1. STATIC ELECTRICITY

2. What is static electricity?
How does it is generated?

3. 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

4. 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.

5. 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

6. 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

7. 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

9. 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)

12. 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

14. 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

16. 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

18. 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

22. 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

23. 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

24. To avoid electrostatic hazards
Prevent charge accumulation / generation
Prevent electrostatic discharge
Prevent flammable gas mixture condition

25. 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

26. Permanent installed washing machine

27. 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

31. 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

33. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. Special precaution at sea
Avoid static generation / discharge during tank washing
Restrict use of steam for tank washing
Keep tanks inerted