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英国论文代写范文精选-LNG storage tank

2016-06-28 | 来源:51due教员组 | 类别:更多范文

51Due英国论文代写网精选assignment代写范文:“LNG storage tank”,这篇论文讨论了关于液化天然气储罐。天然气是一种应对能源危机最有效的手段。它用于发电厂的发电和作为汽车燃料。由于天然气便于运输和对环境没什么影响,人们对它的需求越来越大了,到现在为止它的需求量已经增加了5倍。

Natural gas is one of the most effective means of coping with today’s energy crisis. It has many applications for domestic purposes such as heating, electric generation in power plants and powering vehicles. The demand of natural gas has increased 5 times in the last few decades. This demand is due to the fact that natural gas is easily transportable and environmentally friendly.

Chen et al. predicted the temperature and pressure changes in a liquefied natural gas cryogenic tank. The properties and composition of LNG fuel were simulated inside the tank as a function of time. Boil off is de fined as the gas being released from the liquid. Boil off of LNG in these LNG tank s usually takes place at LNG stations and can cause excessive pressure build up in LNG tanks.

Boil-off is caused by heat added to the LNG fuel during storage and the filling processes. Heat can leak through the shell of the tank, and be added to the LNG fuel during operation. They stated that the boil-off of the LNG is mainly due to the heat gained by the tank from outside ambient temperature.

Also, the heat leakage into the tank leads to an increase of the vapor pressure. Natural gas is stored in a liquefied state at a temperature as low as 16.2 C in order to decrease its volume and to facilitate transportation. In its liquid state, the density of natural gas is 600 times more than Compressed Natural Gas at room temperature and atmospheric pressure. Compressed natural gas is typically stored at pressures up to 24.821 MPa in cylindrical steel tanks. In order to reduce boil-off, Liquefied Natural Gas should be stored in special tanks which have multi-layered insulation that minimize heat leakage.

There are three different classifications for liquefied naturalgass to rage tanks: single containment, double containment, and full containment. A single containment tank is either a single tank or a tank consisting of an inner and an outer tank such that only the inner tank is capable of storing the LNG. A double containment tank is defined as having an inner and outer tank that is both capable of independently containing the LNG. A full containment tank is defined as a double tank in which the outer tank of a full ontainment tank is capable of both containing the liquid cryogen and of controlled venting of the vapor of the cryogen after a leak. Tanks are additionally classified by the elevations from t he ground level: above-ground type, in-ground type and under-ground type. The type of tank treated in this study is an above-ground full containment type tank.

Generally, LNG storage tanks are composed of three parts: inner tank, outer concrete wall and roof. According to the shape of the inner tank, there are two types of LNG storage tank: 9%-Ni type and membrane type. The 9%-Ni type has a self-supporting inner tank which endures the thermal contraction of LNG temperature, and the hydrostatic pressure from the weight of the LNG. Jeon et al.applied a specia l method to predict the tempera ture of the inner walls of the insulation. I n their proposed model, the effects of the outer tank , the insulation layer and as uspended deck were considered.

The geo metrical dimensions of the tank and the properties of the material used in the tank can be found in this Reference. During normal operation, the inner tank is exposed to cryogenic temperatures from the LNG. However, if LNG leaked from the inner tank, it would soak into the insulation. In the event of such an accident, some insulation layers would not be able to function, and the outer concrete tank could be structurally compromised by a quick temperature drop. In conclusion, the safety of the tank can be ensured only through a thorough thermal analysis.

Graczyk et al. performed a probabilistic analysis of the sloshing-excited tank pressures in LNG tankers. They found that ship motion results in violent fluid motion in the tank which causes high tank pressure. The pressure was measured in a series of a model tests, and the important issues of structural responses, such as the significance of spatial and temporal characteristics of sloshing loads as well as the model scaling problem were addressed. Design specification of the tank has to be studied thoroughly in order to do a simulation of the storage tank. The properties of all the parts of a storage tank have been described by Jeon and Park.

They discussed safe and economical construction for the aboveground LNG tank s. As the capacity increases, special attention needs to be given to the design code and an efficient procedure needs to be established to design an LNG tank with structural and cost efficiency. Various analyses have been carried out by KOGAS technology, including static analysis, wind loading, modal and seismic analysis, temperature modeling, leakage modeling, pre-stress/post tensioning, burn-out modeling, relief valve heat flux modeling and soile structure interaction. They used the LUSAS finite element modeling software to design a 200,000 m3 above-ground tank.

They considered an axisymmetric model for the static stress and thermal analysis, and 3D shell elements for modal analysis and seismic analysis. For burn-out modeling, they performed a transient thermal analysis and predicted the time required for the fuel to burn-out completely. Natural convection causes circulation of the LNG within the storage tank which tends to maintain a uniform liquid composition. The addition of new liquid can result in the formation of strata of slightly different temperature and density within the LNG storage tank. “Rollover” refers to the rapid release of LNG vapors from a storage tank caused by stratification. The potential for rollover arises when two separated layers of different densities exist in a storage tank. LNG rollover phenomena received considerable attention following a major unexpected venting incident at an LNG receiving terminal at La Spezia, Italy in 1971.

The main hazard arising out of a rollover accident is the rapid release of large amounts of vapor leading to potential over-pressurization of the tank. It is also possible that the tank relief system may not be able to handle the rapid boil-off rates, and as a result the storage tank will fail leading to the rapid release of large amounts of LNG.

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