STATIC ELECTRICITY
What is static electricity?
How does it is generated?
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
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.
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
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
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
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)
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
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
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
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
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
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
To avoid electrostatic hazards
 Prevent charge accumulation /
generation
 Prevent electrostatic discharge
 Prevent flammable gas mixture condition
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
Permanent installed washing machine
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
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
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
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
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
 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
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
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
Special precaution at sea
 Avoid static generation / discharge
during tank washing
 Restrict use of steam for tank washing
 Keep tanks inerted