Health & Safety inc Dust Control Stand your ground
You don’t need to be a rocket scientist to safeguard against the hazards of static electricity
For any person responsible for the safety of employees, colleagues, plant equipment and plant property, one of the most potentially confusing aspects of providing a safe operating environment is trying to determine if that site’s manufacturing or handling processes have the potential to discharge static sparks into flammable or combustible atmospheres. Electrostatics is a detailed subject area that, for most of us, appears to be a black art accessible only to academics and experienced process safety consultants. Because static ignition hazards occur at the “nuclear level”, it is naturally difficult to visualise how, and why, static electricity is a hazard in industries where flammable and combustible products are regularly processed. There are so many variables that have a role to play in electrostatics, it is almost impossible to predict the net effects of these parameters, in a hazardous prevention context, without feeling the need to conduct controlled tests to determine if a specific process could produce incendive electrostatic discharges. If you consider that a walking a cross a carpet can generate up to 35,000 volts (35 KV) on a person wearing insulating footwear, it is easy to see how normal, everyday processes can generate potentials well in excess of 10,000 volts (10 KV). For a small object like a bucket, which has a typical capacitance of 20 pico- farads, the total energy available for discharge at 10 KV is 1mJ. This is higher than most flammable vapour minimum ignition energies (MIE’s). Scaling up, the MIE available on a human, at 10 KV, would be 10mJ. In powder conveying operations voltages of the order of 1000 KV can easily be generated on parts of the conveying system. Road tankers undergoing loading can reach MIE levels of 2250 mJ.
Complicating matters further, ignitable electrostatic discharges can occur in many forms ranging from spark discharges, propagating brush discharges, bulking brush discharges, to corona discharges. The effort required to assess, determine and combine these variables into a cohesive audit of a potential hazard is, by no means, easy.
Standards for static grounding control: Fortunately, there are several internationally recognised standards that provide guidance on ways to limit electrostatic hazards enabling those of us responsible for worker health and safety minimise the risk of incendive static discharges. Hazardous area operators who can demonstrate compliance with these standards will go a long way to providing a safe working environment and preventing the ignition of ignitable atmospheres. The most comprehensive standards are: NFPA 77: Recommended Practice on Static Electricity (2007). Cenelec CLC/TR 50404: Code of practice for the avoidance of hazards due to static electricity (2003). American Petroleum Institute API RP 2003: Protection against Ignitions Arising out of Static, Lightning and Stray Currents (2008). API RP 2219: Safe Operation of Vacuum Trucks in Petroleum Service (2005).
The standards, particularly NFPA 77 and CLC/TR: 50404, describe a range of processes where static charges can be generated including flow in pipes and hoses; loading & unloading of road tankers; railcar loading & unloading; filling and dispensing portable tanks, drums and containers; storage tank filling and cleaning; mixing, blending and agitation operations; the conveying of powders and other operations. The API RP 2003 standard focuses on road tanker (tank-truck) loading and railcar filling operations, storage tank filling and general operations involving petroleum products. API 2219 provides detailed guidance on protecting vacuum trucks from electrostatic hazards.
The standards outline what factors can be identified and controlled to limit electrostatic hazards and these controls typically depend on:
• Preventing the accumulation of static charges on material, people and equipment.
• Finding ways to limit the generation of static charges. Controlling the generation of static charges is linked to finding ways of minimising the relative velocity of materials, be that flow velocities of liquids in a pipe or the RPM of a mixing process. Still, even lower velocities can separate enough charges to set up the right conditions for an electrostatic discharge. NFPA 77 (5.1.10) states that the transfer of just one electron in 500,000 atoms is required to generate voltages with enough energy to ignite flammable atmospheres.
www.solidsandbulk.co.uk August 2010 • Solids & Bulk Handling 43
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