Underground storage tanks can be either installed in a vault or directly buried. Both offer additional
protection from external impact and fire, but each has unique challenges. Vaults must be properly
ventilated and designed to not create an explosion or asphyxiation risk. Direct burial vessels should not
have any underground leak points and must be protected from corrosion. Both types of installation must
have hydrogen vents routed to a safe location above grade.

The containerized electrolysis unit should be installed per manufacturer instructions, the requirements
of its listing such as to ISO 22734, Hydrogen generators using water electrolysis - Industrial, commercial,
and residential applications, and NFPA 2, Hydrogen Technologies Code. A primary consideration for
indoor installation is the potential for hydrogen releases from the system, both planned and unplanned.
Another consideration is the total quantity of hydrogen that could be released within the container or
indoor area, especially with respect to the available ventilation.

Electrolyzers should be installed per manufacturer recommendations and meet the criteria of their
listing, such as ISO 22734, Hydrogen generators using water electrolysis - Industrial, commercial, and
residential applications. There are several methods such as partitions, enclosures, ventilation, and
purging that can be used to address non-classified electrical equipment.

Exhaust systems (sometimes referred to as ventilation systems) are used to exhaust hydrogen and air mixtures. Normally these are used to vent streams with less than flammable range hydrogen in air.

That is, hydrogen detectors trigger venting or the ventilation systems runs during all hydrogen operations. In these instances, low concentrations of hydrogen are expected, but deflagration is not expected in these systems and no pressure rating/deflagration protection is needed.

However, if it is determined that higher concentrations of hydrogen may be expected in the vent system, pressure-appropriate designs can be implemented, based on conditions.

The most common modes of failure for vent lines is backpressure and thrust forces.
Backpressure failures can be from several causes:

• Inadequate calculation of the backpressure caused by the high flow rates. Vent system design pressure is often only designed for the maximum 10% backpressure that is required by ASME Code. However, it should be noted that the large flowrates from rupture discs and TPRD’s can often have backpressure as much as 50% of the system design pressure.
• Vent stacks are not required to be designed for the full process pressure of the system that they protect, so plugged lines can create pressures much higher than their design. A best practice is to design the vent stack burst pressure above the MAWP where possible, but this is not always practical, especially for 700 bar hydrogen fueling stations.
• Inadequate installation. Vent stacks are often not pressure tested after installation as they should be. This can lead to installation errors not being identified. Examples include inadequate welds or incompletely tightened fittings, especially compression fittings.
• The flow/pressure reaction forces. CGA G-5.5 has equations for determining the
  reaction forces on vent piping and its supports. The reaction forces from this formula, are greater than the pressure times the area. The first fittings and vent stack end supports in a vent system are most susceptible to these reaction forces.

Off gassing of hydrogen from battery sources depends on battery chemistry, usage/duty cycle, age, and other considerations. The specific battery manufacturer should provide recommendations for fire prevention and mitigation in battery charging rooms as the battery itself impacts how an event would be mitigated. Adequate ventilation such that combustible mixtures cannot develop is a general recommendation, as are combustible gas detection shutoffs integrated into system and/or facility safety circuits. However, system-specific design elements could change recommendations and  fire protection strategies. Also, the model Fire Codes will have minimum requirements for battery charging areas.

Assuming this question relates to the roof of the enclosure, there are no design criteria on this topic to the Panel’s knowledge. The key to the design would be ensure that the exhaust ventilation inlet is located at the highest point and that there are no pockets that can capture hydrogen (restrict flow to the exhaust inlet).

Information on Toyota’s repair garage approach is available at Toyota Mirai Hydrogen Fuel Cell EV- Repair Garage Design & Safety. CFD modelling could also be used to anticipate the flow of released hydrogen based on either natural or forced ventilation.

A best practice, even for small hydrogen vents, is to vent to a dedicated vent system outside the building where possible. Several international codes and standards can be used to provide guidance; the Panel recommends discussing the configuration with a local fire official to ensure their required standards are followed. In the U.S. NFPA 2, Hydrogen Technologies Code, has information in Section 13.3 (https://www.nfpa.org/product/nfpa-2-code/p0002code). Additional best practices can be found at Best Practice Overview: Use of an Electrolyzer.

Other considerations when venting the electrolyzer to the room in addition to making sure there is adequate ventilation include proximity of ignition sources near the exhaust release point of the electrolyzer, location of room air inlets and outlet exhaust, and shutdown of the electrolyzer if exhaust is lost. Increasing ventilation to dilute the venting hydrogen can be one method to resolve the situation, but other alternatives are to redirect the vent stream to a chemical exhaust system, a fume hood, or piped directly outdoors.

It is always recommended that the area in which this work would be done be adequately ventilated and in accordance with the Building Code, NFPA 2 and NFPA 45 if applicable. In the Panel’s opinion, it’s recommended that the weight measuring equipment as you’ve described it  be designed for Class 1, Group B, Division 2. Also consult with the local authority having jurisdiction as to the extent of the area classification.

The regulations for electrical classification in Europe and a US jurisdiction such as California are significantly different and should not be assumed to be the same. Consultation with Authorities Having Jurisdiction or a Third- Party expert regarding the application of the US National Electric Code is advised. Some additional important points:

The HSP has concerns with the use of ventilation to provide an option to the required electrical classification. The Panel does not advocate for unclassified equipment in the described area. It is a major challenge to achieve the response time required to sense and implement emergency ventilation from continuous ventilation required to cope with internal leakage. While the leak may be small, it might also be large and overwhelm almost any ventilation rate.

The space inside the container should be classified, and electrical equipment in the container should be Class 1, Div 2. Otherwise consider putting the electrical equipment outside the container.