I've opened this new thread in response to the interest shown (by Proteus and Bunkerbarge et al.) in my flash steam control system, elements of which I have described in other threads.
In 2006 I had some vague idea about building a boiler which didn't fall foul of the Pressure vessel regs. and insurance. I visited the 2006 Warwick Model Exhibition and luckily found Steamboat Phil with his display of flash steam boats, old and new. With Phil's good natured encouragement, off I set to build a "controlled" flash boiler that could sedately power a Windermere launch at a few knots, but not to break any high speed records.
It took two hours to wind the 15 foot x 1/4" copper pipe into a volume about 3" dia. 9" long boiler. That was the easy part. The rest of the story revolves around learning to program PIC controllers, building the Stuart D10 from castings, building the electric pumps and servo-controlled gas valve.
There were many conflicting ideas on how to control a flash boiler - do you control the steam pressure by the pumps or by the gas valve? The PIC controllers were ideally suited for experimenting with the different methods, because, once the controller board was built, the different control methods could be downloaded without having to resort to the soldering iron or lathe.
I've got the stage where I can go from cold (2degC) to engine running at 200RPM in three minutes using 8 grams of butane/propane, tick-over at 200RPM lightly loaded at 6.5psi, using 1.2grams/min and run at 300RPM heavily loaded at 38psi using 4.5grams/min.
The only control signal I transmit is the engine speed demand signal (the boat will have separate channels for valve gear, drains and rudder control) and the PIC based controller monitors the boiler outlet steam temperature and pressure, gas cylinder pressure and the economiser outlet temperature. The controller then calcualtes the feed water requirement and gas valve servo position.
The speed regulation is purely done on feed water and gas supply, since there is no throttle / regulator between the boiler and the engine. If I set the demand RPM to 400 then the pumps gradually increase the feed flow and the gas valve opens to maintain the steam temperature - the speed then increases and hovers around 385 to 415 RPM.
That's the potted story to start the thread.
oh, and by the way, surprisingly, you don't bother controlling the steam pressure!