On H2Tools Best Practices, under Operations, a page titled "The Importance of Purging Hydrogen Piping and Equipment," says the following about bringing a hydrogen system back online: “One should always assume that air is present and verify that the system has been purged when reintroducing hydrogen into a system. It is recommended that purge procedures reduce oxygen levels below 1% prior to putting the system back online.” Does the recommendation apply to vent lines that are continuously purged with nitrogen?

Purging of vent systems is not required and in most instances is not recommended.

A nitrogen purge is generally not needed for a vent system designed in accordance with CGA G-5.5.  However, there are times where this might be considered or required due to the specific design of a system.  Where determined by a risk review to be needed, A continuous purge into a vent system reduces the probability air or oxygen in the piping. Intermittent purging should be evaluated, but if a constant flow of either nitrogen or hydrogen is provided, then the configuration should be adequate. For intermittent purging, an initial full purging with nitrogen or helium is the best safety practice.  If a design and HAZOP condition is that the vent system must be purged with an inert gas for safe operation, then the vent system should be purged prior to putting the system back in service. 

If purging becomes required for a liquid hydrogen vent system, the only acceptable gas per CGA G-5.5 is helium, as this is the only gas that does not solidify at liquid hydrogen temperatures.