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The most damaging slag forming materials are vanadium salts and sodium chloride. If the slag that forms on the brickwork would remain in place, it would not cause any particular trouble; however, the slag does not remain in place. Instead it peels off or melts and runs off taking some refractory with it and exposing a fresh layer of refractory to further slag attack. When deterioration of the brickwork has progressed until only a 75mm thickness of refractory remains, the wall should be replaced. When sufficient slag has accumulated on the deck to cause striking with resultant deposits of carbon the slag should be removed. If less than 38mm of firebrick remain after the slag is removed, the entire deck must be replaced. Another type of slag that results from using fuel oil that is contaminated is usually more damaging than peeling slag. This type of slag is very glassy in appearance and when this slag melts, it usually covers the entire wall or deck. Refractory shrinkage is another cause of furnace deterioration. True shrinkage (permanent shrinkage) is quite rare in firebrick approved for naval use. However, defects can occur even in approved firebrick. In any case, it is important to recognize the appearance of true firebrick shrinkage because of the extremely dangerous condition it could create if it should occur. When the firebrick shrinks, the hot-face dimensions of each brick become measurably smaller than the cold-face dimensions. This condition leaves an open space around each brick, and the entire wall or floor becomes loose. A wall or floor having this appearance is DANGEROUS and should be completely replaced as soon as possible.
Also, during your inspection, look for signs of unequal stresses that arecaused by rapid raising of the furnace temperature while raising steam too rapidly. Emergencies may arise that require the rapid raising or lowering of furnace temperatures, but it is important to remember that the refractory cannot stand this treatment often. As a rule, you will find that raising the furnace temperature too rapidly causes the refractory to break at the anchor support and lowering the temperature too rapidly causes deep fractures in the refractory.
Also, look for signs of mechanical strain caused by poor operation of the boiler. Continued panting or vibration of the boiler can cause a weakened section of the wall to be dislocated so that the refractory to fall out onto the furnace floor. Improper oil-air ratio is the most common cause of boiler panting and vibration. Proper operation of the boiler, with particular attention to the correct use of the burners and forced draft blowers, generally prevents panting and vibration of the boiler.
Inspection should also be made of the lower side of the floor pan. Any overheating indicates a loss of insulation and excessive heat penetration. Under normal conditions the brickwork in a boiler should last for a number of years without complete renewal. Expansion joints should be inspected often for signs of incomplete closure. It is important to keep the joints free of grog, mortar and refractory particles so that the joints can close properly when the boiler is fired.
You can tell if an expansion joint is closing completely when it is heated by inspecting it when it is cold. If the inside of the expansion joint is light in color when the furnace is cold, the expansion joint is closing properly. If an expansion joint does not close properly when heated, the inside is dark and discolored. The same method can be used to tell ifcracks in refractory materials are closing properly when the furnace is fired. If the cracks are dark, showing that they do not close, they should be repaired. Since the first firing on a refractory plastic or castable burner front does more damage than any other single firing the first inspection after installation is a very important one. The unfired burner front may appear to be in perfect condition while actually containing defects of material or workmanship that will show up immediately in the first firing.
After the boiler has steamed for several hours, slabs of plastic about 13mm to 25mm thick may separate from the burner’s front surface and fall off. This is because the surface layer is more densely rammed during installation than the remainder of the material. Radial cracks in the burner fronts may be found on the first inspection. These cracks are not harmful. They are caused by stresses resulting from the normal expansion and contraction of the refractory as it is heated and cooled. After the radial cracks occur, the stresses are relieved and there should be no further cracking of this type. The cracks that eventually result in extensive damage run approximately parallel to the surface. |
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