Are most vent line failures due to backpressure or fatigue?

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.