Examination of Independently Fired Auxiliary Boilers
When examining independently fired auxiliary boilers, certain areas require particularly close scrutiny due to being subjected to high thermal and mechanical stresses, corrosion, and wear.
Examination Type | Boiler Part | Reason for Close Scrutiny |
Internal | Water-Side Tubes & Drums | Inspect for scale buildup, corrosion, and pitting. Scale reduces heat transfer, causing localized overheating and tube failure. Pitting (often from dissolved oxygen) creates stress points that can lead to cracking. |
| Tube Ends/Connections | Highly stressed areas (rolled or welded joints) prone to caustic embrittlement and corrosion fatigue cracking due to concentration of stresses and chemicals. |
| Manhole & Handhole Seats | Check for damage or uneven surfaces which lead to leaks. Leaks promote local caustic concentration and embrittlement in crevices. |
| Furnace Surfaces | Look for signs of stress, fatigue, or overheating, especially in areas adjacent to the burner, which operate under the highest heat flux. |
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External | Furnace, Refractory, & Burner | Examine refractory lining for cracks or damage, which can cause casing distortion and overheating of pressure parts. Check the burner assembly for wear and proper combustion indication (e.g., excessive soot). |
| Welds & Attachments | Pay attention to all external welds, as they are susceptible to thermal and mechanical fatigue cracking due to localized stresses and repeated heating/cooling cycles. |
| Mountings & Expansion Points | Inspect safety valves and blowdown connections for leaks and functionality. Ensure adequate expansion clearance for drums and headers to prevent undue stresses caused by thermal expansion. |
| Casing & Insulation | Check the external casing for air leakage (which impacts combustion efficiency) and insulation condition. |
During boiler examinations, recognizing the distinct features of metal failure is key to determining the cause.
Failure Mechanism | Cause | Distinguishing Features |
Metal Fatigue (due to Caustic Embrittlement) | A specific form of stress corrosion cracking caused by the accumulation and concentration of caustic soda in highly stressed areas (e.g., rolled tube ends, seams). | Crack development is typically intercrystalline (between the metal grains). The cracks are often branched and occur without significant plastic deformation (swelling). |
Corrosion Fatigue | The combined effect of cyclic mechanical stress and a corrosive environment (e.g., dissolved oxygen). | Characterized by transgranular cracking (across the metal grains). The surface usually shows evidence of pitting where cracks initiated. Cracks are often blunt and accompanied by corrosion products. |
Overheating (Plastic Flow) | Occurs when a component (usually a tube) is heated beyond its design temperature (due to scale/deposit buildup or flame impingement), causing the metal to soften and lose strength. | Failure results in a "thin-lipped" burst with significant wall thinning and localized swelling (bulging) around the rupture. Metallurgical analysis shows changes in the metal's microstructure (e.g., spheroidization). |
Direct Overpressure | A failure from exceeding the vessel's design pressure (often due to safety valve malfunction) without a pre-existing overheating condition. | Failure is a sudden and violent rupture. The metal near the fracture point retains its original thickness and does not show significant swelling or plastic flow. The fracture edges are typically sharp and brittle in appearance. | |