Article,
Use of Advanced Techniques to Model the Dispersion of Chlorine in Complex Terrain
Process Safety and Environmental Protection, 79 (2): 89--102 (March 2001)
DOI: 10.1205/09575820151095175
Abstract
The EU Seveso II Directive requires operators of major hazard facilities
to prepare safety reports for sites storing quantities of dangerous
substances in excess of specified levels. The safety report should
include an assessment of the risk associated with the facility, which
will include an evaluation of the effects of releases of dangerous
substances to the environment. The models commonly used for assessing
the dispersion of dense gases in the atmosphere are based on the
`ideal' of flat, unobstructed terrain. For `real' situations
such models may be unduly pessimistic in their predictions and, in
certain circumstances, may even be optimistic. This paper describes
work undertaken by Environmental Resources Management (ERM), in association
with sub-consultants Rowan Williams Davies & Irwin Inc. (RWDI) and
the Health and Safety Laboratory (HSL) to model the dispersion of
chlorine in complex terrain. The work involved physical modelling
of releases in a Boundary Layer Wind Tunnel (BLWT) and the use of
Computational Fluid Dynamics (CFD). The paper focuses on the key
findings of the study, which provide a dramatic insight into how
terrain and buildings can fundamentally alter the dispersion behaviour
of dense gases. The results show how flat terrain models may overestimate
the chlorine hazard range by as much as a factor of 5, whilst the
predicted direction of travel of the cloud may err by up to 90�.
This has implications not only for the assessment of risks associated
with major hazard facilities, but also for land-use planning in the
vicinity of the site and emergency preparedness.
