Static electricity is an excess of electric charge trapped on the surface of an object.It can be either positive (+) or negative (-) and is manifested when some force has separated the positive electrons from the negative protons of an atom. Typical forces include flowing, mixing, pouring, pumping, filtering or agitating materials where there is the forceful separation of two like or unlike materials. The charge remains until it is allowed to escape to an object with a weaker or opposite electrical charge, such as the ground, by means of an electric current or electrical discharge. Examples of static generation are common with operations involving the movement of liquid hydrocarbons, gases contaminated with particles (e. g., metal scale and rust), liquid particles (e. g., paint spray, steam) and dust or fibers (e. g., drive belts, conveyors).
The static electric charging rate is increased greatly by increasing the speed of separation (e. g., flow rate and turbulence), low conductivity materials (e. g., hydrocarbon liquids) and surface area of the interface (e. g., pipe or hose length, and micro pore filters).
Discharge of static electricity can create severe hazards in those industries dealing with flammable substances, where a small electrical spark may ignite explosive mixtures and probability of occurrence of accident is high during exposure of high electrostatic charge densities to a flammable atmosphere.
Reducing the risk of ignition from static discharge is achieved by bonding, earthing, or substitution. Bonding is the process of connecting two or more conductive objects together by means of a conductor. Earthing involves connecting one or more conductive objects to the ground. Substitution involves replacing conductive materials with non-conductive ones.
The implementation of any or all of those methods will reduce the risks of catastrophic fires and explosions. When the discharge cannot be avoided by above means, ignition can be prevented by excluding ignitable vapour-air mixtures through inerting and mechanical ventilation. Inerting is a method of displacing the air with an inert gas to make the mixture nonflammable and mechanical ventilation can be applied to dilute the ignitable mixture well below the flammable range.
A complete process of hazard identification and assessing the associated risk is essential for prevention of fire and explosion event occurring from static electricity.
References
Technical Standards and Guidelines associated with Static Electricity
COMAH- Regulations governing the storage of hazardous chemicals in the United Kingdom.
DSEAR- Regulations governing the use of dangerous substances in the workplace.
ATEX 137- European Directive covering work places with Potentially Explosive Atmospheres.
ATEX 95- European Directive covering equipment used in potentially explosive atmospheres.
BS5958- British Standard Code of practice for control of undesirable static electricity.
ANSI Z41 SD Type II- USA Standard for Industrial Safety Footwear incorporating Static Diissipative Properties.
API RP2003- American Petroleum Institute: Protection Against Ignitions Arising out of Static, Lighting, and Stray Current.
CLC/TR 50404- Cenelec Code of practice for the avoidance of hazards due to static electricity.
EN ISO 20345- European footwear Standard incorporating details of anti-static footwear
NFPA 77- Recommended practice on Static Electricity, published by National Fire Protection Association.
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Electricity-Guidance for Plant Engineer
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