Monday, March 2, 2009

Static Electricity - 5


Fueling Operations
The flowing movement of flammable liquids like gasoline inside a pipe can build up static electricity. Non-polar liquids such as paraffin, gasoline, toluene, xylene, diesel, kerosene and light crude oils exhibit significant ability for charge accumulation and charge retention during high velocity flow. Static electricity can discharge into a fuel vapor. When the electrostatic discharge energy is high enough, it can ignite a fuel vapor and air mixture. Different fuels have different flammable limits and require different levels of electrostatic discharge energy to ignite.

Electrostatic discharge while fueling with gasoline is a present danger at gas stations. Fires have also been started at airports while refueling aircraft with kerosene. New grounding technologies, the use of conducting materials, and the addition of anti-static additives help to prevent or safely dissipate the build up of static electricity. The flowing movement of gases in pipes alone creates little, if any, static electricity. It is envisaged that a charge generation mechanism will only occur when solid particles or liquid droplets are carried in the gas stream.

Mobile Phones and Gasoline Pumps
Although there have been numerous media reports and posted warnings at gasoline pumps about the risk of fire caused by mobile phones, there has not been a confirmed case of an electrical discharge from a mobile phone ever causing a fire or explosion among gasoline fumes. To date, it is simply an urban legend. This legend was further investigated on an episode of Mythbusters (and also on Brainiac), where the protagonists tried to ignite gasoline using a cell phone. The show showed educational and very shocking footage of how most gas pump fires start. In almost all cases, the fire is caused by the person pumping the gas re-entering the car after the fuel has begun to fill the tank, and then step out to take the pump nozzle out. When they grab the pump nozzle, the static discharge occurs from the built up of static electricity on the person, usually from friction that occurred inside the car between the carpet or seat and said person. This discharge can cause the ignition of the highly explosive gasoline vapor by the gas tank opening. This possible fire scenario has led many gas stations to remove the automatic locking mechanism on the gas pump nozzles that were designed to make it easier to fill up an empty tank, as this mechanism also allows a person to step away from the automobile during filling.

Static Discharge in Space Exploration
Due to the extremely low humidity in extraterrestrial environments, very large static charges can accumulate; causing a major hazard for the complex electronics used in space exploration vehicles. Static electricity is thought to be a particular hazard for astronauts on planned missions to the Moon and Mars. Walking over the extremely dry terrain could cause them to accumulate a significant amount of charge; reaching out to open the airlock on their return could cause a large static discharge, potentially damaging sensitive electronics.

Ozone cracking
A static discharge in the presence of air or oxygen can create ozone. Ozone can attack rubber parts. Many elastomers are sensitive to ozone cracking. Exposure to ozone creates deep penetrative cracks in critical components like gaskets and O-rings. Fuel lines are also susceptible to the problem unless preventative action is taken. Preventative measures include adding anti-ozonants to the rubber mix, or using an ozone-resistant elastomer. Fires from cracked fuel lines have been a problem on vehicles, especially in the engine compartments where ozone can be produced by electrical equipment.

Applications of Static Electricity
Static electricity is commonly used in xerography, air filters (particularly electrostatic precipitators), automotive paints, photocopiers, paint sprayers, theaters, flooring in operating theaters, powder testing, printers, and aircraft refueling.


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