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| Computer modelling helps assess and reduce the risk of worker exposure near industrial vents, tanks and stacks. |
By Tracy Canney, ORTECH Environmental
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| Computer modelling helps assess and reduce the risk of worker exposure near industrial vents, tanks and stacks. |
Chemical releases to the air, whether planned or accidental, of heavy gas vapours such as chlorine, ammonia, benzene, and cyclohexane, are of major concern at industrial facilities due to their potential occupational health hazards. One tool that can be effectively used to estimate the on-site concentrations of these types of pollutants is the SLAB heavy gas computer model.
SLAB is capable of determining the pollutant concentration for ground level evaporating pools, elevated vertical jets (or stacks), horizontal jets and ground based instantaneous releases at specified receptors over various averaging time periods. SLAB is commonly applied for accidental release modelling of heavy gas vapours to the atmosphere, (e.g. US EPA's Risk Management Plan). However, another often overlooked application is its use as an industrial health and safety planning tool to estimate employee exposure to short-term releases of heavy gases.
Canadian ORTECH Environmental, as a third party to HSP Engineering of Cornwall, Ontario, used the SLAB heavy gas model to assist a client in estimating employee exposure to a proprietary nitrogen-based, volatile chemical ('contaminant A') during a regularly scheduled maintenance period on a 60 metre tall tower, located 15 metres downwind from a low level stack.
The stack periodically released various quantities of contaminant A during venting operations. The client's primary concern was the employees' exposure level to contaminant A at various floors on the tower during maintenance work. The concentrations were estimated by the SLAB model and compared to the Short-Term Exposure Limit (STEL) for contaminant A to determine if the exposure levels were a health risk.
The SLAB model was run for three different emission rates at durations of both one minute and five minutes. The two sets of meteorological conditions used in the model were a stable condition scenario (stability class F with 1.5 metres per second wind speeds) and a typical condition scenario (stability class B with five metres per second winds).
Both conditions were at an ambient temperature of 10°C. The emission configuration for the stack in SLAB was a vertical jet release with all of contaminant A released in the vapour phase. Receptors, located on the tower at the heights of the working stages, were set at ground level, 15 metres, 40 metres, and 60 metres above ground.
The results predicted that for all emission scenarios, the ground level, 40-metre and 60-metre receptors on the tower were well below the STEL for contaminant A. The height of concern was the 15-metre level, where two of the six emission scenarios had levels above the STEL for the contaminant A (these both occurred under the typical condition scenario).
The client, now armed with the estimates of the release scenarios at all of the receptors, can use this information as a guideline for planning and scheduling maintenance periods on the tower. Based on the conditions modelled, the client can determine what precautions are necessary for personal safety of employees working on the tower. In order to expand to other meteorological conditions and other areas of concern at the facility, additional meteorological scenarios and receptor distances could be analyzed.
The application of the SLAB model is an efficient and effective tool for industrial facilities that may have potential employee exposure to heavy gases. This case study successfully illustrates that SLAB is a useful tool, not only to predict the impacts of accidental releases on the surrounding community, but also as a planning tool in various industrial health and safety issues that are faced by facilities on a daily basis.