Cryogenic Valve Structural Features & Technical Requirements

Cryogenic valves are primarily used in extremely low-temperature environments and are widely applied in fields such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), and the chemical industry. Due to the impact of low temperatures on valve materials and sealing performance, the design of cryogenic valves requires special attention to their low-temperature resistance, sealing effectiveness, pressure-bearing capacity, and structural reliability. This article will explore the structural features and technical requirements of cryogenic valves in detail from multiple perspectives.

Bonnet Design and Packing Protection

The bonnet of a cryogenic valve typically features an extended structure, primarily aimed at protecting the packing and stuffing box to ensure that the packing remains above 0°C during operation. Low temperatures can cause the packing to lose elasticity, leading to leakage issues. In severe cases, the packing and valve stem may freeze, affecting the normal operation of the valve stem and potentially causing damage to the stem. The extended bonnet design not only prevents these issues but also provides additional external space for insulating materials, thereby reducing cold energy loss.

According to the BS 6364 standard, the extended portion of the packing gland for valves used in cold boxes has a minimum length requirement, while for other applications, the minimum length of the extended packing gland should be 250mm.

Small Wall Thickness and Wall Thickness Standards

The body of a cryogenic valve, including the valve body and bonnet, typically adopts a "small wall thickness" design. Different types of valves have specific wall thickness requirements to ensure they can withstand working pressures in low-temperature environments. The wall thickness design of the valve should comply with relevant standards such as ASME B16.34, API 600, and BS 1873. Specifically, the wall thickness of gate valves, globe valves, and check valves must not fall below the requirements stipulated by these standards. Additionally, the diameter of the valve stem should also meet the relevant requirements of standards such as API 600 or BS 1873.

Stuffing Box and Sealing Design

The design of the stuffing box in cryogenic valves is particularly important. To avoid direct exposure of the packing to low temperatures, the stuffing box is usually positioned at the top of the extended bonnet, keeping the packing away from the low-temperature section and ensuring it operates at a higher ambient temperature. This design enhances sealing effectiveness and prevents leakage due to packing failure at low temperatures.

For cryogenic valves with a DN300 or larger, a double-packing structure with an intermediate metal spacer ring is recommended. The choice of packing material is also crucial. Common low-temperature packing materials include flexible graphite and stainless steel304 wire braided packing, suitable for temperatures ranging from -73°C to ambient. For high-pressure cryogenic valves, the sealing gasket between the flange and bonnet typically uses 304 stainless steel flexible graphite spiral wound gaskets or PTFE.

For higher design temperatures (above -46°C), metal ring seals can be used, especially for cryogenic valves with pressure ratings of Class 900 LB and above. The valve body can adopt an internal pressure self-sealing structure, effectively improving sealing performance.

Disc and Plug Design

The design of the disc and plug in cryogenic valves is particularly critical, especially in high-pressure and extremely low-temperature environments, where their sealing and corrosion resistance need special attention. For cryogenic gate valves, the choice of disc material directly affects their operational performance. Rigid discs are suitable for cryogenic gate valves with DN50 or smaller, while elastic discs are preferred for valves with DN50 or larger. Elastic discs can adapt to deformation of the valve body caused by changes in medium temperature and pressure, maintaining good sealing performance.

In globe valves, the plug typically adopts a conical or spherical structure, which helps improve fluid control accuracy. For hard-seal cryogenic valves, Co-Cr-W hard alloy is often welded onto the sealing surfaces of the disc or plug and valve body to enhance wear resistance and sealing effectiveness.

Mid-Cavity Pressure Relief Function

After a cryogenic valve is closed, residual low-temperature liquid inside may absorb heat from the surrounding environment over time, gradually returning to ambient temperature and eventually re-vaporizing. The vaporized liquid expands several times in volume, generating extremely high pressure. Without effective measures, this pressure could cause deformation or rupture of the valve body and bonnet. To avoid such issues, "bi-directional sealing" valves (e.g., gate valves and ball valves) are typically equipped with a mid-cavity pressure relief function. For example, a "relief hole" may be drilled in the gate of a gate valve or in the ball of a floating ball valve. This design prevents "abnormal pressure rise" due to temperature increases, thereby avoiding damage to the valve body and bonnet.

All cryogenic valves are required to have a unidirectional sealing function, and the valve body must clearly indicate the medium flow direction to ensure that the medium flows in one direction only, preventing leakage.

Seat Sealing Design

The design of the seat sealing pair in cryogenic valves is crucial. Depending on the working medium's temperature and nominal pressure, the seat sealing can use metal-PTFE soft seals or metal-to-metal hard seals. PTFE is suitable for medium working temperatures above -73°C, but at low temperatures, PTFE tends to become brittle, and at pressure ratings above CL1500, PTFE may exhibit "cold flow," affecting sealing performance. Therefore, hard-seal cryogenic valves often use Co-Cr-W hard alloy welding technology, integrating the seat and valve body into a single unit to avoid deformation or leakage due to low temperatures.

Low-Temperature Resistance and Material Selection

Material selection for cryogenic valves is critical. To ensure stability and safety in extremely low-temperature environments, valve materials typically include low-temperature-resistant alloy steels, aluminum alloys, and austenitic stainless steels. Hard alloy materials are used for welding components such as the plug and seat, effectively enhancing the valve's wear resistance, corrosion resistance, and sealing performance.

Conclusion

The application of cryogenic valves in extremely low-temperature environments demands high standards, with stringent requirements for their design and manufacturing. From the extended bonnet structure to the packing protection design, from wall thickness requirements to the selection of sealing pairs, every detail is crucial to the valve's safety and reliability. Through meticulous design and strict technical requirements, cryogenic valves can operate stably in harsh low-temperature environments, ensuring the smooth progress of various industrial processes.