Rupture disks are differential pressure devices; they burst based on the pressure difference across the disk element. If a pinhole leak develops in the rupture disk, process pressure will slowly migrate into the interspace. If this space becomes pressurized, the disk will require a significantly higher vessel pressure to burst, effectively rendering the safety system useless.
Perhaps the most crucial requirement outlined in ISO 4126-9 is the management of the space between the rupture disk and the safety valve seat (the interspace). Iso 4126-9 Pdf
The primary goal of ISO 4126-9 is to ensure that a safety system acts as the for pressure equipment. It assumes that the fluid being discharged is in a single-phase flow (either pure liquid or pure gas); if you are dealing with two-phase flow (gas/liquid mixtures), you must refer to ISO 4126-10 for specific guidance. What is Covered? Rupture disks are differential pressure devices; they burst