In high pressure fittings intended for hydrogen applications, knowing the operating pressure is not enough to determine whether a component is truly suitable for the application. Material, geometry, surface quality, machining precision and internal cleanliness must all be evaluated together.
Hydrogen also introduces specific critical issues. Its characteristics require particular attention to connection tightness and material compatibility, especially when the system operates at high pressures and under repeated pressure cycles.
For this reason, fitting quality should not be assessed only in terms of its mechanical ability to withstand pressure, but also in terms of the component’s ability to maintain the design conditions over time.
First Check: Is the Material Compatible with Hydrogen?
When selecting materials for high pressure fittings, possible hydrogen embrittlement must also be taken into account. This is the alteration of mechanical behaviour that can occur when hydrogen penetrates the metal.
The phenomenon does not affect all materials in the same way. Chemical composition, microstructure, treatments, stress state, temperature and operating conditions can significantly influence the sensitivity of the component.
For this reason, it is not enough to choose a material simply because it is generally considered corrosion-resistant or commonly used in industrial systems. It must be assessed specifically in relation to hydrogen service and the actual operating conditions of the system.
Some high-nickel austenitic stainless steels, such as AISI 316/316L (with a nickel content above 12% to reduce susceptibility to embrittlement), or special nickel-based alloys such as Inconel, are frequently used in these applications.
However, alloy selection should refer to relevant international standards, such as ASME B31.12 for hydrogen piping and pipelines or ISO 19880-3 for hydrogen refuelling station components, and should be part of a complete assessment of the entire component.
Types of High Pressure Fittings for Industrial Applications
High pressure fittings can use different technologies depending on pressure, geometry, maintenance requirements and circuit characteristics.
Double-ferrule mechanical fittings make it possible to create connections without welding by correctly tightening the fitting components onto the tube. In applications involving particularly high pressures, cone & thread configurations are also used, where end preparation and installation precision play a crucial role.
Another option is to use welded joints or custom-designed manifolds, reducing the number of mechanical connections within the system.
There is no single solution that is always better. In a dedicated article, we have already explored the comparison between Swagelok fittings and welding for high-pressure joints.
Machining Precision Becomes Part of the Sealing Performance
For high pressure fittings, dimensions and tolerances are not merely values to be shown on a drawing. Contact surfaces, threads, cones, seats, internal passages and transitions between diameters must be manufactured with a level of precision consistent with the function of the component.
A nominally correct geometry can still present critical issues if the mating surface is inadequate or if machining and deburring leave imperfections in areas involved in sealing.
For this reason, turning, milling and other machining operations become an integral part of overall system quality.
At Ricos, we approach these components by considering not only the final shape, but also the conditions required to achieve repeatable geometries and surfaces that are compatible with subsequent assembly and inspection stages.
Internal Cleanliness: The Requirement You Cannot See on the Drawing
In high pressure fittings for hydrogen, precise machining must be accompanied by a level of cleanliness appropriate to the application.
During manufacturing and assembly, chips, metal particles, oils, water, dust, oxides or residues from previous processes may remain inside the component.
Even apparently minor contamination can become problematic if it reaches valves, regulators, seals or other sensitive parts of the circuit.
For this reason, internal cleaning should not be considered a purely cosmetic operation performed at the end of production. It must be part of the process and defined according to the specifications required by the system.
For components intended for high-pressure hydrogen applications, even a detail that cannot be seen from the outside can therefore become critical.
Cleanliness Starts Before the Final Treatment
For high pressure fittings, obtaining a clean component does not simply mean subjecting it to a cleaning treatment after machining.
Quality must be built progressively: correct machining limits burrs and residues, deburring removes unwanted material and subsequent cleaning stages remove contamination remaining on the surfaces.
Drying, handling and protection after treatment must also prevent the component from being contaminated again.
In other words, cleaning cannot compensate for poorly controlled machining.
It is the integration of geometric precision, surface quality and final treatment that makes the component consistent with the requirements specified by the project.
Does Clean Always Mean “Oxygen Clean”?
The required cleanliness level for high pressure fittings must be defined according to the specific application rather than through a single definition that applies to every system.
The term “oxygen clean”, often associated with degreasing and decontamination standards such as ASTM G93, is used in industries where particularly strict limits on hydrocarbons and particulate contamination are required. However, this does not automatically mean that every component intended for hydrogen service must be treated according to the same specifications.
Gas purity requirements established for the specific application, such as ISO 14687 for the protection of fuel cells, together with the characteristics of the system, connected components and customer requirements, determine the actual cleanliness level to be achieved.
For this reason, it is important to define cleanliness criteria during the design phase rather than only after the component has been completed.
Fittings or Custom Manifolds: How Many Connection Points Are Really Necessary?
When designing high pressure fittings, one of the questions to ask concerns the number of joints that are actually required.
In some systems, integrating multiple flow paths into a custom-designed manifold can reduce the number of fittings and simplify the overall circuit architecture.
This does not mean that reducing the number of connections automatically makes the system safer: every component must still be correctly sized, machined, assembled and verified according to the intended operating conditions.
However, it can be a valuable design approach, particularly in complex hydrogen storage and distribution systems. We explored this solution in our article on custom manifolds for hydrogen storage.
Before Pressurization, the Component Must Be Evaluated as Part of the System
High pressure fittings for hydrogen cannot be selected by considering nominal pressure and dimensions alone.
Material, tubing, connection geometry, temperature, operating cycles, cleanliness, machining operations and assembly methods must all be compatible with one another.
This overall perspective makes it possible to address issues such as hydrogen embrittlement, connection tightness and internal contamination without treating them as separate problems.
At Ricos, machining precision and attention to cleanliness are therefore considered from the component definition stage onward, together with the subsequent manufacturing and verification phases.
Are you developing a component or a circuit for high-pressure hydrogen applications?
Contact us to assess the geometry, materials, machining processes and requirements of your project together.
