static electricity. what is static electricity? how does it is generated?

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  • What is static electricity? How does it is generated?
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  • Static electricity Electricity in which the electrons and ions do not move is called static electricity Created by rubbing two objects together, allowing electrons to switch from one object to another Static electricity builds up and then leaps in a spark from one object to another
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  • Static electricity Generated when 2 materials in contact are separated Electrical charges transferred from one to another producing nett +ve and ve charges Opposite charges will attracted to one another As material separated, work done to overcome the attractive force produce PD between materials Very high PD cause electrical breakdown of air, lead to spark produced.
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  • Static electricity onboard Generated during normal operation involving friction: Between 2 solids substances Solid & liquid substances (friction from passage of liquid through hose) Solid substances coming together & separated Various sort of motion person / material When sufficient / accumulated on surface, charged object / person come close to another which not grounded / unequally charged, 2 charges neutralise by jumping the gap Spark produced across this gap will ignite any explosive
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  • General hazards & problems Possible ignition of flammable vapours or powders production of unexpected shocks in humans that result in injury cause industrial handling problems: Adhesion / repulsion of paper in printing industry Damage to delicate integrated circuits Blocking of powders and dusts in pipes fire or explosion for: Combustible substances in flammable range Static charge built up and discharge to a neighbour, usually earthed object ignite the surrounding flammable mixture
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  • Source of static electricity Steaming cargo tank when steam passes through a pipe charge separation takes place between steam droplets and side of pipe Can be avoided by bonding themto the ship structure However, charge on the water droplets will accumulate in tanks Total charge may reach a proportion that sufficient to produce sparks Condition is worst if tank atmosphere is flammable
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  • Source of static electricity Pipeline flow As oil flows in a pipe, some ions in the oil near to pipe surface will left behind Oil and pipe line become electrostatically charged but in opposite Problem reduced with oils having high electric conductivity - electric charge easily recombine to pipe surface / earthed Hazard exists when low electrical conductive oils, such as gasoline, kerosene, naphtha, jet fuel, diesel oil, gas oil, benzene, LO etc. Also known as 'white' oils Charge separation is poor but charge recombination is even poorer Rate of oil flows and presence of filters in cargo lines contribute to build up of static charge Charge dissipate to bottom and side of the tank with long time process (approx 30 minutes)
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  • Source of static electricity Oil mixing with water During loading, any water lying at tank bottom or cargo lines become entrained in oil and spread out. Charge separation occur as water pass through oil and become oppositely charged Degree of charging depends on: rate of flow conductivity of oil Charging continue until all water droplet movements ceased If air is blown into tank, may redistributed charge throughout the tank Increase total residual charge in the oil
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  • Source of static electricity Oil splashing / spraying Upon impact tank structure, oil droplets left some ions on metal surface bounce off Oil become charged by splashing / spraying Hazard of explosion is high both volatile / non volatile types Above flash point Loaded into tanks containing flammable vapour
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  • Source of static electricity Unbonded intermediate flange When oil flows in insulated rubber hose, static charge accumulated on metal flanges hose connected Possibility of accumulation at flange ends usually connected to ship / shore metal flange Charge accumulation at intermediate flange unable to recombine with any charge in oil flows inside the hose
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  • Source of static electricity Stray current Ship alongside at jetty may have different electrical potential due to: Ship / jetty are cathodically protected with impressed current system Faulty electrical equipment earthed either ashore / onboard Galvanic potential difference between ship / shore
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  • Source of static electricity Tank slack during heavy weather Effect of rubbing two metals Release of CO2 During discharge, rapid cooling took place resulting in formation of solid CO2 particle, charged when impact / contact with nozzle Lead to sparking
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  • Condition for static charge leads to explosion Accumulated charge in liquid / solid causing electric field formed in gas mixture Flammable gas mixture Electric discharge must cause spark with sufficient intensity to ignite
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  • To avoid electrostatic hazards Prevent charge accumulation / generation Prevent electrostatic discharge Prevent flammable gas mixture condition
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  • But, unfortunately During cargo loading, electrostatic charge in oil may accumulate and concentrate on oil surface May discharge causing sparks if distance too close to tank structure Any protrude metal will act as lightning conductor earthed probe
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  • Permanent installed washing machine
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  • Steel tape ullage Tape in contact with ullage pipe Tape not contact but operator stands barefoot on wet charged deck Tape and operator not contact with tank, but ullage tape when contact with oil, is charged sparks occur between tape and ullage pipe
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  • Sampling Using conductive line Sparking reduced by attached sample cans to a non conductive line Still contain charged oil and spark could jump from can to mouth of deck opening
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  • Floating conductor in cargo tank Floating object on oil surface may provide short circuit between oil surface and tank structure Tin cans, piece of timber with protruding nails / bolts
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  • To prevent such discharge Keep tank atmosphere non flammable by using IG Use non absorbent & non conductive ullage tape Used permanently installed ullaging device Take ullage through sounding pipes Use sampling can made of non conductive material with non conductive line
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  • Precaution to prevent / minimise electrostatic charge Steaming cargo tank Used fixed apparatus to introduce steam Kept minimum steam velocity Earthed probe should not put in tank Loading cargo Initial stage kept minimum flow rate Until all splashing / surface turbulence ceased Reduce charge generation in pipe flow mixing of water in pipe line
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  • Avoid loading both volatile and non volatile products having temperature above flash point Used of antistatic additive in oil Bonding and earthing Hose flanges including intermediate flanges All metal couplings for tank washing All metal objects onboard bonded together to ship structure
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  • Pyrophoric iron sulphide Formed in cargo tank due to: Presence of iron oxide (rust) Presence of hydrogen sulphide gas Lack of oxygen Use of IG increase the possibility When iron sulphide exposed to air, oxidise to produce iron oxide & release sulphide / sulphur dioxide gas Generate heat until glow red source of ignition
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  • Special precaution in port Avoid stray current connection / flange Mooring/towing lines, cargo connection ready to detach in case of fire Adjust rate of loading avoid generation of electrostatic charge
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  • Special precaution at sea Avoid static generation / discharge during tank washing Restrict use of steam for tank washing Keep tanks inerted
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