Teams moving from natural gas into hydrogen frequently carry their existing filtration specifications across unchanged. Most of those specifications do not survive contact with hydrogen, and the failures are not subtle.
Sealing
Hydrogen's molecular diameter is roughly 0.29 nm against methane's 0.38 nm. Elastomer seals that are entirely leak-tight on natural gas permeate measurably with hydrogen, and standard soap-bubble leak testing cannot detect the rates that matter.
The practical answer is metal-to-metal sealing where the design permits, specialised fluoropolymer or hydrogenated nitrile elsewhere, and helium leak testing to 1×10⁻⁹ mbar·L/s as an acceptance criterion rather than a nice-to-have.
Metallurgy
Hydrogen embrittlement affects high-strength steels by diffusing into the lattice and reducing ductility, with the effect increasing as material hardness rises. Materials perfectly acceptable for natural gas at the same pressure may be unsuitable.
Austenitic stainless steels with controlled hardness perform well. Hardness limits must be enforced through the weld and heat-affected zone, which means the welding procedure is as much a safety document as the material certificate.
Purity thresholds
Fuel cell grade hydrogen to ISO 14687 Grade D limits carbon monoxide to 0.2 ppm and total sulphur to 4 ppb. These are orders of magnitude below industrial gas practice, and they change what counts as a contaminant.
In particular, materials that are inert in every other sense may still outgas at levels that matter here. Every wetted material in the flow path requires characterisation, not assumption.
Electrolyser-specific loads
Alkaline electrolysers carry potassium hydroxide aerosol into the product stream. Removing it requires coalescing media compatible with a strong base, and a drainage design that returns recovered electrolyte to the process rather than discarding it.
PEM electrolysers present a different problem: ionomer fragments and catalyst particulate at very low concentration but very small size, requiring absolute filtration well below 1 µm at the outlet.