Why is the Fluid Den 2 greyed out? I'd like to use it for a system that may have liquid in it at one time and vapor another.

The architecture of the software does not permit easy extensibility to additional densities, and will require significant rewrite, from what we're told.

As it's not a code requirement to look at two different densities together (i.e. subtract a liquid case from a gas case), the cost / benefit ratio is perhaps still a bit too high compared to more pressing design considerations.

The easiest option would be to create two models once the modeling is complete - one labeled vapor, the other labeled liquid.

You may also use WNC for gas case unless a part of the line is liquid and the other one gas.