For many, the achievement of carbon neutrality through clean energy by the year 2050 is one of the most important challenges facing mankind. To achieve this goal many believe that the usage of various hydrogen technologies as an alternative energy source will be key.
This article introduces an R&D-themed approach involving advanced Low-E technologies development at the raw materials properties level. The technical research targets the accomplishment of safer and cleaner operations of any valves in the hydrogen usage field, particularly in severe hydrogen usage applications. This knowledge could also be applied in many severe conditions, including hydrocarbon services.
By Kodai Inoue, Executive Packing Engineer – Nippon Pillar Packing Co., Ltd & Takashi Nishi, Director of Sales Engineering – Nippon Pillar Corporation of America
The Data
Various valves are used in the processes of production, transport, storage, and end use of hydrogen that has wider than typical temperature and pressure ranges. Technical research tests were conducted in various carefully designed gaseous hydrogen and liquid hydrogen applications to acquire more accurate knowledge on the next generation of Low-E technologies; the data can be used to help with the transition to future clean energy applications.
What is the Best Sealing Material?
Figure 2 shows the typical properties of the materials typically used for valve stem packings. In terms of the raw material properties, expanded graphite has various advantages over PTFE, except for friction properties. In particular, the heat resistance and linear expansion coefficient of expanded graphite are significantly advanced in comparison to PTFE, which is why expanded graphite is now widely used for valve stem packings and gaskets for hoods.
Given the extensive temperature ranges required in the hydrogen markets, the research focus has been on expanded graphite and its performance under hydrogen conditions. Figure 3 shows the evaluation results of tensile strength tests on expanded graphite raw materials under various temperature conditions. The tensile strength of expanded graphite after exposure to 500ºC of high-temperature hydrogen showed no significant decrease in tensile strength post-exposure. This suggests that expanded graphite can be used in high-temperature hydrogen environments as it does not react. The tensile strength under the cryogenic condition with helium at -269°C increases by roughly 75% compared to atmospheric temperature. One can therefore speculate that one of the reasons for this result is the solidification of water in the micro gaps inside the expanded graphite sheet. Since it is known that hydrogen does not react with expanded graphite under high temperature hydrogen conditions, it can be used at cryogenic temperatures.
Based on the data collected and depicted in Figures 1 and 2, it was determined that expanded graphite can be used as the main material for valve stem packing under both high-temperature and cryogenic hydrogen conditions.
Packing Optimization
While expanded graphite is suitable for gland packing for vales stems in high temperature and cryogenic conditions, expanded graphite alone is not suitable due to the friction coefficient of expanded graphite, which in some cases makes it difficult to operate the valve handle.
Expanded graphite is also a very flexible but brittle seat material. This suggests that expanded graphite would need lubricant to maximize its functionality as a packing for sealing fluid. Without optimal lubrication, the very low molecular weight of media such as hydrogen and VOCs becomes very difficult to contain and can leak through the seat.
It is therefore essential to put research efforts into optimizing the lubrication of packing at both high and extremely low temperatures.
A Closer Look at PTFE
PTFE is the most widely used lubricant for valve stem packings, as it helps lower the friction coefficient of the packing while reducing packing abrasions. PTFE also improves the sealing performance of braided packings by filling the gaps in the braiding. However, the maximum service temperature of PTFE is lower than that of expanded graphite. If PTFE is used under conditions exceeding 350°C, it will lose the advantages as a lubricant mentioned earlier due to the decomposition. If left unchecked, it can lead to the production of hydrogen fluoride gas; this is known as a harmful gas.
Hydrogen fluoride gas corrodes the valve stem and stuffing box leaving catastrophic damage on the valves. Another lubricant solution from PTFE must therefore be selected for gland packing to accommodate high-temperature conditions.
Finding Optimal Lubricant Source
Materials informatics is the use of information science, such as machine learning, to improve the efficiency of various materials development. In the aforementioned study, Materials Informatics was used to search for an optimal lubricant source. This approach enabled a successful search based on data, rather than relying solely on experience and knowledge. Furthermore, by providing Lids in the packing structure, the lubricant was able to maintain seal performance, even under high temperatures, by suppressing penetration leakage from inside the packing.
The packing optimized by applying the informatics method and the packing containing PTFE were compared under the conditions of the ‘High-Temperature Testing’ procedure specified in the API622 addendum. The results showed the sealing performance of the PTFElubricant packing was extremely poor after exposure to high temperatures, while the optimized packing exhibited stable sealing performance.
When tested under cryogenic conditions, it was determined that more improvement in resistance to wear was necessary. This is likely due to the fact that solidification from atmospheric moisture can wear out the packings in cryogenic conditions.
To mitigate the risk of failure, a minimum amount of PTFE coating was chosen as the lubricant for cryogenic conditions. PTFE is as solid at cryo- genic temperatures as it is at room temperature, however, a simple coating will peel off quickly. To account for this challenge a special coating method was used to form a stronger coating layer on the sliding surfaces of the packing.
The packing performance between the new cryogenic packing design optimized by this method and a conventional graphitic combination packing in ISO 15848-1 durability class CO1 mode at -100°C test temperature was compared using a test bench. The results showed that the sealing performance of the conventional graphitic combination packing was extremely poor after low-temperature sliding compared to the optimized packing design which exhibited stable sealing performance. The optimized packing also showed a stable friction coefficient and there was no abrasion powder observed from the packing after the testing.
For the Future
Carbon neutrality will continue to accelerate toward a sustainable future for mankind. To achieve this, individuals must make effective use of energy sources including but not limited to hydrogen. It is therefore important to continue research and development endeavors to ensure an optimal seal for all kinds of medias.
