The Steam Supply Valve
This valve is the one which opens the steam supply from the boiler to the engine. Triple expansion sgeam engines require a minimum of 100 psi, and preferably 120-200psi. But amteur built boilers are rarely certified above 100 psi.
But compressed air gets to 120 psi with no drama. So guess what will power this engine until I get around to making a high pressure boiler.
So the on-off valve needs to be pretty solid, so it does not explode and send hot fragments of metal in all directions.
Here is the main supply valve as specified and built for my triple expansion steam engine.
It all attaches to the high pressure steam chest and cylinder.