Electrostatic grounding and equipotential bonding in explosive atmospheres 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 connection of metal pipes. Correctly understand static electricity, electrostatic grounding and equipotential bonding The requirement is to do a good job of electrostatic protection and lightning protection engineering design basis. Starting from the basic concept of static electricity, this paper analyzes the requirements of electrostatic grounding and equipotential bonding. Combined with engineering practice, it discusses several implementation methods of equipotential bonding at the flange connection of metal pipes, and analyzes the different requirements and characteristics of electrostatic protection and lightning protection. , and make engineering proposals.

01

The concept of static electricity

Static electricity is an electrical charge that exists on the surface of an object and is relatively static to the observer. Static electricity can be generated by the separation and contact of objects, the attachment of charged particles, electrostatic induction and dielectric polarization. During production, storage and transportation, it is generated and accumulated on materials, equipment, human bodies, pipes and structures.

Electrostatic safety refers to the state and conditions in which there is no economic loss caused by 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 of static electricity dissipated. Usually, as long as the rate of static electricity charging is less 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

Electrostatic grounding resistance

The movement of charges on or between objects does not require very low resistance. In order to discharge the charge on the object, the grounding method is usually used to avoid electrostatic discharge with the help of the equilibrium potential of the earth. Experiments have shown that if the surface resistance of an object is less than 1MΩ, the charge can be moved, so if the electrostatic grounding resistance of the object is less than 1MΩ, static electricity can be released to the ground. Relevant standards and regulations 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.

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 electrostatic charging, or the accumulated static electricity can be released to eliminate static electricity.

03

Equipotential bonding at flange connections of metal pipes

The conductive connection at the flange connection of the metal pipe achieves equipotentiality for the purpose of electrostatic protection and lightning protection.

The conductive connection is different from the electrostatic grounding. The electrostatic grounding is to discharge static electricity by means of the ground, achieve the electrostatic balance of the object, and avoid the accumulation of static electricity of the object. The equipotential connection at the flange connection of the metal pipeline is to avoid the charge imbalance between the flange surfaces of the metal pipeline, forming a potential difference, and avoiding electrostatic gap discharge or lightning gap discharge due to the potential difference. Equipotential connection at the flange connection of metal pipes should pay attention to two aspects: connection resistance and charge conduction current carrying capacity.

04

regulations of relevant standards

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

1) Article 4.2.2 of GB50057-2010 “Code for Design of Lightning Protection of Buildings” [1] stipulates: For the lightning protection induction of the first type of lightning protection buildings, when valves, flanges, elbows, etc. When the transition resistance value at the connection is greater than 0.03Ω, a metal wire should be used for jumper connection. Flanges with not less than 5 bolts may not be bridged in a non-corrosive environment. The second and third types of lightning protection buildings do not have the above requirements for lightning induction.

2) Article 5.3.4 of SH/T3097-2017 “Petrochemical Electrostatic Grounding Design Specification” [2] stipulates that when the metal flange is fastened with metal bolts, it is usually not necessary to provide additional electrostatic jumper connection lines, but it should be ensured that at least There are 2 metal bolts with good conductive contact surfaces.

3) Article 2.7.5 of HG/T20675-1990 “Electrostatic Grounding Design Regulations for Chemical Enterprises” [3] stipulates that when the metal flange is fastened with metal bolts, it is not necessary to set up an electrostatic jumper. Under corrosive conditions, the contact surfaces of not less than two bolts should be trusted and degreased before installation, and anti-loosening nuts should be installed. From practical experience, there is sufficient electrostatic continuity between metal flanges connected by metal bolts, only by bolts. Installation requirements under corrosive conditions, the purpose is to ensure continuity.

4) Article 10.12.1 of GB/T20801.4-2020 “Code for Industrial Pipelines for Pressure Piping – Part 4: Fabrication and Installation” [4] stipulates that for pipelines designed 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 Refueling and Hydrogenation Stations of Vehicles” [5] stipulates that the connection between the two ends of the flange of the processing pipeline in the explosive atmosphere should be bridged by metal wires.

When the flange connection bolts are more than or equal to 5, they may not be bridged in a non-corrosive environment. Metal wire jumpers should be installed at the connections between the hoses and flanges on oil, liquefied natural gas (LNG), compressed natural gas (CNG) and liquefied petroleum gas (LPG) pipelines in explosive atmospheres. Due to poor contact connection (resistance greater than 0.03Ω) at the connections at both ends of the flange, static electricity and lightning sparks may occur, resulting in fire or explosion accidents. Flanges with at least 5 connecting bolts may not be bridged in a non-corrosive environment.

6) Article 9.0.4 of GB50177-2005 “Code for Design of Hydrogen Stations” [6] stipulates that the connections between the hydrogen station and the pipe flanges and valves in the hydrogen supply station should be bridged by metal wires. In a normal environment without rust, the contact resistance of pipe joints, valves, flanges, etc. are all below 0.03Ω. If the pipe joint is rusted, it will increase the contact resistance. The test shows that when there is rust and corrosion between the flanges connected by bolts, in the case of a relatively low lightning current amplitude (10.7kA), the flanges can also discharge in gaps, causing fire or explosion accidents. If the hydrogen station does not frequently check and test the transition resistance of pipe joints and flanges, etc., once the contact joints are rusted, it is very dangerous. Therefore, it is stipulated that all pipes 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 the flange connections on the pipelines of 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 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 of contact resistance, resulting in fire and explosion accidents. In order to prevent Due to poor connection or metal corrosion at both ends of the upper flange of the pipeline, the contact resistance increases, which shall be implemented in accordance with the provisions of this article.

In the actual engineering design and implementation, an objective and scientific engineering plan for equipotential bonding should be determined through a comprehensive analysis, and the following factors should be considered:

1) How should the connection between the flange pairs take into account both anti-lightning induction and electrostatic protection.

2) Are there different solutions for different media properties (hydrogen, oxygen, etc.)? Is it possible to have a unified solution for different explosive atmosphere areas?

3) Is it a corrosive environment, and whether the anti-corrosion measures will affect the conductivity and the extent of the effect?

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

5) What connection method is most suitable?

6) Are the provisions of the standard specification appropriate?

05

On-site FAQ

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

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

1) Some viewpoints believe that conductive jumper wires are also required for metal flanges connected with metal bolts. In fact, the bolts, nuts and washers of metal pipes are made of metal and have good electrical conductivity. If there is no corrosion layer affecting the electrical conductivity, there is no need to bridge the conductive wires. If the contact between the bolt and 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 a 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 wires.

2) Some viewpoints believe that symmetrical grounding is required, that is, repeated grounding is required to improve the reliability of grounding. In fact, there is 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) An electrostatic jumper is made in the fire water pipeline of an oil field. In fact, impure water is a conductive medium, and no anti-static jumper is required.

4) It is not necessary to make electrostatic bridges on both sides of the welding seam of a stainless steel pipe at a site, because the welding seam is conductive.

06

Several ways of flange conductive jumper

6.1 Conductive jumper through bolts or gaskets The metal bolts and gaskets matching the flange are good conductors and can realize conductive jumper. This method needs to consider the corrosion effect of the environment on the flange and bolt material.

For many on-site environments, anti-corrosion measures will be taken, such as: painting with 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 is 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 the conductive jumper can be realized by bolts are as follows:

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

2) Flanges and bolts of corrosion-resistant materials. For example: stainless steel is resistant to corrosion by process media.

3) Conductive anti-corrosion measures. For example conductive anti-corrosion coating or plating, such anti-corrosion measures should be considered in the selection of equipment materials, and should not be used only for conductive bridges.

6.2 Equipotential connection is realized by crimping the grounding piece through the nut. The metal connecting piece is crimped through the bolt and nut matched with the flange, and the connecting piece is crimped with the jumper wire to realize the conductive jumper connection. In this method, attention should be paid 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-resistant metal arrangements or lugs of the same material as the flange bolts should be used, otherwise, corrosion may occur.

6.3 Setting “grounding lugs” to realize conductive jumper In the method of setting special “grounding lugs” or transition connectors on the pipeline or equipment (valve), crimp the connecting piece, and the connecting piece is crimped and jumpered over the metal wire to realize the conductive connection. This connection method is recommended when the flange bolt connection with anti-corrosion measures in a corrosive environment cannot achieve a conductive jumper connection. 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 included with the valve. The type of “earth ear” should be able to 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 electrical conductivity due to corrosion, the bolt anti-corrosion adhesive can be used. 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 the bolt, and can maintain the electrical conductivity of the bolt connection.

07

The basis for the relevant standard specifications

According to the provisions in the literature [1] and with reference to foreign materials, taking the test records of a company in Tianjin as an example, the measured value of the transition resistance at the flange connection is shown in Table 1.

It can be seen from Table 1 that with and without jumper wires has little effect on the transition resistance of the connection, and the transition resistance values ​​of 4 bolts and 8 bolts without jumper wires are not much different. Even in some cases, the transition resistance of 4 bolts is less than that of 8 bolts. To sum up, the factor affecting the transition resistance of the connection is mainly whether the conductive contact is good, not the number of bolts.

08

Questions about the connection resistance specified in the standard specification

As mentioned above, the movement of charges on or between objects does not require very low resistance. In order to discharge the charge on the object, the grounding resistance should be less than 1MΩ. The grounding resistance specified in the specification is not more than 100Ω, which is already very generous. Why does the electrostatic jumper need to be less than 0.03Ω?

Electrostatic grounding of metal bodies such as pipes in explosive environments is to avoid gap discharges that may cause sparks. Since a 100Ω conductive connection 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 the surge caused by lightning induction, and also does not require a connection resistance less than 0.03Ω. In addition, sparks do not occur in non-gaseous media.

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

09

concluding remarks

This paper introduces the concepts of static electricity, static electricity protection and lightning protection. According to the method and requirements of equipotential connection of metal pipeline flanges, and compares different standards, from the perspective of engineering practice, when flange bolted connection has good electrical conductivity, it is preferred to use the bolted connection. Equipotential bonding. For flange conductive jumpers, you need to pay attention to the conductivity of the flange bolts themselves. Not all flanges need additional conductive jumpers. In some occasions, the grounding lug method can be used for conductive jumper connection, and the flange connection and conductive jumper should also pay attention to corrosion problems, and corresponding anti-corrosion measures should 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 production safety of production equipment.