Overview
EN ISO 23251:2020 (ISO 23251:2019) provides guidance for the selection, design and application of pressure-relieving and vapour depressuring systems in the petroleum, petrochemical and natural gas industries. Intended primarily for oil refineries, it also applies to petrochemical plants, gas plants, LNG facilities and upstream production sites. The standard supplements API Std 521 (6th ed., 2014) and identifies additional requirements and clarifications specific to European and international practice.
Key topics and technical coverage
- Scope and applicability - pressure-relief devices (PRDs), rupture discs, safety valves and vapour depressuring systems used to protect equipment from overpressure and enable controlled depressuring.
- Supplements to API Std 521 - provisions that modify or add to API Std 521 including exceptions and additional guidance.
- Failure modes and special cases - guidance on chemical reactions, fire scenarios, causes of vacuum, choke valve failures, check valve reverse-flow evaluation and acoustic fatigue.
- PRD considerations - selection, use of multiple PRDs, interaction between devices, and requirements for individual PRDs.
- Piping and backpressure - line sizing, backpressure effects, reaction forces and routing for relief lines.
- Disposal to flare - combustion properties, flare system design and sizing, ignition of flare gases, and use of liquid-seal drums.
- Administrative and operational controls - conditions under which administrative measures (e.g., car‑sealing or chain‑locking valves) may be used, and limits tied to hydrostatic test pressures.
- Alternative methods - guidance for alternative approaches to fire relief and blowdown system design where justified.
Practical applications
EN ISO 23251:2020 is used to:
- Specify and verify design of pressure-relieving and depressuring systems for refineries, petrochemical plants, LNG terminals and gas processing facilities.
- Determine appropriate PRD selection (safety valves, rupture discs) and relief line sizing for process safety and regulatory compliance.
- Define safe flare disposal arrangements and depressuring plans for emergency scenarios (fire, runaway reactions, equipment failure).
- Inform risk assessments and engineering studies addressing overpressure protection, acoustic fatigue and valve failure modes.
Who should use this standard
- Process safety and piping engineers
- Pressure equipment designers and manufacturers (PRVs, rupture discs)
- Flare system and relief-system designers
- Plant operators, safety managers and regulatory compliance teams
- Vendors preparing equipment specifications or certifying conformance
Related standards and references
- API Std 521 (6th edition, 2014) - primary reference supplemented by EN ISO 23251
- ISO 4126 (safety devices for protection against excessive pressure)
- ISO 15649 (piping)
- ISO 25457 (flare details)
- ISO 28300 (venting of low-pressure tanks)
- EN 764-7 (safety systems for unfired pressure equipment)
EN ISO 23251:2020 is essential for organizations seeking robust, internationally aligned guidance on pressure relief and depressuring system design and operation in hydrocarbon processing facilities.
