The standard addresses the challenges of discharge piping, specifically and Superimposed Back Pressure .
ISO standards are copyrighted intellectual property. They are not typically available for free legal download. Official PDFs must be purchased from: Iso 4126-9 Pdf
Conclusion
The following individuals and organizations should use ISO 4126-9:2019: The standard addresses the challenges of discharge piping,
Title: ISO 4126-9 — Safety Devices for Protection Against Excessive Pressure — Part 9: Requirements for Pilot-Operated Safety Valves Iso 4126-9 Pdf