Refractory linings spalling

Another form of alkali metal attack on the hot faces of refractory linings involves their high temperature reaction with various components of the brick to form expansive crystalline phases which cause brick to bloat on their hot faces and, subsequently, erode or spall. An example Is the case of alumina brick exposed to sodium at temperatures from about 1700°F to 3000°F. Although sodium does not form a low temperature melt with alumina, it reacts with the alpha phase of alumina, corundum, to form beta alumina, sodium aluminate. Beta alumina has a much greater volume than the very dense corundum and, therefore, disrupts the brick bonding matrix, causing eventual bond failure. 

The presence of sodium and vanadium complexes in the fuel oil ash can, under certain plant operating conditions, result in considerable harm to the equipment. Spalling and fluxing of refractory linings is associated with the presence of sodium in the fuel. Above a certain threshold temperature, which will vary from fuel to fuel, the oil ash will adhere to boiler superheater tubes and gas turbine blades, thus reducing the thermal efficiency of the plant. At higher temperatures, molten complexes of vanadium, sodium, and sulfur are produced that will corrode all currently available metals used in the construction of these parts of the plant. TTie presence of trace amounts (ASTM D-1318) of vanadium (ASTM D-1548, IP 285, IP 286) in fuel oil used in glass manufacture can affect the indicator of the finished product. 

Rain Spalling. Severe mechanical damage to the refractory lining of a rotary kiln, indistinguishable from CHEVRONiNG (q.v.), but sometimcs due to sudden contraction of exposed parts of the kiln in heavy rain. 

Molten slag from low meltittg ash penetrates into the pores of the refractory linitig of a boiler or the furnace. Difference in the coefficients of expansion and contraction of the slag and the refractory material causes Spalling of the refractory lining thereby reducing its life. 

Brick Construction Brick-lined construction can be used for many severely corrosive conditions under which high alloys would fail. Brick linings can be installed over metal, concrete, and fiberglass structures. Acid-resistant bricks are made from carbon, red shale, or acid-resistant refractory materials. Red-shale brick is not used above 175°C (350°F) because of spalling. Acid-resistant refractories can be used up to 870°C (1600°F). See Table 25-10. 

Brick lining protection can be used for many conditions that are severely corrosive even to high-alloy materials. It should be considered for tanks, vats, stacks, vessels, and other similar equipment items. Brick shapes commonly used for such construction are made of carbon, red shale, or acid-proof refractory materials. Carbon bricks are useful for handling alkaline conditions as well as acid, while the shale and the acid-proof refractory materials are used primarily for acid solutions. Carbon can also be used where sudden temperature changes are involved that would cause spalling of the other two materials. Red shale bricks generally are not used at temperatures above 118.88°C (300°F) because of poor spalling resistance. Acid-proof refractories are sometimes used at temperatures up to 871°C (1600°F). 

Every refractory has a limited service temperature, or maximum temperature. Above these temperatures brick linings are used. The cutoff of refractory in lieu of brick is about 1800°F. Bricks have the capacity to seal and return to shape after heating without spalling. Sohd monolithic linings do not. In addition, bricks can be made in thicker sections than can be cast from refractory. 

Volume effects of the reactions should also be taken into accotmt (not only in gas corrosion of refractories). The reactions of oxidation of nitride-bonded silicon carbide side lining are positive. The positive volume effect of the reaction may play a positive role, diminishing the open porosity (Table 1.11). However, on the other hand, it may cause tensile strains, which may result in either spalling or cracking of the refractory (Fig. 1.26). 

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