Transfer of Fluids and Dry Bulk
e.g. - lighting is example of large electrostatic discharge → that can cause harm and damage. However - discharge does not need to be large to be dangerous
Common dangerous scenario → when dry bulk flows through a hose or pipe.
- friction against inner walls → can lead to electrostatic charging of hose and substance
- at the outlet → opposite charges between hose and substance can cause sparking
- if substance is combustible → such as fuel → spark can start fire or even explosion. BOOM!
- may come as surprise that many seemingly safe substances → such as flour mixed with plenty of air → can cause explosion when ignited by spark
Due to danger of electrostatic charging + discharge in presence of combustible substances, preventing measures such as grounding or earthing are often carefully followed
Air Flow Across Surface of Planes and Vehicles
- env. with low humidity → moving vehicle can acquire static charges → due to friction between vehicle body and the air
- sometimes when alighting passengers touch vehicles outer metal body → they get non fatal but painful shock due to a small electrical discharge.
- small charge is dangerous → for trucks carrying volatile and flammable substances.
- danger is reduced to some extent → by using modern tyres → containing carbon and embedded metal mesh → which help to dissipate charges to ground
airplane → acquires static charges → due to friction with air, ice crystal, and other particles in air.
- however this build up of static charges on plan → will only stay on the outer metallic surface.
- this is because → charges repel each other → and the furthest they can get away from each other is outermost surface
even if airplane → struck by lightning → charges from lighting → will also only stay on outermost surface
- hence people and equipment inside plane → not affected by lightning
- charges on planes surface → subsequently discharged → to the air → via one of the many discharge rods.
for mid air refuelling between tanker aircraft and receiving aircraft → proper discharge of static charges becomes very imp. !
Spray Painting
If paint is charges +ve → paint droplets will repel each other
- results in more uniform mist
usually → car part being painted → is metallic and grounded
- when positive droplets approach part to be painted → free electrons in metals → attracted to surface. attraction between paint and surface → helps droplets → to land on targeted part → instead of drifting somewhere else.
→ compared to uncharged paint → there is less paint wasted → and resulting coat is more uniform.
- though electrostatic charges are useful in this case → they may also cause sparking and turn mist into fire spray
Electrostatic Precipitator
incineration plants, power stations, chemical plants, factories → often produce → waste gasses → containing particulate pollutants → in the form of smoke, dust, chemical droplets.
some pharmaceutical facilities + hospitals → also generate exhaust gasses → containing harmful bacteria and fungi → that must be removed before gasses are released into atmosphere
- removal can be accomplished effectively → with electrostatic precipitator.
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Sample
suggest how charge may cause (fuel) to ignite
- when there is a sudden discharge of excess charges in the fuel, spark may be produced and could ignite the fire
explain how connecting wire from aircraft to ground prevents build up of positive charge on aircraft
- wire provides conducting path → of low resistance → for electrons to flow from ground to aircraft → discharging the excess positive charge
it is important to say the word “sudden” discharge else it will be marked as incorrect