Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

The drawing in your previous post appears to show an oscillating cylinder so in that case there would be no valve. The "pipe" on top of the boiler may just be a solid rod handle for the filler plug, although it looks like you would need to remove the chimney to unscrew it.

That drawing was from a fire engine patent. Dewey didn't patent Maxwell's horizontal, so its boier internals are a mystery.  However, the horizontal was supposed to have a throttle valve, and the pipe on top of the boiler doesn't look like it could function as a safety valve.