Modeling of Ground Flare Pits

New LNG liquefaction facilities have been configured to include large ground flares to allow for the safe combustion of large volumes of combustible gasses in the event of facility upset conditions. A recent example is a planned LNG development nearby Darwin, Australia. These ground flares can exceed the size of football fields, measuring greater than hundred yards in length. A typical ground flare consists of several hundreds of burners, surrounded by porous radiation fences. Questions have been raised as to whether the plumes may impact the facility, the work force, or nearby roadways in the event that the wind happens to be blowing in the right direction at the right speed.

This study examines the use of Computational Fluid Dynamics (CFD) to accurately model plumes from ground flares and the downwind temperature distribution. The CFD model is first validated by comparing its results with commonly used dispersion models such as PHAST and CALPUFF. The flow features of a ground flare plume rising in crosswind are then discussed. In particular the plume acts as what is commonly referred to as a “jet in cross flow.” In this type of flow, counter rotating vortices are known to form. These vortices are not represented in commonly used Gaussian plume models.

The use of two adjacent flare pits is then investigated. The interaction of the two pairs of counteracting vortices is discussed, as it influences the downwind temperature distribution. The results show that the distance between adjacent flare pits is of importance when planning multi-flare pit operations.