User:Milton Beychok/Sandbox

(CC) Diagram: Milton Beychok
Diagram of the flammable range of methane( the predominant component of LNG vapor).

In its liquid state, LNG is neither flammable nor explosive. For LNG to burn, it must first vaporize, mix with air in the proper proportions (the flammable range is 5 to 15 volume %), and then be ignited.

When LNG is released into the atmosphere because of a leak, spill or any other cause (on land or at sea), it immediately begins to vaporize by absorbing heat from the ground or the sea water. At the usual ambient air temperature of about 15 to 40 °C, natural gas is much lighter than air. However, the natural gas vapor formed when LNG vaporizes is very cold (i.e., −160 °C) and therefore much denser than the ambient air, which means that the initial vapor will remain at ground or sea level. The vapor will begin mixing with air and water moisture in the air will be condensed to form a visible vapor cloud. As the cloud is initially formed, it contains too much natural gas to be flammable. It will linger near ground level until further heat is absorbed and the natural gas rises and mixes with more air. As that occurs, the concentration of natural gas within the cloud will begin to decrease. At some point, the natural gas concentration in the cloud will decrease to where the gas is in the flammable range and becomes ignitable. If an ignition sources is present at that point, only that part of the cloud which is within the flammable range will burn. The vapor cloud will not explode unless it is confined. Any part of the cloud that enters a building or becomes confined in a congested area will become explosive if it encounters an ignition source.

There are possible hazards associated with LNG other than fire or explosions:

• Rapid Phase Transition: The sudden vaporization or phase transition from liquid to vapor that has occurred upon occasion when LNG has been spilled into water has caused a physical explosion. No injuries have occurred from an RPT of LNG but equipment has been damaged. The explosive overpressures caused by RPTs have not been well measured as yet, but indications are that the overpressures have not been high enough to cause personnel injury. *
• Asphyxiation: For human death to occur from asphyxiation, the LNG vapors must reduce the normal oxygen concentration in the air (about 21 volume %) to less than 6 volume %. This would occur when the concentration of LNG vapors in the air is about 71 volume %. Breathing is impaired when the oxygen level in the air is reduced to less than 15 volume % and vomiting occurs when the oxygen level is below 10 volume %, which correspond to the concentration of LNG vapors in the air being about 28 and 52 volume %respectively.
• Freeze burns: A singe incident in which a person experienced a freezer burn by being sprayed with an LNG leak in 1977 when a valve ruptured during the loading of an LNG carrier.
• Roll over in storage tanks: Due to the pressure exerted by the hydraulic head of LNG in a tank, the lower level of LNG in the tank is at a pressure and equilibrium temperature somewhat higher than the LNG at the upper level in the tank and, hence, is somewhat less dense than the LNG at the upper level. Thus, the tank contents are susceptible to the lower level suddenly rising to the top level because of the density difference. This is referred to as "roll over". Should this occur, a small fraction of the LNG would immediately vaporize into gas because it was no longer subjected to any hydraulic head pressure. Since the expansion ratio of LNG vapor to liquid is about 600 to 1, even a small amount of vaporization can generate a large volume of gas. The resulting sudden increase in tank pressure can exceed the capacity of the pressure relief valves and perhaps lead to a failure of the roof or wall of the tank. The first such roll over occurred in 1971 and slightly damaged the LNG tank roof. This problem is mitigated by monitoring the tank temperatures at various levels and by providing pumped mixing systems.

Overall, the LNG industry has an excellent safety record compared to refineries and other petrochemical plants. Worldwide, there are 23 LNG export (liquefaction) terminals, 58 import (regasification) terminals, and 224 LNG ships, altogether handling approximately 168 million metric tons of LNG every year. LNG has been safely delivered across the ocean for over 40 years. In that time there have been over 45,000 LNG carrier voyages, covering more than 100 million miles, without major accidents or safety problems either in port or on the high seas.28 The LNG industry has had to meet stringent standards set by countries such as the U.S., Japan, Australia, and European nations.

According to the U.S. Department of Energy, over the life of the industry, eight marine incidents worldwide have resulted in spillage of LNG, with some hulls damaged due to cold fracture, but no cargo fires have occurred. Seven incidents not involving spillage were recorded, two from groundings, but with no significant cargo loss; that is, repairs were quickly made and leaks were avoided. There have been no LNG shipboard fatalities.29

Isolated accidents with fatalities occurred at several onshore facilities in the early years of the industry. More stringent operational and safety regulations have since been implemented.

• Safety History of International LNG Operations, March 2009