Overview
EN ISO 4126-10:2024 - Safety devices for protection against excessive pressure (Part 10) specifies methods for sizing safety valves and bursting discs for gas/liquid two‑phase flow. It applies to pressurized systems such as reactors, storage tanks, columns, heat exchangers, piping and transportation tanks/containers. The standard defines the sizing procedure, applicability limits and the information needed to determine discharge mass flow and vent‑line performance for two‑phase flow scenarios. Note: pressures in this standard are absolute, not gauge.
Key topics and technical requirements
- Scope and application range: sizing approach for two‑phase (gas/liquid) flow at safety device inlets; examples of covered equipment are listed (reactors, tanks, columns, heat exchangers, piping, transport containers).
- Sizing steps and workflow: structured procedure covering identification of the sizing case, prediction of flow regime, calculation of required mass flow, and computation of dischargeable mass flux and pressure change in vent lines.
- Flow regimes and phenomena: guidance on predicting gas/vapour vs two‑phase flow, level swell, effects of liquid viscosity and foaming.
- Mass flow calculations: methods for mass flow required to be discharged due to overpressure causes (inflow excess, external heating, thermal runaway).
- Two‑phase discharge modelling: key parameters such as the two‑phase discharge coefficient (K), dimensionless mass flow rate (C), and the compressibility coefficient ω are used to calculate downstream stagnation conditions and vent performance.
- Limitations and special cases: explicit limits for flashing flow, condensing flow, multi‑component liquids, dissolved gases, immiscible liquids, and rapid pressure/temperature transients.
- Corrections and practical factors: slip corrections for non‑flashing and straight‑pipe two‑phase flows; consideration of vent‑line geometry and behaviour.
- Regulatory linkage: informative Annex ZA explains the relationship to the EU Pressure Equipment Directive (2014/68/EU) and presumption of conformity where applicable.
Applications and practical use
- Designing and selecting safety valves and bursting discs for equipment subject to two‑phase discharge.
- Performing relief system calculations for process safety analyses, relief network sizing, and emergency vent design.
- Assessing vent‑line performance under realistic two‑phase discharge conditions to ensure proper operation and avoid malfunction (choking, spring‑loading issues).
- Supporting compliance documentation for pressure equipment and CE marking under relevant directives.
Who should use this standard
- Process, mechanical and pressure‑safety engineers
- Relief system designers and consultant engineers
- Valve and bursting disc manufacturers
- Regulatory and compliance specialists responsible for pressure equipment safety
Related standards
- ISO 4126 series (other parts), including ISO 4126‑7 (common data for safety devices)
- EU Pressure Equipment Directive 2014/68/EU (see Annex ZA for conformity guidance)
Keywords: EN ISO 4126-10:2024, safety valves, bursting discs, two‑phase flow, sizing, pressurized systems, relief device sizing, pressure safety, compressibility coefficient.
