The Crucial Role of Earthing Systems

Understanding Earthing Systems: A Cornerstone of Industrial Safety

In the realm of industrial operations, particularly those involving the handling of flammable liquids and powders, the concept of an earthing system is not merely a technical detail but a fundamental pillar of safety. An earthing system, also known as grounding, provides a safe path for electrical current to dissipate into the earth, preventing the accumulation of dangerous static charges that could lead to fires, explosions, or harm to personnel and equipment. This document delves into the critical aspects of earthing, focusing on its application in preventing electrostatic hazards during the transfer of materials.

System Earthing vs. Equipment Earthing

Earthing can be broadly classified into two main categories: system earthing and equipment earthing. While system earthing is crucial for the efficient and proper operation of electrical systems, equipment earthing directly addresses the safety of personnel and plant. Its primary function is to provide a controlled pathway for the dissipation of static electricity, thereby mitigating the risk of uncontrolled electrical discharges in potentially hazardous environments. For safe dissipation of static electricity, a resistance to earth of less than 10⁶ Ω.m is generally considered sufficient across most situations.

The Perils of Static Electricity in Flammable Liquid Transfer

The transfer of flammable liquids presents a significant hazard due to the potential for static charge accumulation. This occurs through charge separation as liquids flow through pipes. If the liquid's specific resistance exceeds 10⁸ Ω.m, detectable and hazardous charges can build up. The risk is amplified if the liquid contains immiscible components or suspended solids, such as crystallisation processes in toluene or the presence of water in toluene. In such scenarios, it is advisable to limit transfer velocities to below 1 m/s.

Measures to Mitigate Static Hazards during Liquid Transfer:

• Ensure pipelines are completely filled to prevent the formation of explosive mixtures.
• Remove contaminants and solids wherever possible.
• Utilise inert gas blanketing.
• When transferring by 'blowing across', use an inert gas.
• Avoid mechanical mixing or agitation of low conductivity liquids.
• Use ball valves with earthed metal spheres.
• Employ low transfer velocities. For partially filled pipes or pipes discharging into containers, specific velocity limits apply based on liquid type and pipe diameter.

Recommended Velocities for Mineral Oil Products and Similar Liquids:

The following table outlines recommended maximum transfer velocities for mineral oil products (like gasoline, petrol, kerosene) and other chargeable liquids (excluding carbon disulphide and ether), considering nominal pipe diameters:

Adherence to these velocities ensures no hazardous charges are generated in homogeneous liquids. However, even velocities below 1 m/s can generate hazardous charges when suspensions of crystals in non-conductive liquids are conveyed.

Specific Guidelines for Ether and Carbon Disulphide:

For ether and carbon disulphide in pipelines up to 25 mm diameter, the maximum velocity should not exceed 1 m/s. Larger pipes require even lower velocities. A general rule for all homogeneous liquids (except carbon disulphide and ether) and all pipelines is that at velocities below 1 m/s, no dangerous charges will be generated.

Additional Safety Measures:

• Flanges should be earth bonded.
• Utilise sub-surface dip pipes or bottom entry filling when discharging into vessels.
• Ensure regular inspection and testing of earth bonding.

Earthing During Powder Transfer

Powder transfer, whether via screw conveying, vacuum transfer, or pneumatic conveying (low pressure/dilute phase or high pressure/dense phase), also poses significant risks of electrostatic discharge. Pneumatic conveying systems, especially low-pressure systems with high velocities (10-25 m/s), can lead to intensive charging of the conveyed material and the pipeline. This can result in discharges between conductive parts or the entrainment of charges into receiving containers.

Classification of Powders by Resistivity:

Powders are categorised into three groups based on their volume resistivity:

• Low resistivity powders: e.g., metals, with resistivities up to about 10⁶ Ω.m.
• Medium resistivity powders: e.g., many organic powders like flour, with resistivities in the range of 10⁶ Ω.m to 10⁹ Ω.m.
• High resistivity powders: e.g., certain organic powders, synthetic polymers, and minerals like quartz, with resistivities above 10⁹ Ω.m.

Reducing Hazards in Powder Transfer:

• Ensure pipelines for pneumatic conveying are made from metal with good earth bonding. Resistance to ground for all conductive components should be less than 10 ohms.
• Ground all operators involved in powder loading, ensuring their resistance to ground is less than 1 x 10⁸ ohms.
• Avoid insulating coatings on the inner surfaces of metal containers and pipelines.
• Use plastic flanges with plastic transfer lines where appropriate.
• Avoid coatings or sheathing on pipelines made of insulating material.
• Use anti-static plastic or paper bags when handling materials in the presence of flammable gases, vapours, or dusts with low minimum ignition energies (< 4 mJ).
• Discharge powder into containers or silos via intermediate loading equipment, such as a cyclone fabricated from conductive material, to reduce velocities and earth charge. Rotary valves, bag dump hoppers, or scroll feeder systems can also be employed.

Earthing for Offloading and Temporary Storage

When offloading flammable liquids, stringent precautions are necessary to prevent static accumulation and protect against lightning. Standard copper strip is typically used for the main earthing system, connected to an earthed copper rod with a total resistance to earth of less than 10 ohms. A bulk loading and offloading procedure should be implemented, mandating that tankers are connected to the earthing point before offloading begins. The electrical resistance between the couplings on a flexible discharge hose must not exceed 10⁶ ohms.

Before bringing temporary storage for flammable liquids or explosive powders online, a thorough assessment of earthing provisions, including associated earth testing, must be conducted. This assessment should cover the storage vessel and all supporting ancillary equipment.

Flexible Pipelines and Earthing

When flexible hoses are used, several measures are recommended:

• If velocities exceed 1 m/s, hoses should be made of conductive material or non-conductive material with embedded fine wire mesh. This mesh must be bonded to the hose's metal flanges or couplings.
• For metal hoses with a liner, the metal mantle and flanges or couplings must be bonded together.
• The electrical resistance between the two couplings of a flexible hose must not exceed 10⁶ ohms and should be measured at regular intervals.
• Use of ball valves with earthed metal spheres is also advised.

Relevant Codes of Practice and Standards

Numerous codes of practice and standards provide comprehensive guidance on the earthing of plant and equipment to prevent electrostatic hazards. These include:

• HS(G)176 The storage of flammable liquids in tanks: Provides guidance on earthing flexible hoses, dip rods, tubes, and tank structures.
• HS(G)140 Safe use and handling of flammable liquids: Recommends earthing of flexible hoses and provides guidance on preventing electrostatic charging.
• LPGA COP 1 Bulk LPG storage: Details earthing requirements for LPG plant, specifying electrical resistance to earth less than 1 x 10⁶ ohms and compliance with BS 5958 Parts 1 and 2.
• Road Traffic (Carriage of Dangerous Substances) Regulations (L16, L17, L18, L19): Provide guidance on the earthing of road tankers carrying various hazardous substances.
• BS 3492:1987 Specification for electrically bonded road and rail tanker hose and hose assemblies for petroleum products: Addresses the requirements for bonded hoses.
• BS 5345: Part 1:1989 and Part 4:1977 (superseded by BS EN 60079-14:1997): Cover selection, installation, and maintenance of electrical apparatus in potentially explosive atmospheres, including earthing requirements for intrinsic safety.
• BS 5908:1990 Code of practice for fire precautions in the chemical and allied industries: Recommends a resistance to earth of less than 1 MW for adequate charge dissipation, and preferably less than 10 Ω for ease of maintenance.
• BS 5958:1991 Code of practice for the control of undesirable static electricity (Parts 1 & 2): Offers detailed practical guidance on earthing, resistance values, and material selection to control static electricity in various industrial situations.
• BS 6651:1992 Code of practice for the protection of structures against lightning.
• BS 7430:1998 Code of practice for earthing.
• BS EN 1127-1:1998, Explosive atmospheres – Explosion prevention and protection: Requires bonding and earthing of all conductive parts that could become hazardously charged.
• FS6023:1989 Explosible dusts: The elimination of ignition sources: Provides recommendations on bonding and earthing.

Connecting a Switch to the Earthing System

When connecting a switch to an earthing system, the earth connection from the switch handle should always be separate from the connection for the switch metalwork. This connection should be as short as possible and made using a standard copper conductor directly to the earthing system.

Earthing in Substation Environments

In substations, all exposed metalwork within the substation must be bonded to the substation earthing system to protect personnel. Additionally, a bridging conductor is installed between each set of gateposts, and a flexible earth strap connects the gatepost to the gate itself, ensuring comprehensive earthing coverage.

Conclusion

The meticulous implementation and regular maintenance of earthing systems are paramount in preventing electrostatic hazards within industrial settings. By understanding the principles of system and equipment earthing, adhering to recommended velocities, using appropriate materials, and following established codes of practice, organisations can significantly reduce the risk of fires and explosions, ensuring a safer working environment for all.

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