SAFETY IN THE PLANT
Benchmarking 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 laboratory testing to determine if a specific process could produce incendive electrostatic discharges.
If you consider that a walking across a carpet can generate 35,000 volts (35 KV) on a person, 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 metal 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 ignition energy available on a human, at 10 KV, would be around 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 carry as much as 2000 mJ of ignition energy.
It can be time-consuming, and expensive, to investigate and determine the level of voltage that can arise as a result of these charging mechanisms. Complicating matters further, ignitable electrostatic discharges can occur in many forms ranging from spark discharges, propagating brush discharges and bulking brush discharges, to corona
SEPTEMBER/OCTOBER 2019 www.reviewonline.uk.co
discharges. The effort required to assess, determine and combine these variables into a cohesive audit of a potential hazard is, by no means, easy.
Which standards should I follow to control static electricity in ignitable atmospheres? Fortunately, there are several internationally recognised standards that provide guidance on ways to limit electrostatic hazards enabling those 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 60079-32-1: “Explosive atmospheres - Part 32-1: Electrostatic hazards, guidance” (2015).
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: 60079-32-1, 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 loading and railcar filling operations, storage tank filling and general operations involving petroleum products. API RP 2219 provides detailed guidance on protecting vacuum trucks from electrostatic hazards.
These standards outline what factors can be identified and controlled to limit electrostatic hazards and these controls typically depend on:
• Preventing the accumulation of electrostatic charges on plant equipment, people and the material transferred.
• Controlling the process to minimise the generation of electrostatic charges.
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.
Effective earthing 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.
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Enquiry No. 27
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