Heat recovery steam generators |


Figure 4 highlights fatigue (41%) and tensile overload (31%) as the primary mechanisms causing reheater tube failures.


Figure 4. HRSG reheater tube failure mechanisms


7% 4%


of failures involving this material would in any case be small.


While materials do not inherently cause failures, of course, a poor material choice, with materials deployed in wrong locations may accelerate failure mechanisms.


41% 31% 3% 14%


Creep Fatigue Corrosion


Fabrication defect Tensile overload FAC


Creep fatigue Corrosion fatigue


Materials


The materials encountered in failed HRSG tubes are presented in Figure 5. Note that the frequency of failures by material also reflects how common the material is across the HRSG fleet. For example, based on field observation T23 is not a commonly used material in heat recovery steam generators, hence the fraction


Figure 5. Materials employed in failed HRSG tubes


5% 21% 47% 3% 15% 9%


Carbon steel SA 213 Gr T11 SA 213 Gr T22 SA 213 Gr T23 SA 213 Gr T91 Stainless steel


For example, carbon steel tubing in areas that are exposed to risk of FAC is more susceptible to wall thinning, due to a lower resistance to wear caused by this mechanism. This explains why, after repeated FAC failures, plant operators often specify repairs that use low-alloy steel (T11, T22, T91) tubes. These materials contain chromium and molybdenum, which improve the resistance to magnetite dissolution and significantly reduce the risk of tube wall thinning. Repairs after FAC tube failures can entail partial replacement of affected tube sections or even installation of completely new modules. HRSG designs from some OEMs have also evolved to use low-alloy materials in critical sections such as the upper tubes of the LP evaporators. Unfortunately, this partial “solution” while less costly than extending use of low-alloy materials to further locations, is often imperfect, with failures occurring in adjoining carbon steel sections after some years of operation.


Significant reductions in the creep life of grade 91 components have been observed. These have often been attributed to incorrect heat treatment resulting in an undesirable material microstructure.


Prevention of tube failures For the most part, tube failures can be avoided by operating within the design envelope and by performing the appropriate O&M actions as specified in plant operating procedures.


As plants age certain additional actions may become necessary or more frequent, such as cold-end cleaning to remove excessive amounts of corrosive deposits on tubes or chemical cleaning to remove deposits in evaporators.


It is essential that the O&M actions include regular in-service and off-line (outage) inspections as well as the monitoring of HRSG process data at reasonable intervals. Effective inspection and monitoring are essential for preventing failures, as they enable the detection of potential issues before degradation leads to a failure.


Tensile overload of reheaters provides an example of where effective monitoring can anticipate failures. Failures due to tensile overload of reheaters are typically a result of thermal shock incidents such as rapid cooling or quenching of the affected tubes; inadequate condensate draining or attemperator over- spraying are almost always the root causes. Ensuring manual drain valves are opened before start-ups, and attemperator spray/isolation valves are not passing are key to avoiding these types of failures.


Tetra Engineering has also noted that some plants lack flow sensors in the reheat and superheater spray loops; accessing this data could help prevent over spraying and subsequent thermal shock.


Inspections during outages can detect incipient cracks at the tube–header welds via non-destructive testing (NDT). Bowed tubes can also be an indication of excessive tensile stresses.


Another example concerns superheater tube creep failures. These are among the most frequently observed tube failures and are almost always attributable to high metal temperatures. Causes include non-uniform gas turbine exhaust gas flows, maldistribution of duct burner flames, burner firing with low steam flow, poorly designed flow correction devices and side or centre baffle damage. Visual inspection can identify any mechanical damage such as missing baffles, with accompanying NDT (hardness and surface replication) used to check for signs of creep degradation. If superheater thermocouples are installed by design, these can be used to track temperatures and to provide input to creep life calculations. If not present, thermocouples can be retrofitted at relatively low cost. Finally, taking a tube sample for laboratory analysis is always an option to get a definitive reading on creep progression. This is ideally done after other data indicates that there may be a potential creep problem or after some longer period of operation (50 k hours or more).


Conclusions


In summary, analysis of a data set constructed using a sizeable sampling of the tube failure mechanisms and locations identified in projects performed by Tetra Engineering over several decades shows that hot-end components, ie, superheater and reheater tubes, are the biggest problem area, with nearly half the failures investigated occurring in these tube locations.


This is consistent with the observed failure mechanisms, wherein creep, creep–fatigue, and fatigue account for about 40% of the failures. The frequency of occurrence of these mechanisms and their propensity to occur in the hot-end components is consistent. Creep requires high temperatures and fatigue will be most common in components subject to high thermal stresses and thermal expansion movements.


The relatively low frequency of FAC failures is somewhat surprising in that it is often cited as a major factor in HRSG failures. It is important to note that the data set is representative only of the projects from which it was compiled. While these projects are relatively broad based in terms of type and age of plants, it should not be taken as a definitive assessment of the distribution and frequency of tube failures in all HRSGs.


[1] D. Moelling, P. Jackson and F. Anderson, HRSG tube failure diagnostic guide - 2nd edition, Tetra Engineering Group Inc., 2004 14 | March 2023| www.modernpowersystems.com


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