Preventive measures against spillage of low temperature cargo in gas carrier
Liquid spills have the potential to cause damage to the ship's structure
and frostbite or chemical burns to any person contacting the liquid or
cold vapour. The liquid will evaporate to form a flammable or toxic
cloud.
The rate at which spilled liquid vapourises and forms a gas cloud
depends mainly on the properties of the liquid spilled, the air temperature, the nature of the surface it is spilled on and the area that
it spreads over.
Initially the cloud will be cold and low lying and drift down wind. It may
be visible as a white cloud, which is condensed water vapour.
Care should be taken to prevent spillage of low temperature cargo because of the hazard to personnel and the danger of brittle fracture. Remember that a flammable or toxic cloud may not be visible.
If spillage does occur, the source should first be isolated and the spilt liquid then dispersed. If these is a danger of brittle fracture, a water hose may be used both to vaporize the liquid and to keep the steel warm.
Fig: consequences of LNG spill over water
If the spillage is contained in a drip tray the contents should be covered or protected to prevent accidental contact and allowed to evaporate. Liquefied gases quickly reach equilibrium and visible boiling ceases; this quiescent liquid could be mistaken for water and carelessness could be dangerous.
Suitable drip trays are arranged beneath manifold connections to control any spillage when transferring cargo or draining lines and connections. Care should be taken to ensure that unused manifold connections are isolated and that if blanks are to be fitted the flange surface is clean and free from frost. Accidents have occurred because cargo escaped past incorrectly fitted blanks.
Suitable drip trays are arranged beneath manifold connections to control any spillage when transferring cargo or draining lines and connections. Care should be taken to ensure that unused manifold connections are isolated and that if blanks are to be fitted the flange surface is clean and free from frost. Accidents have occurred because cargo escaped past incorrectly fitted blanks.
Liquefied gas spilt onto the sea will generate large quantities of vapour by the heating effect of the water. This vapour may create a fire or health hazard, or both. Great care should be taken to avoid such spillage, especially when disconnecting cargo hoses.
Dangers from use of LNG
Due to its special characteristics handling of LNG can be dangerous for the crew, ship and the
environment.
Cryogenic damage :
First of all due to its low temperature the LNG can be dangerous for ship's crew. It can cause frost
bites (cryogenic burns). A possible spill on the deck causes brittle fracture to common ship steel.
Asphyxiation :
LNG can be harmful, because it can be inhaled by crew in case of leakage. That happens in cases
only of big concentration. LNG consist mainly of methane, which is natural product of decay
process, such as decay of garbage and compost. So small concentration is not a problem, it is
actually a product of normal human body processes. If there is a major or ongoing leak, and the
area where its happening is enclosed, the concentration is going up. In this case breathing in such
area can have some adverse, but temporary, medical effects. Individual can feel dizzy, a sensation
together with headaches and feel of tiredness. The medical effects come not from inhale of natural
gas but more from the lack of oxygen in area. Actually it symptoms of inhaling natural gas are the
same with inhaling air with little oxygen.
Greenhouse effect :
The methane is known as a strong greenhouse gas. It contributes 25 times and more to the global
warming compared to carbon dioxide (CO2) in 100 year period. And methane has at least 34
times the effect when accounted for aerosol interactions. Fortunately the natural gas has effects
only for a short time (a net lifetime of 8.4 years in the atmosphere). According to studies methane
concentration has increased 150% since 1750 and it accounts for 20% of the total radiative forcing
from all greenhouse gases excluded water. Any leakage of natural gas subsequently causes
damage to the environment.
Rapid Phase Transition :
Rapid Phase Transition (RPT) occurs when the liquid natural gas comes in touch with water (as in fig.)
This results in very quick evaporation of gas and creates an overpressure similar to an explosion.
The natural gas in this state usually has very high concentration, so it can be inflamed only when
the concentration decreases to 15%. From this aspect the phenomenon is predominately a
physical or mechanical expansion with high pressure energy. The combustion can exist only on
the edges of vapor cloud where the concentration drops to 5%-15%.
Figure. Possible outcome of LNG spill over water
Explosions and Fire :
Last, but not least natural gas leakage can be dangerous due to its inflammability. It comes earlier
than problems with breathing because 5-15% concentration is already flammable or explosive
mixture, while greater concentration is needed to cause health problems. Less than 5%
concentration causes no problem, because there is not enough gas to burn, while with more than
15% there is too little oxygen for burn. The instances with fire and explosions can be divided into
the following categories: pool fire, vapour cloud fire and explosions. The pool fire happens when
the vapors burn immediately after the release. The needed requirement is the concentration to be
in the flammable range near the leakage. Vapour cloud fire happens away from the source of
leakage, but ignited cloud fire can return to initial source of release. Despite the fact that LNG is
non-explosive it can happen in a confined area. In such a case, vapour cloud fire creates an
overpressure and destructive explosion occurs.
Fig:Analyzing and reporting from control room the high level alarm
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