Phil's right, this falls under NFPA 68. Chapter 5.4 (of the 1998 edition) gives the requirements for a duct. It is possible to use a duct up to 20 ft long, but the longer the duct, the more it will affect the required vent area and/or the design pressure of the vessel. Best would be to avoid a duct altogether and have the vessel (at least the part of it that has the explosion vent on it) stick out above the roof.