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Creep / Long term Overheating
Creep is permanent plastic deformation at temperatures and stresses below the metal’s defined yield point on the stress-strain curve. Boiler tube materials have a maximum operating temperature defined by the manufacturer. Differences between the actual operating temperature and the expected design temperature can significantly shorten the life of affected tubes. Three major factors contribute to unexpectedly high tube-metal temperatures during boiler operation: Internal oxide scale growth; loss of wall thickness due to oxidation, corrosion, and erosion; and an extreme operating environment with uncontrolled operating conditions.
Oxide layers formed on the inside surface of tubes act as an insulating barrier between cooling steam and flue gas. As oxide layer thickness grows the tube metal is progressively insulated from cooling steam, and tube metal temperature increases. Increased tube metal temperature raises the oxide scale formation rate. Longitudinal oxide cracks can appear at tube ID and OD surfaces. Note that the ductility of iron oxide is much less than that of steel. Thus, as the tube expanded by creep-deformation, the brittle oxide scale does not follow the expansion, and longitudinal cracks can appear.
Creep/ long-term overheating failures are thick-edged, longitudinal failures located on the hot side of the tube. Thickened, external scale with an “alligator hide” appearance is typical. Tube wastage flats at the 10:00 and 2:00 o’clock positions on the hot side may be present. Failures range in size from small blisters to wide open fish-mouth fractures.
Thick internal oxide scale can be a good indicator of overheating damage. Microstructural features that indicate elevated tube temperatures, such as spheroidized carbides, sigma phase, and graphitization, may be visible. Creep cavitation voids will exist at the crack tip.
The outside surface of superheater and reheater tubes facing incoming flue gas is typically protected by a layer of ash deposits. Aerodynamic factors can cause relatively thinner layers of deposits at the 10:00 and 2:00 o’clock positions of the hot side (12:00 o’clock being the hot side). Thinner deposits cause a number of issues that raise the tube metal temperature and internal stress. First, a thinner ash layer can effect heat transfer rates into the tube metal by exposing the tube to more radiant heat. Second, the oxidation rate of tube material is increased as more metal is exposed. This removes tube material and increases internal stress levels. Finally, more tube metal is exposed to oxidation in a sulfur-containing environment, which leads to higher tube corrosion rates and therefore increased internal stress.
Boiler operating conditions also play a critical role in the development of creep damage. Higher than expected flue gas temperature, displaced fireball, periodic over firing, or uneven firing of fuel burners can increase tube metal temperatures. Tubes can also overheat when downstream from blocked gas lanes and plugged tubes.
Performing life assessment analysis should be the first step in developing a long-term solution to creep issues. Evaluating the oxide scale layer and tube wall thickness will yield a remaining useful life estimate. Decision makers can then move forward in selecting a material, design, or operating solution. Material solutions involve replacing the spent tube with the same material or upgrading to something more resistant. Design solutions include replacing superheater/reheater circuits to equalize heat transfer and avoid radiant cavities. Several operating solutions exist such as chemical cleaning, removing tube blockages, adjusting burner settings, and minimizing excessive velocities.