Materials for mid furnace environments of kraft recovery boilers
Kraft recovery boilers experience localized accelerated corrosion in mid furnace areas. Corrosion patterns may vary from one boiler to other depending upon the operation parameters and general design of the boiler. Local gaseous environments were characterized in areas with corrosion and equivalent areas without accelerated corrosion in a selected boiler over the period of two years. Results from this study indicated that in the areas with accelerated corrosion there was a large variation in the local gas composition at the waterwall surface. Reducing and sulfidizing gases were found in the areas with accelerated corrosion whereas the gas compositions were more oxidizing in the low corrosion areas. Observations during field tests have indicated that the local waterwall temperature may also vary in the mid furnace due to localized combustion and air flow patterns. Mid-furnace environments were simulated in laboratory to evaluate the effect of gas composition and fluctuations on corrosion kinetics of carbon steel tube material. Laboratory tests with SA-210 carbon steel samples with gas composition cycling between oxidizing and reducing/sulfidizing led to an unstable scale formation on the surface. Extent of accelerated corrosion was found to depend upon the extreme partial pressure values of sulfur and oxygen. Carbon steel samples were coated with chromized or aluminized diffusion coatings in the laboratory and were tested under sulfidizing conditions. Both type of diffusion coatings had superior performance compared to the carbon steel samples under similar conditions. Midfurnace corrosion can be controlled by making gaseous environments in the mid furnace more stable. However, more practical solution seems to be to use waterwall tubes with protective outer layer. This may be done by using composite tubes (stainless steel outer layer), thermal spray coatings of resistant alloys or diffusion barrier coatings like chromizing or aluminizing treatment.
Corrosion in lower furnace of kraft recovery boilers-in-situ characterization of corrosive environments
Fireside corrosion in kraft recovery boilers is a continuing problem, especially in boilers using carbon steel tubes in lower furnace areas. Certain areas in the lower furnace of kraft recovery boilers consistently have higher corrosion rates than nearby areas with the same tube material.These differences in the corrosion rate suggest differing local environmental conditions. Corrosive environments were characterized in a kraft recovery boiler, which had well-defined areas with high corrosion rates and low corrosion rates. We monitored three important variables in the selected areas: tube surface temperature, gas composition at the waterwall surface, and smelt composition. Recovery boiler gases were analyzed using an on-line gas chromatograph. Temperature and smelt had no effect on differences in the corrosion rate in the two areas. Smelt was not present in either the low or the high corrosion rate areas. Gas compositions were significantly different in the high corrosion area compared to the low corrosion area. This study demonstrated that direct pyrolysis of black liquor on the waterwall surface results in significantly higher concentrations of sulfur-containing gases in the high corrosion areas, leading to higher corrosion rates of the waterwall.
Review of Literature on Probability of Detection for Magnetic Particle Nondestructive Testing
Abstract : A critical review of the available literature concerning the minimum reliably detectable defect size aNDI for magnetic particle testing (MPT) in aerospace applications is presented. Four probability of detection (POD) studies relevant to detection of fatigue cracks in aircraft components were found over the period 1968 to 2011. As the statistical methods used in these four previous studies were either outdated or otherwise deficient, the original data were reanalysed using currently accepted techniques. A meta-analysis of the results is presented, with emphasis on statistical inferences for the defect size expected to be detected with 90% POD. It is shown that the minimum reliably detectable defect size aNDI = 2.0 mm currently specified by the Royal Australian Air Force for wet fluorescent magnetic particle inspection using the continuous method is consistent with estimates of the average performance of MPT derived from the reanalysis of the literature.