You cannot guess or estimate the density as there is a large range of API that are called heavy oil. You must obtain at least the density from your client.
For your question I am not sure Porocel catalyst people are the best to contact. Typically, there are no relef valves in a Claus plant on the sulfur processing side as they would get plugged off. It is also unlikely that there is a relief valve on the heating side. Process equipment is...
As one of the authors of the article mentioned, there is enough info there or in GPSA. A good reference to consult are the books by Manning and Thmopson, Pennwell Publishers. When rating separators, I would suggest understanding the concepts as this is much trickier than design. As stated...