STATIC STATIC ELECTRICITYELECTRICITY

What is static What is static electricity? electricity?

How does it is How does it is generated?generated?

Static electricityStatic 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

Static electricityStatic electricity Generated when 2 materials in contact are Generated when 2 materials in contact are

separated separated Electrical charges transferred from one to Electrical charges transferred from one to

another producing nett +ve and –ve chargesanother producing nett +ve and –ve charges Opposite charges will attracted to one Opposite charges will attracted to one

anotheranother As material separated, work done to As material separated, work done to

overcome the attractive force produce PD overcome the attractive force produce PD between materialsbetween materials

Very high PD – cause electrical breakdown of Very high PD – cause electrical breakdown of air, lead to spark produced.air, lead to spark produced.

Static electricity onboardStatic electricity onboard Generated during normal operation involving Generated during normal operation involving

friction:friction: Between 2 solids substancesBetween 2 solids substances Solid & liquid substances (friction from passage of liquid Solid & liquid substances (friction from passage of liquid

through hose)through hose) Solid substances coming together & separatedSolid substances coming together & separated Various sort of motion – person / material Various sort of motion – person / material

When sufficient / accumulated on surface, When sufficient / accumulated on surface, charged object / person come close to charged object / person come close to another which not grounded / unequally another which not grounded / unequally charged, 2 charges neutralise by jumping the charged, 2 charges neutralise by jumping the gapgap

Spark produced across this gap will ignite any Spark produced across this gap will ignite any explosiveexplosive

General hazards & problemsGeneral 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

Source of static electricity Source of static electricity Steaming cargo tankSteaming 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

Source of static electricity Source of static electricity

Pipeline flow 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)

Source of static electricity Source of static electricity Oil mixing with water 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

Source of static electricity Source of static electricity

Oil splashing / sprayingOil splashing / spraying Upon impact tank structure, oil droplets left Upon impact tank structure, oil droplets left

some ions on metal surface – bounce offsome ions on metal surface – bounce off Oil become charged by splashing / sprayingOil become charged by splashing / spraying Hazard of explosion is high both volatile / Hazard of explosion is high both volatile /

non volatile typesnon volatile types Above flash pointAbove flash point Loaded into tanks containing flammable vapourLoaded into tanks containing flammable vapour

Source of static electricity Source of static electricity

Unbonded intermediate flange 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

Source of static electricity Source of static electricity

Stray currentStray current Ship alongside at jetty may have Ship alongside at jetty may have

different electrical potential due to:different electrical potential due to: Ship / jetty are cathodically protected with Ship / jetty are cathodically protected with

impressed current systemimpressed current system Faulty electrical equipment earthed either Faulty electrical equipment earthed either

ashore / onboardashore / onboard Galvanic potential difference between Galvanic potential difference between

ship / shoreship / shore

Source of static electricity Source of static electricity

Tank slack during heavy weatherTank slack during heavy weather Effect of rubbing two metalsEffect of rubbing two metals

Release of CO2 Release of CO2 During discharge, rapid cooling took During discharge, rapid cooling took

place resulting in formation of solid CO2 place resulting in formation of solid CO2 particle, charged when impact / contact particle, charged when impact / contact with nozzlewith nozzle

Lead to sparkingLead to sparking

Condition for static charge Condition for static charge leads to explosionleads to explosion

Accumulated charge in liquid / solid Accumulated charge in liquid / solid causing electric field formed in gas causing electric field formed in gas mixturemixture

Flammable gas mixtureFlammable gas mixture Electric discharge must cause spark Electric discharge must cause spark

with sufficient intensity to ignitewith sufficient intensity to ignite

To avoid electrostatic To avoid electrostatic hazardshazards

Prevent charge accumulation / Prevent charge accumulation / generationgeneration

Prevent electrostatic dischargePrevent electrostatic discharge Prevent flammable gas mixture Prevent flammable gas mixture

conditioncondition

But, unfortunatelyBut, unfortunately

During cargo loading, electrostatic During cargo loading, electrostatic charge in oil may accumulate and charge in oil may accumulate and concentrate on oil surfaceconcentrate on oil surface

May discharge causing sparks if May discharge causing sparks if distance too close to tank structuredistance too close to tank structure

Any protrude metal will act as Any protrude metal will act as lightning conductor – earthed probelightning conductor – earthed probe

Permanent installed washing Permanent installed washing machinemachine

Steel tape ullageSteel tape ullage

Tape in contact with ullage pipeTape in contact with ullage pipe Tape not contact but operator Tape not contact but operator

stands barefoot on wet charged stands barefoot on wet charged deckdeck

Tape and operator not contact with Tape and operator not contact with tank, but ullage tape when contact tank, but ullage tape when contact with oil, is charged – sparks occur with oil, is charged – sparks occur between tape and ullage pipebetween tape and ullage pipe

Sampling Sampling

Using conductive lineUsing conductive line Sparking reduced by attached Sparking reduced by attached

sample cans to a non conductive sample cans to a non conductive lineline

Still contain charged oil and spark Still contain charged oil and spark could jump from can to mouth of could jump from can to mouth of deck openingdeck opening

Floating conductor in cargo Floating conductor in cargo tanktank

Floating object on oil surface may Floating object on oil surface may provide short circuit between oil provide short circuit between oil surface and tank structuresurface and tank structure

Tin cans, piece of timber with Tin cans, piece of timber with protruding nails / boltsprotruding nails / bolts

To prevent such dischargeTo prevent such discharge

Keep tank atmosphere non flammable by Keep tank atmosphere non flammable by using IGusing IG

Use non absorbent & non conductive ullage Use non absorbent & non conductive ullage tapetape

Used permanently installed ullaging deviceUsed permanently installed ullaging device Take ullage through sounding pipesTake ullage through sounding pipes Use sampling can made of non conductive Use sampling can made of non conductive

material with non conductive linematerial with non conductive line

Precaution to prevent / Precaution to prevent / minimise electrostatic chargeminimise electrostatic charge Steaming cargo tankSteaming cargo tank

Used fixed apparatus to introduce steamUsed fixed apparatus to introduce steam Kept minimum steam velocityKept minimum steam velocity Earthed probe should not put in tankEarthed probe should not put in tank

Loading cargoLoading cargo Initial stage – kept minimum flow rateInitial stage – kept minimum flow rate Until all splashing / surface turbulence Until all splashing / surface turbulence

ceasedceased Reduce charge generation in pipe flow – Reduce charge generation in pipe flow –

mixing of water in pipe linemixing of water in pipe line

Avoid loading both volatile and non Avoid loading both volatile and non volatile products having temperature volatile products having temperature above flash pointabove flash point

Used of antistatic additive in oilUsed of antistatic additive in oil Bonding and earthing Bonding and earthing

Hose flanges including intermediate Hose flanges including intermediate flangesflanges

All metal couplings for tank washingAll metal couplings for tank washing All metal objects onboard bonded All metal objects onboard bonded

together to ship structuretogether to ship structure

Pyrophoric iron sulphidePyrophoric iron sulphide

Formed in cargo tank due to:Formed in cargo tank due to: Presence of iron oxide (rust)Presence of iron oxide (rust) Presence of hydrogen sulphide gasPresence of hydrogen sulphide gas Lack of oxygenLack of oxygen

Use of IG increase the possibilityUse of IG increase the possibility When iron sulphide exposed to air, When iron sulphide exposed to air,

oxidise to produce iron oxide & release oxidise to produce iron oxide & release sulphide / sulphur dioxide gassulphide / sulphur dioxide gas

Generate heat until glow red – source of Generate heat until glow red – source of ignitionignition

Special precaution in portSpecial precaution in port

Avoid stray current – connection / Avoid stray current – connection / flangeflange

Mooring/towing lines, cargo Mooring/towing lines, cargo connection ready to detach in case connection ready to detach in case of fireof fire

Adjust rate of loading – avoid Adjust rate of loading – avoid generation of electrostatic chargegeneration of electrostatic charge

Special precaution at seaSpecial precaution at sea

Avoid static generation / discharge Avoid static generation / discharge during tank washingduring tank washing

Restrict use of steam for tank Restrict use of steam for tank washingwashing

Keep tanks inertedKeep tanks inerted