The loading and unloading of road tankers with flammable and combustible products, presents one of the most serious fire and explosion risks for site operations within the hazardous process industries.
Powders and liquids with low electrical conductivities are the prime sources of static charge generation because their electrical properties do not easily permit the transfer of excess charges. Instead, non-conductive and semi-conductive liquids and powders retain and accumulate charges after they make contact with conductive objects. The most common interface for charging of non-conductive and semi-conductive product is contact with metal plant equipment including pipes, filters, pumps, valves, barrels, totes, mixers and agitators. When the electrostatically charged liquid (or powder) is deposited into a container like a barrel, tote, or road tanker charging of the container will occur if there is nowhere else for the charges to go.
For sparking to occur in road tanker product transfer operations, other conductive objects must come into close proximity with the charged container of the road tanker. Examples of conductive “objects” include the fill pipe entering the opening on the top of the container, fall prevention systems like folding stairs, and drivers or operators working around the road tanker. The charges on the road tanker’s container attract opposite charges to the surface of the object and rapidly create an electric field between their respective surfaces.
It is the strength of this electric field that causes the “breakdown” of the air between the container and the object. When the air is “broken down” a conductive path for the excess charges to rapidly discharge themselves is created, leading to a static spark discharge.
Standards and recommended practice governing the static control of road tanker product transfers
Regulators are extremely cautious about the ignition hazards presented by static electricity in road tanker product transfer operations. Three standards, in particular, provide clear guidance on what precautions should be taken. NFPA 77, API RP 2003 and IEC 60079-32 state that grounding of the road tanker should be the first procedure carried out in the transfer process. Grounding effectively creates an electrical circuit that connects the road tanker to the Earth and it is this connection to earth which prevents static charges accumulating on the road tanker’s container. The reason the charges can transfer from the road tanker to earth is because the Earth has an infinite capacity to absorb and redistribute static charges, with the positive effect of removing the ignition source from a potentially combustible atmosphere.
The electrical resistance of this circuit from the road tanker to the “ground source” (or “grounding point”) which is in contact with the earth, is a key performance indicator of the entire grounding circuit’s capacity to provide a secure and safe product transfer operation.
Road tanker grounding systems
The standards advise that a grounding system, which can measure and monitor resistance in the grounding circuit, can be utilised. The system should verify if the ground connection to the road tanker is complete before loading or unloading is initiated.
The precautionary guidance regarding the grounding of road tankers in IEC 60079-32 reflects the IEC’s stated goal of providing the latest state of the art guidance. It states that the “earth cable” earthing the road tanker should be part of a static grounding system that continuously monitors the resistance between the road tanker and the designated grounding point located on the loading gantry.
It also maintains that the grounding system should be interlocked with the transfer system to shut down the product transfer operation if this resistance exceeds 10 ohms. It also states that the grounding system should be capable of recognising when it is not connected to the chassis/tank of the road tanker. This ensures that situations where the tank of the road tanker is not connected to the grounding system, for example, where an operator could connect the clamp to an isolated metal mud-guard or wheel-nut, will not result in a permissive condition for the transfer operation, thereby eliminating the risk of electrostatic charging of the road tanker.
System like the Earth-Rite RTR will analyse the capacitance of the road tanker as part of the grounding circuit. If the capacitance presented is in the normal range for road tankers, the grounding system will recognise that it has made a positive connection to a road tanker. From the site operator’s perspective, this eliminates the risk of drivers unknowingly connecting the grounding clamp to parts of the truck chassis that are electrically isolated from the truck’s container.
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