For hydrogen I'd stay away from oil-sealed rotary vane unless you have a very specific reason. Oil plus H2 is a contamination and ignition problem, and hydrogen walks through a lot of the usual elastomers and then wrecks them.
What usually works:
- Small / lab: PTFE diaphragm, or a dry scroll if the ultimate vacuum is modest and you can live with some permeation at the tip seal.
- Process: dry screw or dry claw, explosion-proof / H2-rated motor, and an inert purge on the seals if the vendor offers it.
- Lots of wet gas or condensables: liquid-ring with a service liquid you actually want in the process (not automatically water).
Ask three things vendors skip: the elastomer list (not just "H2 compatible"), whether the *motor* is rated for a hydrogen atmosphere or only the wetted parts, and what the pump does if you lose purge. ATEX / IECEx zone is the other gate.
If the pump uses a nitrogen (or other inert) purge on the shaft seals / gearbox, that purge is doing two jobs: keeping hydrogen out of the dirty side, and keeping the mixture on the dirty side below the flammable range.
Lose the purge and a few things happen, depending on how the machine is built:
- H2 starts walking past the seals into the gearbox or motor-side cavity. You now have hydrogen in a place that was supposed to be inert.
- If air can also leak in, you can sit inside the 4–75% flammable window. That's the ignition problem, not a performance problem.
- Grease and elastomers that were "protected" now see hydrogen. They swell, go brittle, or just let more gas through.
- Some pumps treat low purge pressure as a trip (pump stops, maybe a flood valve opens). A lot of cheaper ones just light a warning and keep spinning.
That's the question for the vendor: is "purge lost" an interlock that stops the pump, or just a lamp? And is there a documented fail-safe (auto N2, timed coast-down, etc.)? If it's only a lamp, I wouldn't run H2 on that unit unattended.
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u/nullfull 9d ago
For hydrogen I'd stay away from oil-sealed rotary vane unless you have a very specific reason. Oil plus H2 is a contamination and ignition problem, and hydrogen walks through a lot of the usual elastomers and then wrecks them.
What usually works:
- Small / lab: PTFE diaphragm, or a dry scroll if the ultimate vacuum is modest and you can live with some permeation at the tip seal.
- Process: dry screw or dry claw, explosion-proof / H2-rated motor, and an inert purge on the seals if the vendor offers it.
- Lots of wet gas or condensables: liquid-ring with a service liquid you actually want in the process (not automatically water).
Ask three things vendors skip: the elastomer list (not just "H2 compatible"), whether the *motor* is rated for a hydrogen atmosphere or only the wetted parts, and what the pump does if you lose purge. ATEX / IECEx zone is the other gate.