Waterwall tube failures

Waterwall tubes - spiral construction


  • Waterwall (evaporator) tubes and bends in two designs of boiler at a European power utility


  • Multiple failures of waterwall tubing, at straights and bends
  • In one case, explosive ejection of tube fragments and a large steam release outside the boiler casing
  • Significant downtime at a critical production period


  • History of corrosion failures in the waterwall tubes and bends
  • Safety issues with regard to explosive ejection of tube fragments and large steam releases
  • Inspection reveals large number of defects remaining in the waterwall tubes


  • To determine the relationship between critical crack size and operating pressure
  • To establish safe exclusion zones


  • Fracture mechanics assessment, including leak-before-break analysis
  • Steam release calculations


  • Following hydrostatic testing at ambient conditions (and the inspections undertaken) the maximum current defect size was established
  • The failures can be explained solely on the basis of the tube pressure stress and predictions are consistent with the observed failure mechanism and final crack size
  • Leak before break analysis provided relationship between operating pressure and critical crack size and hence the likelihood of each failure mode during operation
Leak-before break analyses for different tube-wall thicknesses


  • Critical crack sizes determined as a function of operating pressure and wall thickness
  • Safe operating pressures determined from observed crack sizes
  • Standard gas release calculations found to be inappropriate for steam
    Safe exclusion zones determined on the basis of more accurate release formulae
Exclusion zone calculations, for jet and cloud steam releases


  • Explosive type failures can be eliminated by reduced pressure operation
  • Unit brought back on-line during critical period
  • Fast response by team leads to minimal downtime
  • Insurers satisfied by comprehensive analysis undertaken

Plant Integrity