Structural and Technical Features of Cryogenic Valves

 
Cryogenic valves play a crucial role in extreme low-temperature environments, widely used in the storage, transportation, and processing of liquefied natural gas (LNG), liquefied petroleum gas (LPG), and other cryogenic industrial media. The design and manufacturing of these valves not only need to meet stringent temperature and pressure requirements but also ensure exceptional sealing performance and operational reliability in cryogenic conditions. To cope with various complex cryogenic working conditions, the structure and technical characteristics of cryogenic valves are meticulously designed to ensure safety, durability, and stability under harsh conditions.

Bonnet Structure Design

 
Cryogenic valve bonnets are designed with an extended structure to achieve the following purposes:
Protection of Packing and Gland: The extended bonnet positions the packing gland above the valve body, away from the cryogenic medium, keeping the packing temperature above 0°C. This design effectively prevents the packing from losing elasticity and leaking due to low temperatures, and also avoids damage to the stem surface caused by freezing.
Prevention of Cold Loss: The extended bonnet provides sufficient space for the application of insulation materials around the valve body, reducing cold energy loss and enhancing system insulation efficiency.
Standard Requirements: According to BS 6364 standards, the minimum length of the extended portion of the packing gland for cold box valves must meet the standard requirements to ensure sealing performance and service life. For other cryogenic valves, the length of the extended packing gland must be no less than 250 mm to meet operational needs under various conditions.

Optimization of Valve Body and Stem Structure

 
Special attention is given to the wall thickness and diameter of the valve body and stem during the design of cryogenic valves to ensure strength and reliability under extreme conditions:
Minimum Wall Thickness Design: The wall thickness of the valve body and bonnet is not determined by the ASME B16.34 standard but is selected based on the specific valve type and application:
Gate Valves: The wall thickness of the valve body and bonnet must not be less than the requirements of the API 600 standard.
Globe Valves: The wall thickness of the valve body should meet the minimum requirements of the BS 1873 standard.
Check Valves: The minimum wall thickness should comply with the BS 1868 standard.
Stem Diameter: The stem diameter must be designed to meet the requirements of API 600 or BS 1873 standards to ensure sufficient bending strength and wear resistance under cryogenic conditions, coping with the load and wear caused by frequent operation.

Packing Box, Packing, and Gasket

 
The design of the packing box and packing is critical to the sealing performance of cryogenic valves:
Packing Box Position Design: The packing box is usually placed at the top of the extended bonnet to avoid direct contact with the cryogenic section, allowing the packing to operate at a relatively higher temperature. This design improves sealing effectiveness and reduces packing wear.
Double Packing Structure: For cryogenic valves with a nominal diameter of ≥DN300, a double packing structure with an intermediate metal isolating ring is recommended to enhance sealing and service life.
Packing Material Selection: Flexible graphite packing with 304 stainless steel wire braiding or lip-type PTFE packing (suitable for media temperatures above -73°C) is typically used to maintain good sealing performance in cryogenic environments.
Gasket Application: Appropriate gaskets should be selected based on different pressure levels and media temperatures. For pressure levels ≤CL600, 304 stainless steel flexible graphite wound gaskets or PTFE gaskets are generally used; for pressure levels ≥CL900, metal sealing rings and pressure self-sealing structures are recommended to enhance sealing performance.

Design of Internal Valve Components

 
Internal valve components, such as gate, disc, and ball, are designed with attention to sealing and durability in cryogenic environments:
Gate Structure: For gate valves with a nominal diameter <DN50, a rigid gate is recommended; for diameters ≥DN50, an elastic gate is preferred. The elastic gate can accommodate slight deformations of the valve body caused by temperature and pressure changes, ensuring a good fit and sealing effect between the valve and the seat.
Globe Valve Disc: Cone or ball-shaped structures are commonly used for better flow regulation and sealing performance.
Sealing Surface Material: Hard-sealed cryogenic valves often have Co-Cr-W hard alloy overlay on the sealing surfaces of the gate, disc, and valve body, enhancing wear resistance and sealing performance, suitable for harsh cryogenic conditions.

Valve Seat Sealing Structure

 
Based on the working temperature and pressure of the medium, cryogenic valve sealing pairs can be divided into soft sealing and hard sealing:
Soft Sealing: Generally adopts a metal-PTFE structure, but PTFE materials become brittle at temperatures below -73°C, making them unsuitable for extremely low temperatures or high-pressure (≥CL1500) environments.
Hard Sealing: Cryogenic gate valves, check valves, and globe valves often use valve seats with Co-Cr-W hard alloy overlay, forming an integrated structure to prevent leakage caused by low-temperature deformation and improve sealing stability and reliability.

Bidirectional Sealing and Pressure Relief Measures

 
In the closed state, the residual cryogenic liquid in cryogenic valves may vaporize and expand due to absorbing environmental heat, potentially leading to abnormal pressure increases. Therefore, effective pressure relief measures must be taken:
Relief Hole Design: A “relief hole” is provided on the gate or ball at the inlet end of bidirectional sealing gate valves and ball valves to release pressure in the valve cavity, preventing valve body deformation or rupture.
Unidirectional Sealing: All cryogenic valves must indicate the medium flow direction on the valve body to ensure correct installation and operation, preventing sealing failure due to reverse operation.

Application and Selection Considerations

 
Cryogenic valves are widely used for liquid and gas media transport and control under cryogenic conditions in industries such as LNG, LPG, chemical, and food cold chains. Special attention should be paid to the following when selecting cryogenic valves:
Operating Temperature: Ensure that the packing and sealing materials of the valve perform well at the actual operating temperature.
Pressure Level: Select the appropriate valve structure and sealing method based on system pressure to avoid leakage or valve body damage caused by overpressure.
Installation Direction: Strictly follow the medium flow direction indicated on the valve body to prevent sealing issues caused by reverse operation.

Conclusion

 
Cryogenic valves, with their superior structural design and material selection, demonstrate excellent sealing performance and operational reliability in extreme low-temperature environments. Whether for the storage and transportation of LNG and LPG or other cryogenic processes, cryogenic valves play a vital role, providing solid support for the safe and efficient operation of systems and opening up broader prospects for applications in the cryogenic field.