High Temperature Ball Valves

Metal seated ball valves meet the temperature requirements. You will have to inquire about leakage. I don't intend to plug for a manufacturer, this is one I know of but have no experience with.

Some other potential vendors for 'severe service' valves;-
ValvTechnologies Mogas Industries

*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

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

Technical Valve Services in the UK may be able to assist -

I have positive experiences with the linked below factory, but not with high temperature valves, or for hydrogen. I would suggest that you search for North Sea offshore valve suppliers. Study of details/samples below might help you-

Ladish & ValvTechnologies also both make metal-seated ball valves for this, depending on valve size & metallurgy you need.