Circumferential Cracking of Boiler Tubes (Corrosion Assisted Fatigue Cracking)
What is circumferential cracking?
Circumferential cracking of water wall tubes is a fairly common failure mechanism of waterwalls in pulverized coal power boilers. The horizontally banded cracking arises from the combination of a corrosion mechanism and cyclic stress or loading of the boiler tube on the fireside of the panel. This cracking phenomenon is usually fairly wide spread in regions where conditions are favorable, more often in supercritical than in subcritical units.
Why does circumferential cracking occur?
The primary corrosion mechanism is local sulfidation beneath ash or slag formations on the water wall. It is likely that pyrosulphates such as sodium and potassium pyrosulphate form as low melting temperature eutectics and cause ash or slag to periodically shed from the waterwall surface. This shedding results in an immediate local increase to tube temperature. As ash and slag reforms, tube temperature will decrease. Local changes in the shedding introduce changes in the stress profile in adjacent material and membrane. In units with water cannons, the impingement of water on the tube surface shocks the ash/slag film can also cause shedding to occur. Slag shedding and water impingement give rise to thermal expansion and contraction skin stresses, as the interior and external sides of the boiler tubes are thermally steady and constrained. As corrosion occurs at the crack tip, and stress cycles occur, the crack progresses and, left unchecked, can lead to equipment failure.
How is circumferential cracking avoided?
As this is a dual mechanism failure mode, elimination of either mechanism will stop corrosion assisted fatigue cracking from occurring or prevent progression if already initiated. It is a normal operation cycle of a boiler for ash and slag to form and temporarily shed; however, any coating of the tube surface that inhibits thick slag films from forming prior to shedding will reduce consequential changes in temperature. If a surface is already cracked and cracks in the tube surface are effectively sealed by a coating, the elimination of a path for corrosive media to the crack tip will eliminate further progression and widening of the crack walls through general corrosion.
High Velocity low stress coating systems as applied by IGS’ HVAS process technology and ceramic applications are effective in preventing the initiation and progression of corrosion assisted fatigue and the resulting circumferential tube cracking.
UTEx – 5-460 is a low stress sulfidation resistant thermal spray coating material and is used in combination with ceramic anti slagging top coat.