M.Jamali
I don't know how much experience you have with explosion design, but my impression is that it is limited.
You mention that you two vessels, one stores Propane and the other Butane, you also mention a flare line.
Under normal operating conditions neither of those things is a problem, you also need to add some type of accident. If something happens that results in a vessel leak and then the leaked gas forms a vapor cloud, that you have the basis for an explosion.
Usually, when I do this type of analysis a risk engineer or process engineer has done a risk analysis and provided the needed scenarios. Once you have the energy, an explosion, or more correct a blast wave can be calculated. When I work with this type of projects I usually get the blast parameters (peak pressure and duration) and go from there.
Typically, for a structure, that is my area of experise, it means a transient dynamic (often non-linear) analysis. You mention Ansys, I work with another general FEM-solver, but Ansys can do this. You mention a few other software's, I recognize some of the names but I don't know if they can do this type of analysis.
You ask about standards and there may be some that are relevant for your specific project. But based on the information you have provided I won't speculate regarding what standards to apply. I also know that some of the "Big guys" in the process industri has their of requirements.
Also, in my experience, that are not that many shortcuts in this type of safety related designs.
Good Luck
Thomas