William and Ernesto,
API 2000 details how to calculate venting requirements, both in and out flow. You should also read it's rationale for the calculations as they are made on several assumptions. The location of your storage tank, should be considered. Also, there are some articles on my site,
My Webpage that might be of further interest.
As to a sump pit for molten sulfur, I am not sure that I even know what that application is. Maybe further explanation would elicit some additional responses.