ISO 4126

Covers — safety devices protecting against excessive pressure: safety valves (Part 1), bursting discs (Part 2), combinations (Part 3), pilot-operated safety valves (Part 4), CSPRS (Part 5), and two-phase sizing (Part 10), with type testing and certified discharge coefficients.

Does NOT cover (exclusions) — the following are governed elsewhere:

• The API relief-valve family → API 520 / 526 / 527 (parallel US approach).
• Control valve sizing → IEC 60534 / ISA 75.
• The protected vessel construction → EN 13445 / ASME BPVC Section VIII.

ISO 4126 is the EN/PED overpressure standard; API 520-527 is the parallel US family — device certifications do not automatically cross over.

ISO 4126 sets minimum requirements for overpressure devices:

• Device Types — safety valves, bursting discs, pilot-operated valves, and combinations.
• Sizing — required relief area using certified discharge coefficients, including two-phase (Part 10).
• Type Testing & Certification — performance verification and marking for the device.
• Application — set pressure, overpressure, and back-pressure limits for reliable operation.

ISO 4126 is the ISO/European standard for safety devices protecting against excessive pressure — safety valves, bursting discs, pilot-operated valves, combinations, and two-phase sizing across its parts. It is the counterpart to the API 520/526/527 relief-valve family and is used with PED-regulated equipment and EN vessel/piping codes.

Crucially, its sizing coefficients and type-test certification differ from the API approach: a valve certified to ISO 4126 is not automatically API-compliant, and vice versa. The choice follows the governing code (PED/EN vs ASME/API).

Engineering Note: On PED/EN projects, specify ISO 4126-certified relief devices and use its certified coefficients — do not size a European safety valve with API coefficients or assume an API-stamped valve satisfies the PED without the correct certification.