Explaining the "three inch rule": why model flares don't match full-scale

It was Pohl and co-workers who discovered the so-called "Three Inch Rule" in their work on flares in the 1980s. They found that model flares with pipe diameter less than three inches do not have the same Combustion Efficiency (CE) and stability behaviour as full-scale flares, at least when wind is negligible. Later researchers, including the author, used model flares in wind tunnels to investigate the effects of wind on CE. These studies also found a Three Inch Rule despite the very different flow conditions. This paper uses the published flaring data to unravel the mechanisms behind the Three Inch Rule. The flow and combustion regimes are delineated in terms of the relevant dimensionless parameters. The inadequacy of model flares smaller than 3 inches is given a theoretical basis. This result is significant for regulators of air emissions from flares, who should be wary of accepting evidence from small model flares in drafting new regulations and should revisit regulations based on those results.