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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