Puff or Plume?

Many hazardous gas releases from valve breaks or tank ruptures have a finite time duration, Td, on the order of one to ten minutes. Assuming that the cloud of hazardous gas moves at the wind speed, u, the travel time, Tt, to a receptor at downwind distance, x, is about x/u. Britter and McQuaid (1987) point out that the dispersing cloud acts as a plume for Tt < Td, and acts as a puff for Tt > Td. At about Tt = Td, the back end of the cloud “detaches” from the source. This paper addresses the question of whether, for a given total mass release from the tank in a given time, Td, the downwind concentrations would be much less for a cloud duration of 30 min than for a cloud duration of 30 s. We focus on maximum 20 s averaged concentrations, since that averaging time is typically used for short-term health effects analysis of chemicals such as chlorine. We consider distances of less than 2 or 3 km and wind speeds of about 1 m/s.

For the stated conditions, the solutions of the Gaussian continuous plume and instantaneous puff formulas show that the cloud from the 30 s release is behaving as a puff, with significant along-wind dispersion, over most of its trajectory, while the cloud from the 30 min release is behaving as a continuous plume until it reaches distances of more than 1 km. It is found that, although the puff has a much larger concentration very near the source, the concentrations drop off faster with distance for the puff because of the additional presence of along-wind dispersion. By the time the cloud reaches downwind distances of 1 to 3 km, the predicted maximum concentrations are not very different for the two cases with release durations of 30 s and 30 min.

As a sensitivity study, the SLAB dense gas model was run for several variations in inputs of emission duration, assuming a constant total mass release. The SLAB model will account for the reduced vertical dispersion and enhanced lateral dispersion of the dense gas, whereas the above Gaussian model analysis assumed neutrally-buoyant gas. For the same scenario analyzed in the previous paragraph, SLAB agrees with the Gaussian model that, at distances of 1 to 3 km, the predicted concentrations are quite close for the two release durations (30 s and 30 min). Even close to the source, the concentrations are calculated by SLAB to not reflect the factor of 60 difference in release duration. The difference is closer to a factor of 20 at that location, primarily due to the larger horizontal spread of the initial dense gas cloud for the shorter release duration. Thus it is important, when carrying out studies of the possible maximum health or environmental impact from a hypothetical hazardous gas releases, to carry out several sensitivity studies, varying the inputs within reasonable bounds. The worst case is not always intuitively obvious.