.4 Use bolt anti-corrosion glue

There is a corrosive environment in petrochemical plants. In order to ensure that the bolts will not affect the electrical conductivity due to corrosion, bolt anti-corrosion glue can be used. This glue has the function of rust conversion, can eliminate rust, and can isolate the penetration of external corrosive media, making the bolts It will not be corroded and has a service life of more than 10 years. This method does not hinder the disassembly and assembly of bolts, and can maintain the electrical conductivity of the bolt connection.

07Basis for relevant standards and regulations

According to the regulations in literature [1] and referring to foreign materials, taking the test records of a company in Tianjin as an example, the measured values of the transition resistance at the flange joints are listed in Table 1.

It can be seen from Table 1 that there is little effect on the transition resistance of the connection with or without jumper wires, and there is little difference in transition resistance between 4 bolts and 8 bolts without jumper wires. Even in some cases the transition resistance of 4 bolts is less than that of 8 bolts. In summary, the main factor affecting the transition resistance of the connection is whether the conductive contact is good, not the number of bolts.

08Questions about the connection resistance stipulated in the standard specification

As mentioned above, the movement of charge on or between objects does not require very low resistance. In order to discharge the charge on the object, the grounding resistance is less than 1MΩ. The grounding resistance stipulated in the specification is no more than 100Ω, which is already very generous. Why does the static crossover need to be less than 0.03Ω?

Metal objects such as pipes in explosive environments are electrostatically grounded to avoid gap discharges that may cause sparks. Since a conductive connection of 100Ω is sufficient to balance the charge and potential between flanges, a connection resistance of less than 0.03Ω is not required.

The “anti-lightning induction” in the specification is actually to avoid sparks in the flange gap caused by lightning induction, and also does not require a connection resistance of less than 0.03Ω. In addition, sparks do not occur in non-gaseous media.

The conductivity of the metal contact surface without a corrosion layer must be good enough to discharge static electricity or lightning-induced surges, and it does not need to be measured or judged by contact resistance. It is superfluous to regard the connection resistance less than 0.03Ω as the basis for judging good conductivity.

09 Conclusion

This article introduces the concepts of static electricity, static electricity protection and lightning protection, and compares different standards for the equipotential connection methods and requirements of metal pipeline flanges. From the perspective of engineering practice, when the flange bolt connection has good conductivity, bolt connection is preferred. Equipotential bonding. For flange conductive bridging, it is necessary to pay attention to the conduction of the flange bolt itself, not all flanges require additional conductive bridging. In some occasions, the method of grounding lugs can be used for conductive bridging. Attention should also be paid to corrosion problems in flange connection and conductive bridging, and corresponding anti-corrosion measures need to be taken. Reasonable engineering design and implementation not only meet the requirements of electrostatic protection and lightning protection, but also the main guarantee for the safety of production equipment.