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Short Term Overheat

Short term overheating occurs in the absence of normal cooling effects caused by steam/water contact. The tube metal temperature quickly rises as a result. The tube surface bulges and eventually ruptures in a matter of seconds to minutes.
Tubes that have experience short term overheating have a thick, brittle, and dark oxide scale on external surfaces. Short term overheating produces considerable tube deformation in the form of metal elongation and reduction in wall thickness.  A fish-mouth type, longitudinal rupture with thin-edged fracture surfaces is typical for ferritic steel when it has failed before reaching its upper critical temperature. Microstructure examination can reveal evidence of grain plastic deformation near the fracture face and phase transformation to a hardened microstructure. This transformation occurs due to rapid overheating and quenching during rupture. Tubes exhibit relatively thick ID oxide scale at the failure locations along with possible hard quenched microstructure(s).   

Probable cause
Overheating is caused by conditions that impede heat transfer to the tube base metal. Short-term overheating can occur in steam-cooled and water-cooled tubes under the following conditions:

1)     Plugged by debris, scale, or condensate from incomplete boil out.
2)     Exposed to high heat transfer rates from improper firing of fuel burners.
3)     Low coolant flow due to poor circulation or upstream tube leak.
4)     Conditions that impedes heat transfer i.e. steam blanketing, oxide scale layer.

Common Locations 
This condition occurs in all tubes except economizer and downstream of economizer tubing. 
         Water walls

Corrective action
Corrective actions involve measures to prevent blockages of tubes, control drum water levels, assure coolant circulation and reduce excessive firing rates. Borescope the tube from header to header to check whether there is any flow restriction. Redesign or relocation of inclined or horizontal tubing may be required to prevent film boiling which will contribute to this problem.    Overheating is not detectable by normal NDE methods, since the failure event results from a sudden temperature rise and the metal degradation is rapid. Laboratory metallurgical examination of fracture surfaces and microstructure features can provide information pertinent to the failure investigation.