Types and Characteristics of LNG Cryogenic Valves

LNG (Liquefied Natural Gas) is primarily composed of methane. When natural gas is cooled to -162°C at atmospheric pressure, it transforms from a gaseous state into liquid form, known as LNG. One cubic meter of LNG is equivalent to 625 cubic meters of natural gas in its gaseous state at standard conditions. This significant volume reduction makes the storage and transportation of LNG more efficient and cost-effective.

Types of Cryogenic Valves

Cryogenic valves come in various shapes and sizes, each suited for different applications and pressure ranges. Common types include:

1. Cryogenic Ball Valves: Featuring a ball mechanism and a polytetrafluoroethylene (PTFE) body, these valves are known for their excellent flow characteristics and are particularly effective when a tight shut-off is needed. However, ball valves can be prone to wear and tear along the seals and around the ball itself, making them suitable for applications that require an unrestricted flow path.
2. Cryogenic Relief Valves: Designed to maintain system pressure at a pre-determined level and provide protection from over-pressurization. These valves release steam when safety limits are exceeded, ensuring the system remains safe.
3. Cryogenic Globe Valves: Named for their spherical shape, these valves contain an internal movable disk that rotates at a 90-degree angle to the plane of the valve seat. They offer reliable long-term sealing performance but may not be ideal for applications where high flow rates are essential.
4. Cryogenic Gate Valves: Featuring a wedge-shaped gate that closes in line with the valve seat, these valves result in minimal pressure drop even when fully open. While they provide good flow characteristics similar to ball valves, they are also susceptible to wear and tear.
5. Cryogenic Butterfly Valves: Utilizing a circular disc at the center, these valves are shorter in length compared to other cryogenic valve types. They are lightweight, economical, and simple to operate, making them ideal for systems that require quick opening and closing actions.

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Fire and Explosion Risks of LNG

LNG is a colorless and odorless liquid with a low boiling point of -162°C. When exposed to the atmosphere, LNG boils vigorously, creating vapor clouds due to the condensation of water vapor. These vapor clouds can form flammable mixtures with air in narrow concentration ranges (5.3% to 14% LNG). In confined spaces, LNG vapor poses an explosion risk. If LNG leaks at ambient temperature, it rapidly evaporates, producing large quantities of methane gas. The resulting flammable mixture can spread downwind over considerable distances. In enclosed areas, if the gas concentration reaches 2-3% (the lower explosion limit), all personnel must evacuate the site. The combustion and radiation rates of LNG fires are comparable to those of gasoline fires, making LNG extremely dangerous.

Differences Between LNG and Other Cryogenic Valves

Due to LNG's flammability and explosiveness, stringent safety measures are in place for its storage and transportation. Unlike LO2 (Liquid Oxygen), LN2 (Liquid Nitrogen), and LAr (Liquid Argon) valves, LNG valves must include pneumatic emergency shut-off valves and flame arrestors as per standards such as those from the NFPA (National Fire Protection Association). Additionally, copper and its alloys are avoided in LNG systems to prevent corrosion and metallurgical degradation, which contrasts with the widespread use of these materials in other cryogenic valves. Instead, LNG valves are typically made of 304 stainless steel.

Technical Requirements for LNG Cryogenic Valves

Strength and Rigidity

LNG cryogenic valves must withstand significant thermal stress due to rapid temperature changes. The primary valve components should handle various loads without permanent deformation. Finite element stress analysis and anti-seismic evaluations are used to ensure valve reliability. Low-carbon or ultra-low-carbon austenitic stainless steel is often used for its superior strength, toughness, and resistance to temperature and pressure fluctuations.

Sealing and Safety

To prevent fires and explosions, LNG systems must have zero leakage. Valve design, material selection, and rigorous testing ensure this. Low-temperature tests, such as immersing the valve in -196°C liquid nitrogen and checking for helium leaks, validate the sealing materials and structures. These tests guarantee the valves' excellent sealing performance and reliability, even under high-frequency vibrations and accelerations.

Standards and Testing for Cryogenic Valves

Cryogenic valves are subject to stringent standards and testing to ensure their safety and reliability. Key standards include:

• MSS SP-134: This North American standard covers materials, design, dimensions, fabrication, pressure testing, and non-destructive examination of cryogenic valves with body/bonnet extensions. It specifies that materials in contact with cryogenic fluids should have suitable mechanical properties at the specified minimum temperature and that the design should isolate the stem packing from the cryogenic fluid's temperature effects.
• BS 6364: This British standard specifies the design, manufacture, and testing of valves for cryogenic service. It requires extended bonnets/glands to keep the stem packing at an adequate temperature and prohibits flat-seated discs for globe valves, among other specifications.
• ASME B16.34: This standard includes specifications for valve parts such as flanges, threading, and welding ends. It also covers material selection, pressure testing, and leakage detection.
• ISO 21011: This international standard outlines requirements for the design, manufacture, and testing of valves for cryogenic fluids, covering temperatures from ambient to cryogenic. It specifies materials, design, and testing procedures to ensure valve safety and performance.

Testing Procedures

Cryogenic valves undergo rigorous testing, including:

• Shell Strength Test: Can be hydrostatic or pneumatic, performed at 1.5 times the system's maximum working pressure.
• Shell and Seat Leak Test: For pneumatic tests, the system is pressurized to 1.1 times the maximum working pressure; for hydrostatic tests, to 1.3 times.
• Cryogenic Prototype and Production Test: Conducted in accordance with BS 6364, these tests ensure that valves meet all necessary safety and performance standards.

Conclusion

LNG cryogenic valves are vital for the safe and efficient handling of LNG. Understanding the different types, their specific applications, and the stringent technical requirements ensures the optimal performance and safety of LNG systems. The careful selection and maintenance of these valves are crucial to preventing accidents and ensuring the reliability of LNG operations. By adhering to international standards and rigorous testing protocols, the integrity and safety of cryogenic valves can be assured, thereby safeguarding both people and infrastructure involved in LNG processes.