Closing the Loop: Static Earthing Best Practices for Rail Tanker Transfer Operations
Rail Tanker Transfer Operations Introduction
In the hazardous process industries, commonly referred to as the “Ex” and “HAZLOC” industries, static electricity is generated frequently. Various grades of crude oil, refined petroleum products like LPG, and numerous chemicals fall into a category of materials known as “static accumulators”.
During a typical rail tanker loading operation, the static accumulating product is transferred from a storage tank via a gantry loading system into a receiving rail tanker. The transfer process electrostatically charges the product as it moves through the pump, filtration, and piping system.
If the rail tanker is not directly connected to the earth, electrostatic charges accumulate on its surface, causing its voltage to rise rapidly. The tanker then seeks to discharge excess potential energy, with the most efficient method being a static spark.
Energy Discharged in Static Sparks
Grounded objects near charged surfaces are prime targets for electrostatic sparks. Allowing the uncontrolled accumulation of static electricity in an Ex atmosphere presents a significant ignition risk. If the rail tanker is ungrounded, its voltage can rise to hazardous levels in a very short space of time.
The Minimum Ignition Energy (MIE) is a measure of the minimum amount of spark energy necessary to ignite combustible gases and/or vapours.
NFPA 77:2024
Annex B – Physical Characteristics of Materials
B.1 Combustibility Parameters of Gases and Vapors
C Lowest MIE calculated using the heat of oxidation method.
The Importance of Bonding and Earthing the Rail Tanker
Comparing the energy of static sparks with the minimum ignition energies of petroleum products and flammable chemicals highlights the importance of bonding and earthing the rail tanker and connected equipment, such as hoses and piping. Additionally, at these energy levels, mitigating electrostatic shocks to workers is critical, as involuntary reactions to shocks can lead to accidents, especially when personnel are working at height.
Of the several factors influencing static charging, earthing the rail tanker is one of the most controllable methods to enforce as part of your Standard Operating Procedure (SOP) for rail tanker loading and unloading operations. Proper earthing helps maintain a low resistance to the general mass of the earth, allowing for safer dissipation of static charges.
In North America, dedicated static earthing systems for rail tankers are standard practice. In Europe, practices vary; some sites actively ground rail tankers, while others assume sufficient bonding between the tanker chassis and wheels enables charge dissipation through the tracks.
Industry Codes of Practice
In the case of a rail tanker IEC TS 60079-32-1 (Explosive Atmospheres: Electrostatic Hazards) recommends that the resistance of the bonded connection between rail tracks and the loading gantry should not exceed 1 meg-ohm. However, as rail tracks and gantry connections typically involve heavy-duty metal conductors, a threshold above 10 ohms may indicate a potential fault with the earthing circuit.
The National Fire Protection Association (NFPA) provides specific guidance on controlling electrostatic hazards in Ex areas.
- NFPA 77: Recommended Practice on Static Electricity (Chapter 12.4.2) (2024) highlights the risk of non-conductive wear pads and bearings, which can prevent charge dissipation from the rail tanker to the wheels.
- It recommends a maximum resistance of 10 ohms for earthing and bonding circuits.
- The use of interlocks is also recommended to prevent product transfer if the rail tanker is not grounded.
Specifying a Static Earthing System for Rail Tanker Operations
One of the challenges with static electricity is its imperceptibility. This can lead to a false sense of security among operators conducting rail tanker product transfer operations. An earthing system that provides a clear visual indication, such as a “GO/NO-GO” signal using a traffic light method of indication, enhances safety SOPs and compliance with best practice.
Interlocking the transfer system with the earthing system adds a level of control. This helps prevent product transfer, starting when proper earthing is not established.
Newson Gale recommends the Earth-Rite® PLUS for bonding rail tankers to loading gantries. This system is ATEX and IECEx certified for all gas and liquid vapor groups and helps achieve and maintain a 10-ohm or lower connection between the rail tanker and the product transfer system.
When connected, the Earth-Rite® PLUS automatically verifies the continuity between the rail tanker and the loading gantry true earth ground connection. The system uses a Factory Mutual-approved earthing clamp with tungsten carbide teeth, providing a strong, vibration-resistant connection.
Unlike non-monitored earthing systems that assume static dissipation, the Earth-Rite® PLUS actively monitors the connection using dedicated static earthing terminals. This monitored connection provides a verified earthing path for static charges.
Once a secure connection is established, a cluster of green LEDs flashes continuously to indicate active monitoring. The system also features volt-free double-pole, double-throw contacts that can interlock with pumps and PLCs. If resistance exceeds 10 ohms, the system halts product transfer, reducing static generation and mitigating the risk of electrostatic discharge igniting flammable atmospheres.
The Earth-Rite® PLUS system provides earthing with a clear visual indication, interlocks, and continuous ground loop monitoring, indicating that the resistance remains at 10 ohms or less throughout the transfer operation.
Conclusion
Proper earthing and bonding of rail tankers in Ex environments is essential for managing electrostatic hazards.
Best practices include:
- Using dedicated static earthing systems to provide a monitored, low-resistance connection.
- Following IEC 60079-32-1, NFPA 77, and API RP 2003 recommendations to maintain resistance below 10 ohms.
- Interlocking earthing systems with transfer operations to prevent loading when earthing is not established prior to product transfer.
By incorporating these measures into standard operating procedures, facilities can mitigate ignition risks associated with static electricity and promote safer rail tanker loading and unloading operations.