For depressurization, set time =15 min and let HYSYS find the Cv
For firecase , you can use the 50 btu/lb latent heat/ or the calculated latent heat to find the required orifice. you may use this orifice in dep utility and fine tune the orifice size by running multiple runs of the utility.
For adiabatic, i assume you are running a depressurization case and the approach more or less is similar to setting the time and finding Cv. API 521 sets the guidelines for final pressure and time.
See help from your senior engineers in your organization as dep/relief system design is a serious field.