The Use of the Liquefied Natural Gas - Case Study Example

AN INVESTIGATION OF THE LIQUEFIED NATURAL GAS (Student Name) (Course No.) (Lecturer) (University) (Date) Introduction There is an increased demand for the use of the Liquefied Natural Gas (LNG) in many industrial operations as well as homes in the world today. This is primarily attributed to its availability and ease of transportation through a combination of road and water transport even to the most remote areas. Also, the LNG is preferred because of its clean burning thus reducing the negative environmental impacts. Due to its combustibility and its ability to explode, there are growing concerns over the safety of the people working in such areas and the environment, suggesting that safety precautions should be taken in the production, storage and transportation of the liquefied natural gas (Mokhatab, 2014, p. 28). The LNG comprises of a mixture of liquid hydrocarbon with methane as its primary constituent. Other elements found therein include; sufficient quantities of ethane, propane and nitrogen and other elements in trace amounts depending on the source of the natural gas. With a boiling point of between -1660C and -1570C and an average density of between 430 kgm-3 and 470kgm-3, both influenced by the exact composition, LNG occupies 600 times less space as its gaseous state hence preferably kept in storage tanks especially in areas where pipelines are not available. The aim of this paper is to explore the dispersion of the liquefied natural gas in a tank farm and the factors that affect it during leakage (Benartik et al., 2010, p. 1-6). Effects of heat transfer during leakages in the gaseous form In many instances in the storage tanks where LNG is stored, leakages can occur due destruction of the storage tank walls or piping problems. The two types of leakages that may occur as a result of damages to the storage tanks are; continuous and transient leakages. When the leaking time is longer than the dispersion time of the seeping substance in the atmosphere or on the ground, it is referred to as the continuous leakage. Additionally, transient leaking occurs when the leaking time is shorter than the dispersion time of the leaked material after the occurrence of the leakage. In the event where LNG leakage occurs from a storage tank due to the damage of the tank wall or issues related to piping, the floor of the safety dike supplies it with heat changes causing rapid vaporization at the surface, causing the diffusion of a flammable gas composed primarily of methane into the atmosphere (Zhu, 2014, p. 220). This type of diffusion is known as gravity diffusion because it involves a heavier gas than ambient air. Methane has a boiling point of -1620C and a specific weight of 1.48 in relation to the ambient air. This means that the dilution process of the gas in the atmosphere is not as easy as in the case of a lighter gas with a density approximately equal to that of the atmosphere. This means that the leakage of the liquefied natural gas evaporates, it goes back to the gaseous state which is the natural gas. Depending on the temperature of the liquefied natural gas relative to the temperature of the surrounding air, the leakage of the LNG tank due to damage can result in the formation of the vapour cloud. If for instance, the temperature of the leaking gas is the same as that of the surrounding air, there will be no cloud formation. However, the nature of the leakage occurring in this case is different. The temperature of the leaking natural gas is -1490C while the temperature of the surrounding air is 200 C. This means that the densities of the two gases are different, the density of LNG being higher than the density of air. When the density of the liquefied natural is higher than the density of the surrounding air, the result is the formation of the cloud and its dispersion into the air (Matanovic et al, 2014, p. 293). Moreover, the formation of the vapour cloud depends on the flow rate of the leaking liquefied natural gas from the tank. The higher the flow rate of the leaking natural gas, the more the formation of the vapour cloud and its dispersal into the air (Takeno et al., 1995, p. 1-9). The formation of the vapour cloud caused by the difference in temperature, hence densities of the leaking liquefied natural gas from the storage tank can result in fires in cases where igniting substances are in the vicinity. Flammability refers to the ability of a substance to ignite. The ignition of any fluid depends on the flammability limits. The lower explosive limit refers to the minimum concentration of a gas or vapour in air, capable of producing a flash of fire in the presence of an igniting substance. If the concentration of LNG in air is less than the lower flammable limit, combustion cannot take place because the gases are too lean to burn. The upper flammable limit of LNG refers to the maximum concentration of the gas in air that can burn in the presence of an ignition source. The flammability of LNG depends on the concentration of the vapour in air and its temperature. When the temperature, pressure and concentration of LNG in air increases, the result is reduced lower flammability limit and increased upper flammability limits, causing explosion of the vapour cloud. LNG cloud dispersion properties and how it react with air (state of concentration) After the occurrence of leakages from the storage tank, the spilt liquefied natural gas evaporates from the surface into the atmosphere. The temperature of the LNG as it leaks from the storage tank determines the formation of the vapour cloud. If the temperature of the leaking LNG from the tank is the same as the atmospheric temperatures, chances of formation of the vapour cloud becomes nil. However, in the case provided, the temperature of LNG is -1490C and the temperature of ambient air is 200C resulting in the weight of LNG being higher than that of air, thus resulting in the formation of the LNG cloud. One of the factors that influences the liquefied natural gas cloud vapour dispersion are obstructing objects. Objects that block the flow path of the evaporated LNG vapour affect cloud dispersion by increasing the atmospheric disturbances. The presence of buildings, for instance, causes eddy currents thus resulting in increased concentrations behind the buildings than would have occurred in a case where the flow occurs in an open field with no obstructions. Additionally, the dispersion of the LNG cloud is also influenced by the surface roughness. The roughness of the surface over which the cloud vapour flows affects its dispersal since it causes turbulence. Also, in many dispersion models, the surface roughness length is used as a parameter. An increase in the surface roughness increases the turbulence thus reducing the dispersion rates of the cloud vapour. Also, the LNG cloud cover dispersal is also caused by the fluctuation of the wind. According to meteorological data, the direction of wind is not constant at any particular time but keeps changing. Therefore, with all factors held constant, wind and turbulence can result in the dispersal of the LNG cloud by some degree. Over long periods of time, however, the wind flux will result in the spreading of the concentration of the cloud. The shape of the LNG vapour cloud is influenced by many factors. One of the factors that influence the shape of the LNG cloud is the wind velocity. The speed of the wind also affects the dispersion rate of the vapour cloud formed. If the wind velocity is low, the vapour cloud forms a loop. This is because the reduced speed of the wind concentrates the cloud at a given point for a significant amount of time. High wind speeds result in the fanning of the vapour cloud because it is blown towards the direction of the wind. The speed of the wind increases the hazard risk in case an igniting substance is reached because it spreads over a wide area (Cleaver et al., 2007, p. 432). Another factor that affects the shape of the cloud is the temperature of the gas released relative to the ambient air temperature. For instance, the LNG released from the storage tank, having a lower temperature than that of the air, becomes denser than the air, thus the cloud formed tends to sink and spread over the surface depending on the speed of wind. The shape of the cloud also depends on the shape of the storage tank. In many instances, storage tanks are cylindrical, thus helping in reducing the bulging of the cloud in the event of a leakage. Additionally, the shape of the vapour cloud formed following the leakage determines the number of people and structures covered by the cloud, hence the more spread the cloud, the higher the damage to property and death of people from vapour cloud explosion. Properties One of the properties of the LNG vapour cloud is that it undergoes gravity spreading. Gravity spreading occurs when the heavy dense LNG cloud spreads its own weight, other than the dispersion caused by the wind. The dispersal of the cloud is not totally dependent on the wind speed but can spread upwards on its own when the speed of wind is low. When continuous leakage of the LNG occurs, the mass of the evaporated vapour is higher at the edges of the cloud than in the middle at the gravity spreading zone. Another property of the LNG cloud is the difference in concentration. Measured concentrations of the cloud at 1% forms alternating shoulders and dips. Higher concentrations occur at the edges of the cloud than at the middle of the cloud. This explains why fires and explosions occur when the edges of the cloud interact with ignited surfaces. Other properties of the cloud include its effect on the air flow, speed and density, lower cloud height and a bigger cloud width and the robust heat changes from the ground to the liquefied natural gas pool and cold natural gas vapours causing the increase in disturbances and the entrained air (Ohma et al, 2004, p. 4). Heat transfer process in LNG cloud In cases where leakages of the LNG occur in storage tanks due to damages, source trails form releasing large amounts of cryogenic liquefied gas onto the ground or water surfaces in the presence of wind, depending on the surrounding. Originally, quick horizontal spreading of the cloud occurs as a result of the increased hydrostatic head of the cloud. The result is a low plume with a bigger width that has a more pronounced effect when the wind speed is low. In cases where the speed of the wind is low or moderate, the cloud is moved upwards although the gas finally loses its upward flow and is rolled back to the source. As a result of this velocity change, is the production of a horseshoe-like vortex, binding the horizontal parabolic gas plume (Nasr and Connor, 2014, p. 281). The gravitationally created collapse of the cloud converts the potential energy into kinetic energy. Some of the kinetic energy is transferred to the surrounding ambient fluid where it is degenerated by the atmospheric disturbances. The occurrence of the energy transfer is at the head of the spreading cloud and in its wake. When the gravitational velocity is lower than the average ambient wind velocity, substantial amounts of air is entrained due to mixing and the cloud is advected downwind at the same velocity as that of the ambient air. As a result, the cloud forms a parabolic shape with a regular cloud depth at the centre. Surface heating of the cloud and the heat from the condensation of the water vapour and the entrained air reduce the negative buoyance of the cloud. This results in a reduction in the sudden Richardson Number thus facilitating both dispersion by the atmospheric turbulence as well as the downwind advection of the fluid. As the positivity of the buoyancy increases and the cloud becomes less stable, the cloud lifts off the ground (Smill, 2015, p. 121). In cases where the leakages that occur are large and the movement of the wind is low or moderate, the uplifting of the cloud does not occur unless the lower flammability limit is reached. However, for very small -scale spills, the surface heating may be far- fetched. Dispersion distance of LNG cloud from leakage source. The leakage of the liquefied natural gas from the storage tanks results in the formation of a cloud of vapour that can have numerous negative consequences on both the people and the environment. In many occasions, the consequences of the cloud have been felt in areas far from their production, especially when the cloud interacts with an ignited surface of object along the way resulting to fires, affecting both people and buildings within the cloud at the point of ignition (Warey, 2006, p. 179). As a result, many countries in the world, including the United States of America, have laid regulations, requiring the LNG developers to ensure that the flammable vapour cloud caused by the leakages that may occur in the storage tanks or the pipes do not extend over wide ranges of distances from the source or in cases of LNG production, the vapour cloud does not extend beyond the facility. In the event of leakages from storage tanks, there are different physical phenomena that may occur. Flashing for instance, occurs if the pressurized liquid is at temperatures above the boiling point of the ambient air. Atomization of the liquid stream may also occur as it expands to atmospheric pressure. Of great importance, however, is the distance of dispersion of the cloud following the leakage. The distance of dispersion of the cloud is important in determining the safety of the people and buildings within the surrounding areas of the storage tank. Generally, the safety distances of liquefied natural gas cloud distance depend on the activity levels rather than on the storage capacity of the tank. The activities that, therefore, determine the safety distances are the loading and offloading of the LNG. The extent of the leakage, however, depends on the cross sectional area of the damaged gap, the volume and pressure. Liquefied petroleum gas releases typically have longer safety distances than gaseous releases, usually, 2-3 times longer. There also occurs a difference in the safety zone with reference to half the lower flammable limit and the lower flammable limit. In an event whereby half the lower flammable limit is used as the maximum safe gas concentration, the safety zone increases by two fold or even more (Ruffeing, 2011, p. 25). In such instances as jet fires, the safety distance depends on the length of the jet fire rather than the distance to flammable gas concentration before ignition. The reason for this is the increase in the hazardous heat radiation distance, causing it to be longer than the distance to the non-ignited lower flammable liquid concentration. In simplicity, distance diffuses the concentration of the cloud vapour. As a result, the effects of cloud explosion are thus more profound in the area near the source term than further away. This only applies to the simple models. However, when such factors such as blockage and terrain are also considered, more complex tools are required. The shape of the cloud also has significant effects on the hazard risk. When the LNG cloud forms a blanket and spreads over a large area, ignition can caused cloud explosion and detrimental effects over a large area. Different models have also been developed to help in determining the effect of distance on the liquefied natural gas vapour cloud. One such model is the use of FLACS, with a flash utility that allows the user to outline a pseudo- release location where the beginning of the release is viewed as a single-phase jet. At the source, the single-phase jet is a mixture of gas and air at a temperature lower than that of the source liquid due to evaporation. The jet mass flow rate becomes higher than the leak flow rate because of the entrained air. However, the use of the flash utility assumes a scenario where there is no obstruction by objects during the expansion process. In the case of blockages by buildings, different approaches are used (Benjamin et al., 2008, p. 4-18). How dispersion affect nearby workers and surrounding area Owing to the different properties of the liquefied natural gas, it is of potential hazard to the people working in such facilities as well as to the surrounding environment. One of the effects of the leakages on the nearby workers is asphyxiation. This refers to the inhalation of the LPN under low supply or in the absence of oxygen. The occurrence of asphyxiation involves the dilution of the oxygen concentration in the breathing zone of people to below 15% oxygen concentration by the LNG vapours. This results in the impaired behaviour. If the oxygen concentration is diluted below 10%, it causes nausea and vomiting while the dilution below 6% oxygen concentration results in death. The LNG vapour concentrations required to reach these levels include 28.2%, 52.2% and 71.3% respectively. These concentrations occur only near the point of release of the LNG vapour and thus can only affect the workers (Woodward and Pitblado, 2010, p. 9). Another effect of the cloud dispersion is the enclosed vapour cloud explosions. When there is a build -up of the natural gas vapours in a building or in an enclosed space, explosions may occur. In many cases, such explosions are caused by the leakages in the natural gas lines or pipes that occur in a building. When LNG is at a temperature below the boiling point, the atmosphere within a storage tank is purely boil off gas with no oxygen content. In such a case, the damage to a valve cannot allow in sufficient air causing the vapour space to be combustible. However, when the vapour from a passing LNG cloud leaks into a building, an explosion occurs. Air intakes in buildings are higher than most dense vapour clouds and the occurrence of such explosions in buildings is not common (Anderson and Johan, 2013, p. 10-12). Another threat that results from cloud dispersion is the occurrence of fire, these are, the flash fires, jet fires and pool fires. Jet fires are caused by the release of LNG under high pressure in power plants and mainly affect the workers on the site and not the general public. Jet fires can cause damage t property and even loss of life at the plant site. Another type of fire caused by the leakage and hence the cloud dispersal of the LNG is the flash fire (Kytomaa and Morrison, 2013, p. 6). The spillage of the LNG from a storage tank onto the ground or on water results in the evaporation of the gas, causing the formation of a dense cloud that is driven upwards by the wind. If any part of the vapour cloud interacts with an ignition source and burns, the result is the production of a flash fire which burns both upwards and downwards from the ignition point. Flash fires are transient with a short duration of a few seconds. However, flash fires have lethal effects on the people within the cloud. Additionally, the amount of incident radiation that reaches a surface from the flash fires is lower than that which occurs in the jet or pool fires over the same distance. This is mainly because the pool and jet fires take longer. In some cases, the flash fire may burn back to the pool or ignition may occur at the pool resulting in the pool fires. Pool fires are also fatal. Another effect of the cloud dispersion on the workers in power plants are freeze burns. These occur when a person comes into contact with the ignited LNG cloud vapour. The effect of thermal radiation on any given body is determined by the intensity of the radiation and exposure time to the radiation. Thus, the severity of the burns caused by the cloud vapour on a person depends on the period of exposure and the magnitude of the fire. Different types of burns thus occur when individuals are exposed to vapour dispersal (Koopman and Ermak, 2007, p. 15). Depending on how severe the burn is, different degrees of injury can be established. The first degree of injury is a slight skin burn affecting the epidermis with tenacious redness but with no formation of blisters. The second degree of injury involves the intermediary skin burn is characterized by the formation of blisters. The depth of the blister may be shallow, found only on the epidermis or may include both the dermis and the epidermis. The third degree of burns involves the severe burning of the skin causing the formation of char. Risk assessment of the LNG leakage in tank farm Risk assessment is defined as the valuation of the potential loses that may occur as a result of the leakages of the LNG from the storage tanks. For the effective management of an organization or a project, it is important to manage the risks to be able to quantify the amount of risks associated with a project and develop ways of mitigating such risks before they occur. Quantitative risk analysis for instance, involves providing the management with the channels where losses are likely to occur and ensuring that measures are put in place to mitigate these risks before they occur, hence saving the costs of managing the risks after occurrence. Where LNG is kept in the storage tanks, it is also important to carry out thorough risk assessments according to the set regulations to avoid the losses that may occur due to fires (Tusiani and Shearer, 2007, p. 122). Risk assessment involves the overall process of risk analysis and evaluation, done by comparing the risks against the targets and mitigating until the desired level of risk reduction is achieved. The risk analysis process is carried out in a formal manner involving risk identification, frequency analysis, consequence assessment, risk summation and risk management. In the case of LNG leakage from a storage tank, the hazards that can be identified include the flash, jet and pool fires that may be caused by the formation of the cloud vapour. Also, other risks may be associated with the inhalation of the liquefied natural gas vapour causing intoxication and the eventual death of the workers at the storage site (Scandpower, 2012, p. 30). One of the main areas of concern in the case of LNG leakages from storage tanks is the resulting fires from the leakages on the storage tanks. It is, therefore, important that the sizes of the leakages be determined in advance to analyse the possible risks that may be associated with them. The fires that result from the leakages depend on the diameters of the gaps that are found on the storage tanks. The larger the diameter of the gap, the larger the amount of LNG leakage produced and the larger the magnitude of the fire produced (Chakrabarty et al., 2016, p. 235) . This means that the resulting fires from the different sizes of the gap cause varied risks. In risk assessment, it is important that such storage tank leakages are identified earlier and mitigated before they occur, thus reducing the exposure of the workers as well as the surrounding populations. Another important area of concern in the risk assessment of the LNG leakage in storage tanks is the ignition capacity of the stored substance. Liquefied natural gas is not flammable unless it interacts with a source of ignition. In situations where the ignition is remote and takes a longer time, flash fires and vapour cloud explosions may occur that could result in detrimental damages. In places where the storage tanks are situated above the ground, ignition sources should be avoided near the storage tanks to prevent the occurrences of fires that result from the leakages. The distance of the surrounding areas from the storage container leak is another important area of risk assessment in the event of leakages from the storage tanks. As a positive risk assessment measure, the storage tanks that contain the liquefied natural gas is commonly situated in isolated areas far away from the settlements. This is because of the potential hazards that may occur especially if congested settlement areas are situated close to the LNG plants from the ignition of the cloud vapour thus resulting in the loss of life and destruction of property. Conclusion With regards to the above discussion, it is clear that the liquefied natural gas causes different risks to the people as well as the surrounding environment, thus, requiring that it should be handled with a lot of care right from its production through to the storage. Although the LNG storage tanks are well constructed using concrete and metallic cylinders, leakages still occur and can be minimised by conducting the regular checking of the storage tanks to ensure that they are in good conditions at any time to ensure the safety of the plant workers and the surrounding environment. References Anderson, P and Johan, S. (2013). Thermal Exposure from Burning Leaks on LNG Hoses: Experimental Results, p. 10-12. Benartik, A, Senovsky, P and Pitt, M. (2010) LNG as a potential alternative fuel e Safety and security of storage facilities, Elsevier Ltd, p. 1-6) Benjamin, R, Sam, M and Zhang, Y. (2008). 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