For PED law, is H2 a stable or an unstable gas?
We have a piping with design pressure of 56 barg and ND 50, which falls in II category if H2 is considered as stable, or in III cat. if H2 is considered as an unstable gas.
An unstable gas typically decomposes over time, causing an increase in internal pressure while doing so, or even combustion.
Pure Hydrogen gas safely contained in a vessel doesn't do this.
The PED Classification term you're probably looking for is Group 1 Dangerous or Group 2 Other (Not contained in Group 1) rather than Stable or Unstable.
See if this classification tool can help you. It's free to use: