Boiler Tube Erosion Resistance via Gradient Alloy Coating
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Solution Overview
Problem
Circulating fluidized bed boilers experience severe tube erosion and failure due to exposure above the refractory ledge, leading to leaks and requiring frequent maintenance, as existing erosion-resistant coatings do not effectively prevent tube gouging.
Innovation Solution
Increasing the sidewall thickness of boiler tubes in erosion-prone areas and applying a non-homogeneous alloy coating with discrete layers of varying hardness to gradually taper to the nominal diameter, providing enhanced erosion resistance and preventing premature failure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thermal spray coatings are applied to protect boiler tubes from erosion, then tube leakage between outages is prevented, but tube gouging still occurs requiring welding and grinding repair during each outage
Solution Approach 1:
The patent applies a non-uniform coating thickness distribution, with the thickest coating applied at the refractory ledge interface where erosion is most severe. The coating thickness gradually tapers to zero at a distance above the ledge, creating a gradient structure that provides maximum protection where needed while avoiding discontinuities in less severe areas.
Solution Approach 2:
The thickened coating section is applied in advance during tube installation or previous maintenance, creating a protective buffer zone before erosion occurs. This preliminary protective action allows the coating to absorb erosion damage during operation, preventing both leakage between outages and gouging during outages.
2Object-affected harmful factors
If tube shielding is deployed to mitigate erosion, then tube protection is improved, but the step or discontinuity at the top edge of shielding becomes a new source of failure
Solution Approach 1:
The patent employs a curved, gradual taper of the coating thickness rather than a sharp discontinuity. The coating thickness transitions smoothly from maximum at the refractory ledge to zero at a distance above it, eliminating abrupt edges that would create stress concentrations and new erosion sites.
Solution Approach 2:
The patent changes the geometric parameter of coating thickness continuously across the tube surface, creating a gradient structure. This gradual parameter change avoids sudden transitions that would create discontinuities, while still providing the protective function against erosion.
3Ease of manufacture
If uniform thickness coating is applied to boiler tubes, then manufacturing is simplified, but erosion resistance in high-wear areas is insufficient
Solution Approach 1:
The patent applies a non-uniform coating thickness distribution, with the thickest coating applied at the refractory ledge interface where erosion is most severe. The coating thickness gradually tapers to zero at a distance above the ledge, creating a gradient structure that provides maximum protection where needed while avoiding discontinuities in less severe areas.
Data Source
AI summary
The erosion-prone sections of the tubes in a circulating fluidized bed boiler are provided with a locally thickened sidewall without forming discontinuities on the outer surface of the tubes. This can be accomplished, for example, by replacing the erosion prone portion of the tube with a section having a smaller inside diameter, but the same outside diameter, or by replacing the erosion prone portion of the tube with a section having a thicker sidewall, but the same inside diameter, and smoothing over the outside discontinuity with an alloy coating. A useful alloy coating is also disclosed which can be used for this and other applications.