Overheating is one of the most serious causes of deterioration of boilers. Overheating of the boiler tubes and other pressure parts may result in oxidation, accelerated corrosion, or failure due to stress rupture. Although overheating can occur during normal boiler operations, most often it results from abnormal conditions, including loss of coolant flow or excessive boiler gas temperatures. These abnormal conditions may be caused by inherently faulty circulation or obstructed circulation resulting from water tubes partly or wholly plugged by sludge or dislodged scale particles. Over-firing or uneven firing of boiler burners may cause flame impingement, short term overheating, and subsequent tube failure. The results may be oxidation of the metal, deformation of the pressure parts, and rupture of the parts, allowing steam and water to escape.
Boiler tubes may be damaged by poor circulation. Under certain conditions of load and circulation, a tube can become steam-bound long enough to overheat locally and fail. If circulation is periodically reestablished, the hot portion of the tube is quenched by relatively cool water. This often causes thermal fatigue cracks, which may eventually result in tube failure. This condition can also result in caustic or chelate corrosion. Steam binding may be caused by the insulating effect of slag deposits on the outside of the lower part of the tube. This demonstrates the importance of avoiding, as much as possible, non-uniform slagging of waterwalls. Steam superheaters can become overheated and severely damaged during start-up if cold boilers are fired at an excessive rate before a sufficient flow of steam is established to keep the superheaters cool. They can also become overheated if the steam vented from the superheater outlet is not sufficient to provide steam flow through the superheater during warm-up or low -load operations. The overheating results in warped tubes and oxidation of the tube metal, leading to early tube failure.
The faulty operation of steam-separating devices may result in deposition of boiler water solids in the superheater tubes, with subsequent damage to the tubes from overheating as the deposits impede heat transfer.