What must all steam generating parts exposed to fire or hot gases be covered by?

The correct answer is that all steam generating parts exposed to fire or hot gases must be covered by water. This is crucial because the primary function of water in a steam-generating system is to absorb heat and prevent overheating of the components. If parts are exposed to high temperatures without water, they risk damaging or failing due to thermal stress. Water also acts as a coolant, maintaining operational safety and efficiency.

In contrast, while materials like fire brick or ceramic tile may provide thermal insulation or protection in various contexts, they are not a substitute for the cooling and heat absorption that water provides in steam generation systems. Expansion joints, while important for accommodating thermal expansion in piping and other components, do not directly protect steam generating parts from exposure to heat. Thus, the presence of water is critical in maintaining the integrity of the system.