Design and Selection of Industrial Valves for LNG Industry

 
Liquefied Natural Gas (LNG), as a clean and efficient energy source, is widely used across the globe. However, due to its need to be stored at an ultra-low temperature of -162°C at atmospheric pressure and its highly flammable nature, handling and transporting LNG imposes stringent requirements on equipment. Particularly in the design and material selection of industrial valves, careful consideration must be given to the unique low-temperature and high-pressure environment of LNG. This article delves into the design principles, material choices, and specific requirements of LNG valves in practical applications, offering a comprehensive analysis of how to ensure system safety and stability under harsh conditions.

Design Requirements for LNG Valves

 
To ensure the safe transport and handling of LNG, the design of valves must meet a series of strict requirements. These requirements encompass not only the selection of materials but also the structural design and sealing performance of the valves, all aimed at coping with LNG's low-temperature, high-pressure environment and flammability. The following sections will explore the key design requirements of LNG valves, including temperature tolerance, sealing efficiency, and how optimized designs can enhance the overall performance and safety of the valves.

1. Cryogenic Adaptability and Structural Design

 
LNG valves must function reliably in extremely low-temperature environments. Given the ultra-low storage temperature of LNG, valve bodies must withstand the thermal stress caused by drastic temperature changes, as well as additional stresses from connected pipelines. This necessitates valves with high strength and durability. To ensure sealing performance and minimize cold energy loss, LNG valves typically feature extended bonnet designs, which keep the temperature at the bottom of the packing gland above 0°C. This design not only protects internal components from the effects of low temperatures but also extends the valve's service life.
 
Moreover, sealing design is crucial for valves operating in cryogenic conditions. Valves like gate and globe valves often incorporate an upper sealing structure to prevent seal failure due to low temperatures. The connection between the valve body and bonnet is typically achieved through bolting, welding, or pipe joints, ensuring stability and safety without the use of threaded connections, which could loosen or leak, leading to safety hazards.

2. Sealing Performance and Pressure Relief Measures

 
In LNG systems, valves must exhibit exceptional sealing performance. To prevent abnormal pressure buildup in the valve cavity, valves are often designed with pressure relief holes or channels, and alternative pressure relief methods may be employed if necessary. These pressure relief measures are usually located at the inlet end to ensure that the valve's sealing performance remains uncompromised even under extreme conditions.
 
The design of the sealing surfaces in LNG valves must account for material deformation in low-temperature environments, commonly using metal-to-metal or metal-to-soft sealing structures. Soft sealing surfaces must be supported by metal seats to prevent cold flow deformation caused by low temperatures, ensuring the reliability and durability of the seal.

3. Material Selection and Processing Techniques

 
Material selection is critical to the performance of LNG valves. The materials used in LNG valves must not only maintain high strength in low-temperature environments but also possess a stable microstructure to prevent brittle failure at low temperatures. Particularly for valve components that operate below -100°C, such as the valve body, bonnet, disc, seat, and stem, deep cryogenic treatment is required before final machining to ensure their mechanical properties meet design specifications.
 
In addition, the welded structures of LNG valves must undergo rigorous low-temperature testing to ensure the reliability of the welds and welding performance. When valves are operated frequently in low-temperature environments, internal components need to have excellent resistance to seizing, galling, and wear, ensuring long-term stability and reliability in service.
 
The design of the packing section is also highly stringent, typically employing dual packing structures with metal isolation rings to enhance sealing performance and prevent media leakage and stem freezing. If the packing loses elasticity, it could lead to media leakage, stem freezing, or even seizing, significantly impacting the safe operation of the system. Therefore, the selection and design of packing must ensure long-term stability in low-temperature conditions.

4. Anti-Static Design

 
As LNG is a flammable gas, there is a risk of fire caused by static electricity during its use. To avoid safety hazards due to static buildup, LNG valves are generally designed with anti-static features, ensuring that the valve components can effectively conduct electricity, preventing static accumulation and thereby ensuring the safe operation of the system.

Applications and Selection of LNG Valves

 
LNG valves play a crucial role in the production, transportation, storage, and regasification of LNG. Given that LNG remains in a liquid state at low temperatures and is highly flammable, the selection and application of valves must be precise to ensure the safe and efficient operation of the system.

1. On/Off Valves

 
In LNG systems, on/off valves, such as gate valves, butterfly valves, and ball valves, are commonly used. Gate valves are widely applied in high-pressure LNG systems, and despite potential leakage issues under high pressure, they remain the preferred choice for systems above 300 pounds due to user familiarity with their operation and characteristics. Butterfly valves are typically used in low-pressure systems below 150 pounds, known for their simple structure and ease of operation. However, butterfly valves are not suitable for high-pressure systems due to their poorer sealing performance under high pressure. Ball valves, with their short operation time and low flow resistance, are often used in ambient temperature systems, especially in applications requiring rapid opening or closing.

2. Flow Control Valves

 
For LNG systems requiring precise flow control, butterfly valves, globe valves, and needle valves are common choices. Butterfly and globe valves are suitable for large-diameter, low-pressure pipelines, offering ease of operation and maintenance. Needle valves, though providing high precision in controlling small flow rates, must be carefully considered when used in applications with significant pressure differentials between the inlet and outlet.

3. Check Valves

 
To prevent backflow in LNG systems, check valves are typically used. For small-diameter pipelines (nominal diameter < DN40mm), lift check valves are commonly employed, while for larger-diameter pipelines (nominal diameter > DN50mm), swing check valves or specially designed split-disc check valves are preferred. These check valves must be designed with consideration of material strength and reliability in low-temperature environments to ensure effective prevention of backflow under extreme operating conditions.

The design and manufacturing of LNG valves require careful consideration of multiple factors, including cryogenic adaptability, sealing performance, material selection, and application context. The structural design, sealing methods, and material processing techniques of valves must undergo rigorous validation to ensure their safety and reliability in low-temperature, flammable environments. As LNG plays an increasingly significant role in the global energy landscape, valve manufacturers must continuously enhance their technological capabilities, developing high-performance valves that meet the specific demands of LNG applications, thereby ensuring the safe and stable operation of the liquefied natural gas industry.