There is no maximum flow-rate that can be vented to the atmosphere, but the hazard analysis should consider the potential risk of dispersion, radiation, and overpressure as part of the vent system design. Flare systems are often used at large hydrogen production facilities as one means to prevent a large unignited cloud from forming but will depend on the specific application.

Each system must be evaluated individually, and it depends on the amount and location of possible
releases. Routing vent lines to a vent stack is the most common approach when venting directly to
atmosphere is not acceptable.

Rupture discs open very rapidly. Historically, rupture discs opening at high pressure (1000 psig and above) have caused the most damage due to deflagration/detonation. Timing a rupture disc would not be possible. Additionally, how would you have a pilot light on a moving tube trailer? Even with a pilot light at the end of the stack may blow out due to the initial high velocity.

See questions #71 & #72 from Venting for Gaseous and Liquid Hydrogen CHS Webinar Q&A

Low-pressure vents at mostly low hydrogen purity are not as large safety risk as high-pressure pure hydrogen vents. These vents should still go to a vent stack, but it will probably be small in diameter and thus the tee vent at the top can be small.

If the purge requires high flow, if purging horizontally, the reaction forces of the flow exiting and the hydrogen cloud should be modeled based on NFPA 2 to ensure the safety of the surrounding area. 

There are no differences in the process design of the vent stack since the venting requirements will follow the same sizing and pressure rating requirements regardless of vent configuration. However, vent systems often create liquid air on their exterior due to the cold venting temperatures. Since this liquid air will drip off the stack, it should be diverted such that it does not directly impinge on the outer vacuum jacket of the vessel. The jacket is often constructed of carbon steel which is not rated for the cold temperature of the liquid air, and as a result, may crack and cause loss of vessel integrity. Horizontal tanks will usually place the vent stack such that liquid dripping from the bottom of the stack will drain to the ground. This is more difficult due to the geometry of a spherical tank so the stacks must be sited and supported differently and/or a shield be placed to prevent impingement. 

The main advantage of a “tee” style design is that the thrust loads at the vent exits are balanced. This means that an unequal force that might push the vent stack over is not present. Generally, the tee is also of the same size as the main vent line, thereby doubling the vent area for less pressure drop. The main disadvantage of a tee stack is that they generally vent with a horizontal discharge which means a hydrogen cloud closer to the ground and higher radiation exposure at grade. The tee is also often equipped with a downward facing miter cut to reduce the probability of rain or snow from entering the stack. However, a downward miter cut can direct the exit flow downwards, normal to the miter, if the vent is at high velocity. This can propel the hydrogen even closer to the ground than anticipated. If miters are used, the vent pipe should be oriented slightly downwards, but with the miter cut facing upward. This will help prevent moisture from entering the stack and if supersonic flows occur, the vent flow will be directed further upwards. CGA G-5.5 Hydrogen Vent Systems shows recommended orientations of vent stack outlets.

Generally flaring is not recommended. Normally GH2 is not flared for most hydrogen equipment as the piping diameters are smaller. The largest stacks are the LH2 vent stacks on trailers and on tanks for the main safety valves are 3”. For GH2 systems the flare stacks are generally smaller in diameter. 

Flaring is a deliberate ignition of a hydrogen stream. If the hydrogen stream is to be ignited, the timing of the ignition must be exact at the very beginning with a flame igniting the hydrogen before the cloud gets too large and represents a deflagration/detonation danger. 

For relief devices, this is very difficult due to the large instantaneous flow rate Flaring is also not typical as:

• A steady and constant flow is needed to maintain ignition
• Reignition explosively is possible if flameout occurs
• Timing of the initial ignition could cause a large cloud to be ignited

If a flare system is used, it must

• Dispose of H2 safely
• Prevent explosions
• Have a steady flow rate or controls that assure ignition is maintained. Variable
  velocities indicate a flare stack may not be advisable.
• Control the flare to assure
  • Pilot ignition
  • Flameout warning systems
  • Limit the backflow of air into the stack
  • Flame dip does not occur
• Variable velocities can cause
  • Flame blowout/burn-off - High velocity
  • Flame Dip -Allows air into the larger vent system- Low velocity

API 521 is a code that addresses flaring, besides the ANSI document.

There is some information on vent stack flaring below. The ANSI/AIAA G-095A-2017 Guide to Safety of Hydrogen and Hydrogen Systems former NASA NSS 1740.16 document addresses vent stack flow rates for flaring.

This document states “Quantities of hydrogen of 0.113 to 0.226 kg/s (0.25 to 0.50 lb/s) have been successfully vented from a single vent 5 m (16 ft) high”. .226 kg/s is a very large flow rate (340,000 scfh/8000 nm3/hr). Per NASA Figure A4.1, there is no flame dip shown (flame receding into the vent stack) below a 3 in stack size, which is consistent with the best practices. 

The 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.

Even if a small experiment is being run inside a fume hood, the best practice is to use a dedicated vent line for hydrogen which vents hydrogen to a safe location outside.  This is especially recommended for planned venting.   This practice avoids situations where flammable mixtures could develop. Each system is unique and should be evaluated and approved for  use independently. For example, the relative flow rates could be considered to understand the concentration within the vent duct. Also, it is not recommended that safety relief devices be vented into a hood due to the high flow.
 

Gaseous hydrogen can be stored forever as long as the system integrity is maintained. However, liquid hydrogen is “use it or lose it” and will boil from system heat leak and build pressure unless it is used or vented. This is not usually an issue for continuous use or low-pressure applications which can use hydrogen gas pressure directly from the tank.  

For intermittent or high-pressure applications such as vehicle fueling, this can be more challenging.  If a gas compressor is used and if demand is regular enough, often the compressor can recover the boil-off gas. This is more difficult for a pump, but again, if usage is sufficient, then the liquid being removed from the tank can minimize or eliminate the need to vent.
 

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.