OK, let's go in a slightly different direction. To summarize, I proposed
putting a water tank over a burner, said water tank having a steam valve
on top. As I understood it, this is dangerous because

a) when the valve is opened and pressure is lowered a catastrophic amount
of water will instantly convert to steam

b) if the tank gets real hot somewhere and is sloshed such that water hits
the hot spot, a catastrophic amount of water will instantly convert to steam

Did I get those right?

So how *do* you build a boiler so that neither of these terrible things happen?
In plain simple words, please, no math allowed. Imagine I'm a bright 10th
grade physics student, for example.

I've looked at boiler designs and they have a firebox which is in thermal
contact with boiler tubes which contain water which is converted to steam.
How does this differ from a water tank over a burner flame?

GWE