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How static electricity is generated onboard a chemical tanker?

Static electricity is generated by friction that occurs between different materials during relative motion. Electrostatic charges can then accumulate in materials which are poor conductors of electricity or which are good conductors but are insulated. If two such bodies with accumulated static electricity charges are brought close together, and if the difference of potential is great enough, the accumulated charge will jump between them.

Anyone who has received a mild shock when touching a doorknob after walking across a carpeted room has experienced such a phenomenon. In such cases the individual has become charged by friction between the carpet and the soles of the shoes and the potential accumulated in the body is discharged to the knob. This, though annoying, is harmless. But if a strong discharge of static occurs as a spark where a flammable atmosphere is present, there is a risk of igniting the atmosphere.

chemical tanker navigation at sea
The primary concern about static electricity is the possibility of generating an incendive spark within a flammable atmosphere. Inerting a tank can prevent the existence of a flammable gas mixture so that no hazard will exist.

Static electricity can be generated due to the passage of a liquid through a hose or pipeline, and turbulence within a tank. In normal circumstances the charge generated is released instantaneously to earth (the ship's structure) because the liquid conducts it, and design features of cargo tank internals will avoid its build up. The problem is greater on very large tankers equipped with large individual tanks than it is on the smaller size of chemical carriers with extensive subdivision and multiple cargo tanks.

Problems from static electricity are most likely to arise when loading cargoes known as static accumulators, often highly refined petroleum products. It is important, quite apart from cargo quality requirements, to make sure that lines which have been flushed with water have been thoroughly drained and that the bottom of the tank is dry before starting to load a static accumulator cargo.

At the initial stage of the loading operation, it is important that the loading rate is limited. Until the bottom longitudinals and tank suction are covered, loading speed of the liquid in the pipeline should not exceed a linear velocity of I metre per second (m/s), which corresponds to the following loading rates:

Pipeline diameter Loading rate

200 mm 115 cubic metres per hour
150 mm 65 cubic metres per hour
loomm 30 cubic metres per hour

Thereafter, loading may be increased to a maximum pipeline speed of 7 m/s. Experience indicates that hazardous potentials in respect of static electricity do not occur if the velocity is below 7 m/s. However, where well documented experience demonstrates that higher velocities have been safely used, an appropriately higher limit than 7 m/s may be employed.

We have summarized below the process of static electricity may happen in a chemical tanker in 5 different steps :

1. An electrostatic charge is generated in the liquid as it flows turbulently through the loading pipeline into the ship'~tank.
In most liquids the charge is released instantaneously to earth* because the liquid conducts it.

2. But in some cases, the charge is accumulated in the liquid because the liquid has a low electrical conductivity. Such liquids are called static accumulators, and are generally found among more highly refined products. An electrostatic field is formed inside the tank.

3. A non-bonded projecting object, or something introduced into the tank, can become a potential electrode or spark promoter, collecting the charge from the liquid.

4. When close enough to an earth* the spark promoter instantaneously releases its charge in a spark through the atmosphere of the tank.

5. Such a spark will almost certainly have enough energy to ignite a flammable vapour. In chemical tanker operations, a flammable atmosphere may be unavoidable.

Following detail pages explain all liquid chemical hazards & precautionary measures while carrying at sea.

  1. Controlling sources of static electricity onboard chemical tanker

  2. Accumulation of electrostatic field, charge relaxation and surface voltage inside a ships tank

  3. Toxicology and associated hazards onboard chemical tankers

  4. Hazards of vapour given off by a flammable liquid while carrying at sea

  5. Reactivity of various noxious liquid chemicals

  6. Most corrosive chemicals carried onboard chemical tankers

  7. Posoning hazards & first aid treatment

  8. What is putrefaction process of liquid chemicals ?

  9. Specific gravity,Vapour pressure and boiling point,Electrostatic charging & measuring Viscosity
  10. General precautions onboard chemical tankers

  11. Mooring precautions onboard chemical tankers

  12. Berth precautions onboard chemical tankers

  13. Cold weather countermeasures, avoiding electric storms

  14. Restriction on using radio equipments and other mobile devices in cargo working areas

  15. Handling precautions for carcinogens or cyanide-like substances

  16. Handling precautions for Benzene & Methanol

  17. Securing cargo tank lids and required safety precautions

  18. Means of access (gangways or accommodation ladders) safety precautions

  19. Preparations for hot work and safety precautions

  20. Safe method of gas freeing after a tank cleaning onboard chemical tankers

  21. Precautions against static electricity

  22. Handling precautions for nitrogen from shore station

  23. Cargo tank entry safety precautions

  24. Ship to ship transfer safety precautions

  25. How to deal with chemical fire onboard ?

Following reference publications provide useful guidance and international regulations for carrying hazardous chemicals at sea.

Our detail pages contain somewhat larger lists of resources where you may find more useful information.

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