Vent stacks must be designed for a fire at the outlet. The mesh is designed to ensure no blockage of the vent stack by animals/insects. 

Additionally, the mesh must be designed for pressure drop to ensure code-compliant back pressure on the relief devices.

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.

Regardless of the piping method, reaction forces should be reviewed and supports designed for the reaction forces. 

Pressure testing of the vent system is recommended to ensure the vent system will withstand relief device activation.

The deflagration pressure is dependent upon many variables.

However, some general concepts are:

1. Deflagration pressure is proportional to operating pressure
2. Deflagration pressure is inversely related to initial temperature
3. Deflagration pressure is based on concentration and H2/O2 ratio
4. In general, an internal deflagration is unlikely to exceed about a 10:1 pressure ratio and an internal detonation pressure is unlikely to exceed 20:1. 

The main advantage of a “tee” style design is that the thrust loads at the vent exits are balanced. This means that an unequal force that might push the vent stack over is not present. Generally, the tee is also of the same size as the main vent line, thereby doubling the vent area for less pressure drop. The main disadvantage of a tee stack is that they generally vent with a horizontal discharge which means a hydrogen cloud closer to the ground and higher radiation exposure at grade. The tee is also often equipped with a downward facing miter cut to reduce the probability of rain or snow from entering the stack. However, a downward miter cut can direct the exit flow downwards, normal to the miter, if the vent is at high velocity. This can propel the hydrogen even closer to the ground than anticipated. If miters are used, the vent pipe should be oriented slightly downwards, but with the miter cut facing upward. This will help prevent moisture from entering the stack and if supersonic flows occur, the vent flow will be directed further upwards. CGA G-5.5 Hydrogen Vent Systems shows recommended orientations of vent stack outlets.

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.

The pressure regulator controls the liquid flow from the tank to the pressure build (PB) vaporizer. As the tank pressure falls, the pressure regulator opens. When the set pressure of the regulator is reached, then the regulator closes. The pressure build circuit depends largely on the required use pressure and the system house-line pressure drop, which is driven by peak flow rate, pressure, and temperature. The set point of this regulator is typically about ~10 to 20 psig or ~1 barg above the use pressure since the house-line regulator needs this pressure to supply hydrogen to the application and control accurately.  The PB design circuit requires knowledge about peak use flow from the tank, which in turn drives the size of the vaporizer heat transfer and the flow rate through the PB circuit. The flow through the PB circuit is based on the replacement rate of the withdrawn liquid with gas required to maintain constant tank pressure. As the liquid is removed from the bottom of the tank, the volume of the liquid removed must be replaced with the same volume of gas. Since density varies significantly with pressure (for both saturated liquid and gas), the flow through the pressure build unit is greatly affected by gas and liquid density, which is affected by the operating pressure/temperature of the tank.

Based on the question, it’s not clear if the reference to stress corrosion cracking (SCC) has been proven or is only suspected. It can be difficult to identify the nature of the cracks (SCC vs H2 embrittlement without analysis and microscopy). In addition, the question does not mention that the cylinders contain hydrogen gas, but it’s assumed since it was submitted to the Panel. 

In this case, it is the experience of Panel members that H2 cylinders rarely used Cu-Ni alloy discs due to concerns over embrittlement. Most discs for high pressure cylinders are copper to resist hydrogen embrittlement. Corrosion and/or SCC from the atmospheric environment are concerns and care should be taken to protect the disc if conditions for SCC might exist, such as the Teflon coating mentioned. The outlet of the vent stack should also be capped to avoid atmospheric contaminants and water/snow/ice.  It’s important that the cap does not interfere with the ability of the disc to function. If the gas is hydrogen and the inquiry is directed toward hydrogen embrittlement, then more information is needed on the alloy. Is it copper-based with nickel as an alloying element or nickel-based with copper as an alloying element? Either way, if the inquiry is about resistance to hydrogen embrittlement from contact with H2, then 300-series stainless steel burst discs are often used at lower pressures (e.g., for LH2 equipment) and copper burst discs are used for high pressure gas. Nickel alloys are not generally recommended for H2 service, but specific alloy qualification is needed. Limited available data show these alloys to be highly susceptible to hydrogen.
 

It depends on the service. Variables include disk design, corrosion resistance, number of cycles, and how to close the operating pressure is to the rupture disc set pressure. One approach is to ask the manufacturer for their recommendation. Typical practice for U.S. Department of Transportation vessels and discs is to replace them at the tube requalification (i.e., every 5 to 10 years in the U.S. depending on the retest method used). Unless there are specific reasons for frequent replacement, it's usually best not to replace discs too often since even replacing the discs potentially introduces risk. Reasons to replace the disks more frequently could include corrosion on the disk and fatigue from repeated pressure cycles. (The closer to the setpoint these are used—greater than 70-80%—the more fatigue and likelihood for premature failure).