ISO 4126-4:2013

Overview

ISO 4126-4:2013 - "Safety devices for protection against excessive pressure - Part 4: Pilot operated safety valves" is a product standard that specifies general requirements for pilot operated safety valves (POSVs). It applies to POSVs with a valve flow diameter of 4 mm and above and set pressures of 0.1 bar gauge and above. The standard covers design, production testing, type testing, performance determination, sizing principles, and marking/sealing. It is a product standard only and does not cover specific applications or installation.

Key topics and technical requirements

• Scope and definitions: Terms such as set pressure, reseating pressure, blowdown, back pressure (superimposed and built-up), flow area and coefficient of discharge are defined to ensure consistent interpretation.
• Design requirements:
  • Guiding arrangements for consistent operation and seat tightness.
  • Secure fastening of seats and a minimum main valve lift (not restricted to less than 1 mm).
  • Locking/sealing of external adjustments to prevent unauthorized changes.
  • Pilot venting to a safe place for toxic or flammable fluids.
  • Provision of drain connections where liquid can collect.
  • Material selection for sliding surfaces to resist corrosion, wear and galling.
  • Design stress limits referenced to appropriate standards (e.g., EN 12516, ANSI/ASME B16.34).
  • Design to ensure certified capacity discharge even after foreseeable damage (discharge at ≤ 1.1 × maximum allowable pressure).
• Testing and certification:
  • Production tests: hydrostatic, pneumatic, seat leakage, adjustment of set/cold differential test pressure, and pressure seal checks.
  • Type tests: operating and flow characteristic determination, coefficient of discharge measurement and certification.
  • Requirements for determining certified (de-rated) capacities and test reporting.
• Performance & sizing: Methods for defining relieving pressure, certified discharge capacity and for sizing POSVs based on measured or calculated coefficients of discharge.
• Marking and sealing: Minimum marking data and sealing requirements to ensure traceability and tamper evidence.

Applications

ISO 4126-4:2013 is used by manufacturers, testing laboratories, certification bodies, and safety engineers involved in:

• Development and manufacture of pilot operated safety valves for pressure vessels, piping systems, compressors and storage equipment.
• Factory testing and type approval of POSVs.
• Specifying valve performance for procurement and conformity assessment.
• Ensuring safe venting arrangements for hazardous fluids.

Who should use this standard

• Valve manufacturers and design engineers
• Quality and testing laboratories
• Third‑party certifiers and regulatory compliance teams
• Process safety and mechanical integrity engineers

Related standards

• ISO 4126 series (Parts 1–11), especially ISO 4126‑7:2013 (Common data)
• EN 12516 and ANSI/ASME B16.34 (referenced for design stress and valve shell considerations)

Keywords: ISO 4126-4:2013, pilot operated safety valves, safety devices, excessive pressure, valve testing, valve sizing, coefficient of discharge, set pressure, back pressure.