This is a big question. In my opinion, any time you make changes to a emergency relief system you need to re-evaluate the entire design, considering things like flow capacity, backpressure and its impact on both new and existing equipment venting into the system, potential for backflow through vent systems, ability to safely handle liquid in the vent system, flammability of the mixture in the vent, and probably a lot of other things that don't immediately come to mind. I think you need to look at it with the same degree of attention used for the original design. CCPS has two books related to the topic, and they are not short books:  "Guidelines for Pressure Relief and Effluent Handling Systems", 540 pages, published in 1998, and "Safe Design and Operation of Process Vents and Emission Control Systems", 336 pages, published in 2006. There is a CCPS project to update (2nd Edition) the 1998 book, it is due to be published in October 2014 according to the Wiley web site.