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STATIC ELECTRICITY

STATIC ELECTRICITY

 

 

Static electricity is an imbalance of electric charges within or on the surface of a material. Electrical charges are generated when two objects or materials that have been in contact with each other are separated.

Static can occur in a number of plant operations, including Liquid flowing through pipes, splash filling tanks, drums, spraying, moving of dry solid powders along with chutes , conveyors etc. It can also happen when materials rub against each other such as drive belts and even people's clothing. If the charge does not have a path to earth it accumulates, since this charge cannot go anywhere, it is called ‘static electricity’. Eventually, the accumulated charge becomes enough to create a spark which transfers the charge from the insulating material to earth.

For static electricity to cause a fire there needs to be a means of charge generation, accumulation of charge, enough charge to create a spark and a flammable atmosphere.

The ability of a flowing liquid to generate a charge depends on the velocity and the electrical conductivity of the liquid. Water is a good conductor so normal pipe flow doesn’t generate charge. Hydrocarbon oils are poor conductors and steps are taken to minimize flow velocities to less than 1meter/second. Oil/water mixtures are worse as the droplets of oil accumulate charge as they bump into each other as the bulk liquid flows along.

Incident: During the manufacture of resin, any water created during the reaction was routinely drained off from the bottom of the reaction vessel. Typically, 50 liters of water was drained off from the kerosene /resin product layer before transfer to the next step in the process. The water was drained off into a 20-liter bucket, usually hung from the drain valve by its handle. The operator present observed the material being drained into the bucket, first water then a Kerosene/water emulsion and stopped when the product layer came through. The operator had successfully drained the first two buckets of loads of water and emulsion from the vessel. Partway through the third bucket, he noticed flames in the bucket, but since the bucket was hanging from the valve handle, he could close the drain valve. Luckily the operator was not injured, he was able to raise the alarm. The remaining content of the vessel emptied onto the floor creating a pool fire, which took an hour to extinguish and resulted in R4.5 million worth of damage to the plant. Static electricity was identified as the most likely source of ignition.

How can we prevent static electricity? Keep liquid velocities low, avoid using rubber, plastic, glass, and other electrical insulators, if dispensing liquids into open containers, ensure earth continuity between source and container, wear conductive footwear and possibly antistatic clothing to prevent charge accumulation on people.

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