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.

Pressure relief systems may use reclosing devices like relief valves, non-reclosing devices like rupture discs, or a combination of both in parallel. Some systems may also be equipped with emergency blowdown systems that are operated by control systems. Selection of the proper devices is dependent on the system design and relative hazards. Variables that affect the selection include the type and size of vessel(s), location, pressure, and inventory.

The compressed gas industry is sensitive to the consequences of a premature activation of non-reclosing relief devices and the associated risk. More early activations have occurred than activations in real fire events. CGA S1.3, Pressure Relief Device Standards-Part 3-Stationary Storage, Containers for Compressed Gases allows for non-reclosing devices, but also recommends having a reclosing device as primary.

API 520, Sizing, Selection, and Installation of Pressure-relieving Devices Part I - Sizing and Selection, provides guidance on relief device selection and installation aimed at process plants. What might make sense in a process plant that has the potential for flammable liquid pool fires that might expose a gas storage vessel to an external fire for an extended period may not apply to other facilities.

Specific considerations not necessarily discussed in either CSA or API standards include:

· A prolonged fire exposure to a vessel may heat the vessel to a level where it is too weak to withstand the relief device set point. For this scenario, a reclosing device would not protect the vessel from reputing whereas a non-reclosing device might.

· Rapid depressurization of a vessel containing high pressure hydrogen can lead to cold temperatures at the nozzle of the vessel and to a lesser extent to the entire vessel. In an external fire case, the cold temperature would likely be mitigated. However, cold temperatures could develop in non-fire venting cases. For metal vessels, the strength of the vessel increases as the vessel cools, thereby reducing susceptibility to failure. But if the vessel is made from carbon or low alloy steel, the vessel may become vulnerable to brittle fracture.

· A depressurization with a non-reclosing device may form a large vapor cloud. Non-reclosing devices are typically larger and depressurize the vessels at a faster rate. There is a high probability that a vapor cloud will form and find an ignition source, resulting in a deflagration. The resultant fireball and overpressure can cause damage and injure people.