Liquid hydrogen has certain advantages in storage and transportation. Compared to hydrogen, liquid hydrogen (LH2) has a higher density and requires lower pressure for storage. However, hydrogen has to be -253°C to become liquid, which means that it is quite difficult. Extreme low temperatures and flammability risks make liquid hydrogen a dangerous medium. For this reason, strict safety measures and high reliability are uncompromising requirements when designing valves for the relevant applications.
By Fadila Khelfaoui, Frédéric Blanquet
Velan valve (Velan)
Applications of liquid hydrogen (LH2).
At present, liquid hydrogen is used and tried to be used in various special occasions. In aerospace, it can be used as a rocket launch fuel and can also generate shock waves in transonic wind tunnels. Backed by “big science,” liquid hydrogen has become a key material in superconducting systems, particle accelerators, and nuclear fusion devices. As people’s desire for sustainable development grows, liquid hydrogen has been used as fuel by more and more trucks and ships in recent years. In the above application scenarios, the importance of valves is very obvious. The safe and reliable operation of valves is an integral part of the liquid hydrogen supply chain ecosystem (production, transportation, storage and distribution). Operations related to liquid hydrogen are challenging. With more than 30 years of practical experience and expertise in the field of high-performance valves down to -272°C, Velan has been involved in various innovative projects for a long time, and it is clear that it has won the technical challenges of liquid hydrogen service with its strength.
Challenges in the design phase
Pressure, temperature and hydrogen concentration are all major factors examined in a valve design risk assessment. In order to optimize valve performance, design and material selection play a decisive role. Valves used in liquid hydrogen applications face additional challenges, including the adverse effects of hydrogen on metals. At very low temperatures, valve materials must not only withstand the attack of hydrogen molecules (some of the associated deterioration mechanisms are still debated in academia), but must also maintain normal operation for a long time over their life cycle. In terms of the current level of technological development, the industry has limited knowledge of the applicability of non-metallic materials in hydrogen applications. When choosing a sealing material, it is necessary to take this factor into account. Effective sealing is also a key design performance criterion. There is a temperature difference of almost 300°C between liquid hydrogen and ambient temperature (room temperature), resulting in a temperature gradient. Each component of the valve will undergo different degrees of thermal expansion and contraction. This discrepancy can lead to hazardous leakage of critical sealing surfaces. The sealing tightness of the valve stem is also the focus of the design. The transition from cold to hot creates heat flow. Hot parts of the bonnet cavity area may freeze, which can disrupt stem sealing performance and affect valve operability. In addition, the extremely low temperature of -253°C means that the best insulation technology is required to ensure that the valve can maintain liquid hydrogen at this temperature while minimizing losses caused by boiling. As long as there is heat transferred to liquid hydrogen, it will evaporate and leak. Not only that, oxygen condensation occurs at the breaking point of the insulation. Once oxygen comes into contact with hydrogen or other combustibles, the risk of fire increases. Therefore, considering the fire risk that valves may face, valves must be designed with explosion-proof materials in mind, as well as fire-resistant actuators, instrumentation and cables, all with the strictest certifications. This ensures that the valve operates properly in the event of a fire. Increased pressure is also a potential risk that can render valves inoperable. If liquid hydrogen is trapped in the cavity of the valve body and heat transfer and liquid hydrogen evaporation occur at the same time, it will cause an increase in pressure. If there is a large pressure difference, cavitation (cavitation)/noise occurs. These phenomena can lead to the premature end of the service life of the valve, and even suffer huge losses due to process defects. Regardless of the specific operating conditions, if the above factors can be fully considered and corresponding countermeasures can be taken in the design process, it can ensure the safe and reliable operation of the valve. In addition, there are design challenges related to environmental issues, such as fugitive leakage. Hydrogen is unique: small molecules, colorless, odorless, and explosive. These characteristics determine the absolute necessity of zero leakage.
At the North Las Vegas West Coast Hydrogen Liquefaction station,
Wieland Valve engineers are providing technical services
Valve solutions
Regardless of the specific function and type, valves for all liquid hydrogen applications must meet some common requirements. These requirements include: the material of the structural part must ensure that the structural integrity is maintained at extremely low temperatures; All materials must have natural fire safety properties. For the same reason, the sealing elements and packing of liquid hydrogen valves must also meet the basic requirements mentioned above. Austenitic stainless steel is an ideal material for liquid hydrogen valves. It has excellent impact strength, minimal heat loss, and can withstand large temperature gradients. There are other materials that are also suitable for liquid hydrogen conditions, but are limited to specific process conditions. In addition to the choice of materials, some design details should not be overlooked, such as extending the valve stem and using an air column to protect the sealing packing from extreme low temperatures. In addition, the extension of the valve stem can be equipped with an insulation ring to avoid condensation. Designing valves according to specific application conditions helps to give more reasonable solutions to different technical challenges. Vellan offers butterfly valves in two different designs: double eccentric and triple eccentric metal seat butterfly valves. Both designs have bidirectional flow capability. By designing the disc shape and rotation trajectory, a tight seal can be achieved. There is no cavity in the valve body where there is no residual medium. In the case of the Velan double eccentric butterfly valve, it adopts the disc eccentric rotation design, combined with the distinctive VELFLEX sealing system, to achieve excellent valve sealing performance. This patented design can withstand even large temperature fluctuations in the valve. The TORQSEAL triple eccentric disc also has a specially designed rotation trajectory that helps ensure that the disc sealing surface only touches the seat at the moment of reaching the closed valve position and does not scratch. Therefore, the closing torque of the valve can drive the disc to achieve compliant seating, and produce a sufficient wedge effect in the closed valve position, while making the disc evenly contact with the entire circumference of the seat sealing surface. The compliance of the valve seat allows the valve body and disc to have a “self-adjusting” function, thus avoiding seizure of the disc during temperature fluctuations. The reinforced stainless steel valve shaft is capable of high operating cycles and operates smoothly at very low temperatures. The VELFLEX double eccentric design allows the valve to be serviced online quickly and easily. Thanks to the side housing, the seat and disc can be inspected or serviced directly, without the need to disassemble the actuator or special tools.
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Post time: Aug-11-2023
