O-rings for hydrogen refueling stations

Where are O-rings used in a hydrogen refuelling station?

O-rings and related seals are used in several places in a hydrogen refuelling station. On the dispenser side, seals are found in valve blocks, hose interfaces, filter housings, connectors and instrumentation points, among others. At the vehicle interface, nozzles, receptacles and breakaways are especially relevant.

High pressure valves, check valves, shut-off valves, ORFS fittings, pressure regulators, sensors and storage or compressor interfaces can also contain O-rings or alternative seals. The load differs per component. A static filter seal requires something different from a dynamic valve seal or a nozzle that is frequently connected and disconnected.

Not every seal in a hydrogen station is automatically a classic elastomer O-ring. In dynamic valves, cryogenic sections or ultra-low-leak connections, PTFE, PCTFE, spring-energized or metallic seals may be a better fit.

Pressure and temperature: H35, H70 and precooling

The main pressure classes for hydrogen refuelling stations are H35 and H70. H35 refers to systems around 35 MPa or 350 bar. H70 refers to systems around 70 MPa or 700 bar. In practice, maximum working pressures and component ratings are higher than the nominal designation, because the system must be able to handle pressure build-up, temperature fluctuations and safety margins.

For O-rings, therefore, it is not only the maximum pressure that matters. Above all, the combination of pressure, temperature, cycles and depressurisation rate determines whether a seal remains reliable. During fast 700 bar filling, hydrogen is often precooled. As a result, temperatures around -40 °C can become relevant for seals in the refuelling interface.

Low temperature can make elastomers stiffer. High pressure increases the risk of extrusion. Rapid decompression can cause blisters or internal cracks. Long-term compression can cause compression set, reducing the sealing force.

Key risks for hydrogen seals

Permeation and microleakage

Hydrogen can diffuse relatively easily into polymers. As a result, gas can migrate through an elastomer, even if the O-ring is mechanically seated correctly in the groove. This is called permeation. In hydrogen refuelling stations, this is especially relevant at high pressure, with small sealing cross-sections, long standstill periods and applications with very low leakage requirements.

Microleakage can also occur through the interface between the O-ring, groove and mating surface. In that case, the cause is not only the material, but also squeeze, surface quality, tolerance, installation or contamination.

Rapid gas decompression

Rapid gas decompression, often abbreviated as RGD, is an important failure mechanism in high-pressure hydrogen. Under pressure, hydrogen can dissolve into the elastomer. If the pressure drops quickly, the absorbed gas wants to expand and escape. If this happens faster than the material can degas, blisters, cracks or loss of sealing can occur.

That is why AED- or RGD-resistant compounds are relevant in high-pressure positions. The test conditions remain decisive. A compound must be validated for the correct pressure, temperature, decompression rate and application.

Extrusion and compression set

At high pressure, an O-ring can be pushed into the gap between two metal parts. This is called extrusion. The risk increases with an excessive extrusion gap, insufficient hardness or inadequate support. PTFE backup rings can mechanically support the O-ring and help limit extrusion.

Compression set occurs when an O-ring does not recover sufficiently after long-term compression. In a hydrogen refuelling station, this can lead to leakage after pressure changes, thermal cycles or maintenance.

Material selection for O-rings in hydrogen refuelling stations

There is no universal best O-ring material for hydrogen. EPDM, HNBR, FKM, FFKM, PTFE and metallic seals can all be relevant, but only within the right application. The compound and validation count at least as much as the polymer family.

A hydrogen-validated EPDM compound can be a logical candidate in many static HRS positions, especially when low temperature, compression set and RGD resistance have been demonstrated. HNBR can be interesting when mechanical robustness is important. FKM is mainly relevant at higher temperature or with co-media such as oil, but must be properly validated at H70 and during rapid decompression.

PTFE, PCTFE and metallic seals come into play when classic elastomer O-rings do not provide sufficient certainty. Think of dynamic valves, cryogenic supply-side components, very low leakage requirements or fire-safe static connections.

Standards and guidelines

Several standards apply to hydrogen refuelling stations at the same time. A material may appear suitable, but that does not mean the complete component has been approved.

Important frameworks include ISO 19880-1 for hydrogen refuelling stations, ISO 19880-3 for high pressure valves, ISO 19880-5 for dispenser hoses, ISO 17268 for nozzles and receptacles, SAE J2601 for fueling protocols and precooling, ISO 14687 and EN 17124 for hydrogen quality, ISO 3601 for O-ring dimensions and housing dimensions, and PGS 35 for Dutch hydrogen refuelling stations up to 700 bar. In addition, PED, ATEX, ISO 11114-2, ISO 11114-4, DVGW ZP 5101, ISO 23936-2 or NORSOK M-710 may be relevant.

Standards help define the requirements. Final approval must take place at component level, including leak tests, temperature tests, pressure cycles, RGD tests, ageing tests and traceability checks.

Groove design, hardness and backup rings

With hydrogen seals, geometry is just as important as material. A good compound can still fail if the groove is too full, the squeeze is incorrect, the O-ring is stretched too much or the extrusion gap is too large.

ISO 3601 is a logical starting point for O-ring sizes and groove design. In hydrogen applications, this must be supplemented by compound-specific verification. The supplier must be able to indicate which squeeze, groove fill, surface quality and hardness are suitable for the pressure, temperature and movement in the application.

In H35 and H70 systems, harder compounds and backup rings are often required. A PTFE backup ring supports the O-ring on the low-pressure side and helps limit extrusion. Installation remains important: sharp edges, contamination, torsion or incorrect installation aids can quickly lead to leakage in hydrogen applications.

Selection advice per component

Selection starts with the component position. Pressure class, temperature profile, static or dynamic use, co-media, groove design, leakage requirement and desired service life then follow. Only after that does a material name become meaningful.

Testing, validation and maintenance

At hydrogen refuelling stations, validation is often more important than fast delivery. Especially with H70, nozzles, receptacles, high pressure valves and dispenser interfaces, the seal must demonstrably function under the correct conditions.

A test package can consist of leak tests, permeation tests, RGD tests, temperature cycles, pressure cycling, hydrostatic or burst testing, durability cycling, accelerated aging, compression set measurement, cleanliness control and batch traceability.

Maintenance requires the same discipline. Replacing an O-ring in a hydrogen station is not an ordinary standard task when the seal is part of pressure equipment or a safety-critical component. Cleanliness, traceability, the correct installation aid and inspection for damage are necessary.

Contact O-ring-stocks.eu for technical advice on hydrogen-compatible O-rings, PTFE backup rings and sealing materials.

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