Cryogenic Globe Valves: Essential for Low-Temperature Conditions

In the modern industrial sector, with the widespread use of low-temperature media such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), liquid oxygen, liquid nitrogen, and liquid argon, cryogenic globe valves play a crucial role as key devices for controlling these low-temperature fluids. The performance and reliability of cryogenic globe valves are of vital importance. Their special design and manufacturing processes enable them to adapt to extremely low-temperature environments, ensuring reliable sealing and thermal insulation under low-temperature conditions, thereby safeguarding the safety and efficiency of industrial production.

During the manufacturing process of cryogenic globe valves, rough-machined parts are placed in a cooling medium (such as liquid nitrogen) for several hours (2 - 8 hours). The primary purpose of this special low-temperature treatment process is to relieve the stresses generated in the parts during the machining process. Through this treatment, the stability of the material's performance in low-temperature environments can be ensured, allowing it to operate normally at working temperatures as low as -196℃ and preventing material performance degradation or failure due to stress concentration.

The selection of materials for cryogenic globe valves is extremely critical. The main components such as the valve body, bonnet, and stem are typically made of stainless steel. Stainless steel has good low-temperature toughness, strength, and corrosion resistance, enabling it to maintain stable performance in low-temperature environments. The valve disc is made of copper, which has good ductility and sealing performance at low temperatures, allowing it to fit tightly with the valve seat and ensure sealing effectiveness. In addition, an epoxy phenolic laminated glass fabric board is used between the upper and lower sections of the stem to provide thermal insulation. This material has good thermal insulation properties, effectively reducing the transfer of heat flow and preventing heat exchange between the upper and lower parts of the stem, thereby minimizing the impact of thermal stress on the valve's performance. The flange sealing surface uses an aluminum gasket, the packing is made of polytetrafluoroethylene (PTFE), and the cylinder employs double special sealing. These material choices are all aimed at achieving reliable sealing and low friction in low-temperature environments to ensure the normal operation of the valve. The entire valve is insulated with polyurethane foam, with an insulation layer thickness of up to 200mm, further enhancing the valve's thermal insulation performance and preventing the loss of cold energy.

The structural design of cryogenic globe valves is a key factor in their stable operation in extremely low-temperature environments. A rational structural design not only ensures the valve's sealing performance but also effectively reduces heat transfer and improves thermal insulation. The following are the main structural design features of cryogenic globe valves.

There are two main types of bonnet designs for cryogenic globe valves: bolted bonnet and welded bonnet. The bolted bonnet connects the valve body and bonnet with bolts and nuts, using a wound gasket (manufactured from 316 stainless steel with flexible graphite) for sealing. This design is convenient for disassembly and maintenance and is suitable for applications requiring frequent inspection. The welded bonnet, on the other hand, is connected by threads and uses full-weld sealing. This design offers higher sealing performance and structural strength, making it suitable for applications with extremely high sealing requirements. The bonnet is designed with a slender structure to extend the heat bridge, preventing frosting of the stem and upper parts of the bonnet due to overcooling of the packing gland area, which could affect usage. This slender structure also facilitates the winding of cryogenic insulation materials, preventing the loss of cold energy, and makes cryogenic construction easier. It keeps the packing gland outside the cryogenic layer, allowing for easy tightening of the gland bolts or addition of packing without damaging the cryogenic layer when needed.

The stem is designed with a two-section structure, separated by an insulating pad (epoxy phenolic laminated glass fabric) in the middle to reduce heat flow and achieve thermal insulation. The valve disc is made of a softer copper material, while the valve seat is made of a harder stainless steel material. At low temperatures, austenitic steel may undergo phase transformation, leading to uneven sealing surfaces. In such cases, the soft valve disc can deform under the operating force to fit tightly against the valve seat, compensating for material deformation caused by thermal and organizational stresses. This solves the sealing problem under high-pressure, low-temperature conditions. The combination of hard and soft materials fully utilizes the performance characteristics of different materials at low temperatures, ensuring the valve's sealing reliability under extreme low-temperature conditions.

Gaskets play a vital sealing role in cryogenic globe valves. Since the valves need to operate at ambient temperature, low temperature, and under temperature changes, the gasket material must have reliable sealing and recovery properties. Generally, gasket materials with minimal performance changes are selected, such as asbestos packing impregnated with PTFE or molded plastic packing. These materials can maintain good elasticity in low-temperature environments, ensuring sealing performance. The packing is made of PTFE, which has good chemical stability, a low friction coefficient, and excellent sealing performance, enabling it to maintain reliable sealing effects for extended periods in low-temperature environments.

The design requirements for cryogenic globe valves are strict and diverse to ensure their reliability and safety in extremely low-temperature environments. The following are the main design requirements.

Cryogenic globe valves must be able to operate stably for extended periods in low-temperature media and surrounding ambient temperatures. This means that the materials and structural design of the valves need to withstand the long-term test of low-temperature environments without experiencing performance degradation or failure due to temperature changes.

The valve should not become a significant heat source in the low-temperature system. Heat inflow not only reduces the system's thermal efficiency but can also cause rapid vaporization of the internal fluid, leading to abnormal pressure increases and potential hazards. Therefore, the design of cryogenic globe valves must focus on thermal insulation, using rational structural design and material selection to minimize heat inflow to the greatest extent possible.

Low-temperature media should not adversely affect the operation of the handwheel or the performance of the packing seals. The globe valve components in direct contact with low-temperature media should have explosion-proof and fire-resistant structures to ensure safe operation in low-temperature environments. Additionally, since globe valve components operating at low temperatures cannot be lubricated, structural measures must be taken to prevent friction components from scoring, ensuring the normal operation of the valve.

From a metallurgical perspective, apart from austenitic steel, copper, aluminum, and other metals with a face-centered cubic lattice, most steels exhibit low-temperature brittleness in low-temperature states, which reduces the valve's strength and service life. Therefore, when selecting main materials, it is essential to choose materials suitable for low-temperature operation first. Aluminum does not exhibit low-temperature brittleness at low temperatures, but due to its low hardness and poor wear and scratch resistance of aluminum and aluminum alloy sealing surfaces, its use in low-temperature globe valves is somewhat limited, being selected only in low-pressure and small-diameter valves.

In the design of cryogenic globe valves, the extended bonnet structure is an important design improvement that brings significant advantages to the valve's performance and operation. The following are the main advantages of the extended bonnet structure and its performance in practical applications.

The design of the extended bonnet structure is primarily aimed at keeping the valve operating handle and packing gland structure away from the low-temperature zone. This not only prevents operators from suffering frostbite due to contact with low-temperature components but also ensures that the packing gland and gland cap are used at normal temperatures, preventing the packing's sealing performance from deteriorating and extending the packing's service life. At low temperatures, as the temperature decreases, the elasticity of the packing gradually disappears, and its leak-proof performance declines. If the medium seeps into the packing and stem area and freezes, it will not only affect the normal operation of the stem but may also cause the packing to be scratched by the up and down movement of the stem, leading to severe leakage. The extended bonnet structure effectively avoids these problems, enhancing the valve's safety and reliability.

The long-neck structure facilitates the winding of cryogenic insulation materials, preventing the loss of cold energy. Since low-temperature pipelines generally have a thick layer of cryogenic insulation, the long-neck bonnet makes cryogenic construction easier and keeps the gland cap outside the cryogenic layer. This design allows for easy tightening of the gland cap bolts or addition of packing when needed, without damaging the cryogenic layer, greatly improving the convenience of maintenance. When selecting the length, in addition to meeting the requirements of standards and the special requirements of the design unit, it is also necessary to consider whether the designed cryogenic layer thickness is greater than this length. If the cryogenic layer thickness is greater than the bonnet length, the bonnet should be appropriately extended to ensure the matching of the cryogenic layer and bonnet, further improving the thermal insulation effect.

During actual use, cryogenic globe valves may encounter problems such as inability to operate. A detailed fault analysis has been conducted regarding this issue. The analysis found that the problem may be related to the valve's structural design, material properties, low-temperature treatment process, or operating environment. By improving the structural design, such as optimizing the clearance between the stem and valve disc, enhancing thermal insulation measures, and selecting more suitable materials, the valve can operate normally in the test system. These improvement measures not only solve the current fault problem but also enhance the overall performance and reliability of the valve, providing a strong guarantee for stable operation under low-temperature conditions.

As an indispensable key device in the low-temperature industrial field, the advancement of the design and manufacturing processes of cryogenic globe valves directly affects the efficiency and safety of industrial production. Through special low-temperature treatment processes, rational material selection, meticulous structural design, and strict quality control, cryogenic globe valves can achieve reliable sealing and thermal insulation in extremely low-temperature environments. They meet the storage, transportation, and usage requirements of low-temperature media such as liquefied natural gas, liquefied petroleum gas, liquid oxygen, liquid nitrogen, and liquid argon.