See H2Tools, Best Practices: Purging, for a description of different purging approaches for hydrogen systems.

Purging is not recommended as a continuous part of vent stack operation. However, maintenance activity is a transient event and it’s prudent and recommended to purge a vent system prior to performing maintenance. It’s always possible that hydrogen could be leaking internally from a valve or other component and therefore create a hazard. Of particular note, care must be taken that proper isolation of the vent system is performed such that the vent system can’t be inadvertently used during maintenance. Since vent systems and stacks rarely have isolation valves to prevent unintended isolation of relief devices, proper maintenance on the vent system may require the entire system or plant to be taken offline.

Purging can be accomplished in several ways including by using pressure cycles, sweeping, or vacuum. Sweeping is the least reliable but can be effective on simple pipe runs. In most cases, vent systems are open to the atmosphere and the ingress of air from the outlet is likely. However, the vent system should be designed to handle fire or explosion internally. This
generally is not difficult and is a best practice that can also be found in several standards which are incorporated by reference in Codes. 

While air that enters from the vent stack outlet is expected to be within the vent stack, any leaks or openings in the vent system should be repaired such that they don’t act as a venturi and aspirate air while venting. A continuous source of air ingress should be avoided. 

No, but it depends on the application. Nearly all vents less than 4” in size are not purged with N2. This is primarily due to: 1) large flows required to dilute hydrogen below the flammable range, 2) the cost of the nitrogen, 3) the potential blockage of the stack when being inserted a vent header/stack serving a liquid hydrogen system, 4) the potential for backpressure (depending on the source) to damage or restrict operation of relief devices, and the lack of incidents with non-purged system.

However, a nitrogen flow can be a means considered for specific systems warm GH2 system as part of a hazard assessment. For example, a nitrogen purge might be appropriate for a large diameter vent header that operates at very low pressure such that it might not be able to be designed for an internal deflagration. 

If a nitrogen purge is to be used on a liquid hydrogen system, then the vented hydrogen should have a means to be warmed above -320 F to prevent liquefaction or freezing of the nitrogen. N2 is not allowed for the purging of LH2 systems per CGAG-5.5.

At NASA Cape Canaveral, they used a natural gas line connected to the vent stack outlet with a thermal sensor to make sure the pilot was lit. They may also have had a sensor to ensure the H2 fire was lit. In cases where substantial quantities of unused hydrogen is vented and the timing and amount of the flow rate is known and controlled, flaring might be useful. NASA guidelines stipulate that flaring is appropriate for hydrogen vent rates surpassing 0.2 kg/s (~0.44 lb/s). Flare systems themselves must incorporate pilot ignition, flameout warning mechanisms, and a means to purge the vent line, ensuring comprehensive safety measures are maintained throughout the process.

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.