Depressuring studies and the HYSYS depressuring utility

So many people seem to be struggling with this although there is a lot of info available to do tell you how to do this. Just for the record, the Aspentech support site has a document that is updated regularly on how to better use the depressuring utility, read it please! And below in short a couple of rules of thumb in case you really do not want to go and read the entire document:

Case 1 - The blowdown case

• Vessel geometry MUST be entered such that you properly represent
• The volumes occupied by each phase
• The metal mass in contact with each phase
• The heat transfer area for each phase
• Note that the "metal mass correction" is introduced as extra surface area!
• This gets more complicated if you represent more thant one piece of equipment in a single depressuring utility.
• Use the Adiabatic model for heat flux and the Detailed model for heat loss
• The PV term contribution should be close to 100%
• The recycle efficiencies should be less than 0.1% for the dominant phase

Case 2 - The Fire case

• Choose your heat flux equation
• If you choose API Fire, choose "None" for the heat loss model and do NOT apply the heat to the outside wall
• For the Stefan Boltzmann method you really should add the detailed heat loss model and do not forget to applied the heat to the outside wall
• For the others you may want to choose a different heat loss model, but you really need to know what you are doing!
• The PV term contribution and the recycle efficiencies have little impact. So leave the PV term at default and leave the recycle efficiencies at 100%

Do not try to be the hero of precision and use a very small time step!!! This applies in particular to blowdown cases. In version 2006 and previous there is an issue that causes the results to be all wrong if you decrease the time step to below 0.001 second or so (assuming the default compositional period of 10). The issue is solved in version 2006.5. That new version has a couple of improvements on default values, but the default on recycle efficiency remains 100%.

I would really welcome all suggestions to make the utility more user friendly, although you have to realise there is a fine line between user friendliness and making the simulator take all responsability for a sensitive study like a depressuring study.