Watch your head theirs Ideas flying all over this thread
The pump I'm talking about is for a closed loop system for pumping the low pressure condensate back into the higher pressure boiler. If you make a open system you don't need a pump, but either your boiler will need to store all the water for one trip or you will need a high pressure storage tank.
Daox it sounds like the die grinder just isn't powerful enough. Adding a flywheel can smooth out power delivery via inertia, but I don't think that's the problem. Just need more power. I'm amazed that just the field windings take so much power that a die grinder can keep up.
When I was talking about the pump being belt driven I was talking about it being driven by the steam expander or turbine, so when X amount of steam goes through the expander Y amount of water is always pumped back in. I was thinking we would need to get roughly the right size pump then fine tune the flow rate by changing the pulleys. I like the bicycle chain and sprocket idea, rig a couple rear cassettes to replace the pulley and you now have many ratios to choose from.