Mario,
Forgive me if I have got this totaly wrong, as you know gas is not my strong point, but as I understand the situation after a chat I had with Mike Lambert as FRA Pneumatics rep. his view is that buffer tanks or any other part of the system opperating above 3.4 bar may in theory need to tested and certified.
(see below)

8.7.
All pneumatic components used with pressures greater than 50 psi (3.4 bar) must be rated / tested to at least the maximum pressure available in that part of the system.
You may be required to provide documentation / certification to support this.

(for example, could such doumentation / certification be provided for Davids empty butane bottle?)