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Fire hazards and precautions against statistic electricity in liquefied gas carrier
Static electricity can cause sparks capable of ignition a flammable gas. Some routine operations can cause electrostatic charging, and precautions to minimize the hazard are given below.
Putting on Anti-electrostatic Clothes and Shoes
(1) When work is carried out on the upper deck or in areas where inflammable gas is likely to exist, anti-electrostatic clothes shall be put on. Pure cotton or wool socks and underwear are preferable, but at least those that do not generate sparks shall be used.
(2) Such clothes shall not be put on or off in areas where inflammable gas is likely to exist, such as on deck near tanks and compressor room.
(3) To avoid the collection of static electricity on a human body, conductive shoes shall be used to discharge static electricity from a human body to the hull. But conductive shoes become ineffective on the dry or newly coated deck.
Use of Electrostatic Discharge Plate
(1) Everybody who is coming out of the accommodation on to the upper deck shall touch a Electrostatic Discharge Plate provided with near the entrance without fail to discharge static electricity charged on his body.
(2) If the handle of a door is grounded, no gloves shall be used for going in and out at the door.
The extent to which materials, whether solid, liquid or vapour, can generate and retain a static charge depends on their electrical resistance. If the resistance is high, a charge can be built up. It is also possible for a charge to build up on materials in a system with low resistance (e.g. metals) that are electrically insulated from each other. The cargo system of a gas carrier is electrically bonded to the ship’s hull to prevent charge build-up. It is important that such bonding connections are maintained in an efficient condition.
Hoses are normally bonded to their flanges by the metal reinforcement, and thus provide a continuous path to earth though the ship’s manifold and the hull. If an insulating flange is inserted at the shore manifold, the intermediate flanges and metal reinforcement will still provide that continuous path.
A significant static electrical charge can be caused by high fluid velocities, change from liquid to vapour / liquid droplet flow, small particles carried in a vapour steam and by impingement.
In an un-bonded system static electricity could be generated by:
(1) Flow of liquid through pipes
(2) Flow of liquid / vapour mixtures through spray nozzle
(3) Flow of a vapour containing particles (e.g. rust) through piping.
The risk of causing ignition by static electricity is reduced if the system is correctly bonded or if flammable mixtures are avoided.
High velocity water droplets in a jet of steam may become charged in passing through a nozzle and could produce a charged mist. For this reason steam should not be injected into a tank, compartment or piping system which contains a flammable mixture. Steam may sometimes be used to provide external heat to defrost or dry a bonded system containing flammable liquid or vapour, but only if the surrounding atmosphere is non-flammable.
When liquid carbon dioxide under pressure is released at high velocity, rapid evaporation causes cooling and particles of solid carbon dioxide may form. The solid particles in the cloud of CO2 may become electrostatically charged. For this reason carbon dioxide should not be released into spaces containing a flammable mixture.
Ship / Shore Insulating, Earthing and Bonding
In order to provide protection against static electrical discharge at the manifold when connecting and disconnecting cargo hose strings and metal arms, the terminal operator should ensure that they are fitted with an insulating flange or a single length of non-conducting hose, to create electrical discontinuity between the ship and shore. All metal on the seaward side of the insulating section should be electrically continuous to the ship. And that on the landward side should be electrically continuous to the jetty earthing system.
The insulating flange or single length of non-conducting hose must not be short-circuited by contact with external metal; for example, an exposed metallic flange on the seaward side of the insulating flange or hose length should not make contact with the jetty structure either directly though hose handling equipment. Simply switching off a cathodic protection system is not a substitute for the installation of an insulating flange or a length of non-conducting hose. Cargo hoses with internal bonding between the end flanges should be checked for electrical continuity before they are taken into service and periodically thereafter.
A ship / shore bonding cable is not effective as a safety device and may even be dangerous. A ship / shore bonding cable should therefore not be used.
Note: although the potential dangers of using a ship/shore bonding cable are widely recognized, attention is drawn to the fact that some national and local regulations may still require a bonding cable to be connected. If a bonding cable is demanded, it should first be visually inspected to see that it is mechanically sound. The connection point for the cable should be well clear of the manifold area. There should always be a switch on the jetty in series with the bonding cable and of a type suitable for use in a hazardous area.
It is important to ensure that the switch is always in the “off” position before connecting or disconnecting the cable. Only when the cable is properly fixed and in good contact with the ship should the switch be closed. The cable should be attached before the cargo hoses are connected and removed only after the hoses have been disconnected.
Reactivity of liquefied gas cargo
- Safety checklist for gas carrier
- Cargo Machinery Room Precautions
- Fire hazards and precautions - Sources of Ignition in Liquefied Gas Carrier
- Matters that require attention to onboard work
- Fire hazards and precautions - Atmosphere Control For Gas Carrier
Toxicity and associated health hazards in liquefied Gas Carrier
Liquefied gases - How to remove all cargo liquid from tanks
External links :
International maritime organization
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