LNG Valve Challenges and Solutions

In today's energy sector, liquefied natural gas (LNG) is increasingly gaining attention as an efficient and clean energy source. With the rapid development of the LNG industry, the demand for cryogenic valves is also growing swiftly. However, the unique properties of LNG, its flammability, explosiveness, and extremely low temperature, coupled with the reality that LNG plants are often located in coastal environments with salt spray, mean that the quality of cryogenic valves is directly related to the safe operation of equipment. As a result, the selection and design of cryogenic valves have been placed on a higher priority agenda.

Liquefied natural gas (LNG) is the product of natural gas liquefied at extremely low temperatures (as low as -160°C). Compared to gaseous natural gas, LNG has the significant advantage of being easier to transport and store. In the processes of LNG production, transportation, storage, and regasification, cryogenic valves play an indispensable role. These valves include LNG globe valves, gate valves, butterfly valves, check valves, ball valves, safety valves, and throttle valves, all of which are part of the control equipment for multiphase flow with phase change at low temperatures and high pressures. Due to the differences in liquefaction and storage and transportation processes, the design and purpose of these valves vary.

In the traditional field of cryogenic fluid control, a large number of process control valves are used in complete process flows, with globe valves, gate valves, and ball valves being the main on-off valves. These valves have the characteristics of low flow resistance, the ability to transport two-phase gas-liquid flow, resistance to valve clogging, and large flow control. However, when applied to the LNG field, traditional valves face many challenges.

Take gate valves and ball valves as examples. Traditional gate valves and ball valves form a bidirectional seal when opened or closed, creating a blind spot within the valve chamber. LNG is a low-temperature saturated liquid or two-phase gas-liquid flow. After valve operation, the LNG in the blind spot will rapidly vaporize due to the presence of ambient heat sources, causing a sharp increase in temperature and pressure. This not only leads to easy damage to the upper multiple seals and the lower main seal surface but may also trigger more serious safety hazards such as valve explosion. To solve this problem, traditional cryogenic gate valves and ball valves used for LNG usually add a pipe outside the valve body to connect the blind spot with the outlet section of the ball valve and drain the low-temperature fluid. However, this method destroys the advantages of the main seal surface of gate valves and ball valves, such as bidirectional sealing, bidirectional cutoff, and bidirectional control, and can only use one main seal surface, failing to achieve bidirectional sealing. Moreover, since the LNG at both ends of the pipe is highly prone to vaporization and the LNG fluid can easily flow backward, the external pipe connection does not effectively solve the problem of bidirectional cutoff. At the same time, the external pipe is prone to damage and leakage due to strength issues, and it also has disadvantages such as difficulty in adding insulation layers on the outside of the valve body and asymmetrical valve appearance.

After vaporization, LNG is a flammable and explosive gas, mainly composed of methane (CH₄). Traditional cryogenic valves, with numerous sealing points such as bidirectional main seals, multiple seals between the valve body and valve cover, and pipeline flange connection seals, are prone to CH₄ leakage. Under the low-temperature condition of -162℃, the sealing gaskets and sealing surfaces often come into direct contact with LNG, making the sealing materials highly susceptible to low-temperature brittleness and leakage, posing significant safety hazards.

When LNG valves are in operation, there is a large temperature difference between the upper and lower parts. For example, in globe valves and gate valves, the valve body is in contact with LNG, while the valve stem rotating actuator, upper valve body, and upper valve stem components are in contact with the external atmospheric environment, resulting in a temperature difference of about 200℃ between the two ends of the valve. This temperature difference leads to significant thermal stress within the components, especially between the valve stem and the upper valve body. Since LNG valve bodies are generally made of cast steel, which has a fast heat transfer rate, a longer upper valve body and valve stem are required to delay heat transfer, preventing the rotating actuator and other components from becoming too cold to function properly or to prevent personnel from frostbite. Additionally, because the valve body is made of cast steel and the valve stem is made of a rigid forging, the two have significantly different thermal expansion coefficients. Under low-temperature conditions with a large temperature difference, significant thermal stress can lead to valve body cracking, valve stem deformation, and main seal surface damage. Therefore, when traditional cryogenic valves are used in the LNG field, a longer valve stem is required to reduce local thermal stress strain, making the entire valve larger in volume to accommodate cold shrinkage and address the issue of significant thermal stress.

LNG is a low-temperature fluid, and the pipeline transportation pressure is generally below 0.2MPa, in a saturated or overheated state. During transportation, heat is continuously provided to LNG through valves and pipelines from the outside, causing continuous vaporization of LNG and the formation of two-phase flow. When two-phase flow encounters sudden cutoff, it can easily lead to a sharp increase in the pressure of the remaining LNG in the pipeline and exceed the critical point. When the pressure rapidly exceeds the critical pressure of 4.6MPa and the temperature exceeds the critical temperature of -82.59°C, it poses a significant safety hazard to the entire transportation system. Therefore, the design pressure of general LNG valves or LNG systems is greater than 6MPa, which increases the design difficulty of the entire LNG system, making the equipment bulky and large in volume.

Faced with the many problems of traditional cryogenic valves in the application of the LNG field, valve manufacturers and researchers have continuously explored and innovated, developing a series of new LNG valve technologies to meet the high requirements of the LNG industry for cryogenic valves.

The double-seat ultra-low-temperature top-entry valve is a new type of LNG valve. This valve has a special seat design that can ensure the depressurization of the middle cavity and maintain good downstream sealing performance. Through this design, the blind spot problem of traditional valves can be effectively solved, avoiding the sharp increase in pressure and seal surface damage caused by LNG vaporization.

To overcome the shortcomings of traditional cryogenic valves in sealing, new LNG valves have adopted high-performance sealing materials. These materials can maintain good sealing performance under low-temperature conditions of -162°C and are not prone to low-temperature brittleness. At the same time, by optimizing the sealing structure and reducing the number of sealing points, the risk of CH₄ leakage is reduced, and the safety of the valve is improved.

To address the thermal stress problem of LNG valves, new valves have adopted thermal compensation and insulation design. By adding insulating materials between the valve stem and the upper valve body, heat transfer is reduced, and thermal stress is lowered. At the same time, flexible connections or expansion joints are used to effectively relieve stress caused by different thermal expansion coefficients, improving the reliability and service life of the valve.

The structure of new LNG valves has been optimized to adapt to the special properties of LNG. For example, by shortening the length of the valve stem and reducing the volume of the valve, advanced materials and manufacturing processes are used to ensure the performance of the valve under low-temperature conditions. In addition, new valves have adopted more rational flow path designs to reduce flow resistance and improve transportation efficiency.

With the continuous progress of technology, intelligent control technology has also been introduced into LNG valves. By installing sensors and controllers, the status of the valve, such as temperature, pressure, and flow parameters, can be monitored in real-time, and the valve opening can be automatically adjusted according to preset programs to achieve precise control. Intelligent control not only improves the convenience of valve operation and control accuracy but also enables timely detection of potential fault hazards, enhancing system safety.

In LNG systems, the correct selection of LNG valves is key to ensuring the safe operation of the system. Here are some suggestions for selecting LNG valves.

Different LNG process stages have different requirements for valves. For example, during the LNG liquefaction process, throttle valves may be needed to control flow and pressure; in transportation and storage stages, gate valves or ball valves are required for on-off control. Therefore, the appropriate valve type should be chosen based on specific working conditions.

The material of LNG valves should be able to withstand low-temperature environments and have good corrosion resistance. Generally, the valve body is made of cast steel or stainless steel, and the valve stem is made of a rigid forging. For components in direct contact with LNG, materials that are resistant to low temperatures and corrosion should be selected, such as low-temperature steel or nickel-based alloys.

Due to the flammable and explosive nature of LNG, the sealing performance of valves is crucial. Valves with reliable and durable sealing performance should be chosen, and the number of sealing points should be minimized. New sealing technologies and blind spot-free designs can effectively improve valve sealing performance.

LNG valves have various operating methods, including manual, electric, and pneumatic. The appropriate operating method should be chosen based on actual operating conditions and the degree of automation required. For example, in situations requiring remote control, electric or pneumatic valves should be selected.

As a key piece of equipment in the LNG industry, the development of LNG valve technology is of great significance for ensuring the safe and efficient transportation and storage of LNG. Although traditional cryogenic valves face many problems when applied to the LNG field, new LNG valve technologies have made significant progress through continuous innovation and development. In the future, with the continuous progress of technology and the changing market demands, LNG valve technology will continue to develop towards higher safety, higher efficiency, smaller size and weight, stronger intelligence, and broader applicability, providing strong support for the sustainable development of the LNG industry.