Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Wilesco has been quoted as saying their safety valve pops at 1.8 bar which would be 26 psi.

I have not tested the current Wilesco safety valves, but would assume about 1.5 bar/22ish psi these days.

The 1.8 bar/26 psi number, origins from the original instruction sheets, accompanying engines carrying the earlier weighted safety valves:
[ Guests cannot view attachments ]