The recognized and generally accepted good engineering practices (RAGAGEP) for employing a nitrogen purge into a hydrogen vent stack is that inerting is not generally used for nitrogen purge in a hydrogen vent stack because most inert gases freeze at liquid hydrogen temperatures. The vent stack should be designed for a fire and the internal overpressure caused by a deflagration. If inerting is used, it should be with helium, although a continuous purge with helium is not practical due to availability and cost. CGA G-5.5, Section 6.2, states that “[h]ydrogen vent systems do not require inerting of the vent stack or flare to ignite gases exiting the vent system. If inerting is chosen, vent stacks connected to a liquid hydrogen source shall not be inerted with any other gas than helium as other gases can solidify at hydrogen temperatures.” CGA G-5.5 Section 5.5 says a reduced L/D (length over diameter ratio) of the vent line reduces the potential for an explosion in the vent stack. 

However, Section 6.2.12 also states that “[h]ydrogen vent systems within the scope of this publication (gaseous and liquid hydrogen at user sites) are unlikely to sustain deflagration or detonations, regardless of the L/D ratios. The relatively simple geometry of the system (few turns, few tie ins) and operating scenarios are not conducive to forming detonable hydrogen-air concentrations within the system and limit potential ignition sources external to the stack discharge. In the unlikely instance that a deflagration or detonation occurs, experience has shown that a system designed for 150 psig (1030 kPa) will sustain the event without bursting.” It is important to note that the vent stack should not have an opening in the vent system that can pull air into the vent stack (e.g., an open drain connection), as this substantially increases the risk of a fire, deflagration, or detonation in the vent stack.