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BS EN ISO 16903:2015 pdf free download

BS EN ISO 16903:2015 pdf free download.Petroleum and natural gas industries – Characteristics of LNG, influencing the design, and material selection.
5.3.4 Expansion and dispersion of gas clouds
Initially, the gas produced by evaporation is at nearly the same temperature as the LNG and is denser than ambient air. Such gas is, at first, subjected to gravity spreading by flowing in a layer along the ground until it warms sufficiently by absorbing heat from the soil and mixing with the ambient air.
The dilution with warm air increases temperature and decreases the molecular weight of the mixture.
As a result, the cloud is in general denser than ambient air until diluted well below the flammable limit. Only in case of high water content of the atmosphere (high humidity and temperature) can the condensation of water during the mixing with the cold LNG vapours heat-up the mixture as such that it becomes lighter than air and results in a buoyant cloud. Spillage, expansion, and dispersion of vapour clouds are complex subjects and are usually predicted by computer models. Such predictions should only be undertaken by a body competent in the subject. Following a spillage, ‘fog’ clouds are formed by condensation of water vapour in the ambient air. When the fog can be seen (by day and without natural fog) and if the relative humidity of the ambient air is sufficiently high, the visible fog is a useful indicator of the travel of the vaporized gas and the cloud gives a first indication of the extent of flammability of the mixture of gas and air as the visibility of the cloud is a function of the humidity and ambient temperature, not a function of the natural gas release.
In the case of a leak in pressure vessels or in piping, LNG sprays as a jet stream into the atmosphere under simultaneous throttling (expansion) and vaporization. This process coincides with intense mixing with air. A large part of the LNG is contained in the gas cloud initially as an aerosol. This eventually vaporizes by further mixing with air.
5.3.5 Ignition
A natural gas/air cloud can be ignited when the natural gas concentration is in the range from 5 % to 15 % by volume.
5.3.6 Poolfires
The surface emissive power (SEP) of a flame from an ignited pooi of LNG with greater diameter than 10 m can be very high and shall be calculated from the measured values of the incident radiative flux and a defined flame area. The SEP depends on pool size, smoke emission, and methods of measurement. With increased footing, the SEP decreases. The Bibliography contains a list of references which can be used to ascertain the SEP for a given circumstance.
5.3.7 Development and consequences of pressure waves
In a free cloud, natural gas burns at low velocities resulting in low overpressures of less than 5 x 10 Pa within the cloud. Higher pressures can occur in areas of high congestion or confinement such as densely installed equipment or buildings.
5.3.8 Containment
Natural gas cannot be liquefied by applying pressure at ambient temperature. In fact, it shall be reduced in temperature below about -80 °C before it liquefies at any pressure. This means that any quantity of LNG that is contained, for example between two valves or in a vessel with no vent, and is then allowed to warm up increases in pressure until failure of the containment system occurs. Plant and equipment shall therefore be designed with adequately sized vents and/or relief valves.
Designers need to pay attention to eliminating the potential to shut in even small volumes of cryogenic liquid, including attention to details such as cavity venting of ball valves.
5.3.9 Rollover
The term rollover refers to a process whereby large quantities of gas can be emitted from an LNG tank over a short period. This could cause over pressurization of the tank unless prevented or designed for. It is possible in LNG storage tanks for two stably stratified layers or cells to be established, usually as a result of incomplete mixing of fresh LNG with a heel of different density. Within cells, the liquid density is uniform but the bottom cell is composed of liquid that is denser than the liquid in the cell above. Subsequently, due to the heat leak into the tank, heat and mass transfer between cells and evaporation at the liquid surface, the cells equalize in density and spontaneously mix. This spontaneous mixing is called rollover and if, as is often the case, the liquid at the bottom cell has become superheated with respect to the pressure in the tank vapour space, the rollover is accompanied by an increase in vapour evolution. Sometimes, the increase is rapid and large. In a few instances, the pressure rise in the tank has been sufficient to cause pressure relief valves to lift.BS EN ISO 16903 pdf download.

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