The Influence of Inadequate Water Circulation on Boiler Tube Failures
By Robert E. Hargrave, P.E.
Welcome to our archives. In July, we are sharing an article by Robert Hargrave, PE, which originally appeared in the September/October 1994 issue of Industrial Water Treatment. His article boiler water treatment, and in particular the causes of boiler tube failures.
Inadequate waterside circulation can promote premature boiler tube failures. The predominant failure modes associated with insufficient circulation are internal corrosion gouging and overheating damage. Sluggish circulation can affect a single tube circuit due to restriction or blockage; or general regions of a boiler because of operation or unit design. Horizontal or inclined tube circuits are most commonly affected by poor circulation.
Away from the radiant heat zone of a boiler, stratification occurs when fluid velocity in a tube is insufficient to maintain turbulence or thorough mixing of water and steam. Buoyancy effects tend to force the liquid phase to stratify along the bottom and the vapor phase along the top of a horizontal tube run, forming a steam “blanket.”
Separation of the steam and water phases (steam blanketing) can lead to corrosion gouging and failure of a tube by providing a mechanism for concentration of normally soluble species. For example, free caustic in the boiler water can concentrate along a waterline in a horizontal tube by repeated splashing and evaporation, resulting in parallel longitudinal gouges. If the upper portion of the tube is exposed to a heat flux, boiling action can also contribute to concentration of aggressive compounds and result in localized corrosion damage.
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