It is best to avoid planned blowdown of large amounts of hydrogen inventory at high flowrates if possible.  Low flow releases from vent systems are normal and occur for purging, delivery operations, and maintenance activity.   A challenge with high flow blowdown of a hydrogen system is that venting large quantities of hydrogen can itself be a hazardous activity.   Large blowdowns at high rates from vent systems can lead to jet fires and explosions after release to the atmosphere.

Flaring can be an option.  However, if flare stacks are used, they must ignite before the hydrogen reaches the end of the vent stack, so that a delayed ignition of the hydrogen does not occur, as this could create damaging overpressure.   A flare system is a complicated design for hydrogen. It is not normally a best practice unless the timing of the release is always known, and the flare cannot be extinguished until the hydrogen flow is stopped. Flares are generally only used at large production facilities which have the necessary infrastructure. 

A best practice for any storage system is to site the storage vessels away from any flammable substances and/or protect the vessels with barriers or insulation. It’s inherently safer to avoid   fire exposure onto the vessels, especially since relief devices may not be well suited to protect a vessel in the case of an impinging fire.  Similarly, there may be other methods to limit the H2 released by reducing the size, type or quantity of safety devices on a storage system. 

A best practice, when the storage vessels are not subject to an engulfing fire, is to use reclosing safety devices, such as spring loaded or pilot operated safety valves.  These do not empty the entire contents of the tubes, but open just to maintain the pressure within design criteria. 

Where it may be impossible to completely eliminate engulfing fires, rupture discs or thermally activated pressure relief devices (TPRD) are often preferred since once they activate, they will continue to vent until all pressure is released.  This is important since the fire may weaken the vessel while still at the reclosing devices’ setpoint, causing a vessel failure and a large sudden release of its content. However, non-reclosing relief devices can also be prone to inadvertent or spurious activation.  This can result in unnecessary and unwanted releases which can cause hazardous situations from high reaction forces and large quantity of the release. 

1. Understand any reactions the hydrogen can add to what is being vented. For instance, O2/H2 vented in the same stack would not be a good idea.
2. Understand all the flow and operating parameters of the streams to ensure no back flow into the hydrogen system or vice versa.
3. Ensure the venting/flaring system can handle the hydrogen flow parameters.

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.

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.