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Requirements for Cryogenic Butterfly Valves
Created on 07.23
Chapter 1: Technical Requirements for Cryogenic Butterfly Valves
1. Structural Configuration and Design Principles
Cryogenic butterfly valves shall adopt a triple-offset metal-seated sealing structure to ensure reliable sealing and long service life under cryogenic service conditions. The three offsets of the triple-offset butterfly valve are defined as follows:
- Shaft Offset:
- Spherical Offset:
- Sealing Surface Offset:
The disc shall be equipped with a stainless steel sealing ring, and the seat shall be constructed of stainless steel with surface hardening treatment applied. Both the disc sealing ring and the seat shall be manufactured from solid stainless steel material.
The valve body shall be furnished with a drip tray to prevent uncontrolled condensate flow, protect the insulation layer, increase heat exchange surface area, reduce the length of the extended neck, and ensure proper operation of the packing box.
2. Valve Body and Stub-End Soft-Seal Welding
The valve shall be supplied with a minimum stub-end length of 100 mm. The stub-end length shall ensure that field welding operations at both ends do not affect the sealing materials. The stub-end shall be manufactured from seamless pipe, longitudinally welded pipe, or forgings, with welding and corrosion protection completed at the factory. The corrosion protection requirements for the stub-end shall be identical to those for the valve body.
The outside diameter of the valve and stub-end shall match that of the connecting pipeline, and the stub-end material shall be consistent with the pipeline material.
For valves supplied with stub-ends, the shell pressure test shall first be conducted at 1.5 times the maximum allowable working pressure at 20°C. After successful testing, the stub-ends shall be welded, followed by an integral shell pressure test of the valve and stub-end assembly at 1.5 times the stub-end design pressure.
3. Extended Stem
The connection design between the stem and the disc shall ensure that no permanent deformation occurs during opening under full differential pressure at design pressure. In the event of failure, the first point of failure shall occur external to the valve body, and the secondary failure loading force shall be significantly greater than the primary failure load.
For on-off valves equipped with actuators, the stem design must accommodate the maximum output torque of the actuator. The stem shall incorporate a blow-out proof design and adopt an integral through-shaft configuration.
For valves with packing glands, strict surface finish requirements shall apply to both the stem surface and the packing chamber bore. The clearance between the gland accessory and the packing chamber shall be less than the clearance between the gland accessory and the stem.
4. Bonnet Extended Neck
The bonnet extended neck length refers to the extension from the uppermost support point to the packing box of the bonnet, which shall satisfy the vapor space requirements. Bonnet connections shall be made by bolting, welding, or integral bonnet construction. Integral bonnets are permissible only for valves DN50 and smaller; threaded bonnet connections are not permitted.
The design of the bonnet extended neck shall ensure that the packing operates at ambient temperature. The primary function of the extended bonnet is to maintain the packing box at a distance from the cryogenic medium, ensuring that the packing remains within the ambient temperature range, thereby guaranteeing reliable stem sealing performance.
5. Seat
The sealing pair of the butterfly valve shall adopt either double-offset or triple-offset geometry to satisfy either unidirectional or bidirectional sealing requirements. Hardfacing alloy shall be deposited by welding on the sealing surfaces of both the closure member and the seat. The thickness of the hardfaced layer after machining shall be no less than 1.6 mm. For service temperatures below -100°C, the hardfaced components shall be subjected to cryogenic treatment after welding.
6. Material Requirements
Material selection for cryogenic butterfly valves is critical to ensuring safe valve operation. The primary requirements are as follows:
- Cast materials shall not be accepted as substitutes for forged materials.
- The carbon content in carbon steel used for pressure-retaining parts shall not exceed 0.23%; for forgings and castings, this may be relaxed to 0.25%.
- Copper or copper alloys shall not be used.
- Austenitic stainless steel materials shall be supplied in the solution-annealed condition.
- Bolts and nuts for the disc and gland of valves constructed of austenitic stainless steel, Monel alloy, and nickel-based alloys shall have chemical compositions identical to that of the valve body.
- The packing box and yoke flange materials shall be identical or equivalent to the valve body material.
- Graphite packing shall be corrosion-resistant pure graphite (purity ≥ 99.8%).
- Bolting materials shall meet the requirements of ASTM A320 B8M Class 2.
- Gaskets used in salt-spray corrosive environments shall be supplied with salt-spray test reports for the metallic materials.
- Austenitic stainless steel castings intended for ultra-low temperature service (below -101°C) shall be subject to impact testing.
- All stainless steel materials shall satisfy the 304/304L dual-certification requirement—meeting both the chemical composition of 304L and the mechanical properties of 304.
- The supplier shall provide PMI (Positive Material Identification) documentation for all alloy valves, including body and bonnet.
Cryogenic Material Design Principle: Carbon steel and other body-centered cubic (BCC) lattice metals are subject to low-temperature cold embrittlement, whereas the impact toughness of austenitic stainless steel and other face-centered cubic (FCC) lattice metals is essentially unaffected by low temperatures. Three cryogenic temperature grades are commonly applied in design: -46°C (low-temperature carbon steel), -101°C, and -196°C (300-series austenitic stainless steels).
7. Fire-Safe and Anti-Static Design
The valve shall incorporate fire-safe and anti-static design features. The maximum resistance of the entire static discharge path shall not exceed 10² Ω. The anti-static design shall provide electrical continuity between the stem and the valve body to dissipate static charge and eliminate potential hazards. All resilient-seated valves shall be equipped with anti-static devices.
When zero leakage and fire-safe performance are required, the valve shall be designed in accordance with API 607 (except for rubber-lined valves). The seat sealing surface typically employs a dual-sealing configuration of non-metallic and metal:
- Primary Seal:
- Secondary Seal:
Fire testing shall be conducted in accordance with BS 6755 or API 607, and the manufacturer shall provide API 6FA or API 607 fire-safe certificates.
8. Sealing and Operation
The supplier shall provide detailed drawings of all sealing areas of the valve. O-ring seals shall be given preference. Where packing is employed, spring-loaded packing shall be provided.
Butterfly valves 8 inches and larger shall be furnished with gear operators. For manually operated valves, the maximum operating force at the rim of the handwheel or lever during cryogenic operation and performance testing shall not exceed 360 N.
The valve shall operate smoothly in all positions throughout a service life of five (5) years.
9. Valve Locking
Locking devices shall be provided at both the fully closed and fully open positions. Manually operated valves shall be capable of being locked with a padlock.
The gearbox for hand-operated valves shall be of the helical gear type, fully enclosed, and filled with suitable lubricant.
10. Welding and Non-Destructive Examination (NDE)
Socket welding shall not be accepted. Flanges welded directly to the valve body shall not be accepted.
NDE Requirements:
Valve Type | Examination Requirement |
900 class and above cast stainless steel valves | 100% RT |
600 class and above cast or forged stainless steel valves | 100% MT or PT |
All other valves | Minimum 10% of each batch, at least one valve; 100% RT + 100% PT for butt-welded ends |
Post-weld seat surfaces | 100% PT |
11. Pressure and Leakage Testing
Valve testing items shall include:
- Shell test
- No-load operation test
- Seat leakage test
- Disc pressure-withstand capacity test
Test Requirements by Valve Type:
Valve Type | Shell Test | Backseat Test* | Low-Pressure Seat Test | High-Pressure Seat Test |
Gate Valve | Required | Required | Required | Optional |
Globe Valve | Required | Required | Optional | Required |
Plug Valve | Required | N/A | Required | Optional |
Check Valve | Required | N/A | Optional | Required |
Floating Ball Valve | Required | N/A | Required | Optional |
Butterfly Valve / Trunnion Ball Valve | Required | N/A | Required | Optional |
Note: All valves with backseat sealing capability (except bellows-sealed valves) shall be subjected to backseat testing.
Hydraulic pressure tests shall be conducted using hydraulic oil or water-emulsion agents to prevent rusting. After hydrostatic testing, the valve shall be thoroughly purged and dried until the dew point meets the specified requirements. Electrical heat tracing may be applied to the valve body during the purging stage to accelerate residual water evaporation.
Leakage Criteria:
- 1.1 × Pneumatic Pressure Test:
- Soft-seated valves: Per ISO 5208 Rate A.
- Metal-seated valves: Twice ISO 5208 Rate D.
- Low-Pressure Gas Test:
- Soft-seated valves: Per ISO 5208 Rate A.
- Metal-seated valves: Per ISO 5208 Rate D.
Low-leakage testing shall be performed in accordance with ISO 15848-1. For metal-seated butterfly valves, leakage rate requirements shall comply with API 598.
12. Cryogenic Testing
Following ambient-temperature testing, ultra-low temperature testing (at -196°C) shall be performed in accordance with BS 6364, including:
- Cryogenic operating performance test
- Cryogenic sealing performance test (packing + flange gaskets + seat)
- Cryogenic cyclic life test
Cryogenic Test Procedure:
- Immerse the valve in a liquid nitrogen tank (with the bonnet and packing gland located above the liquid nitrogen level).
- Maintain immersion for sufficient duration to ensure stable temperature equilibrium of the valve body.
- Monitor temperatures at various points using thermocouples.
- Conduct pressure testing with helium gas at cryogenic temperature.
- Cycle the valve through 20 open-close operations, measuring operating torque at the first and last cycles.
- Gradually pressurize to the maximum allowable working pressure and inspect all sealing points.
- Maintain pressure for 15 minutes, followed by slow depressurization.
Safety Precautions:
- Prevent cryogenic burns; operating personnel shall be equipped with protective clothing, safety shoes, and cryogenic gloves.
- Ensure adequate ventilation during helium gas discharge to avoid asphyxiation hazards.
- Establish a restricted danger zone during pneumatic pressure testing, and prohibit unauthorized personnel from entering.
Chapter 2: Low-Temperature Triple-Offset Butterfly Valves for LNG Service
1. Triple-Offset Concept and Structural Features
The three offsets of the low-temperature triple-offset butterfly valve for LNG service are defined as the shaft offset, spherical offset, and sealing surface offset:
- Shaft Offset:
- Spherical Offset:
- Sealing Surface Offset:
2. Performance Characteristics
- Superior Sealing Performance:
- Friction-Free Operation:
- High Flow Capacity:
- Short Operating Stroke:
- Wide Application Range:
- Long Service Life:
- Modulating Capability:
3. Top-Entry Configuration
LNG ultra-low temperature butterfly valves are classified into two top-entry configurations:
(1) True Top-Entry Butterfly Valve
The disc is of the top-entry design, allowing the entire disc assembly, stem, and sealing ring to be removed from the top of the valve body for maintenance. In-line maintenance is achievable regardless of valve size. The body-to-bonnet connection is a single joint, with only one potential external leakage path.
(2) Side Top-Entry Butterfly Valve
The disc adopts a side-entry arrangement, with the body extended on one side and equipped with a maintenance access opening. Maintenance requires personnel to reach into or enter the access chamber, resulting in lower maintainability. This configuration has two connections (body-to-bonnet and access opening-to-cover), creating two potential external leakage paths.
Chapter 3: Hydraulically Controlled Check Butterfly Valve
1. Application and Performance Specifications
The hydraulically controlled check butterfly valve combines both shut-off and check functions in a single valve, replacing two separate valves. It closes in two stages—quick-close and slow-close—according to a pre-set program, effectively eliminating and suppressing water hammer effects caused by sudden power outages or emergency pump shutdowns, as well as preventing reverse flow-induced pump turbine reversal.
Performance Specifications:
Parameter | Value |
Nominal Diameter (DN) | DN600 ~ DN3000 |
Nominal Pressure (PN) | 0.6 ~ 1.6 MPa |
Operating Temperature | ≤ 80°C |
Suitable Media | Water, oil products, and other non-corrosive fluids |
2. Construction and Working Principle
The valve assembly consists of three main components: the valve body, hydraulic actuator, hydraulic power unit, and electrical control cabinet. The accumulator design replaces the conventional counterweight, offering a compact, lightweight, and simple structure with comprehensive functionality.
Valve Operating Timing Parameters:
Parameter | Value |
Opening Time | 20 ~ 120 seconds |
Quick-Close Time | 2 ~ 30 seconds (angle approx. 65° ± 10°) |
Slow-Close Time | 2 ~ 60 seconds (angle approx. 25° ± 10°) |
Hydraulic Operating Principle:
- Normal Operation:
- Valve Opening:
- Valve Closing:
- Position Hold:
- Manual Operation:
3. Installation, Operation, and Maintenance
Installation Precautions:
- Clean the valve body, disc sealing surfaces, and internal bore prior to installation.
- Flange bolts shall be tightened evenly and in a symmetrical sequence.
- Ensure the sealing surface is oriented upstream and the valve shaft is positioned downstream relative to flow direction.
Accumulator Maintenance:
- Do not charge with oxygen, compressed air, or any flammable gas.
- Nitrogen charging pressure:
- DN ≤ 600: 6.5 ~ 8.5 MPa
- DN ≥ 700: 7.0 ~ 9.0 MPa
- Inspect bladder gas pressure weekly during the first month, then monthly thereafter.
Hydraulic Oil Management:
- Use hydraulic oil No. 20 or No. 30, or anti-wear hydraulic oil No. 46.
- Sampling and analysis shall be performed quarterly; filters shall be cleaned periodically.
- The oil tank shall be thoroughly cleaned during oil changes; mixing of different oil grades is strictly prohibited.
Chapter 4: Technical Codes and Standards
The technical requirements for cryogenic butterfly valves specified herein shall comply with the following standards:
Standard No. | Title |
API 609 | Butterfly Valves: Double-Flanged, Lug, and Wafer Types |
59A (NFPA) | Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG) |
ISO 14313 | Petroleum and Natural Gas Industries — Pipeline Transportation Systems — Pipeline Valves |
ISO 5208 | Industrial Valves — Pressure Testing of Metallic Valves |
BS 6364 | Valves for Cryogenic Service |
BS 6755 | Testing of Valves |
MESC SPE 77/200 | Valves for Low Temperature and Cryogenic Service |
ISO 15848-1 | Industrial Valves — Measurement, Test and Qualification Procedures for Fugitive Emissions |
Conclusion
After decades of development, butterfly valve technology has evolved from conventional butterfly valves to triple-offset metal-seated butterfly valves, from ambient-temperature service to cryogenic service, and from single shut-off functions to combined shut-off and check functions. The design philosophies, material systems, and testing methodologies have been continuously refined.
Cryogenic triple-offset butterfly valves, with their superior sealing performance and friction-free characteristics, have become the preferred solution for LNG and other cryogenic applications. Hydraulically controlled check butterfly valves, through accumulator technology and staged closing sequences, effectively ensure the safe operation of pumping station systems.
Engineering professionals in valve selection and design processes shall strictly adhere to the applicable codes and standards, with full consideration of service conditions and material suitability, to ensure the safe, reliable, and long-term operation of valves.
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