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Fly Ash Erosion
Fly ash erosion is caused by non-uniform or excessive gas flow, which accelerates a large volume of fly ash particles and directs them onto the tube surface. This phenomenon removes tube metal and oxide directly, which can lead to catastrophic failure if not properly monitored. The main factors that affect the rate of removal are particle velocity, angle of impact, particle composition and shape, and the tube’s erosion resistance.
Tube erosion is enhanced by distortion or misalignment of tubing rows; misalignment or loss of gas flow guides and baffles; operation above the maximum continuous design rating, above design excess air flow, or with non-uniform flow of flue gas; and fouling or plugging of gas passages by ash buildups. Changing fuel to one with higher ash content can result in more erosion and failures.
Fly ash erosion produces polished flat spots via the removal of oxide scale and tube material. A longitudinal, thin edged fracture results when the erosion rate is high. The fracture may be thick edged due to high hoop stresses. Erosion damage is usually limited to a small area.
The root causes of fly ash erosion can be verified by determining the reason for nonuniform or excessive gas flow at the damage location. Misalignment of tubing or flow guides and buildup of ash can be seen and corrected without any further testing requirements. Verification of excessive gas flow requires reviewing operating practices and conditions, and possibly conducting gas velocity tests. "Cold air" velocity tests have achieved good correlations between areas of high velocity and locations of failures.
Corrective actions involved either reducing the amount and velocity of ash striking the tube or increasing the amount of wear resistance of the tube. Changing boiler operation conditions such as reducing load, lowering excess air levels, balancing air flows, modifying soot blowers, and preventing ash accumulations are possible actions to reduce the ash velocity. Structural changes such as baffles, fences, shields, and plates may also be made, but care must be taken to prevent transfer of the erosion problem to another location. Cold air tests should be conducted after such changes. Pad welding and flame spray are short term actions which increase the amount of wear resistance of the tube. Staggered tube arrangements may also be replaced with inline tube geometry.
Visual examinations and ultrasonic (UT) tube wall thickness measurements are listed to detect and monitor fly ash erosion. UT surveys should be conducted after a change in fuel supply or after a failure in order to determine the extent of erosion and to prevent a subsequent failure within a short time. Periodic UT surveys can provide data for predicting remaining useful service life and planning corrective actions. An estimate of remaining service life should be made on tubes in similar conditions.
Fly ash erosion can occur at:
(1) Gaps between the tube bank and the duct walls.
(2) Gas bypass channels where the velocity of the flue gas can be much higher than that of the main flow.
(3) Protrusions or misalignment of tubing rows.
(4) Areas adjacent to large accumulations of ash
All tubes are vulnerable; however the following are the most common: