Even though they do not possess moving parts, it is amazing how many ways a boiler can fail and how seemingly trivial actions can have such serious repercussions. Boilers do deteriorate over time, but they can also fail prematurely. 
The "moving parts” of a boiler are the circulating water, the flue gas, and the thermal expansion of the steel and insulation. The flux of heat from the combustion products across the various steel pressure boundaries adds another layer of complexity.