The difference between cryogenic valves and non-cryogenic valves mainly stems from the variation in their applicable temperature environments. Cryogenic valves are typically used at temperatures of -40℃ and below, while non-cryogenic valves are employed in normal-temperature or medium-to-high-temperature scenarios (above -10℃). They can be specifically distinguished in terms of materials, structure, sealing performance, operation, and application:
1. Applicable Temperature and Core Requirements
Cryogenic valves: Designed for extremely low temperatures ranging from -40℃ to -270℃ (e.g., liquid nitrogen at -196℃, liquefied natural gas at -162℃). Their core requirement is to maintain structural stability and reliable sealing under low-temperature conditions while avoiding the impact of low temperatures on operational safety.
Non-cryogenic valves: Suitable for normal temperatures (-10℃ to 120℃) or medium-to-high temperatures (above 120℃), such as in steam and hot oil systems. There is no need to consider issues like material brittleness or component shrinkage caused by low temperatures; instead, the focus is on meeting strength and basic sealing requirements at the corresponding temperatures.
2. Material Selection: Resistance to Low-Temperature Brittleness is Key
Low temperatures can cause most materials to become "brittle" (a phenomenon known as low-temperature brittleness), and non-metallic materials may harden or crack. Thus, material selection is the most fundamental difference between the two types of valves:
Cryogenic valves:
Valve body/bonnet: Materials with excellent low-temperature toughness are mandatory. Austenitic stainless steels (e.g., 304, 316) are preferred because they retain toughness even at -196℃ without exhibiting low-temperature brittleness. For extremely low temperatures (e.g., liquid helium at -269℃), titanium alloys or nickel-based alloys may be used.
Sealing elements: Non-metallic seals must use low-temperature-resistant materials (e.g., modified polytetrafluoroethylene, perfluoroether O-rings) to prevent leakage caused by low-temperature hardening. Metal seals, such as those made of copper alloys or stainless steel, compensate for shrinkage through "low-temperature pre-tightening".
Valve stem: Stainless steel or precipitation-hardened stainless steel is used to prevent deformation or fracture at low temperatures.
Non-cryogenic valves:
Valve body/bonnet: Materials such as cast iron, cast steel (e.g., WCB), and carbon steel can be used. These materials are cost-effective and have sufficient strength at normal or medium-to-high temperatures but will become brittle and crack at low temperatures, making them unsuitable for low-temperature applications.
Sealing elements: Ordinary rubber (e.g., nitrile rubber, EPDM) or conventional polytetrafluoroethylene is sufficient, as they meet the required elasticity and sealing performance at normal temperatures.
Valve stem: Carbon steel, chrome-molybdenum steel, etc., are used. In medium-to-high-temperature scenarios, emphasis is placed on the high-temperature strength of the material.
3. Structural Design: Targeted Solutions for Low-Temperature Challenges
Low-temperature media can cause component shrinkage, and "cold loss" (the vaporization of low-temperature media due to heat absorption) must be avoided. Therefore, the structure of cryogenic valves is more complex:
Special designs for cryogenic valves:
Long-neck structure: The bonnet is designed with a long neck (100–300mm in length) to separate operating components such as handwheels and stuffing boxes from the low-temperature zone. This not only prevents operators from frostbite when in contact with low-temperature parts but also reduces cold transfer to the outside through the valve stem (avoiding external frosting or icing that could affect operation).
Anti-shrinkage compensation: Connecting bolts between the valve body and bonnet are preloaded to prevent loosening and leakage of the sealing surface caused by component shrinkage at low temperatures. Some sealing surfaces are designed with "elastic compensation structures" (e.g., bellows seals) to offset the effects of shrinkage.
Anti-cavitation and flow guidance: Low-temperature liquids (e.g., LNG) are prone to vaporization (flash evaporation) during throttling. The internal flow channel of the valve must be smooth to prevent cavitation damage to the sealing surface caused by turbulence.
Anti-static design: Static electricity is conducted through metal components (e.g., conductive springs between the valve stem and valve body) to prevent hazards caused by static accumulation in low-temperature flammable and explosive media (e.g., LNG).
Designs for non-cryogenic valves:
No long-neck structure is required, and the valve body can be directly connected to operating components.
Sealing relies on conventional bolt preloading, with no need for low-temperature shrinkage compensation.
Medium-to-high-temperature valves may focus on "high-temperature-resistant sealing" (e.g., using metal graphite gaskets) but do not require design considerations for "cold loss".
4. Sealing Performance: Stricter Requirements for Low Temperatures
Cryogenic valves: Most cryogenic media (e.g., LNG, liquid oxygen) are flammable, explosive, or toxic. Leakage can cause rapid volume expansion due to vaporization (e.g., LNG can expand 600 times in volume after leakage), so "zero leakage" must be achieved. Some valves use "bellows seals" (metal bellows between the valve stem and valve body) to prevent failure of conventional packing seals at low temperatures.
Non-cryogenic valves: Sealing requirements depend on the medium. For example, tap water valves allow minimal leakage, and steam valves need to reduce leakage but do not require "zero leakage". They typically use packing (e.g., asbestos, graphite) or ordinary O-rings to meet requirements.
5. Operation and Maintenance: Adaptation to Low-Temperature Environments
Cryogenic valves:
Operating components (e.g., handwheels, actuators) are kept away from the low-temperature zone via the long-neck structure to avoid freezing and jamming.
Regular "cold tightening" is required: After low-temperature operation, component shrinkage may cause bolts to loosen, requiring re-tightening.
Low-temperature lubricants (e.g., silicone-based grease) must be used, as ordinary lubricating oil will solidify and fail at low temperatures.
Non-cryogenic valves:
There are no low-temperature restrictions on operation, and ordinary engine oil or grease can be used for lubrication.
Maintenance focuses on medium-induced corrosion (e.g., in acid-alkali environments) or high-temperature aging (e.g., replacement of rubber seals), with no need to address low-temperature-related issues.
6. Application Scenarios
Cryogenic valves: Exclusively used in low-temperature medium systems, such as LNG storage tanks and pipelines, liquid nitrogen/liquid oxygen transportation, and aerospace low-temperature experimental equipment.
Non-cryogenic valves: Cover most conventional scenarios, including tap water pipelines, industrial steam systems, hot oil transportation, and ordinary gas pipelines.
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