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: 

• https://www.aiche.org/ili/academy/courses/ela210/hydrogen-laboratory-safety
• https://www.aiche.org/ili/academy/courses/ela253/introduction-hydrogen-safety-first-responders    

Frequency and severity off consequences are situational and subject to the safety review team’s best judgement. One measure of severity is an estimate of the energy released if ignited. Assuming the worst-case mix to be stoichiometric, the energy content of a 500 mL of hydrogen in air is about 0.2 Wh (700 Joules), comparable to the energy release of a wooden, blue-tipped matchstick (~1kJ or 1 Btu). This may not be very significant in a safe location such as an operating fume hood. See Risk assessment and risk ranking at H2Tools, Best Practices: Ranking Risks, for more information. 

Using tools inside a fume hood that may have a flammable gas mixture should be prohibited. A properly operating hood of the right capacity should keep the mixture of hydrogen in air inside the hood below the Lower Flammability Limit (LFL) of hydrogen further reducing any risk. 

If the use of tools is necessary, the source of hydrogen should be isolated before the work begins even if the concentration of hydrogen is expected to be below the LFL. It is best practice to leak test equipment before introducing hydrogen to minimize the probability of leaks. If spark resistant tools are suitable for a specific task for working with hydrogen systems, use of such tools will lower the probability of producing a spark. 

The key concern with any hydrogen release is the risk of creating a flammable mixture. There should be no environmental issues if you properly vent hydrogen to a safe area where it is diluted in air below the flammability limit before contacting an ignition source. Very small quantities of hydrogen are frequently releasing into a fume hood. Releases have to be small enough so that the vent air is sufficient to dilute to below the lower flammability limit. The fume hood face velocity should be in excess of 100 ft/min (30 m/min). 

Larger quantities of hydrogen should be released through a properly designed and constructed hydrogen vent stack. Standards and codes such as CGA G 5.5 and NFPA 2 provide guidance for vent stack design and installation.

If the concentration of hydrogen is less than the Lower Flammability Limit (LFL) of 4% in an inert gas, it is unlikely that a leak of this gas mix will form a flammable mixture as it dilutes into air. For example, industry uses ‘forming gas’, a mixture of 4 to 5% H2 in nitrogen, as an oxide reducing agent in materials processing furnaces and soldering operations. This mixture can also be used in conjunction with a hydrogen detector for leak testing gaseous hydrogen equipment.

Sprinkler systems and other fire suppression means are prescribed per building and fire codes to limit fire spread to other materials. In the case of a hydrogen leak and fire, it is best practice to isolate the hydrogen source, and let any residual hydrogen gas burn out. Even if the initial fire is extinguished, additional leaking hydrogen may accumulate and ignite with the potential for an explosion. 

Outside storage is generally considered safer and is required for large amounts of gas. Stationary storage should be located outside at a safe distance from structures and ventilation intakes, and protected from vehicle impact. 

Hydrogen storage separation distance requirements are typically based on the quantity and pressure of the hydrogen or the piping diameter, depending on the type of storage. Consideration should be given to distances between multiple containers to prevent interaction during an unintended hydrogen release. More detailed guidance can be found in the applicable codes and standards such as NFPA 2, Hydrogen Technologies.

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 potential sources of delayed ignition. Hydrogen is easily ignited, and the larger the cloud, the more likely it is for it to find an ignition source. The cloud itself may serve as an ignition source as the force of the gas release may cause dust or other contaminants to mix in the air creating a static charge which ignites the hydrogen. Similarly, the force of the release can cause surrounding materials and equipment to create an ignition source from impingement. Examples include doors being pushed against surrounding equipment, crushed stone being blown against equipment leading to an impact and spark, etc. The release may also be larger than the hazard assessment or codes anticipated, thereby extending the vapor cloud into areas with other ignition sources such as unclassified electrical equipment, hazardous activity by personnel, or surrounding equipment. 

The oxygen in sprayed water would not be a hazard for delayed ignition

Absolutely. Vent systems will experience a variety of transient conditions of pressure, temperature, and thrust load, so stress analysis to anticipate the strength and flexibility needed are important for safe design. These issues are often overlooked and only become an issue when they are called upon to operate in emergencies. 

It is a best practice to include the vent system in the process hazards analysis (PHA)