ISO 23251:2019

Overview

ISO 23251:2019 provides guidance for pressure-relieving and vapour depressuring systems used in the petroleum, petrochemical and natural gas industries. Intended primarily for oil refineries, it also applies to petrochemical facilities, gas plants, LNG facilities and upstream oil & gas production. ISO 23251 supplements API Std 521 (6th edition, 2014), specifying where API 521 applies and where ISO-specific methods or standards replace API references. The standard helps users select the most appropriate pressure-relief (PR) and depressuring solution for the risks and circumstances of particular installations.

Key topics and technical requirements

ISO 23251 focuses on practical design and selection issues for pressure-relieving and depressuring systems, including:

• Scope and definitions for PR devices (PRVs, safety valves, rupture disc devices) and system terms aligned with API Std 521.
• Supplements to API Std 521: specifies exceptions and clarifications where ISO methods apply instead of API documents.
• Chemical reaction and two-phase flow: references ISO 4126-10 as an additional methodology for emergency vent sizing in chemical reaction and mixed-phase scenarios.
• Fire scenarios and fire relief design: uses ISO 28300 for tank venting guidance and the ISO 4126 series for valve sizing methods in fire cases.
• Piping, backpressure and line sizing: applies ISO 15649 and ISO 4126 series for piping design, backpressure and sizing of relief lines.
• Multiple PRDs and system interactions: guidance on combining devices, reaction forces and evaluation of multiple relief scenarios (EN 764-7 referenced for safety systems).
• Disposal to flare: design, combustion properties, ignition and sizing of flare systems with ISO 25457 referenced for flare details and methods.
• Updated design considerations: allowance for administrative controls (e.g., chain-locking) under specific conditions, added requirements for choke valve failures and acoustic fatigue, and strengthened guidance on check valve reverse-flow evaluation.

Applications and who uses it

ISO 23251 is used by:

• Process, safety and mechanical engineers designing pressure-relief and depressuring systems
• Plant designers and EPC contractors in refineries, petrochemical plants, gas plants and LNG facilities
• Operations and maintenance teams responsible for PR device selection, testing and inspection
• Equipment vendors supplying PRVs, rupture discs, flare systems and associated piping
• Regulators and compliance officers referencing international best practice

Practical uses include PRV and rupture-disc selection, emergency depressuring strategy, flare system design and sizing, piping layout for relief lines, and evaluation of failure modes (choke valves, check valves, acoustic fatigue).

Related standards

• API Std 521 (6th edition, 2014) - primary reference supplemented by ISO 23251
• ISO 4126 series (including ISO 4126-10) - safety devices and emergency vent sizing for two-phase flow
• ISO 25457 - flare details and flare system methods
• ISO 15649 - piping for petroleum and natural gas industries
• ISO 28300 - venting of atmospheric and low-pressure storage tanks
• EN 764-7 - safety systems for unfired pressure equipment

Keywords: pressure-relieving systems, depressuring systems, PRV, rupture disc, flare system design, oil refinery safety, petrochemical pressure relief, LNG depressuring.