Effective control of static electricity through grounding and bonding
Static electricity or the build up of electrostatic charge is present all around us. In everyday life, a static spark is seen as a nuisance: in a flammable atmosphere, its effect can be catastrophic. Many plant fires and personnel injuries can be directly linked to a static spark igniting a vapor, gas or dust atmosphere. There are, however, various protective measures that can be adopted across industry to control this ever-present threat to people, plant and processes.
When implementing safety measures in potentially explosive atmospheres, there are many issues to consider. Eliminating potential ignition sources is the best starting point, both in terms of good engineering design and general operating procedures. However, in any type of flammable atmosphere there may be hidden dangers present, in the form of "isolated conductors". These are conductive objects which are either inherently or accidentally insulated from earth, so as to prevent any static electricity generated from safely dissipating, thus resulting in accumulation of charge on the object. These isolated conductors include metal flanges, fittings or valves in piping systems; portable drums, containers or vessels; tanker trucks, rail cars and even people I Isolated conductors are probably the most likely source of static ignition incidents in industry.
To understand the extent of the danger and how it may be controlled, the fundamentals of static electricity, and how it is manifested must be considered. In any industrial process where there is movement, the coming together and separation of materials will generate static. This could be liquid flowing through a pipe, powder dropping down a chute, a mixing process, or a person walking across a floor. The extent of the charge generation current is usually very low, typically no greater than 0.1 mA. If the object or piece of plant is in good enough contact with earth, this charge will be lost as it is generated. However, if the object is insulated from earth, the charge will start to accumulate.
Paints, coatings, gaskets, seals and other non-conductive materials can all be sufficiently insulating to prevent safe static dissipation. Static charge can quickly build up to a very high potential, sometimes with voltages in excess of 30kV. Depending on the capacitance of the object, this may result in significant levels of energy available for discharge, well above the minimum ignition energy (MIE) of the surrounding flammable atmosphere.
Typical MIEs vary according to whether the flammable atmosphere comprises vapor, dust or gas, but many commonly used solvents have MIEs of well below 1 millijoule (see Tables A & B). If the isolated conductor then comes into proximity with another object at a lower potential, much of this energy could be released in the form of an incendiary spark. Of course, in order for an ignition of the flammable atmosphere to occur, there would also need to be a suitable concentration of fuel (vapor, dust or gas) in the air; but for the purposes of safe plant design, the very fact that there is an identified flammable atmosphere should suggest that ignition is possible or likely.
A more extensive version of this article, including specific applications information is available through the Newson-Gale website. Request a copy of the “Applications & Bonding Applications” brochure!