- ISO 4126 is a family of standards for safety devices for protection against excessive pressure.
- ISO 4126-1 is the product standard normally referenced for safety valves, while ISO 4126-4 covers pilot operated safety valves.
- ISO 4126-3 addresses safety valves or CSPRS installed in series with bursting disc safety devices upstream of the valve.
- ISO 4126-9 supports application and installation review, while ISO 4126-10:2024 covers sizing of safety valves and bursting discs for gas/liquid two-phase flow.
››› ISO 4126 Overview
ISO 4126 helps buyers define valve type, performance, testing and documentation before procurement
ISO 4126 Safety Valve Standards should be used as a technical framework rather than a simple keyword on a purchase order. For a reliable RFQ, the buyer still needs protected equipment data, set pressure, required capacity, medium, temperature, allowable overpressure, back pressure, inlet and outlet piping, material requirements and documents.
The most common procurement confusion is treating ISO 4126 as one single valve standard. In practice, ISO 4126 includes separate parts for safety valves, bursting discs, combination assemblies, pilot operated safety valves, controlled safety pressure relief systems, common data, application and installation, and two-phase flow sizing.
What this page helps you confirm
- Which ISO 4126 part is relevant to the valve or relief device in your RFQ.
- How ISO 4126 connects to real safety valve datasheets and sizing inputs.
- When to review pilot operated valves, bursting disc combinations or CSPRS.
- Which documents should be requested before production, FAT and shipment.
- How ISO 4126 relates to ASME, API and DIN / EN references in international safety valve RFQs.
››› ISO 4126 Parts
Key ISO 4126 references for safety valve and relief device RFQs
Use the references below to identify which ISO 4126 part matches the relief device, installation case or sizing question in your RFQ. Official ISO records should be checked before final compliance decisions, especially when a project specification names a specific edition.
ISO 4126-1
Safety Valves
Core product reference for safety valves. Use this when the RFQ asks for ISO safety valves, type testing, production testing, marking, set pressure and discharge performance documents.
ISO 4126-2
Bursting Disc Safety Devices
Relevant when the pressure protection package uses bursting discs for corrosion isolation, zero leakage, fast opening or as part of a safety valve combination assembly.
ISO 4126-3
Safety Valve + Bursting Disc Combination
Used when a bursting disc is installed upstream of a safety valve or CSPRS. Review installation distance, pressure monitoring, marking, capacity effect and maintenance philosophy.
ISO 4126-4
Pilot Operated Safety Valves
Relevant for pilot operated safety valves used in clean gas, large capacity, high set pressure or selected back pressure applications. Pilot cleanliness and operating conditions must be checked.
ISO 4126-5
Controlled Safety Pressure Relief Systems
Relevant when a project specifies CSPRS instead of a conventional spring-loaded or pilot operated safety valve arrangement.
ISO 4126-7
Common Data
Useful for shared definitions, calculation data and supporting information referenced by other ISO 4126 parts.
ISO 4126-9
Application and Installation
Supports installation review for safety devices excluding stand-alone bursting disc devices. It is useful for inlet/outlet piping, isolation, back pressure and system layout discussions.
ISO 4126-10
Two-Phase Flow Sizing
Relevant for gas/liquid two-phase relief from reactors, tanks, columns, heat exchangers, piping and transport containers.
EN ISO 4126
European CE / PED Projects
In European projects, EN ISO 4126 may be referenced with PED, EN flanges and valve testing requirements. Confirm harmonized standard and notified-body expectations early.
››› How to Use ISO 4126
Do not select the valve by standard number alone
ISO 4126 can identify the product family and performance reference, but the valve cannot be quoted correctly without operating data. A complete RFQ should still define relief scenario, capacity basis, medium, temperature, back pressure, installation and documentation.
For example, a spring-loaded steam valve, a pilot operated gas valve, a valve protected by an upstream rupture disc, and a two-phase reactor relief case may all reference ISO 4126, but they require different engineering checks.
Minimum engineering checks
- Confirm valve type: spring-loaded, pilot operated, bursting disc combination, CSPRS or thermal relief.
- Define protected equipment, MAWP, set pressure, allowable overpressure and operating pressure margin.
- Provide required relieving capacity and calculation basis, not only pipe size or flange size.
- State medium phase: gas, vapor, steam, liquid, flashing liquid or gas/liquid two-phase flow.
- Review back pressure, inlet pressure loss, discharge route and installation limitations.
- Request sizing calculation, material certificate, set pressure report, pressure test and seat test.
››› Practical Cases
ISO 4126 safety valve application cases with typical RFQ data
These cases show how ISO 4126 requirements usually appear in real RFQs. Final selection must still be confirmed by protected equipment datasheet, sizing calculation and project specification.
Case 1: ISO 4126-1 Spring-Loaded Safety Valve for a Pressure Vessel
Protected equipment: separator, receiver, reactor or storage vessel. Typical data: medium, set pressure, MAWP, required capacity, relieving temperature, back pressure, material, connection, seat tightness and test certificates.
Case 2: ISO 4126-4 Pilot Operated Safety Valve for Clean Gas
Protected equipment: gas receiver, compressor discharge, storage vessel or PRV station. Typical data: gas composition, set pressure, back pressure, required capacity, pilot cleanliness, operating margin and leakage requirement.
Case 3: ISO 4126-3 Bursting Disc Upstream of a Safety Valve
Protected equipment: corrosive, toxic, fouling or zero-leakage service. Typical data: burst pressure, safety valve set pressure, installation distance, pressure monitoring between disc and valve, capacity correction and replacement policy.
Case 4: ISO 4126-10 Two-Phase Relief From a Reactor
Protected equipment: reactor, column, heat exchanger, tank or transport container. Typical data: vapor/liquid composition, mass fraction, relieving pressure, temperature, two-phase flow model, discharge route and disposal system.
Case 5: EN ISO 4126 for CE / PED Pressure Equipment
Protected equipment: European pressure vessel or skid package. Typical data: PED category, notified-body requirement, EN flange standard, material certificate, conformity documents and valve test records.
Case 6: International RFQ Comparing ISO 4126 With API / ASME
Protected equipment: global EPC package. Typical data: ISO 4126 reference, ASME code basis, API 520 sizing basis, API 527 seat test requirement, flange standard and documentation acceptance criteria.
SHOP-FLOOR VERIFICATION
From Code Requirement to Shop-Floor Verification
Standards define technical requirements, but purchase-order compliance depends on design, materials, inspection scope and recorded test results. This actual ZOBAI photograph documents safety valve pressure-testing workstation. It is facility evidence, not a substitute for project-specific certificates or an agreed inspection and test plan.
››› RFQ Checklist
Information to send when your project specifies ISO 4126
Complete RFQ data helps confirm whether ISO 4126-1, -3, -4, -9 or -10 is the right reference for your safety device and whether the quoted valve can protect the equipment.
- Protected equipment: pressure vessel, boiler, pipeline, tank, compressor, reactor, heat exchanger or skid.
- Applicable reference: ISO 4126-1, -2, -3, -4, -5, -7, -9, -10, EN ISO 4126 or project standard.
- Valve type: spring-loaded safety valve, pilot operated valve, bursting disc combination, CSPRS or two-phase relief device.
- Design pressure / MAWP, set pressure, overpressure, operating pressure and operating margin.
- Medium name, phase, molecular weight, density, viscosity, vapor/liquid fraction and special hazards.
- Required relieving capacity, sizing basis, relief scenario and calculation method.
- Back pressure, inlet pressure loss, discharge route, silencer, flare, scrubber, vent or drain system.
- Required documents: datasheet, drawing, sizing report, MTC, set pressure certificate, pressure test and seat tightness test.
››› Related Standards & Selection Pages
Continue your ISO 4126 safety valve standards review
Use these pages to connect ISO 4126 with international code references, product families, sizing, testing and final RFQ preparation.
Safety Valve Standards Guide
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
DIN / EN Safety Valve Standards
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
ASME Safety Valve Standards
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
API 520 Safety Valve Sizing
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
API 521 Pressure Relief Systems
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
API 527 Seat Tightness Test
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
Safety Valve Sizing
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
Pilot Operated Safety Valves
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
Spring Loaded Safety Valves
Open this related resource to compare requirements, prepare RFQ data and support safety valve selection.
››› FAQ
ISO 4126 Safety Valve Standards FAQ
Short answers for common procurement and engineering questions related to ISO 4126 safety valves.
What is ISO 4126 used for in safety valve projects?
ISO 4126 is used as a standards family for safety devices that protect against excessive pressure. In safety valve procurement, it helps define product type, performance, testing, installation and documentation requirements depending on the applicable part.
Which ISO 4126 part applies to safety valves?
ISO 4126-1 is the core product standard for safety valves. If the RFQ uses a pilot operated safety valve, ISO 4126-4 may be relevant. If a bursting disc is installed upstream of the valve, ISO 4126-3 should be reviewed.
Is ISO 4126 the same as API 520?
No. ISO 4126 and API 520 can both appear in pressure relief projects, but they are not the same document. ISO 4126 is an international standards family for safety devices, while API 520 is commonly used for sizing and selection of pressure-relieving devices in many process industry projects.
When should ISO 4126-10 be reviewed?
ISO 4126-10 should be reviewed when the relieving flow may be gas/liquid two-phase flow, such as reactor relief, flashing liquid, fire case with liquid inventory, columns, heat exchangers, piping systems or transport containers.
What data should be included in an ISO 4126 safety valve RFQ?
An ISO 4126 safety valve RFQ should include protected equipment, applicable ISO 4126 part, valve type, medium, phase, set pressure, design pressure, operating pressure, relieving capacity, temperature, back pressure, connection standard, material, test requirement and required documents.
Can ISO 4126 be used for EN / PED projects?
Yes, EN ISO 4126 is often used in European pressure equipment projects together with PED requirements, EN flange standards, material documents and project-specific conformity documents. The exact compliance route should be confirmed with the buyer, EPC contractor or notified body.