This type of failure is usually indicated by a "thick-lipped"
burst of the boiler tube. Long-term overheating can result from
excessive deposition, flame impingement, mild flow restrictions,
or poor water or flue gas circulation patterns.
Probably the most common of these is excessive deposition, which
prevents proper heat transfer and excessive metal temperatures.
This prolonged overheating of the tube causes metal degradation
to the point that in can no longer handle the operating pressure
and a "thick-lipped" failure occurs.
overheating usually occurs in superheaters, reheaters and
waterwalls tubes as a result of gradual accumulation of
deposits or scale, partially restricted steam or water flow,
excessive heat input from burners or undesired channeling of
fireside gases. Horizontal or inclined tubes subjected to steam
blanketing are also prone to long-term overheating failures.
Tube metal operating temperatures above 454°C, or slightly above
the oxidation limits of the tube steels, can lead to blistering,
tube bulging or thick-lipped creep rupture failures .