What is the governing case for dispersion whether it is high flow and low wind velocity or less flow and low wind velocity?
Answers
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18/04/2008
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Graham Bennett, DNV , graham.bennett@dnv.com
The answer to this question also depends upon the nature of the material being dispersed. Dense gases, or vapour clouds with significant droplet entrainment also react differently to wind/stability effects than a neutral or buoyant gas cloud. The further away from the release location, the less impact the momentum dispersion effects of a released jet become. In other words, close to the release point, the release direction and behaviour will be influenced by the release rate of the material and its orientation, as you move further away, atmospheric effects such as wind speed and stability, plus terrain or building effects can also come into greater effect. Therefore, it is critically important that the correct locations are modelled, and that a tool such as the DNV PHAST programme is used, which includes a Unified Dispersion Model which can take into account such variations along the release axis
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14/04/2008
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KengYong Chan, Air Products & Chemicals, Inc, chanky@apci.com
This is an interesting question and the conventional approach is always to use the worst case process conditions (high flow) and perform dispersion using the "typical" or low wind velocity. The "typical" low wind velocity is 1.5 m/s, as this is used by major industries and US RMP modeling.
It is also good to analyze and compare the low flow and high wind velocity conditions to see the extent of cloud dispersion.
Alternatively, one can obtain the average wind velocity from the meteorological station or airports and use this velocity and worst case flow for the dispersion.
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