I am in the process of fine tuning a relief valve inspection / reliability program and am looking for information / criteria to use in order to determine when a valve needs to be evaluated for reliability.

For example the blowdown setting when installed was X. After being pulled for recertification it is now tests on the pretest X +/-.

Can anyone provide information on final test blowdown requirements for section VIII relief valves? Are there any rules of thumb for pretest criteria for blowdown?
 
I  am specifically looking for criteria on adjustable and fixed blowdown settings.

Thanks for your help

Fzob