- THE MAGAZINE
Cases in PointSeveral recent field problems will illustrate this phenemon. Deep erosion occurred at the metal line of a brand new high alumina castable lining in a steel-casting tundish. This erosion could have had potentially life-threatening consequences if the wear had not been detected before a runout of molten steel occurred. Initially, a layer of less reactive insulating powder was added on top of the molten steel to maintain temperature (1530-1550°C). But then a highly reactive flux powder containing borax, fluorite, lime and graphite was added on an “as needed,” discretionary basis, which apparently caused rapid erosion. The use of the flux powder should have been controlled (and minimized or eliminated) to avoid localized accumulation and concentrated chemical attack.
In another example, the lining in an iron tilter consisted of high alumina, low cement castable, fortified with carbon and SiC, and backed by 1 in. of alumino-silicate insulating board. The tilter lining exhibited less than optimum life, due to crack development and iron penetration along the bottom centerline and lower sidewall. It was discovered that the heat flow of the lining was never calculated or verified, and the steel shell of the tilter was being heated beyond the 700°F service limit, causing irreversible growth of the steel and instability in the lining.
Another field example involved the installation of a chemical reactor lining, where masons mixed a mullite mortar with the specified high-grade Cr2O3 mortar to improve the workability of the mortar and ease the installation of bricks. The result was a lining that was less refractory than specified, because the joints were more susceptible to penetration/ reaction. This unauthorized, subtle deviation in the lining could cause a reduction in the lining life due to localized or general wear.
William Parker recently discussed interesting case histories at the St. Louis Section Refractories Symposium. (See Proceedings, St. Louis Section, Refractories Symposium, 2000, p. 127.) In one instance, a chemical plant in Venezuela purchased a new boiler that was rated to operate at 2000°F. The lining (9 in. fireclay castable and 4.5 in. of 2300°F insulating firebrick) kept failing and was replaced numerous times. It was discovered that someone at the plant had added 3 in. of external mineral wool insulation covered by corrugated sheet metal, apparently to reduce the radiated heat in the vicinity of the boiler for the comfort and/or safety of the workers. The external insulation caused overheating of the metal anchors in the castable, beyond their maximum recommended service temperature.