ISO 4126-3:2020

ISO 4126-3:2020 - Overview

ISO 4126-3:2020 defines requirements for in‑series combinations of safety valves (or CSPRS) and bursting disc safety devices installed close‑coupled (within five pipe diameters) upstream of the valve. The standard covers design, application, installation, marking and documentation for the assembled product (bursting disc, safety valve/CSPRS and any connecting spool piece). It also provides a method to establish and certify the combination discharge capacity factor (Fd) used when sizing relief systems.

Key topics and technical requirements

• Scope and definitions: Clear terms for combination, bursting disc assembly, bursting pressure, Fd, coefficients of discharge, set pressure and related symbols.
• Design and construction: Requirements that each component comply with applicable parts of the ISO 4126 series (e.g., ISO 4126‑1, ‑2, ‑4, ‑5) and that any spool piece meets relevant design standards.
• Installation and inlet line: Combination must be installed within five pipe diameters of the valve inlet; inlet line pressure drop (non‑recoverable losses) must be calculated and meet ISO 4126‑9 requirements.
• Combination performance & testing:
  • Methodology for determining the combination discharge capacity factor (Fd) by testing, including test rig, procedure and acceptance criteria.
  • Conditions and parameters applicable to the safety valve and the bursting disc during tests.
  • Alternatives to testing and rules for certifying and applying a certified Fd, including limits on use for sizes larger than those tested.
• Marking, identification and documentation: Requirements for marking the bursting disc, valve and combined assembly and for supplying test certificates when a certified Fd is used.
• Storage and transport: Preparation and handling guidance for the assembled product.

Applications and practical value

ISO 4126-3 is essential when specifying or installing close‑coupled pressure relief combinations where a bursting disc is used upstream of a safety valve/CSPRS to:

• Protect valves from corrosive or fouling services
• Prevent leakage or limit product loss after disc burst
• Ensure accurate relief sizing using a validated combination discharge factor

Typical users:

• Pressure‑relief system designers and process safety engineers
• Valve and bursting disc manufacturers and assemblers
• Plant engineering, operations and maintenance teams
• Third‑party certifiers and regulatory compliance personnel

Related standards

• ISO 4126‑1: Safety valves
• ISO 4126‑2: Bursting disc safety devices
• ISO 4126‑4: Pilot operated safety valves
• ISO 4126‑5: CSPRS
• ISO 4126‑6: Application and selection of bursting disc devices
• ISO 4126‑9: Application and installation (inlet line requirements)

ISO 4126-3:2020 supports reliable pressure relief design by defining how to assemble, test, mark and apply safety valve + bursting disc combinations and by standardizing the procedure to derive the combination discharge factor used for accurate relief sizing.