What happens when heating water in a pressure vessel?
Look at the phase diagram of water to see what happens. Josiah Willard Gibbs invented these diagrams o help understand what happens as a substance is heated under pressure, and transitions from solid to liquid to gas and supercritical fluid (neither quite gas nor liquid).

Given sufficient heat, the vessel will burst. Above ~650 K, the water can no longer be liquefied by pressure, and behaves more like an ideal gas, so pressure continues to increase.