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How to control static electricity inside a ships tank during loading of chemicals ?

Sources of static electricity and how to eliminate ?

Loading of bulk liquid chemicals in a ships tank involved static electricity hazards. Controlling the main reasons of static electrical fields generators can eliminate potentials threats.
Splashing and spraying of static accumulators should generally be avoided, as they lead to the formation of charged mists or foams. A charged mist can be ignited even if the temperature does not reach the flash point: so splash filling and spraying are a danger even with relatively high flash point cargoes.

Filling of a ship's tank should normally be through a pipeline which ends near the bottom of the tank so that, in the early stages of loading, the liquid is gently laid on the bottom. When the rising liquid covers the pipeline outlet, turbulence in the tank is considerably reduced and fewer static charges are generated.

Safe pumping rate

The faster the liquid flows through the pipeline to the ship's tank, the higher is the electrostatic charging. To avoid excessive turbulence within a static accumulator cargo, the velocity of liquid entering a tank should be very low until the inlet is well covered . Low velocity also limits any mixing with water that might be present in the tank bottom.

After the inlet has been submerged, the flow velocity may be increased, but it should still minimise turbulence and avoid breaking the liquid surface.


Presence of water

Most static accumulators are not miscible with water. The presence of water produces two sources of static electricity. First, friction occurs at the surface of the water droplets dispersed within the cargo liquid, so far more static charges are generated than if the cargo liquid did not contain the water. Second, the charged droplets settle through the liquid and gather at an interface, producing a high voltage at the liquid surface. This process may continue even after tank filling has ceased.


Gas bubbling up through the filled tank

After loading, pipelines are often blown through using air, nitrogen or other gases. When the gas enters the tank from the bottom it will rise through the liquid in small bubbles, generating a high voltage at the surface. If it is necessary to blow through after loading a static accumulator, the amount of gas allowed to enter the ship's tank should be kept to a practical minimum.




Relaxation time downstream of filters

Micropore filters made of paper, cloth, felt, chamois or a metal grid, particularly if deep and thick, are prolific generators of electrostatic charges. Strainers such as perforated metal baskets are not. The liquid is highly charged when it leaves the filter in the loading line. For such charge to be relaxed, the liquid has to flow quietly in the pipeline for some time, before entering the ship's tank.

Practical experience has shown that 30 seconds is sufficient. Filters are usually located ashore so the transit time is adequate, but if the distance between the filter and the ship's tank is not great enough, either the flow rate should be reduced, or the pipe length increased or its diameter enlarged, or a relaxation tank should be provided in between the filter and the storage tank.


Unearthed conductors

A conductor having no electrical contact with earth can become charged and rise in voltage through induction (without physical transfer of charges) and collection (with physical transfer). An unearthed conductor floating on the surface of a charged liquid actually collects charges from it. A conductor located in a charged mist becomes charged to approximately the same voltage as the mist, even though it cannot collect any charge.

In summary, a rise in voltage is possible without charge transfer to an unearthed conductor. If a spark then jumps between the unearthed conductor and an earthed metal surface all the energy the former has accumulated flows instantly into the spark, which therefore has a higher chance of being incendive.

Avoiding the presence of unearthed conductors in ships' tanks is of fundamental importance to prevent incendive sparks, because they provide the electrode from which a spark can jump. The following are examples of unearthed conductors which might be present inside a ship's tank:


thin metal scraps, including rust: they do not float, but can be buoyed up by charged foam;

a metal coupling at the end of a non-conductive cargo hose used for filling the tank;

a metal rod or the tube on a gas sampling meter;

a metal sampling can or thermometer holder lowered on a non-conductive rope;

a tank washing machine on the end of a hose having a broken bonding cable, particularly when the hose is empty;

dropped tools falling through a tank filled with a charged mist from water washing: the mist might be invisible.


Projections and probes in tanks

Tanks are sometimes equipped with sounding pipes which extend down from underdeck towards the liquid surface. Other examples of projections and probes are high level alarms, spraying nozzles and fixed tank washing machines.

If the liquid being loaded is at a high surface voltage, an incendive brush discharge to an unbonded projection may take place. The need to avoid such a situation will have been taken into account during the design of fixed projections inside a cargo tank, and all requirements for safety as to materials of construction, earthing, insulation and static electricity generation will have been checked while the ship was being built.

It is important that any routine servicing should be performed in accordance with manufacturer's instructions, but no on board modifications to the equipment itself should be contemplated.


Gauging and sampling of tanks

Whilst loading a static accumulator cargo,- conductive objects which are not bonded to the ship's structure such as metal sampling cans, gauge tapes and thermometers should not be lowered into a tank. A period of 30 minutes should elapse after filling has stopped for the charge to be relaxed before any unbonded metallic or other conductive equipment is introduced. A metal sounding rod, suspended on a rope, will not be earthed.

The metal becomes charged when it is immersed in the charged liquid and, when it is then lifted, a metal to metal spark may jump between the rod and the rim of the tank opening, with a high probability of being incendive. If the surface voltage of the liquid is very high, it is possible to get an incendive brush discharge to the equipment when it first approaches the surface of the liquid during lowering.

Completely non-conductive equipment could in theory be used, but in practice it is difficult to ensure that such material remains non-conductive because it is habitually exposed to dirt and moisture. It is therefore better strictly to observe the waiting time in all cases.

The restriction of this waiting time may be avoided only if the gauging and sampling equipment is lowered inside a sounding pipe that extends all the way down and is connected to the bottom of the tank, because the voltage inside the sounding pipe is small. It is important to note that a shorter sounding pipe is not safe.


Washing of tanks

During tank washing, a charged mist is produced and is present throughout the space. Such mist persists for a few hours after washing has come to an end. If an unearthed conductor is lowered into the charged mist, it becomes charged to a voltage which may be high enough for an incendive spark to jump to some part of the tank structure.

The limitations on water flow rate per nozzle, per machine and per tank have been established by extensive research, and should not be exceeded. If the water contains cleaning additives or is recycled, or the washing medium is other than clean water, washing should be conducted in a non-flammable atmosphere: i.e. the tank should be made inert. The practical aspects of tank washing are therefore important to observe.


Steaming

Steam issuing from a nozzle will generate a mist of charged water droplets. Therefore steam should never be injected into a tank that may contain a flammable atmosphere.


Bonding and earthing

A spark cannot jump between two conductors which are either electrically bonded together or both earthed, because they are kept at the same voltage.

Effective bonding is achieved by connecting a metal cable between objects. The cable is sometimes permanently fixed to one conductor and bolted or clamped to the other. At the removable end, contact should be metal to metal and care should be taken to make sure paint, dirt or rust does not hamper it.

The cable should be strong enough to have good resistance to wear and tear.

Bonding and earthing cables should be inspected periodically and their resistance checked with a meter. Many hoses used in marine operations are made electrically conductive. A pair of flanges bolted together can be relied upon for being electrically continuous, as can flexible joints of metal loading arms, so bonding or jumping wires around them are not needed.

A different electrical phenomenon is experienced when a tanker is connected to a shore installation by a conductive hose or a metal loading arm. Ship, hose, dock and water form the elements of a battery and a large current can flow through the low resistance hose, even if the voltage difference between ship and shore is small. But this is not static electricity.

When the hose is disconnected the current is suddenly interrupted and an electrical arc can be formed between the flanges. There is a risk of igniting a flammable atmosphere existing at the manifold. To prevent such a hazard, the ship has to be insulated from the shore pipeline by means of an insulating flange or a length of non-conductive hose. This keeps the circuit of the battery open, and prevents a spark. However, there is no need to connect a tanker to the dock by a bonding cable, since both are earthed by the water.



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

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


  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. Precautions against static electricity


  21. Handling precautions for nitrogen from shore station


  22. Cargo tank entry safety precautions


  23. Ship to ship transfer safety precautions


  24. 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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