Be careful with very low liquid content systems in the HYSYS depressuring utility

Just a brief warning here:
If you use the HYSYS Depressuring utility for a fire case with very low (but non-zero) liquid content, you may be getting the wrong results. More concrete: if your liquid percent level is less than 5%, beware and use the workaround described below.

The HYSYS model used for the heater that receives the fire heat flux is a "Liquid Heater" model, the default configuration of this liquid heater puts the top of the heater at 5%. If the liquid level drops below this, you will only see a pro-rated heat flow to the fluid. But the Fire equation itself already caters for the liquid level, you don't want to do this twice.

To avoid this problem, do the following:
Open the PFD of the depressuring utility subflowsheet
Open the view of the vessel
Go to the Dynamics tab
Go to the Heat exchanger page
Change the type of heater from liquid heater to vessel heater
You are done!

17 March 2011: I recently found out that even for cases with a zero liquid content you may run into trouble in some rare cases. If the flash produces a tiny amount of liquid, it may not show in any of the result fields, but nonetheless the liquid level is not exactly zero. In such cases the pro-rating of duty is used and you get a zero duty where you may expect full duty. A case we recently saw had an erratic behaviour where duty was applied half of the time you started the run.