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ISO 28921-1:2022 — Industrial Valves — Isolating Valves for Low-Temperature Applications
Design, manufacturing and production testing of cryogenic and low-temperature service valves | Valve engineering guide
Introduction to ISO 28921-1
ISO 28921-1:2022 specifies requirements for design, material selection, manufacturing, and production testing of isolating valves for low-temperature applications. This second edition extends the scope to larger sizes (DN 950-1800, NPS 38-72) and higher pressure classes (PN 400, Class 2500) compared to the 2013 edition. Developed by ISO/TC 153 in collaboration with CEN/TC 69, this standard covers gate valves, globe valves, check valves, butterfly valves, and ball valves designed for service temperatures as low as -196°C (liquid nitrogen temperature).
Low-temperature valves are critical components in LNG (liquefied natural gas) plants, air separation units, cryogenic storage, and aerospace propellant systems. At -196°C, conventional carbon steel becomes brittle and fails catastrophically — specialized materials and design are essential.
Key Design Requirements
Material Selection and Design Features
The standard mandates specific material requirements for low-temperature service. Metallic materials must meet Charpy V-notch impact test requirements at the minimum design temperature or lower. Austenitic stainless steels (304/304L, 316/316L) are the standard choice for -196°C service, while 9% nickel steel and aluminum alloys are specified for specific applications. Internal non-metallic materials (seats, seals, gaskets) must be proven suitable for the intended temperature range through thermal cycling tests. The standard also specifies extended bonnet (stem extension) lengths to ensure that the stem seal operates above freezing even when the valve body is at cryogenic temperature.
| Component | Material Requirement | Temperature Limit | Testing Required |
|---|---|---|---|
| Body/bonnet | SS 304/304L, SS 316/316L, 9% Ni steel, or Al alloy | -196°C to +150°C | Charpy impact at Tmin – 5°C |
| Stem | SS 316/316L, SS 630 (17-4 PH), or Inconel 718 | -196°C to +250°C | Tensile + impact + intergranular corrosion |
| Seat rings | PTFE, PEEK, reinforced PTFE, or stellite | Depends on material | Thermal cycling (10 cycles min) |
| Gaskets | Expanded PTFE, flexible graphite, or spiral wound | -196°C to +400°C | Compression + leakage at temperature |
| Bolting | SS 304/304L or low-temperature alloy steel | -196°C to +150°C | Charpy impact + tensile |
Body/bonnet wall thickness for low-temperature valves must be at least the thickness required for the corresponding pressure class, with additional allowance for corrosion, erosion, and the reduced ductility at low temperatures. The standard references ASME B16.34 and ISO 7005 for pressure-temperature ratings.
Engineering Design Insights
Extended Bonnet and Stem Design
The extended bonnet (also called the cold column or neck tube) is the most distinctive feature of cryogenic valves. Its length must ensure that the stem seal temperature stays above 0°C when the valve body is at -196°C, preventing ice formation on sealing surfaces. The required extension length depends on material thermal conductivity — for stainless steel, typical lengths range from 150 mm (DN 50) to 500 mm (DN 600). The standard provides detailed equations for calculating minimum extension length based on heat transfer analysis.
The stem design must accommodate differential thermal contraction between the stem (typically stainless steel) and the body. At -196°C, the stem contracts approximately 3 mm/m relative to room temperature. For a 1 m valve height, this means 3 mm of stem retraction that must not compromise the sealing force or operational torque.
A critical production test specified in Annex A is the low-temperature seat leakage test: the valve is cooled to the minimum design temperature, the body is pressurized with nitrogen or helium at the rated pressure, and seat leakage is measured using a flow meter or bubble test. For soft-seated valves, the allowable leakage rate is typically 0.1-0.5 cc/min/inch of seat diameter. Metal-seated valves allow higher rates specified by the manufacturer.
Production Testing and Quality Assurance
The standard defines production testing (100% of valves undergo hydrostatic shell test, seat leakage test at room temperature, and low-temperature seat leakage test on a sampling basis) and type testing (per ISO 28921-2 for design validation). The sampling plan requires testing one valve from each lot of 50 or fewer valves. If any valve fails the low-temperature test, the entire lot is rejected, and the cause of failure must be investigated.
Practical Valve Manufacturing Application
A manufacturer of cryogenic ball valves for LNG plant applications implemented ISO 28921-1:2022 requirements in their production line. The valves, designed for -196°C service in LNG transfer lines, featured extended bonnets of 300 mm length for DN 150 size, austenitic stainless steel bodies (CF8M), and PTFE seats with glass-fiber reinforcement. Production testing following Annex A demonstrated that 98.5% of valves passed the low-temperature seat leakage test on first attempt (allowable leakage of 0.3 cc/min/inch of seat diameter for soft-seated valves).
A critical engineering insight from the production experience was the importance of the thermal cycle rate during low-temperature testing. The standard specifies cooling at a rate not exceeding 1°C per minute to avoid thermal shock, followed by a stabilization period of at least 1 hour after reaching the minimum test temperature. The total test cycle time of 8-10 hours (including warm-up) constrained production testing capacity. To optimize throughput, the manufacturer implemented a sampling plan based on the standard’s lot acceptance provisions: testing one valve from each production lot of 50, with 200% lot testing for initial validation and 100% lot testing reduced to 25% after 12 consecutive lots passed without failure.
The extension of the standard’s scope to DN 1800 valves in the 2022 edition addressed the growing demand for LNG unloading arms at large-scale import terminals. Valves of this size weigh up to 15 tonnes and require specialized handling equipment. The standard’s provisions for body wall thickness, extended bonnet length, and cold box installation are directly applicable to these large-scale applications.
Frequently Asked Questions
Q: What is the difference between ISO 28921-1 and the previous edition?
A: The 2022 edition extends the size range to DN 1800 and pressure to PN 400/Class 2500, adds definitions for shell and drip plate, excludes safety and control valves, adds type testing requirements from ISO 28921-2, and updates the low-temperature test procedure.
A: The 2022 edition extends the size range to DN 1800 and pressure to PN 400/Class 2500, adds definitions for shell and drip plate, excludes safety and control valves, adds type testing requirements from ISO 28921-2, and updates the low-temperature test procedure.
Q: Can butterfly valves be used at -196°C?
A: Yes, but special design considerations are required including extended bonnet for the shaft seal, cryogenic-compatible seat material (typically PTFE or PEEK), and thermal compensation for differential contraction between the disc and body.
A: Yes, but special design considerations are required including extended bonnet for the shaft seal, cryogenic-compatible seat material (typically PTFE or PEEK), and thermal compensation for differential contraction between the disc and body.
Q: What is the purpose of the drip plate in cryogenic valves?
A: The drip plate (added in the 2022 edition) is positioned below the bonnet flange to prevent condensed water or ice from falling onto the extended bonnet and causing ice bridging, which could impair valve operation.
A: The drip plate (added in the 2022 edition) is positioned below the bonnet flange to prevent condensed water or ice from falling onto the extended bonnet and causing ice bridging, which could impair valve operation.
Q: How is the extended bonnet length calculated?
A> The minimum length Lmin = k · (Tamb – Tmin) / Qmax, where k is the thermal conductivity of the bonnet material, Tamb is ambient temperature, Tmin is the minimum service temperature, and Qmax is the allowable heat flux into the process to prevent seal icing.
A> The minimum length Lmin = k · (Tamb – Tmin) / Qmax, where k is the thermal conductivity of the bonnet material, Tamb is ambient temperature, Tmin is the minimum service temperature, and Qmax is the allowable heat flux into the process to prevent seal icing.
