HRSGs and boilers |


Getting the cycle chemistry right


The key to boiler and HRSG plant availability and reliability Barry Dooley Structural Integrity Associates, Southport, UK


Worldwide, we are continuing to see damage in fossil fuelled power plant boilers, combined cycle HRSGs and steam turbines attributable to lapses in cycle chemistry. Principle damage issues include:


Chemistry influenced boiler tube failures (BTF)


and HRSG tube failures (HTF), in particular: ● Flow-accelerated corrosion (FAC) – still the leading cause of tube failures in HRSGs and a major safety issue for feedwater systems in fossil fuelled power plants


● Under-deposit corrosion (UDC) – mainly hydrogen damage


Corrosion product transport due to inadequate feedwater and lower-pressure-circuit


chemistries, in particular: ● Deposition in HRSG HP evaporators and boiler waterwalls


● Complications arising from an air-cooled condenser (ACC) in the cycle (with the possibility of transported Fe rising to hundreds of ppb in the condensate)


ST deposits/damage/failure, in particular: ● PTZ (phase transition zone) pitting, stress


corrosion cracking (SCC) and corrosion fatigue (CF) cracking – all leading steam turbine (ST) problems


● Issues arising from lack of shutdown protection, ie absence of dehumidified air for the steam turbine and nitrogen blanketing for boilers/HRSGs


● Deposition in LP steam turbine stages – mainly chloride in PTZ


What can be done to address these problems, which are significant contributors to reduced power plant reliability and availability? One approach, pioneered by the present author plus colleagues, involves the concept of “Repeat Cycle Chemistry Situations” (RCCS), which are essentially repeated deficiencies in cycle chemistry operating practices, procedures and controls.


Data collected from some 300 or so power plants worldwide suggest that the failures and damage mechanisms listed above are not random but are found in plants operating with


two or more RCCS relating to: ● corrosion products ● HRSG HP evaporator and fossil boiler water wall deposition


● contaminant ingress (without remedial actions) ● drum carryover ● lack of protection during shutdown ● inadequate on-line alarm instrumentation ● not challenging the status quo ● non-optimal chemical cleaning ● high level of air in-leakage


The imperative is to identify RCCS and eliminate them from plant/chemistry operations. This is proving surprisingly difficult to do.


The table opposite summarises instances of RCCS based on a sub set of 276


comprehensive plant assessments worldwide, carried out over the period 2008-2025. The table provides a ranking of cycle chemistry activities that are not optimally applied. Examples of RCCS reported in the categories shown in the table include the following:


Corrosion products (total iron) ● Levels not known ● Too high (eg, above IAPWS (International


Association for the Properties of Water and Steam) guidelines or local guidance values


● Techniques used: incorrect detection limit; only soluble corrosion products analysed; use of Millipore filters


● Measurements taken at same time/shift each day


● Sampling systems not adequate


HRSG HP evaporator & fossil waterwall deposits ● Samples not taken ● No knowledge of deposit level or deposition rate


● Deposits excessive. Not linked with chemistry and FAC in fossil feedwater or HRSG lower pressure circuits


● Samples taken but not analysed comprehensively


● Unit needs cleaning but management has delayed or cancelled


● Use of FFS (film forming substances) with no prior review of HP evaporator


Figure 1. Hydrogen damage in HRSG HP evaporators. Ingress of chloride when evaporator has deposits. How and why are they allowed? 10 | March 2026 | www.modernpowersystems.com


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