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