What are the Major causes of static electricity build up during liquid cargo operations ?

OIL FLOW

Flow in the pipeline is turbulent, this turbulence scoops up an inner layer of charged molecules and distributes it throughout the bulk of the petroleum in the pipe,whilst leaving the outer layer at the interface with the pipe wall. Thus the charge layers are seperated and the petroleum in and emerging from the pipeline becomes charged. The faster the flow in the pipe the greater the turbulence and the greater the charging. Due to the turbulence if there is build up of charge difference-voltage then spark may be produced due to static electricity

AIR FLOW

The passage of air will not generate static electricity, but if air is passed into the bottom of the tank and then releases water to the surface with it present in the oil in very small quantity the subsequent settling of water back through a low conductivity distillate can result in powerful electro-static charging which will persist until the disturbed water has settled.

PIPELINES

Charged petroleum flows through Jump Hoses – rubber flexible pipeline at initial/final stages of travel. The charge present in the liquid is picked up by the metal flanges which are normally in contact with the liquid therefore a dangerous voltage may accumulate on the exterior of the flange.

WATER IN PETROLEUM LIQUID

When water droplets settle through a depth of petroleum in a tank then it causes seperation of the two charge layers.such charging will continue throughout the period of water settlement and therefore may persist long after pumping into the tank has stopped.

TANK CLEANING, GAS FREEING

Water droplets issuing at high velocity with a jet of wet steam can become charged by contact with the nozzle through which the jet is issuing, this gives rise to a charged mist in the tank any unbonded conductor in the tank may accumulate electrostatic charge by settlement of the mist upon it.

LINE DISPLACEMENT 

Uses of compressed air or inert gas are prohibited in order to clear the pipelines since it may produce flammable vapour unless appropriate precautions are taken. This again may cause water at the bottom of the tank as turbulent. Normal practise is to purge by nitrogen gas. 

DISCHARGING 

The initial loading rate is kept minimum until the shore readiness is received. This is to avoid discharging distillate oils, which may produce flammable vapour.