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. 

The answer is dependent upon the nature of the system and a hazard assessment which evaluates a balance of risk. 

Keeping the hydrogen in the vessel is better so the hydrogen release does not compound the original hazard. Large flowrates from vessels can create significant risk of vapor cloud explosion, jet explosion, or radiation exposure. Vent systems can also fail from poor design or effects of the incident, so they may not work as intended to vent to a safe location. 

However, once exposed to an impinging or engulfing fire, there is a risk that vessel walls may weaken and result in a vessel failure. A vessel failure is generally looked at as a worst-case scenario in most situations and should be prevented if possible. In those cases, the relief systems play a key role, and depressurization of the system is a means to prevent failure. 

The most common means to depressurize a system or vessels are through the use of non-reclosing devices such as rupture discs or TPRD’s, or special instrumented systems that purposely depressurize the system in an incident. It should be noted that these devices and systems are not foolproof and could activate spuriously or in unintended situations which can lead to large releases and create significant hazards. The risk must be evaluated by a hazard assessment.

There are many designs of storage systems where multiple vessels might be needed to obtain the required storage quantity. Regulations differ between vessels and modules which are intended for stationary or transportation purposes. Similarly, there are differences in codes globally. The issues of requiring shutoff valves on individual vessels and requiring TPRD’s are linked since generally vessels need to be in communication with the pressure relief system. A valve on each vessel will require one or more TPRD on each vessel.

There are trade-offs between the risk of having a large number of valves and TPRD’s, each of which is a potential leak/release source, and having a large number of vessels manifolded together with fewer valves/TPRD’s but then larger banks that contain larger quantity of hydrogen that will be released in a single event. The main disadvantage of multiple vessels being connected to a single TPRD is that a much larger release is possible since each vessel can’t be isolated. This may also require a much larger device to be able to vent the multiple vessels which then means a higher release rate with the associated larger vapor cloud, risk of explosion, and radiation profile. The decision is dependent upon the risk assessment and might be
different depending on the location and application. When a tube has its own, individual TPRD and isolation valve (which must be closed during transport), a smaller release of hydrogen would occur.