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ISO 28921-2:2015 — Industrial Valves for Low-Temperature Applications — Type Testing of Isolating Valves
A comprehensive technical guide to type testing requirements for cryogenic isolating valves according to ISO 28921-2
Introduction to ISO 28921-2 and Its Importance
ISO 28921-2:2015 specifies type testing requirements for isolating valves intended for low-temperature applications, particularly in cryogenic service conditions. This standard is part of the ISO 28921 series covering industrial valves for low-temperature applications, with Part 2 focusing exclusively on type testing methodologies. The standard applies to valves designed for operating temperatures below -29 °C, extending down to -196 °C (the boiling point of liquid nitrogen), making it essential for industries such as LNG processing, air separation, cryogenic storage, and aerospace propellant handling.
For engineers designing cryogenic systems, ISO 28921-2 compliance is not merely a certification checkbox — it is the primary assurance that valve sealing integrity will be maintained under thermal contraction cycles that can exceed 3 mm per metre of valve body length.
The standard establishes a comprehensive framework for verifying that valve designs can withstand the extreme thermal gradients, material embrittlement risks, and differential contraction challenges inherent in low-temperature service. Unlike ambient-temperature valves, cryogenic isolating valves must maintain bubble-tight sealing while their components undergo significant dimensional changes and material property transitions. ISO 28921-2 addresses these challenges through a structured type testing protocol that simulates the most demanding operating conditions.
Type Testing Requirements and Conditions
Valve Selection and Representativity
A critical aspect of ISO 28921-2 is the requirement for representative valve selection. The standard mandates that the valve submitted for type testing must be representative of the design family for which certification is sought. This includes consideration of valve size range, pressure class, materials of construction, sealing element design, and end connections. The standard provides detailed criteria for selecting which valve sizes within a product range require individual testing and which can be covered by extension rules.
| Parameter | Requirement per ISO 28921-2 | Engineering Consideration |
|---|---|---|
| Test temperature | -196 °C (liquid nitrogen) | Simulates worst-case LNG and cryogenic conditions |
| Test pressure | 1.1 × design pressure (cold) | Accounts for reduced material strength at cryogenic temperatures |
| Thermal cycles | Minimum 2 complete cycles | Validates sealing after repeated thermal shock |
| Leakage rate | Class A (bubble-tight) per ISO 5208 | Zero visible leakage for gas service |
| Cooling rate | Controlled, typically 1-3 K/min | Prevents excessive thermal stress gradients |
| Soak time | Minimum 1 hour at test temperature | Ensures full thermal equilibration of all components |
A common pitfall in cryogenic valve testing is insufficient soak time. Even after external surfaces reach the target temperature, thick-walled sections and trim components may still be significantly warmer, leading to false passing results during seat leakage tests. Always verify internal temperature through instrumented test valves.
Sealing Element Qualification
ISO 28921-2 places particular emphasis on the qualification of sealing elements. The standard recognizes that polymeric and elastomeric seat materials undergo dramatic changes in mechanical properties at cryogenic temperatures, including increased hardness, reduced elasticity, and potential embrittlement. The type testing protocol requires that sealing elements be subjected to the full thermal cycle while maintaining their functional integrity. For each sealing element design and material combination, the standard requires documentation of material certificates, low-temperature property data, and dimensional stability verification.
Test Equipment and Instrumentation Requirements
The standard specifies detailed requirements for the test setup, including cryogenic Dewar vessels or test chambers capable of maintaining the valve fully immersed in liquid nitrogen or equivalent cooling medium. Instrumentation requirements include temperature sensors at multiple locations (valve body, bonnet, stem, and seat areas), pressure transducers with appropriate range and accuracy, and flow measurement devices for leakage quantification. ISO 28921-2 also mandates calibration traceability to national or international standards for all measurement instruments used during type testing.
Modern cryogenic valve test facilities increasingly employ automated data acquisition systems that record temperature, pressure, and leakage data throughout the thermal cycle. This not only improves test reproducibility but also provides valuable finite element validation data for design optimization.
Engineering Design Insights for Cryogenic Valves
Successful type testing per ISO 28921-2 requires careful attention to several design aspects. Material selection is paramount — body and bonnet materials must maintain adequate impact strength at minimum design temperature, typically requiring austenitic stainless steels (CF8M, CF3M, or 316/304L wrought) with certified Charpy V-notch impact values at or below the minimum service temperature. Stem extension length must be sufficient to protect the stem seal area from freezing, with heat flux calculations demonstrating that the packing area remains above 0 °C under steady-state cryogenic conditions.
Seat design for cryogenic service presents unique challenges. Floating ball valves, trunnion-mounted ball valves, and gate valves each have characteristic sealing behaviours at low temperature. ISO 28921-2 type testing validates that the selected seat geometry — whether using spring-energized polymer seats, metal-to-metal seals, or hybrid designs — maintains the required sealing force across the full temperature range accounting for differential thermal contraction between seat materials and the valve body.
Never assume that a valve passing ambient-temperature hydrostatic testing will automatically pass low-temperature gas seat testing. Differential contraction between PTFE seats and metal bodies can reduce sealing interference by 0.1-0.3 mm at -196 °C, which is often sufficient to create a leakage path that is invisible at ambient conditions.
Frequently Asked Questions
Q1: What is the difference between ISO 28921-1 and ISO 28921-2?
ISO 28921-1 covers the general design requirements for isolating valves for low-temperature applications, including material selection, design features, and manufacturing requirements. ISO 28921-2 focuses specifically on type testing procedures, acceptance criteria, and documentation requirements for design qualification.
ISO 28921-1 covers the general design requirements for isolating valves for low-temperature applications, including material selection, design features, and manufacturing requirements. ISO 28921-2 focuses specifically on type testing procedures, acceptance criteria, and documentation requirements for design qualification.
Q2: Can a valve qualified under ISO 28921-2 be used for all cryogenic services?
Type testing per ISO 28921-2 using liquid nitrogen (-196 °C) qualifies the valve design for any service temperature down to -196 °C, provided the materials of construction are suitable for the specific process fluid. Additional compatibility considerations apply for services such as liquid oxygen (oxidizer compatibility) or hydrogen (permeation and embrittlement).
Type testing per ISO 28921-2 using liquid nitrogen (-196 °C) qualifies the valve design for any service temperature down to -196 °C, provided the materials of construction are suitable for the specific process fluid. Additional compatibility considerations apply for services such as liquid oxygen (oxidizer compatibility) or hydrogen (permeation and embrittlement).
Q3: How does ISO 28921-2 relate to other valve testing standards?
ISO 28921-2 builds upon the general valve testing framework of ISO 5208 (pressure testing) and ISO 15848 (fugitive emission testing). For cryogenic applications, it provides additional low-temperature-specific requirements that supplement these general standards. The standard also references ISO 17292 for metal ball valves and ISO 10434 for bolted bonnet gate valves.
ISO 28921-2 builds upon the general valve testing framework of ISO 5208 (pressure testing) and ISO 15848 (fugitive emission testing). For cryogenic applications, it provides additional low-temperature-specific requirements that supplement these general standards. The standard also references ISO 17292 for metal ball valves and ISO 10434 for bolted bonnet gate valves.
Q4: What documentation is required for ISO 28921-2 compliance?
Full type testing documentation includes: a type test plan, material certificates with low-temperature impact test results, dimensional records before and after testing, temperature and pressure recordings throughout the test cycle, leakage test results at ambient and cryogenic temperatures, and a declaration of conformity for the valve design family.
Full type testing documentation includes: a type test plan, material certificates with low-temperature impact test results, dimensional records before and after testing, temperature and pressure recordings throughout the test cycle, leakage test results at ambient and cryogenic temperatures, and a declaration of conformity for the valve design family.
