- In a recent survey conducted by EUCG Inc. the five leading causes of boiler tube failure were identified.
They were:
1. Coal ash corrosion
2. Long term overheating/creep
3. Soot blower erosion
4. Fly ash erosion
5. Corrosion Fatigue
The procedure for repairing a failed tube was fairly consistent. Many plants use either the window weld or pad weld as an acceptable repair, although the tube would be replaced during a planned outage. Increasing the tube wall thickness and using tube shields are effective methods for protecting against unplanned outages.
Soot blower systems were highly problematic. Many plants experienced problems with condensation in the soot blower system. Also there were maladjustments of the superheater temperature, improper use of water lances and lower slope leaks resulting from large releases of slag.
Maintenance practices have a direct effect on the number of tube leaks and unplanned outages. The survey indicated that simple lack of maintenance or the improper setting of soot blower pressures was a major problem. Poorly angled blowers, the type of nozzle and the wobble of long retractable soot blowers also contributed to tube thinning.
Not surprisingly, the solutions involved shielding the thinning areas, repositioning the blowers or upgrading.
When a repair is required a number of steps and factors must be considered. The tube failure needs to be inspected and the failed areas identified. Consideration must be given to the economic consequences of delaying the outage. The risks in dealing with the severity of the failure and its location are evaluated. Finally the outage can be used as an opportunity to inspect other areas for problems and to perform additional maintenance without extending the shutdown schedule.
The survey respondents used each and every technique of Non Destructive Testing to pin point and evaluate the leaks and to identify any significant damage to other adjacent areas of the failure. Ultrasonics, Dye Penetrant, Magnetic Particle, Visual and X-Ray were all sighted as important tools when locating and evaluating boiler tube failures.
