Health & Safety including Dust Control
Effective grounding and bonding is presented in the standards as the primary means of protection from electrostatic hazards and is the most straight forward, secure and cost-effective means of ensuring static hazards are managed and controlled correctly. Eliminating the accumulation of static charges will eliminate the static hazard.
Where the rate of charge generation exceeds rate of charge dissipation => Charge Accumulation => Static Hazard
Grounding and Bonding - what are the key benchmarks? The ground (earth) has an infinite capacity to absorb charges and “grounding” (earthing) is the act of connecting a body to an electrode (or other buried structure) that has a verified contact resistance to the ground. Grounding provides a path for static charges to rapidly flow to ground, reducing the voltage of the object to zero and thereby eliminating the presence of an ignition source.
The general acceptable maximum resistance to ground is 25 ohms in the U.S and 10 ohms across Europe. “Bonding” connects objects so that they are at the same electrical potential preventing discharges when they are positioned in close proximity to each other. If bonding is carried out, it is preferable to ensure that one of the bonded objects is connected to ground, thereby ensuring all parts of the bonded system are at zero electrical potential.
Given that grounding is the primary source of static hazard prevention it is important to understand what parameters can be indentified as providing a satisfactory level of protection. The key to static hazard protection is ensuring that the path between the charged object and the static ground is of a sufficient quality to dissipate the static charges safely and rapidly to ground. The majority of plant equipment at risk of static charge accumulation is made of metal. Metals are excellent conductors and the natural resistive properties of metals ranging from copper through to steel means that electrical resistance to the transfer of charges from the body is low, provided that the body has good contact with ground. If the metal body is not grounded, this positive characteristic can quickly become a negative as isolated metal conductors are the primary source of static spark ignition hazards.
The maximum value of resistance present in metal circuits, which includes the body at risk of static charge accumulation, should be equal to or less than 10 ohms and is the benchmark value of resistance recommended by all four standards. If a resistance of 10 ohms or more is detected then there is a likelihood that the grounding cable has been compromised and should be checked for corrosion or breakages.
Therefore it is important to ensure that the static dissipative path, the path that channels the charging current to ground, is 10 ohms or less, and stays that way for the duration of the process.
What the standards say: Road tankers (Tank Trucks): NFPA 77, CLC TR: 50404 and API RP 2003 recommend that the first procedure in road tanker material transfer operations is to ground the tanker prior to any other operation being carried out by the driver. Interlocking static grounding systems, with ground status indicators, should also be specified so that if the road tanker is not protected from static discharges due to incorrect grounding, the system will not permit the flow of product thereby eliminating the generation of electrostatic charges. The static grounding system should monitor the resistance in the grounding circuit ensuring it does not rise above 10 ohms. CLC TR: 50404 specifies 10 ohms or 100 ohms as being suitable for convenience in monitoring, however 10 ohms would be the established standard for large companies with a good track record in static control safety.
Road Tanker static grounding system
To illustrate, a 50 m length (164 feet) of 2 mm diameter steel cable, in good condition, should have an overall resistance approximating to 7.2 ohms over its entire length.
Railcars (Tank cars): API RP 2003 and NFPA 77 identify parts of the railcar that could become isolated from the railway tracks. Non-conductive wear pads and bearings can be located between the container and the chassis and it is recommended that the container is grounded prior to, and during, the material transfer process. This will prevent the accumulation of static charges on the container and eliminate the risk of discharges from the container to the fill pipe, and discharges to people or other grounded bodies. NFPA 77 (8.8.2) states:
“Many tank cars are equipped with nonconductive bearings and nonconductive wear pads located between the car itself and the trucks (wheel assemblies)….. Therefore, bonding of the tank car body to the fill system piping is necessary to protect against charge accumulation”.
44 Solids & Bulk Handling • August 2010
www.solidsandbulk.co.uk
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