NFPA 2 provides Tables in Chapters 7 and 8 that specify the hazardous area classifications surrounding vent stack outlets. These are based on typical vent systems and flows, but are only applicable for smaller systems. The designer of a vent system should apply the principles of documents such as IEC 60079-10-1 (also required by NFPA 2) or NFPA 497 to evaluate larger vent releases where the…

Welded joints are always best, but they cannot always be used as a connection to tanks and tubes, as mechanical joints are needed for maintenance. Supports for the reaction forces can help ensure the mechanical joints in the piping does not pull apart. 

If large diameter or thick-walled tube is installed with compression fittings, the use of hydraulic swaging is recommended.

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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…

We are not certain what an inverted vent top is. If this means the hydrogen flow is pointed downward in any way towards grade, then yes it must be avoided. Less dangerous vent gases can be pointed downward, especially those that mix with air rapidly (nitrogen/oxygen/argon). Regardless, reaction forces must be taken into account for any relief valve activation or flow.

Nearly all hydrogen storage tanks and hydrogen storage systems will need some type of pressure relief system to protect the vessels from overpressure. If there are pressure relief devices, some means to vent the hydrogen to a safe location will be needed. An exception to this is hydrogen cylinders due to their relief device type (lead-backed rupture discs (CG-4/5) and
the need to transport…

The distances provided are minimums. While they might be sufficient for most vents from small systems, larger vents will require both a dispersion and radiation analysis to determine the height needed.

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. 

CGA G-5.5 provides several options for vent stack outlets but not all options, nor does it tell when one outlet type is better than another design.

Figure 7, is one design, but in my opinion, not the best design. For instance, for warm gas, typical no-flow, normal scenarios (like a rupture disc on a vent system), Figure 5, a capped vent pipe is the simplest.

My preference depending…

We would not open the vent system to inspect the internal piping without a good reason.

It is recommended to check for water in the vent stack trap

1. At startup and daily during startup.
2. On LH2 tank system, every delivery
3. After the 1st rainstorm after a system is installed
4. LH2 vent stacks after establishing the baseline above
   1. After every large…

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…