Failure Analysis and Metallurgy in Heat Recovery Steam Generator Units

Early identification and mitigation of metallurgical issues ensures that heat recovery steam generator (HRSG) units run at safe and optimal performance levels.

Metallurgical issues arise from intrinsic design and operation of HRSG units.  Thermal gradients due to cycling may not be accounted for in the design of the unit and may result in cyclic failures.  Furthermore, incorrect water treatment can lead to HRSG tube failures in less than six months from initial operation.

Root cause failure analysis is essential to assess component integrity, remaining life assessment, and avoid recurrence of failures.  Intertek’s Asset Integrity Management (AIM) team offers root cause failure analysis and metallurgical services from experienced staff, in combination with complete field engineering and inspection services.

In our experience, typical HRSG tube failures are associated with cycling and water chemistry issues. Common failure mechanisms include:

Flow Accelerated Corrosion (FAC): Single phase or dual phase FAC failure modes occur due to poor control of water chemistry which affects elbows, bends, economizers and evaporators.  Proper identification is needed for mitigation by altering water chemistry and/or selecting different alloy types.

Fatigue and Corrosion Fatigue: This type of HRSG tube failures fatigue is a result of strain induced in thermal transients as well as chemistry issues that may result in high pH levels in water.  Some of the components that are affected include reheaters, evaporators, economizer and tubes.  Vibration and strains should be minimized and water chemistry controlled in order to counteract this type of thermal fatigue.

Under Deposit Corrosion (UDC): As a result of poor layup and bad water chemistry, UDC, a type of hydrogen damage, causes cracking and affects tubes and pipes. UDC is caused by poor layup and bad water chemistry that creates corrosive environments.

Creep: High temperature damage mechanism observed in components exposed to the combined action of temperature and stress. Cracking at welds (Type IV), superheaters, reheaters, and other high temperature components can be mitigated through material degradation monitoring (when possible), proper material selection and reductions in service stresses and temperatures. 

A combination of all the mechanisms listed above is observed in aging plants that are subjected to cycling. 

Due to the differences between failure mechanisms, proper identification is paramount to ensure the correct mitigating actions are put in place.   All corrective actions and mitigations can be unique to each individual mechanism therefore root cause failure analysis is recommended when failures are detected. 

Laboratory

Our facilities in Santa Clara, CA, in collaboration with other Intertek laboratories, are equipped for complete root cause failure analysis investigations of HRSG components.  Intertek’s in-house capabilities include: 

  • Metallographic sample preparation for microstructural analysis
  • Optical microscopy and fractography
  • Scanning electron microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDS) for deposit composition analyses
  • Quantitative chemical analysis for alloy verification
  • Inspection services: pulsed eddy current (PEC), in-situ field metallography and replication, hardness testing and weld inspections

 

 
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