There are several concerns with “snuffing” a hydrogen fire from a vent stack. Most importantly, snuffing a hydrogen fire before the hydrogen is isolated can lead to the buildup of a hydrogen vapor cloud, which may then re-ignite, especially with hot surfaces available from the previous fire. The largest hazard is an explosion of the vapor cloud caused by delayed ignition.  It’s always better to isolate the hydrogen at its source to extinguish the fire as fuel runs out. 

Snuffing systems have been used in the past for vent system outlets mainly due to the negative   perception of a visible hydrogen flame at the top of the vent stack, particularly at night.  The success of these systems was marginal since high and sustained rates of inert gas were required to snuff the flame and sufficiently cool the piping outlet to prevent the venting flow from reigniting.  Generally, it’s preferred to design the vent system such that it can withstand a worst-case continuous fire on the outlet without affecting its integrity or surrounding exposures.  If those criteria are met, then it’s inherently safer to allow the vent to burn than to try to snuff it. 

Dispersion and radiation analysis should be conducted to ensure that the hydrogen cloud will not interfere with the flight path of aircraft. In addition, there may be maximum height requirements due to airport requirements depending on the location of the stack.

Guidance for location of vent stacks is provided by NFPA 2, Hydrogen Technologies Code, which also references CGA G5.5, Hydrogen Vent Systems, for additional guidance. Minimum distances to vent stack outlets should be determined from dispersion and radiation analyses. The height of the vent stack and orientation of the release will affect the minimum separation distance.

Vent stacks should always be grounded in accordance with electrical standards which will reduce the probability of, but not eliminate, vent stack fires. There are numerous design features, such as toroidal rings, that have been suggested to reduce vent stack fires. However, given the many sources of ignition that can potentially ignite vent stack releases, it is virtually impossible to eliminate all such fires so proper design of the vent stack to be able to withstand over-pressures and continuous flame are critical to the design.

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.

There are dozens of safety considerations for safe design of hydrogen vent stacks. Their primary function
is to vent the hydrogen safely, so vent stacks should be designed such that the gas dispersion and
radiation profile (if ignited) do not impact surrounding equipment, buildings, or people. Documents such
as CGA G5.5, Hydrogen Vent Systems, provide numerous details regarding design. Vent stacks should be
designed to handle expected thrust loads, weather conditions, and hydrogen ignition scenarios. Various
building and electrical codes describe basic grounding requirements.

The key concern with any hydrogen release is the risk of creating a flammable mixture. There should be no environmental issues if you properly vent hydrogen to a safe area where it is diluted in air below the flammability limit before contacting an ignition source. Very small quantities of hydrogen are frequently releasing into a fume hood. Releases have to be small enough so that the vent air is sufficient to dilute to below the lower flammability limit. The fume hood face velocity should be in excess of 100 ft/min (30 m/min). 

Larger quantities of hydrogen should be released through a properly designed and constructed hydrogen vent stack. Standards and codes such as CGA G 5.5 and NFPA 2 provide guidance for vent stack design and installation.

Vent stacks must be designed for a fire at the outlet. The mesh is designed to ensure no blockage of the vent stack by animals/insects. 

Additionally, the mesh must be designed for pressure drop to ensure code-compliant back pressure on the relief devices.

Yes, numerous incidents have occurred where frozen air (which contains oxygen) has built up within a hydrogen process or vent system. These incidents with vent systems incorporate more than just a vent stack, but include a vent system consisting of additional atmospheric equipment (such as a tank) where the equipment stays cold and allows air into the system in contact with a cold hydrogen stream. 

Vent systems are at risk since they are “open” to the atmosphere and certain flow conditions might result in the aspiration of air into a cold hydrogen flow which then leads to the freezing of the air. 

A small quantity of solid air can create an explosive hazard which then leads to cascading failures from the initial incident. Solid air in hydrogen is also shock sensitive which can lead to unexpected ignition.