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.
Situations where extinguishing a hydrogen leak prior to stopping flow is safer are rare. Hydrogen releases have a high potential for inadvertent re-ignition and subsequent explosion. Some vent stacks might be equipped with an extinguishing system, but these often can be more hazardous than allowing a properly designed vent stack to continue to burn until the source is isolated.
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.
After moving people to a safe location, if it safe to do so, isolate the source of hydrogen feeding the fire. Burns and explosions are hazards when exposed to a hydrogen fire. For more best laboratory preventative safety practices as well as first responder response to a hydrogen incident See both CHS training resources:
Hydrogen flames can be nearly invisible in daylight, especially at low flowrates. The concentration of hydrogen does not have much effect on the color of the flame. Many hydrogen incidents or fires will have a bright orange hue, or even yellow flames. The color is primarily caused by contaminants that is either naturally in the air in certain environments, swept into the air during the release (such as duct), or surrounding materials which are also burning
There are several levels of documents which can be used to assist with the design, sizing, selection, and installation of the pressure relief device settings for LH2 tanks.
Pressure vessel design codes, such as the ASME Boiler and Pressure Vessel Code will provide minimum requirements for design of pressure vessels (including LH2 tanks), relief devices, and relief systems. However, these codes will not provide the sizing criteria nor anticipate all of the potential demand cases that might be imparted upon a vessel.
In the US, the model fire codes require compliance with NFPA 2, which then references documents such as CGA S1.2 and CGA S1.3 for sizing criteria. These documents have been customized by the industrial gas business specifically for cryogenic fluids such as LH2. API Standard 520 “Sizing, Selection and Installation of Pressure-Relieving Devices in Refineries” of is also a helpful document to provide additional guidance.
For LH2 storage tanks, usually the highest process demand is an engulfing fire with a loss of vacuum insulation to atmosphere. This failure mode can result in additional heat flux from air condensation in the annular space which must also be addressed.
It is not required to proactively vent the contents of an LH2 tank when exposed to fire. Relief devices are required to prevent the accumulation of internal pressure to unsafe levels. Within the ASME BPV, this is 121% of Maximum Allowable Working Pressure for scenarios involving fire exposure. It is common practice, but not required, that at least one device be non-reclosing (e.g. a rupture disc) for both managing the high flow required as well as to relieve the contents of the tank. Reclosing relief devices will maintain pressure in a fire and are more likely to lead to a vessel rupture if the fire ultimately weakens the pressure vessel.
LH2 tanks are unlikely to BLEVE due to the vacuum insulation outer jacket (usually carbon or stainless steel) preventing direct impingement of fire onto the main pressure vessel, as well as the internal cryogenic contents maintaining the main pressure vessel walls at a cooler temperature until the contents have been relieved by the relief devices.
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