A furnace plays a vital role in many industrial processes. It is a crucial component that is used to contain and regulate the controlled combustion of hydrocarbons, such as oil or natural gas, to produce heat.
This heat is necessary for a variety of operations, including heating fluids, cracking hydrocarbons and generating steam.
Additionally, the outer casing of a furnace — the furnace shell — may incorporate features such as refractory lining or insulation materials to optimize heat retention and minimize energy loss. They are typically engineered to meet stringent industry standards for safety, reliability and environmental compliance. Furnace shells also serve to protect personnel and equipment from the high temperatures generated during operation.
Case study: Cracking and corrosion
Optical micrograph (left) and scanning electron micrograph (right) of intergranular cracking and corrosion in a polished cross section of the furnace shell
If a furnace shell were to crack, it would not only disrupt production schedules, but would pose a plethora of safety hazards. Recently, a client in the Gulf Coast region contacted KnightHawk Engineering’s (KHE) materials lab to conduct a metallurgical failure analysis on a section of its furnace wall that was cracked. The client suspected the failures may have been related to thermal fatigue.
Extensive cracking was detected in a plain carbon-steel furnace shell. The refractory of the furnace was approximately 10 inches thick, and for the first 30 years of the furnace’s operation, the furnace did not have any NOx control measures in place. The cracks on the furnace shell all seemed to be near locations where thermal stresses during startup and shutdown would be maximized, and the furnace had been cycled several times per year for its entire operational life.
However, KHE determined that the failure of the furnace shell was caused by nitrate stress corrosion cracking (SCC), and not by thermal fatigue. The presence of stress corrosion was clear from the crack branching and the intergranular nature of the cracking. There are relatively few mechanisms that result in intergranular failure of low-strength plain carbon steel, and of those mechanisms, the only one likely to take place in a furnace shell is nitrate SCC. However, for nitrate SCC to occur there has to be a source of nitrates, and that is where the lack of NOx control measures is important.
The NOx that was produced by the burners for the first 30 years of operation had partially condensed on the ID of the furnace shell and reacted with the insulation to form magnesium and sodium nitrate salts. Then, when the furnace was cycled and cooled to below the dew point, the surface of the metal was wetted and the nitrate salts entered into solution, resulting in the formation of a stress corrosive environment. Upon startup, the areas of greatest stress were the areas where thermal stress was at a maximum, and so the SCC occurred most prominently in these areas.
KHE concluded that the cause of the cracks was due to nitrous oxide deposits. Nitrous oxide deposits on carbon steel surfaces can lead to intergranular SCC of the metal, especially when the metal is routinely cycled through the dewpoint. Despite the initial appearance of the failures — thermal fatigue — a detailed metallurgical analysis was able to determine that the actual cause of the cracking was corrosion related.
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