ISO 4126-5:2013

Overview

ISO 4126-5:2013 - Safety devices for protection against excessive pressure: Part 5: Controlled Safety Pressure Relief Systems (CSPRS) - defines product requirements for controlled safety pressure relief systems regardless of fluid type. It applies to systems with main valves having a flow diameter ≥ 4 mm and intended for use at pressures ≥ 0.1 bar gauge. No temperature limitation is imposed. This is a product standard (not an application/installation standard) and complements other parts of the ISO 4126 series.

Key technical topics and requirements

• System definition and components: CSPRS consist of a main valve plus a control unit (pressure tapping line, pressure sensor, sensing line, control module, control line). Redundancy of control paths is required.
• Operating principles: Distinguishes relieving (valve opens when control force is released) and loading (valve opens when control force is applied) principles, plus open-circuit and closed-circuit control philosophies.
• Design requirements: Guidance on valve end connections, minimum spring requirements, materials selection and actuator/vent arrangements.
• Testing and verification:
  • Production testing: hydrostatic and pneumatic testing, seat leakage, pressure seals, adjustment of set (cold differential) test pressure.
  • Type testing: determination of operating characteristics (opening/reseating times, dead times), flow characteristics, and coefficient of discharge.
  • Certification procedures for discharge capacity and CSPRS performance.
• Performance, sizing and marking:
  • Procedures for determining CSPRS performance and sizing for relief duties.
  • Requirements for marking and sealing of certified devices.
• Terminology and safety parameters: Definitions for set pressure, reseating pressure, overpressure, blowdown, back pressure (superimposed and built-up), flow area/diameter, and certified discharge capacity.

Practical applications

ISO 4126-5 is used to ensure that CSPRS products meet consistent safety, test and certification criteria. Typical applications include:

• Pressure protection components supplied to process industries (chemical, oil & gas, power generation, water/wastewater).
• Systems where controlled relief or safety shut‑off is required to prevent overpressure of equipment and piping. Note: the standard specifies product requirements only - selection, installation and application considerations are covered in other parts of ISO 4126.

Who should use this standard

• Valve and CSPRS manufacturers for design, testing and certification.
• Test laboratories and certification bodies performing type and production testing.
• Procurement and plant safety engineers specifying product performance and accepting certified CSPRS.
• OEMs and pressure equipment manufacturers integrating CSPRS into systems.

Related standards

• ISO 4126 series (Part 1, Part 2, Part 3, Part 4, Part 6, Part 7, etc.) - see ISO 4126-7:2013 for common data and other parts for application/installation, bursting discs, pilot-operated valves and sizing guidance.

Keywords: ISO 4126-5, CSPRS, controlled safety pressure relief systems, safety devices, pressure relief, set pressure, coefficient of discharge, production testing, type testing, valve sizing.