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.

The suspected cause was a mixture of oxygen and hydrogen that passed downstream from the electrolysis unit into several storage vessels. Hydrogen-oxygen mixtures are very hazardous. Subsequent ignition resulted in internal pressure that exceeded the limits of the storage system. The design of electrolyzers, detection of upset conditions, and preventing the accumulation of oxygen within the hydrogen system is important for safe operation.

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.

This decision would depend on the system design, system operation, and a hazard assessment. Likely it would be better to run all hydrogen vents to a common vent or flare system, but this might also restrict the ability to isolate smaller sections for maintenance. 

There is currently a published ASME BPV Code Case describing pressure design requirements for pressure design of electrolyzers. If the Code Case is adopted by the jurisdiction where a new electrolyzer will be installed, the new electrolyzer will have to meet the requirements in the Code Case. The responsible ASME committee is working to revise the Code Case and intends to incorporate the Code Case into ASME BPV Section VIII, Division 1. If the requirements of the Code Case are so incorporated, then jurisdictions that adopt this Section will also automatically adopt the requirements for new electrolyzers. The incorporation is unlikely to occur before the 2027 edition of the ASME BPV Code.

The answers are in context of PEM and alkaline electrolysis operating at or below 30 bar and below 85 deg C°. A general suggestion: Ask component suppliers about material compatibility, but do an independent investigation to confirm. As a general resource,  safety data sheets (SDSs) sometimes provide material compatibility information. Specific recommendations follow. 

• Hydrogen: Hydrogen material compatibility information can be found at Material Compatibility Hydrogen Tools (h2tools.org), including the very detailed technical reference developed by Sandia National Laboratories.
• Alkaline Water Electrolysis Systems: Cell stack electrolytes are typically potassium hydroxide, sodium hydroxide, or sodium chloride solutions. The pumps, piping, gas/liquid separators, and other components must be compatible. An example of an MSDS that provides information about material compatibility can be found at ERCO Worldwide: Potassium Hydroxide Solution.  Other resources include publications by the National Association of Corrosion Engineers and the Materials Technology Institute.
• Oxygen: Oxygen compatibility is a big concern, especially at pressure. ASTM subcommittee G04.02 affords no-cost access to apt standards and cleaning practices for oxygen. Start here: ASTM International Jurisdiction of G04.02. Other resources include CGA G-4.4, Oxygen Pipeline and Piping Systems. Only certain materials are rated for pressurized oxygen. Cleaning to remove particles and oils is very important to reduce fire hazards - remember, almost anything can be fuel in oxygen. 
• Water: Pure water feed to electrolyzers is important. A good approach is to consult with the water purification equipment supplier for recommended materials for the feed water supply components. High purity water corrosion products can contaminate PEM membranes and degrade electrolyte. 
• The use of plastic tubing in H2 and O2 pressure applications is usually precluded. See the AICHE CHS H2 Laboratory Safety course, which discusses a PNNL laboratory incident. Metal tubing is preferred. While plastic tubing may be desirable for non-conductivity and flexibility, one should only consider plastic tubing after a full hazard analysis to assure there are effective protective safeguards (e.g., ventilation, flow limits, protective enclosures, active leak detection, isolation/depressurization) in place. 
• H2 and O2 gases dissolve in significant quantities in liquids at 30 bar. Materials in these services will need to be compatible with the gas as well as the fluid. Note that these gases will readily come out of solution when pressure is reduced and directed to a drain. Open drains in well-ventilated areas are strongly recommended.

Pay particular attention to material compatibility of safety devices, such as pressure relief valves and pressure sensors. It is important to follow the guidance for proper design of vent systems given in CGA G 5.5 for H2 and EIGA Doc 154 for O2. These standards cover topics such as where back pressure is to be avoided and safe vent locations.
 

Requirements for local jurisdictions vary, so the AHJ should be consulted, but NFPA 2:2023, Hydrogen Technologies Code, Chapter 13 has requirements for installation of hydrogen generators up to 100 kg H2/h. Section 13.3.1 General says permitted water electrolysis systems are to be listed to ISO 22734:2019, Hydrogen Generators Using Water Electrolysis - Industrial, Commercial, and Residential Applications, or approved by the local authority having jurisdiction (AHJ). 

For laboratory or small demonstration scale of less than 3 kg/day, UL 61010-1, Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use - Part 1: General Requirements, is also permitted. ISO 22734 is a product safety standard for packaged or factory-matched water electrolysis systems, either alkaline or PEM or AEM. It does not address high temperature solid oxide systems, but there are apt product safety standards for those also. Typically, a manufacturer will present certification documents stating the water electrolysis system, either fully packaged or provided as factory matched equipment designed and built to be integrated on site, meets the requirements of ISO 22734 or an equivalent. 

The equipment should bear a marking plate that attests to conformity with the electrolysis safety standard. If a different certification is provided, one may need to show equivalency with ISO 22734, or if small, UL61010-1. In US and Canada, the nationalized equivalent is ANSI/CSA B22734, and there are also nationalized versions in Australia, China, and Great Britain. One can ask the providers to supply a Declaration of Conformity (common in the European Union) or certificate asserting compliance. One can also hire a third party Nationally Recognized Test Lab (NRTL) to review the equipment conformity to the electrolysis safety standards. 

Be aware that there may be an applicable ASME BPVC section VIII code case published in 2023 that will impact how water electrolysis cell stacks are to be labeled and approved - some AHJs may accept listing/labeling at the system level (probably a safer approach), while others may look at individual components. Be aware also of installation requirements for hydrogen equipment regarding piping and integration with gas compressors, storage, fueling dispensers. Hazardous area classification and safe venting of hydrogen and oxygen gases must be carefully reviewed and made compliant with the local building and fire code.

The British Standards Institute (BSI) has published BS ISO 22734:2019, a British nationalized version of the packaged water electrolyzer safety certification standard. This standard can be used by a Notified Body (BSI is one of many operating in the UK and in Europe) to certify electrolyzer safety to established norms for this equipment. This standard addresses safety of containerized hydrogen generators using low temperature water electrolysis technologies PEM and KOH. It includes requirements to assess hazardous area classification using IEC 60079-10 and to employ methods to mitigate hazardous areas using air dilution and/or equipment designed for hazardous areas per the ISO 60079 series in accordance with the ATEX directive. The standard provides requirements for on-board GH2 storage.

The vent system for the excess hydrogen should be vented in accordance with NFPA 2 and CGA G-5.4 and G-5.5. The oxygen also must be vented safely and should be in accordance with NFPA and CGA G-4.4. 

Please be extremely cautious with compressing hydrogen. NEC/NFPA 70 and its Articles 500/505 address electrical equipment in flammable atmospheres. Please also consider the information in NFPA 2, Hydrogen Technologies Code. NFPA 2 has many prescriptive requirements for use, generation, and compression of hydrogen with properly rated equipment and containers. There have been many accidents where equipment and systems not rated for hydrogen have: 

• leaked near ignition sources such as static electricity and standard electrical equipment, 
• used vessels not intended for hydrogen, 
• used vessels improperly purged of air or other reactive gases, and d) located equipment in areas with inadequate air ventilation that allows hydrogen to accumulate. 

Valuable insight comes from an incident that occurred in May 2019, where an outdoor hydrogen tank exploded at a research and development venture complex during test of a water electrolyzer coupled to a renewable energy system. The estimated 100 lb. TNT equivalent blast killed two and injured six. The cause of the tank explosion was reported to be auto-ignition of a hydrogen-oxygen gas mixture within the storage tank. The gas source, a 9 bar H2 and O2 rated pressurized alkaline water electrolyzer, was under test to evaluate intermittent renewable energy duty cycle performance. 

Technical reports state that potential electrolyzer cell membrane degradation permitted excessive oxygen gas crossover rate through the electrolyzer cell membranes into the product hydrogen gas. This condition may have been made worse by an extended period of low electrolysis gas generation rate prior to the incident, resulting in low hydrogen flow rates insufficient to dilute diffused oxygen below O2 in H2 flammability/explosibility limits. Corrective actions reported include use of catalytic gas purifiers to remove O2 from the product H2 and waste O2 lines.     

Lessons learned: 

1. Understand interrelation of electrolyzer membrane gas permeability, membrane degradation, and dynamic operating range when establishing process safety controls; 
2. Consider automatic gas storage isolation and stopping gas generation when safety limits are exceeded, such as flammable gas mixture, excessive cross-cell differential pressure, low dynamic range, and other applicable process safety limits.