Oxide scale exfoliation and regrowth in TP347H superheater tubes

Abstract

This paper provides an introduction to a comprehensive model being developed to predict and control oxide scale exfoliation from the steam‐side of superheater and reheater tubes in steam boilers. The model deals with the main phenomena involved in scale growth and failure in steam, and incorporates major variables related to boiler design and operation. The considerations used to calculate oxide growth under the specific constrains of small diameter tubes carrying high‐pressure steam and operating with large temperature gradients under temperature and pressure cycling conditions, as well as the evolution of stresses and strains in the scales, are indicated but only a cursory description is given of the details of the analytical treatments. An example is presented of calculations made with the model to predict the extent of blockage expected in a single superheater loop as a function of time and outlet steam temperature under several realistic service conditions. The results suggest that problems due to scale exfoliation would be expected early in the operating life of superheater tubes made from austenitic steel TP347H.

Acoustic emission analysis on tensile failure of steam-side oxide scales formed on T22 alloy superheater tubes

“…6 Second, the exfoliated oxide scales may accumulate inside the tube bends and even result in tube blockage. [7][8][9] The flow of steam inside the tubes will then be restricted, which may result in short-term tube overheat and even failure eventually. In addition, if the exfoliated oxide scales are carried by the steam into the steam turbines, erosion damage of the first stage nozzles and the rotating blades will occur.…”

Soft Sensor for Oxide Scales on the Steam Side of Superheater Tubes under Uneven Circumferential Load

“…Assume that the temperature drop of steam is 100 ∘ C while all other parameters remain the same. The stresses are calculated with (10)- (11). The optimization progress of SVM parameters for the radial stress in 0 ∘ is shown in Figure 7.…”

“…As the load along the circumferential direction of the superheater tube is uneven, the oxide growth temperature is different circumferentially and so is the thickness of the oxide scales. Sabau et al [11] pointed out that the steam temperature gradually increases along the flow direction of the tube and calculated the growth temperature distribution in the axial direction.…”

Failure evolution of steam‐side oxide scales grown on T91 superheater tubes by acoustic emission technique

“…Ferritic‐martensitic T91 steels have been utilized for power plant applications widely because of their superior creep resistance at high temperature . During the operation of boiler, the steam‐side oxide scales formed on T91 alloy tubes are stressed by many sources, such as the oxide growth stress (due to the new oxide formed within existed oxide scales) and the thermal stress (induced by the thermal expansion difference of oxide scales and the substrate) . Under such stress, oxide scales may be exfoliated from the tube surface as they do not have the ductility of the underlying metal .…”

“…[1] During the operation of boiler, the steam-side oxide scales formed on T91 alloy tubes are stressed by many sources, such as the oxide growth stress (due to the new oxide formed within existed oxide scales) and the thermal stress (induced by the thermal expansion difference of oxide scales and the substrate). [2][3][4] Under such stress, oxide scales may be exfoliated from the tube surface as they do not have the ductility of the underlying metal. [5] Exfoliated oxide scales can be collected in the lower tube bends, resulting in short-term overheating failure of tubes.…”