A fire tube boiler, generating steam for a fruit processing plant, failed catastrophically. Figure 1 shows a typical fire tube boiler layout. Coal is conveyed and combusted in the furnace while heat is extracted via forced air convection. The heated air is carried in three passes through the boiler shell, heating the water surrounding the firetubes. Stay bars are welded to the shell of the boiler and reversal chamber of the furnace. They are designed to carry the loads associated with pressurizing the boiler shell and the effects of metal thermal expansion. After 5 years in service a stay bar at the rear end of the boiler failed catastrophically. Upon closer inspection cracks were detected on most of the stay bar joints on the rear end side.
Figure 1: Schematic of typical stoker fired fire tube boiler
An investigation was requested to determine the root cause for failure of the stay bar.
Figure 2 shows the projection of half of the rear end of the boiler. The locations of the stay bars attached from the reversal chamber to the rear end tube plate are seen. The position of the fractured stay bar is labelled.
Figure 2: Drawing of rear end of boiler and positions of rear end stay bars
Figure 3: Fracture surface of stay bar
|Magnetic particle analysis||Cracks present on most stay bar weld joints on rear end|
|External inspection||No metal distortion in region of failureFire tubes, stay bars and tubeplates free of distortionFeedwater supply line corroded in places|
|Corrosion on feedwater tubes||Due to oxygen bubble corrosion|
|Corrosion product and scale product on stay bar|
|Poor welding of rear tube plate to shell||Significant stress corrosion|
|Fracture surface shows ductile failure; rest of surface is brittle and highly corroded (Figure 3)|
|Compositional analysis||Stay tube and rear end tube plate material within specificationCarbon content of tube plate material is low compared to specified maximum|
|Microstructural investigation||Porosity defects in stay bar weldsWeld fusion poor, lack of penetrationTube plate and stay bar material within specification|
|Scanning electron microscopy||Cracks with extensive branching||Corrosion is primary mechanism of crack extension|
|Electron Dispersion Spectroscopy||High presence of sulphur and calcium within the crack|
|Microhardness||Drop in hardness from the weld area tending towards the expected hardness of the stay bar material in a normalised condition.|
Figure 4: Cross section of failed stay bar joint (sample A)
Figure 5: Crevice created by poor weld fusion and subsequent corrosion cracking