Modelling Dense Gases Flashcards
What is the assumption we have made thus far relating to the plume?
The plume has the same average molecular weight as the surrounding air. This has meant that the buoyancy with respect to ambient air depends only on the temperature/
What about if the gas is a lot less dense than the surrounding air?
The plume moves upwards and disperses rapidly. This is not much of a problem as would have to prevent ignition in relief ducting.
What about if the gas is a lot more dense than the surrounding air?
A heavy discharge will mean that it tends to settle towards the ground and disperse rather slowly.
What was found in the experiment carried out for the modelling of dense gases?
The high density vapour “slumped” towards the ground and then spread in a way more like a pool of liquid rather than gas.
What will eventually happen to the high-density vapour?
Once the vapour was mixed with enough air it will start following the usual dispersal model.
What are the consequences of the high-density vapour dispersing slowly?
The initial effects of the release are spread over a smaller area but concentrations are extremely high and danger to plant personnel is greater.
What other release is similar to that of dense vapour?
Release of pressurised volatile liquids.
Why is the release of pressurised volatile liquids similar to that for dense gases?
They flash, which absorbs heat from the released contents. The resulting cold vapour is of high density and may even be able to re-condense. The original liquid may also have been sprayed out with the released vapour. This results in a mist i.e. dense cloud that keeps close to the ground.
What is a potential other consequence of the release of pressurised volatile liquids?
It may be accompanied by a spill of boiling liquid. The initial temperature before release and/or the drop in temperature caused by the flash can make it possible for the liquid to persist at 1 atm.
What is the relationship between the thickness of the pool and the time for which the liquid is there?
The thicker the pool, the longer the liquid is there.
What about for low molecular weight compounds?
The phenomenon for boiling liquid spill and mists would also apply for low molecular weight compounds such as HF and NH3.
What about the liquid and vapour of low molecular weight compounds?
The vapour itself may disperse quickly if it has not cooled too much, but pools of liquid may take a fair while to boil off while producing high concentrations of vapour around them.
At what elevation is wind speed typically measured?
10 m
What is the equation for the wind speed measured at 10 m and the true wind speed at greater heights?
u (z) = u (10 m) * (z/10 m)^p
where p varies with local conditions
What are different values of p for different environments?
- 0.14 - 0.16 for smooth rural conditions and neutral stability
- 0.28 for forests where z_infinity = 400 m
- 0.2 for urban conditions
