Understanding Cryogenic Valves: Essential Features and Applications

Overview

Cryogenic valves are crucial for liquefied natural gas (LNG). Industries store LNG at very low temperatures, as low as -162°C. LNG is highly flammable, so valves must handle these conditions. Cryogenic valves are also used in air separation plants to manage cold fluids.

What Are Cryogenic Valves?

Cryogenic valves control very cold fluids, including liquid nitrogen, oxygen, and helium. They are used in aerospace, medicine, food processing, and air separation plants.

Key Features of Cryogenic Valves

• Material: Made from materials that can handle extreme cold. Standard materials are stainless steel and special alloys.
• Extended Bonnet: Keeps parts of the valve away from the cold fluid. This prevents freezing.
• Leak Prevention: Designed to provide a tight seal. This prevents leaks.
• Pressure Relief: Some cryogenic valves include pressure relief features to handle sudden pressure changes.

Types of Cryogenic Valves

• Cryogenic Ball Valves: Used for quick shut-off and control. They offer a tight seal and are easy to use.
• Cryogenic Gate Valves: Ideal for full flow with minimal pressure drop. Used when the valve needs to be fully open or closed.
• Cryogenic Globe Valves: Offer precise flow control. Suitable for throttling applications.
• Cryogenic Check Valves: Prevent backflow. Ensure one-way fluid flow.
• Control Valves: Regulate flow, pressure, and temperature in cryogenic processes.

Why Are Cryogenic Valves Important?

Cryogenic valves handle and transport cryogenic fluids safely. They prevent leaks and keep the fluid at low temperatures. Without them, there would be more risks and inefficiencies. The right type of valve is essential for each specific application.

Choosing Cryogenic Valves

• Temperature Range: Ensure the valve works at your specific temperature.
• Pressure Rating: The valve should handle the pressure of the cryogenic fluid.
• Material Compatibility: Choose a valve made from materials that work with the cryogenic fluid.
• Valve Type: Pick the right type based on your needs.
• Manufacturer's Reputation: Choose reputable manufacturers known for quality. Ensure they offer a wide range of cryogenic valves.

Effects of Low Temperatures on Sealing

Non-Metallic Sealing Components

Non-metallic materials are elastic at room temperature. They need lower pressure to seal well. At low temperatures, these materials contract more than metals. This decrease in sealing pressure can lead to failure. Rubber loses its elasticity below its glass transition temperature. It becomes hard and loses its sealing properties. Rubber also swells in LNG, making it unsuitable for LNG valves. For temperatures below -70°C, we do not use non-metallic sealing pairs. Instead, we use metal and non-metal composite structures.

Metallic Sealing Components

At low temperatures, metals become stronger and harder, less plastic, and tough, which can cause low-temperature brittleness. We use ferritic stainless steel for temperatures above -100°C. We use austenitic stainless steel, copper, and aluminum alloys for temperatures below -100°C. We rarely use aluminum alloys due to poor wear and scratch resistance. We prefer austenitic stainless steel like 304 and 316L because it remains tough at low temperatures. Austenitic stainless steel can transform into martensite at low temperatures. This leads to changes in volume and internal stress, which can result in sealing failure. To minimize deformation, use high-stability materials like 316L. Parts should undergo low-temperature treatment before precision machining.

Effects on External Sealing

Valve Stem Packing

Rubber and non-metallic materials become brittle at low temperatures. Therefore, sealing rings cannot be used in designs for the stem-to-body connection. Instead, we use stuffing boxes and bellows sealing structures. We use bellows seals where media leakage is unacceptable. Stuffing box sealing structures are easy to manufacture and repair. Packing usually cannot work below -40°C. The stuffing box should operate close to ambient temperature. As the temperature drops, packing elasticity decreases. This reduces the risk of leaks. Media leakage can cause ice formation at the packing-to-stem interface, potentially hindering stem operation and damaging packing.

Middle Flange Gasket

Cryogenic valve flange seals use gaskets. These harden at low temperatures. Gaskets must seal reliably at everyday, low, and varying temperatures. As temperatures drop, bolt length, gasket thickness, and flange thickness shrink. Design considerations include using materials with high linear expansion coefficients for bolts and low coefficients for flanges. Reduce gasket thickness and use materials with small linear expansion coefficients. Increase bolt tensile deformation. We commonly use austenitic stainless steel for cryogenic valves below -100°C for valve body and bolt materials. The ideal gasket material has low hardness at average temperature, good elasticity at low temperature, and sufficient mechanical strength. Common materials include spiral-wound gaskets with asbestos, PTFE, or flexible graphite fillers.

Common Standards

MSS SP-134

Valves for Cryogenic Service Including Requirements for Body/Bonnet Extensions

• Specifies requirements for extended bonnet lengths.
• Outlines measures to prevent abnormal pressure build-up.
• Specifies testing procedures including low-temperature tests.

MSS SP-134 standard official

BS 6364

Specification For Valves For Cryogenic Service

• Details requirements for extended bonnet lengths and operation angles.
• Outlines testing procedures and material specifications.

Click get more information about the BS 6364 standard

SHELL MESC SP 77

Valves in Low Temperature and Cryogenic Services

• Specifies requirements for extended bonnet lengths and operating angles.
• Outlines sealing, bolting, and inspection requirements.

Click Get more information about the SHELL MESC SP 77 standard

Conclusion

Cryogenic valves differ from general-purpose valves in design, manufacturing, and inspection. Choose sealing materials based on the medium and lowest working temperature. Use flexible sealing structures to compensate for low-temperature effects. All cryogenic metal materials must undergo deep cryogenic treatment before precision machining. Test new designs under normal and cryogenic conditions to ensure reliability.

FAQs

1. What are cryogenic valves, and why are they important?
   • Cryogenic valves control cryogenic fluids at extremely low temperatures, ensuring safe and efficient operations.
2. What materials are cryogenic valves made from?
   • Cryogenic valves are typically made from stainless steel and specialized alloys to withstand extreme cold.
3. What are the different types of cryogenic valves?
   • The main types are ball, gate, globe, and check valves, each serving different control and flow purposes.
4. What features should I look for in a cryogenic valve?
   • Look for materials that withstand cold, an extended bonnet to prevent freezing, and a design that prevents leaks.
5. How do I select the right cryogenic valve for my application?
   • Consider temperature range, pressure rating, material compatibility, valve type, and manufacturer's reputation.

For more information on selecting the best cryogenic valves for your needs, visit our detailed guide or contact our experts at SNBV Flow.