Spraying water onto a vent stack, either for gaseous hydrogen or liquid hydrogen, is not recommended. While this is prohibited within the code for liquid hydrogen due to the much greater hazard of plugging the vent system, it also presents hazards for gaseous vents as well. The water can enter the vent system and plug due to ambient conditions. In addition, if the water was sufficient to extinguish the fire before the hydrogen flow is stopped, then a flammable or explosive cloud may form which can reignite unexpectedly leading to a greater hazard. 

Water is only recommended to cool equipment adjacent to a hydrogen fire.

It depends on the facility and risk assessment, but generally multiple pressure and temperatures to one vent stack is not the best practice unless all are similar in pressure and temperature, and the streams have compatible composition and flow rate. Care must also be taken to prevent reverse flow and misdirected flow between portions of the system. Additionally, one vent stack can become a single mode of failure for an entire process or facility. Specific considerations for vent systems include the following: 

1. Design stacks for hydrogen fires at the vent stack outlets.
2. Locate to assure no harm to people or equipment from thermal radiation.
3. Hazard review should be completed for the venting node(s).
4. Potential single mode of failure should be analyzed.
5. System should not allow air to enter while exhausting H2 gas (venturi effect).
6. Vent outlet design should direct venting hydrogen to a safe direction meeting requirements for radiation and dispersion.
7. Vent stacks should be grounded.
8. Supports should be designed to resist reactions from high velocity flow.
9. Stacks and vent piping should be designed to resist overpressure due to internal deflagration.

Several codes and standards address vent systems, but not all topics are fully covered in each. Here is a list of codes and standards that address hydrogen vents: 

1. CGA G-5.4, Standard for Hydrogen Piping Systems at User Locations, and G-5.5, Hydrogen Vent Systems.
2. EIGA Doc. 75/07/E, Determination of Safety Distances; Doc. 211/17, Hydrogen Vent Systems for Customer Applications; Doc. 121/14, Hydrogen Pipeline Systems.
3. IFC 2209.5.4, Venting of Hydrogen Systems.
4. EIGA 211/17, Hydrogen Vent Systems for Customer Applications.
5. ASME B31.12, Hydrogen Piping & Pipelines.
6. NFPA 2, Hydrogen Technologies Code. 
7. ANSI/API 521, Guide for Pressure Relieving & Depressurizing Systems.

ANSI/AIAA G-095A, Guide to Safety of Hydrogen and Hydrogen Systems (formerly NASA Hydrogen Safety Standard).
 

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.

There are no specific code resources that specifically cover hydrogen liquefaction plants, but they must be built to the general building, electrical, machinery, piping, and mechanical codes for process plants. Codes such as NFPA 2, Hydrogen Technologies Code, for installation and emergency response may also be used for reference. It may also be beneficial to break down the requirements into process safety and storage. For general process safety, there is good guidance for large plants. The Center for Chemical Process Safety provides guidance that is not specific to liquid hydrogen but instead addresses process safety. Other resources include the Hydrogen Safety Panel (HSP) and the Center for Hydrogen Safety (CHS).