Don't be afraid of hydrogen
This article addresses the sealing of hydrogen using static flat gaskets made of fiber materials (FA). Hydrogen, known as the 'oil of the future,' presents a new medium for many designers and practitioners. They will need to deal with it in their designs, plant layouts, procurement scenarios, and assembly activities, especially concerning the sealing of components. The article aims to raise awareness for this issue and provide information to make the right decisions regarding material selection and assembly situations.
Static Seals - Soft Material Seals
One of the most common forms of sealing is static sealing. In this case, the components to be sealed do not move relative to each other. In these connections, significant pressures are exerted on the gasket material installed between the flanges – the high-pressure gasket.
To achieve a seal, the gasket material must exhibit adaptive behavior, migrating into the roughness of the flange surface and compensating for its irregularities. On the other hand, the high forces must not destroy the material—a typical engineering compromise. To address this compromise, Klinger has developed a manufacturing process: the calendering process, in which a mixture of fibers and fillers with elastomer as a binder is processed into a sealing plate on a hot roller under tremendous pressure.
The result is a highly resilient gasket, typically capable of withstanding over 200 MPa (approximately 2 tons per cm2) at room temperature, with the smallest pores and the ability to adapt to surface roughness through the compression of the pores and elastomer. By compressing, for example, through screws, surface leakage and leakage through the gasket material are prevented – the higher the sealing force, the tighter the connection.
Tightness requirements for gas supply
The DIN-DVGW type approval according to DIN 3535-6 from April 2019 specifies corresponding values. The specific leakage rate must be 0.1 mg/(s x m). This is based on FA sealing materials with a gasket thickness of 2.0 mm, an internal pressure of 40 bar, and a surface pressure of 32 MPa. Nitrogen is used as the test gas. So far, we have been using fossil fuels such as natural gas (mainly methane) as well as gases like propane and butane as standards for our energy supply. The current leakage requirements are sufficient for these gases, but what about hydrogen?
Is hydrogen different from the usual fuel gases?
Hydrogen gas has a low density, and its atom has a very small spatial extent. It is the smallest atom in the periodic table of elements. Therefore, it can theoretically traverse even the smallest channels more effectively than larger atoms. However, in reality, hydrogen exists in an atomic state only during its production and immediately forms hydrogen molecules (H2) by bonding with the nearest hydrogen atom. These molecules can be visualized as dumbbell-shaped. Nevertheless, the conventional fuel gases such as methane CH4 (the main component in natural gas), propane C3H8, and butane C4H10 are all significantly larger.
In recent years, leak testing for sealed connections has increasingly shifted from using nitrogen to helium (He) as the test gas. We now have the second smallest atom in the periodic table of elements as the standard test gas, enabling us to detect the smallest leaks. Our gas particles are not rigid but move due to Brownian molecular motion. When comparing the kinetic diameters of the relevant gas particles, we observe that the helium atom and the hydrogen molecule exhibit comparable sizes.
In the table of kinetic diameters (www.arnold-chemie.de ) we find
Hydrogen H2 2.3 - 2.9
Helium He 2.6-2.7A
and for methane CH4 3.8
where 1 A = 0.1 nm
We observe that hydrogen is indeed 'smaller' than methane but falls within the same range as our test gas helium. Previous real comparative measurements also show that while there are differences in leakage amounts between hydrogen and helium, these differences are in the same order of magnitude. Additionally, it is worth mentioning that hydrogen burns faster than natural gas. Therefore, in gas burners, the distances between the burner nozzle and flame are smaller. As a result, flame detection technology, material selection for the burner nozzle, and other parameters need to be adjusted. Furthermore, hydrogen, unlike other gases, exhibits a negative Joule-Thompson effect. However, all these factors are not relevant to the tightness of connections.
What practical experience do you have?
Hydrogen has been a common raw material in the chemical industry for many years. According to the German Chemical Industry Association (VCI), hydrogen is exceptionally significant and forms the starting point for important chemical value chains. Currently, about 12.5 billion cubic meters of hydrogen are used annually in Germany (according to vci.de). Even the previously used town gas had approximately 50% hydrogen content. Hydrogen is not chemically aggressive and does not attack common fiber, graphite, and PTFE sealing materials. This demonstrates that we are indeed familiar with handling the medium and have been successfully implementing the corresponding sealing strategies for a long time.
A look at the potential dangers of hydrogen
As with all combustible gases, there is also a risk of unintended combustion in the form of an explosion with hydrogen. The explosive limits of various combustible gases must be considered in this context. The lower explosive limit (LEL) in air is 4 vol.% for hydrogen and 4.4 vol.% for methane, which are quite similar. However, the upper explosive limits are quite different, with 77 vol.% for H2 and 16.5 vol.% for CH4.
Within CEN/TC 58 - Safety and control devices for burners and appliances burning gaseous or liquid fuels - there is Working Group 15, which focuses on hydrogen and prepares information for international standardization. The presentation 'CEN/TC 58 WG 15 evaluations 2022-04-14' addresses, among other things, the comparison of the combustible gases methane, propane, and butane with hydrogen and a hydrogen/natural gas mixture of 20% to 80%, with a focus on gas installation devices. Gas installation devices, such as burners in heating systems, appliances, and household devices, represent a significant potential application area for future hydrogen use. Therefore, the working group has conducted a risk assessment. Extensive calculations and initial measurements have been carried out to gain insights. The danger posed by combustible gases is influenced not only by their leakage behavior but also by their ignitability. The working group has also assessed and described such influences in calculations.
Result
- With our well-known and high-quality fiber-based sealing materials from history, we have had positive experience with the safe sealing of hydrogen.
- Independent leakage measurements show that we are also in the usual ranges for fuel gases for hydrogen.
- A look at the potential of the explosion hazard also shows that we are operating within a known framework for hydrogen, which has been safely controlled for many years.
We, as a sealing manufacturer, can confidently say that we do not need to fear hydrogen as a future carbon-free energy carrier. If the constructions are sound, and the installation is carried out professionally, hydrogen will be a safe pathway to decarbonization. The era of hydrogen can commence.