| Heat recovery steam generators


that many new large HRSGs cycle or operate in flexible duty.


The relatively simple design and less severe operating conditions relative to radiant boilers running with coal or oil create a more benign environment for HRSG heat exchanger tubes. Nonetheless, failures do occur, and tube failure rates naturally tend to increase as plants age. In addition, the cycling operation regimes that are being imposed on plants are driving higher failure rates and changes in the failure modes. Another factor affecting the evolution of tube failure modes and frequency is the demanding steam cycle process conditions encountered in the newer large combined cycle plants. When Tetra Engineering published the first edition of its Tube failure diagnostic guide, in 2004,[1]


the


authors anticipated the higher pressures and temperatures that emerging HRSG designs would impose on tubes. The result would be greater risk of high temperature damage due to creep, corrosion fatigue and damage due to stresses that result from differential thermal expansion. Tetra Engineering’s observations over the two decades since have largely confirmed that prediction.


Insights from the database Based on company projects, Tetra Engineering has gathered statistics on tube failures in HRSGs from over 100 different sites in North/ South America, Europe, Middle East, and Asia. The HRSGs under consideration encompass a diverse range of models and sizes from nearly every manufacturer.


The data are drawn from approximately 24 years of Tetra Engineering projects. Whilst not an exhaustive or complete accounting of every relevant project in that period, it does provide a useful indication of the frequency of occurrence of failures by HRSG location and by mechanism.


Figure 1. Locations of HRSG tube failures


9% 21% 10%


Creep Fatigue Corrosion


12% 5% 12% 7%


Superheater Reheater Preheater


HP economiser LP economiser HP evaporator LP evaporator Other


● Creep failures are due to the degradation of the steel microstructure from tensile stresses and temperatures outside the design envelope over longer periods. It is a well-documented cause of damage in boiler and piping pressure parts and represents 12% of failures in the assessments.


● Fatigue failures (23%) are generally caused by excessive temperature gradients that typically occur during operating transients such as start-up and load changes.


24%


Fabrication defect Tensile overload FAC


Creep fatigue Corrosion fatigue Other


12% Failure locations


Tube failures occur in all heat exchanger modules, from the cold-end economisers to the hot-end superheaters and reheaters. As seen in Figure 1, most of the failures tallied in the data set occurred in reheater and superheater tubes (24% and 21%, respectively), followed by high pressure evaporator tubes (12%). Evidently, the tubes in the hot-end steam-carrying components are the biggest problem area. These are exposed to higher temperatures and pressures, making them more vulnerable to degradation through mechanisms such as creep and thermal fatigue compared to their cold- end counterparts.


Damage mechanisms


The nature of service (baseload or cycling), fuel type and water quality will also influence the frequency of occurrence for a given damage mechanism. Mechanisms driven by operation include: creep; fatigue; creep–fatigue; corrosion; corrosion–fatigue; tensile overload; and flow-accelerated corrosion (FAC). Fabrication defects are another source of failures, the incidence of which is also influenced by operating conditions. The proportion of failures attributable to each damage mechanism is illustrated in Figure 2.


Figure 2. HRSG tube failure mechanisms


4% 5% 5% 9% 23% 11% 12%


● Creep–fatigue (5%) occurs in components experiencing both creep degradation and cyclic stresses. Failures occur at tensile stresses that are lower than would be required if either degradation mechanism acted alone.


● Corrosion in its various forms (such as cold- end surface corrosion or under-deposit attack) represents 20% of the failures.


● Corrosion–fatigue (4%) is the result of the combined action of alternating or cycling stresses and a corrosive environment. It should be noted that fatigue failures identified in some of the root cause assessments could include a corrosion component driving crack growth, hence it may be more relevant to combine fatigue and corrosion fatigue into a single class of failures.


● Flow-accelerated corrosion failures (9%) are caused by accelerated dissolution of the protective internal magnetite layer, leading to progressive wall thinning as base metal reacts to form new magnetite continuously.


● Fabrication defects (11%) vary from incorrect welding procedures to incorrect material installation. These failures are avoidable with even modest surveillance during manufacture.


● Tensile overload failures (11%) result from inducing stresses greater than the material’s ultimate tensile strength, primarily through rapid contraction caused by quenching or rapid cooling of tubes. The tube location (and consequent environment) influences the susceptibility to each damage mechanism. Figure 3 indicates that for superheater sections, creep and fatigue make up more than half of the failures.


11% 20%


Figure 3. HRSG superheater tube failure mechanisms


4% 8% 38%


8% 11% 19%


Creep Fatigue Corrosion


Fabrication defect Tensile overload FAC


Creep fatigue Corrosion fatigue


www.modernpowersystems.com | March 2023 | 13


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