Overview
ISO 4126-7:2013 - "Safety devices for protection against excessive pressure - Part 7: Common data" provides the shared data, definitions, symbols, equations and correction factors used across the ISO 4126 series for safety valves. Published as the second edition (2013), this part avoids repetition in Parts 1–6 by supplying common technical material for the determination of performance, sizing and testing of safety valves and related pressure-relief devices.
Key keywords: ISO 4126-7:2013, safety valves, excessive pressure, pressure relief, discharge capacity, coefficient of discharge.
Key Topics and Technical Requirements
- Scope and terminology: Unified definitions (e.g., set pressure, relieving pressure, back pressure, certified capacity) and the pressure unit (bar, gauge or absolute).
- Symbols and units: Standardized symbols (A, K, p, T, Z, Re, etc.) and units used throughout ISO 4126 Parts 1–6.
- Performance determination:
- Calculation of coefficient of discharge (Kd) from test data and how to report it (three significant decimal places, rounding down).
- Criteria for critical and subcritical flow and associated formulae for flow regimes.
- Discharge capacity equations for steam and gases under critical flow and for non‑flashing liquids in turbulent flow.
- Sizing of safety valves: General sizing rules, separate guidance for gas/vapor relief and capacity calculation methods (including limits on use of the ideal gas formula).
- Thermodynamic properties and correction factors:
- Steam data and steam pressure coefficient tables.
- Charts/tables for compressibility factor (Z), viscosity correction factors (Kv), and capacity correction for subcritical flows (Kb).
- Warnings: do not use the ideal gas formula when relieving temperature > 90% of critical temperature or relieving pressure > 50% of critical pressure; condensation is not considered in that method.
- Mechanical requirements: Minimum requirements, materials, marking, dimensions and testing tolerances for helical compression springs and disc springs used in safety valves.
- Examples and annexes: Worked capacity calculation examples and bibliographic references.
Practical Applications and Users
This standard is applicable for:
- Valve manufacturers - to design, test and certify safety valve performance and to define certified discharge capacity (including the 0.9 de-rating convention).
- Process and mechanical engineers - for accurate sizing and selection of relief devices in pressure equipment, steam systems and gas installations.
- Safety engineers, inspectors and certification bodies - to verify calculations, test results and component conformity to ISO 4126 series.
- Plant designers and OEMs - to integrate relief systems consistent with international practice and to apply correction factors for real fluids.
Related Standards (within ISO 4126 series)
Use ISO 4126-7:2013 when you need a consistent, authoritative set of common technical data and correction methods for sizing, testing and certifying safety valves and related pressure-relief devices.
