Overview
ISO 4126-10:2024 - "Safety devices for protection against excessive pressure - Part 10: Sizing of safety valves and bursting discs for gas/liquid two-phase flow" provides a standardized, practical method to size safety valves and bursting discs when relieving gas/liquid two‑phase flow. It covers pressurized systems such as reactors, storage tanks, columns, heat exchangers, piping systems and transportation tanks/containers. The standard uses absolute pressures (not gauge) and extends established relief sizing practice to the more complex two‑phase flow case.
Key topics and technical requirements
- Scope and fluid states: Lists possible fluid states at the safety device inlet that can create two‑phase flow (e.g., subcooled liquid flashing, saturated boiling, vapour condensing, gas/liquid mixtures, dissolved gases).
- Sizing methodology: Based on the omega (ω) parameter method, extended with a thermodynamic non‑equilibrium parameter to model nonequilibrium phase change during relief.
- Structured sizing steps:
- Identification of the sizing scenario
- Prediction of flow regime at the safety device inlet (gas/vapour vs two‑phase)
- Calculation of the mass flow rate required to be discharged (including scenarios: excess inflow, external heating, thermal runaway)
- Calculation of the dischargeable mass flux through the safety device and vent system, including pressure change
- Verification of vent line performance under plant conditions
- Advanced considerations: Iterative calculation for coupled device size and flow state; allowance for slip between phases; inclusion of outlet velocity and pressure losses upstream/downstream of the device; compressibility coefficient and numerical methods for ω.
- Limitations and special cases: Addresses flashing flow, condensing flow, multi‑component liquids, dissolved gases and boundaries of the method’s applicability.
- Practical aids: Examples in Annex B (mass flow to be discharged), Annex C (dischargeable mass flux and vent line pressure drop) and an Environmental factor (Annex D).
Practical applications and users
- Who benefits: Pressure safety engineers, process designers, relief system and valve manufacturers, piping and vessel designers, safety assessors, regulatory authorities and inspectors.
- Where it’s used: Relief device sizing for chemical plants, petrochemical refineries, power plants, cryogenic and thermal storage, transport tanks, and any system where two‑phase discharge can occur.
- Why use it: Provides a consistent, industry‑recognized approach to prevent overpressure in systems where two‑phase flow changes required relief area and discharge behavior compared to single‑phase sizing.
Related standards
- ISO 4126 series (other parts on safety devices)
- ISO 4126-7:2013/Amd 1:2016 (single‑phase flow formulae referenced and converted to SI units)
For design or legal compliance, obtain the full ISO 4126-10:2024 publication from ISO or your national standards body.
Open preview (PDF)Safety devices for protection against excessive pressure — Part 10: Sizing of safety valves and bursting discs for gas/liquid two-phase flow
