Not sure about the Hydrogen pressure, but most likely it will be auto-ignited upon leakage due to operating temperature.
Extra attention on sealing material/quality is highly suggested.
Graphite packing usually consist of graphite (93 to 99% purities) with bonding agent such as Zinc, PTFE, etc. Please note that Zinc start to become permeable above 200 degC and PTFE above 210 degC. These numbers often differed or debated amongst packing supplier and end-user.
Therefore I would suggest to choose the purest graphite possible.
Not all TapcoEnpro, JamesWalker, ERIKS, etc. sealing components are suitable for high temperature application. Ask for burning and/or aging test result if possible.
I had some incidents on 93% graphite being exposed to 480degC and showing gland extrusion/intrusion after only several cycles.
Companies mentioned above by Tbe Obturator and TugBoat are quite ok. IMI group (they have acquired some ball valve companies) are worth considering as well.
Regards,
D