How are pressure vessels heated?

I'm working on a project in which I'll be dealing with pressure vessels. As of now I've drawn/conceptualized everything on theory basis. The diagrams are made on the basis of theory, but then I came through a situation where I'll require to heat water under high pressure at around 300-350 Celsius. So then I decided to use pressure vessels and to use LPG gas for heating it (LPG has flame temperature of around 1100 Celsius, but please note that I want to heat the water kept in vessel at just 300-350 Celsius)


Now, I'm stuck in thinking that how do you heat pressure vessels? Are there any special port/connections provided to heat them/the content inside them? Or are any special type of heaters used to heat them? *Would be great if you can tell everything you know about heating it*


Optional to read(this could be wrong): Are we supposed to directly heat them on flame? Just like cooking your food on the gas-stove running using LPG? If this is so, then won't it take too much time because the walls of vessels would be very thick?


