Following are the sources of static electricity on ships:
Loading and unloading
An electrostatic charge is generated within the liquid as it flows through pipelines. The amount of charge generated will depend on the ability of the liquid to conduct electricity, a property known as its electrical conductivity. liquid flowing into a cargo tank can be charged by friction within the loading pipeline and remain charged within the cargo tank. The static charge within the cargo will slowly d issipate as the difference in potential between the cargo and the tank structure equalises. This process is called charge relaxation and its speed depends upon the conductivity of the liquid. Should the cargo contain water droplets, friction occurs when these droplets settle by gravity through the liquid in the tank (assuming that the liquid has a density less than that of water). Similarly, if the liquid contains a non-dissolved gas, the liquid could become charged when the gas bubbles rise to the surface of the liquid in the tank
Injecting steam into a cargo tank during tank cleaning can cause a build up of static within the condensed water droplets.
Forced air gas freeing devices can cause a static charge to build up on the body of the equipment.
Cargo tank cleaning
A static charge will be produced when water is forced, under high pressure, through the nozzle of a tank cleaning machine. As a result, the water mist inside the cargo tanks may become charged. A charge can also build on the nozzle of the tank cleaning machine unless the machine is electrically grounded.
Objects such as ullage probes or sampling equipment may already contain an electrostatic charge prior to being lowered into a tank. Lowering and raising such equipment in and out of a cargo tank may also generate a static charge on the line.