For continuous venting of GH2, do you recommend continuous purging with nitrogen?

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.