Firing conditions and heater temperature are the main causes of deterioration of the materials that form the internal lining of the heater. The severity of the deterioration will vary with the heater temperature, which in turn is determined by the process operating conditions.

The purpose of the internal materials, including refractory or insulating linings, is to provide heat protection to the structural steel framing, roof structures, and tube sheets, and to improve the thermal efficiency of the heater. At high temperatures, refractory will deteriorate after long-term exposure by spalling, failure of the binding material, melting, and loss of structural strength. When the insulating value of refractory or insulating material is reduced, the supporting steel is subjected to high temperatures and may deteriorate rapidly as a result of oxidation, scaling, and possible metallurgical changes.

Fluxing may occur when fuel ash and refractory are in contact at a moderately high temperature, producing a slag that may be fluid. Metal oxides, including those of vanadium, molybdenum, and sodium, are fluxing agents. At least three deteriorating actions of this slag formation can be recognized:

a.   Melting. The flux melts at a lower temperature, thereby causing the refractory to become liquid and flow, which reduces the refractory thickness.
b.   Penetration. The flux can penetrate into the sound refractory, thereby compromising its properties.
c.   Chemical action. The flux can react with the refractory and chemically degrade it much like metal thickness being reduced by corrosion.

The general effect of slagging is to decrease the thickness and reduce the insulating effect of the refractory, and thereby allowing a high metal temperature on the supporting steel parts.

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