Abstract
The dispersion of vapour of liquefied natural gas (LNG) is generally assumed to be from a liquid spill on the ground in hazard and risk analysis. However, this cold vapour could be discharged at height through cold venting. While there is similarity to the situation where a heavier-than-air gas, e.g., CO2, is discharged through tall vent stacks, LNG vapour is cold and induces phase change of ambient moisture leading to changes in the thermodynamics as the vapour disperses. A recent unplanned cold venting of LNG vapour event due to failure of a pilot, provided valuable data for further analysis. This event was studied using CFD under steady-state conditions and incorporating the effect of thermodynamics due to phase change of atmospheric moisture. As the vast majority of processing plants do not reside on flat planes, the effect of surrounding topography was also investigated. This case study highlighted that integral dispersion model was not applicable as key assumptions used to derive the models were violated and suggested guidance and methodologies appropriate for modelling cold vent and flame out situations for elevated vents.
Highlights
It is common to use the integral jet dispersion model to assess hazard distances of the discharge of flammable gases for elevated discharges, such as a tall vent stack or from an elevated flare during cold venting
The use of the integral jet dispersion model assumes that the discharge occurs in uniform ambient wind field with no orographic effect and there are no heat sources or sinks involved in the thermodynamics in the dispersion processes
The scenario involved the discharge of liquefied natural gas (LNG) vapour at speed and at a height through an elevated vent stack
Summary
It is common to use the integral jet dispersion model to assess hazard distances of the discharge of flammable gases for elevated discharges, such as a tall vent stack or from an elevated flare during cold venting. By virtue of its height and location, vent or flare stacks, in general, are designed to be inherently safe: disperse harmlessly in the atmosphere and not posing flammable or toxic hazard to personnel or the process facilities. This is done via a few methods such as locating the stack at a far enough distance from the facility, a high enough stack or optimal diameter of stack to allow high velocity venting. This ensures turbulence has sufficient time and intensity to dilute the discharged gas to harmless concentrations
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