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Generation of electrostatic charge inside a ship's tank during loading of chemicals

Electrostatic charge accumulation inside a ship's tank
The liquid flowing into the tank can be charged by friction with the loading pipeline and within the mass of the liquid, particularly if it is passed through a micropore filter, where the contact surface is large. If the liquid is allowed to fall freely into the tank (splash filling), friction with the air through which it falls adds further charge to it.

When the liquid is not miscible with water but contains water droplets, friction occurs when those droplets settle by gravity through the liquid in the tank. Similarly, if the liquid contains an undissolved gas, the liquid is charged when the gas bubbles rise through it. In order of importance, the principal charging mechanisms during filling of ships' cargo tanks are line filtering, splash filling and the settling of water droplets.

When a charged liquid flows into a ship's tank, it attracts a charge of the same size as, but of opposite polarity to, the inner surface of the tank wall. Simultaneously a charge equal to that in the liquid (in magnitude and polarity) is repelled to the external surface of the tank wall, where it is immediately neutralised because the ship is earthed through the water.

Inside the tank a difference in voltage exists between the charge on the wall and that in the liquid. The voltage at the earthed wall is zero. The voltage in the liquid increases with the distance from the wall. There is a voltage distribution in the tank space, called an electrostatic field. In time, the charge in the liquid migrates to the wall of the tank, where it combines with the charge of the opposite sign on the inner surface, the electrostatic field decreases and ultimately disappears. This process is called charge relaxation. Its speed depends upon the conductivity of the liquid.

When water droplets or other charged particles (impurities) settle by gravity through the liquid in the ship's tank, a vertical electrical current is established and a high voltage may result at the surface of the liquid, called surface voltage.

Charge accumulation and relaxation in liquids

In the mass of the liquid, charge generation competes with charge relaxation: if the former is faster, charge is accumulated. The higher the conductivity of a liquid, the faster it relaxes electrostatic charges. Electrical conductivity is a property of a given liquid. It is measured in picosiemens per metre.

Charge accumulation does not occur in liquids having conductivity well above 10 picosiemens per metre. Such liquids are called non-accumulators. At conductivity below 10 picosiemens per metre, however, the accumulation of charge may be significant. Liquids of low conductivIty are called static accumulators. For safety, the border is conventionally put at 50 picosiemens per metre.

Charged foam, generated when splash filling some liquids, may retain its charge much longer than the bulk liquid, because the thin film in the foam bubbles provides only a very narrow path for the flow of the charge relaxation.

Generation of charged mists

Steam issuing from a nozzle can form a charged cloud of water droplets. A charged mist is also formed during the washing of tanks with high velocity water jets. Strong friction takes place at the nozzle, along the jet and by impact against the tank wall. Mists can remain charged much longer than bulk liquids: relaxation occurs only as fast as the droplets agglomerate and settle, since air is practically a non-conductor. Such high voltages can be produced that sparks can occur even in air.

Potential electrodes for sparks to jump from When an insulated or unearthed electrode is immersed into an electrostatic field, it becomes charged through the same mechanism described for the tank wall, but the charge has no path to earth. A spark can then jump from the electrode to the tank wall. If the voltage is sufficient and if the atmosphere is flammable, ignition will occur. Examples of such objects are a metal sampling can lowered by a rope, or a thin metal scrap buoyed by foam. If the voltage is big enough, the same process can take place even if the object is a non-metallic solid. At high surface voltages a spark can jump to the liquid surface itself. This is called a brush discharge.

Long slugs of water produced by the high capacity washing machines once used in VLCCs are thought to have caused incendive discharges to tank structures in past accidents.

Related Info:

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

  1. Precautions against static electricity

  2. Toxicology and associated hazards onboard chemical tankers

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

  4. Reactivity of various noxious liquid chemicals

  5. Most corrosive chemicals carried onboard chemical tankers

  6. Posoning hazards & first aid treatment

  7. What is putrefaction process of liquid chemicals ?

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

  10. Mooring precautions onboard chemical tankers

  11. Berth precautions onboard chemical tankers

  12. Cold weather countermeasures, avoiding electric storms

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

  14. Handling precautions for carcinogens or cyanide-like substances

  15. Handling precautions for Benzene & Methanol

  16. Securing cargo tank lids and required safety precautions

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

  18. Preparations for hot work and safety precautions

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

  20. Handling precautions for nitrogen from shore station

  21. Cargo tank entry safety precautions

  22. Ship to ship transfer safety precautions

  23. How to deal with chemical fire onboard ?

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

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Risk with noxious liquid cargo contact

The biggest risk of a chemical cargo spill

Chemical spill via Annex II overboard line

Cargo hose disconnection - Personal Safety on Chemical Tankers

Cargo handling equipments for handling noxious liquid substances in bulk

Cargo handling safe practices onboard modern chemical tankers

Product information required for various chemical cargo prior loading

Restriction on discharge of cargo residues into sea from chemical tankers

Static electricity -How they generate & required safety precautions

Chemical handling suitable equipments for operational personnel onboard

Ship inspection at foreign ports -An important guideline

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