As is mentioned in the question, it should always be assumed that vent stack fires will occur. The vent stack must be designed to withstand a possible deflagration and the heat from a continuous vent stack fire. The stack should also have sufficient height and be located such that thermal radiation is safe for surrounding personnel, equipment, and buildings. The codes and standards have prescriptive and performance-based approaches. In most cases, the designer can use the prescriptive approach (i.e., the 10 ft above grade) rather than performance based (meeting API 521 thermal radiation guidelines). The example of the bleed valve is a good example where that is likely appropriate. In CGA G-5.5, both approaches are shown. However, each vent stack design should be evaluated   to decide if there is a risk of high flow that might require a larger or taller stack. In general, apply the prescriptive approach to the lower flow vent systems and check the radiation via API 521 for the higher flow/high risk stacks and increase the height accordingly. See example below for calculations.  

Refineries and large petrochemical plants will frequently have flare systems for H2 and other flammable materials. One of the major purposes of these flare systems is to prevent a large unignited cloud from forming since that could result in an explosion hazard and large deflagration overpressures if there is a delayed ignition. However, these are in large facilities which have the infrastructure to support flare operations. ,

API 521 is the only standard with which the Panel is familiar that addresses flaring. This document provides good guidance. The reactivity and flammability dissuade flaring when the flaring poses more hazards than benefits, as stated in API 521 (Section 5.2.1), which can be argued is the case with hydrogen for typical industrial gas applications.

Flares are very rarely used at user locations downstream of the production plants.  Releases, either operational or emergency, tend to be smaller at these facilities. Flowrates also tend to be highly intermittent which raises the risk of flame blowout and reignition. The capital and operating cost can also be relatively high and there may also not be the utilities required to support a flare system. Another consideration is that in many jurisdictions, an air permit is required to address potential air pollution regulations, particularly with regard to NOx.  

There are situations where a flare could be advisable, particularly if a vent system is installed in a process where there could be large releases.  However, a better approach might be to address the source of the release and attempt to mitigate that in some other way. The effectiveness and need should be evaluated through a risk assessment.

There are companies that specialize in flare systems. The HSP is not in a position to comment about specific products and potential applications should be discussed and validated with those manufacturers.   
 

If liquid hydrogen usage is sufficiently high at the fueling station, there may be no need to vent any boiloff generated from the LH2 storage tank. Boil-off gas should be minimized through system design, but where needed, the boil-off hydrogen along with any other hydrogen released must be vented through a local vent stack which is constructed to safely vent the hydrogen in accordance with CGA G5.5 and NFPA 2. These standards anticipate the possible ignition of hydrogen in a vent stack and have provisions for that to happen safely. If economical, the boil-off gas can also be captured by using a gas compressor to store the gas for dispensing into vehicles.