HRSGs and boilers |
Figure 3. Pitting leading to micro-cracks
zone is important because it is where the expansion and cooling of the steam leads to condensation.
A number of processes take place in this zone, such as precipitation of chemical compounds from superheated steam, deposition, evaporation, and drying of liquid films on hot surfaces, and these lead to the formation of potentially corrosive surface deposits.
Figure 4. Example of single-phase FAC Source: Dooley & Lister, PPChem 2018, 20(4), 184.
It is necessary to understand the processes of transport, moisture droplet nucleation, the formation of liquid films on blade surfaces, and concentration of impurities, which are vital to understanding how to avoid corrosion-related damage, and to improve unit efficiency/capacity. Deposition of contaminants and oxides on the PTZ surfaces are the initiating features of pitting if shutdown protection is not provided in the form of dehumidified air. Most of any SCC and corrosion fatigue initiates at pits.
Figure 3 shows examples of pitting leading to microcracks.
Each of the following RCCS lead to PTZ deposits and failures and contribute to damage mechanisms in the PTZ and, once again, it’s important to note that all have been observed in hundreds of case studies, are covered in detail by an IAPWS TGD, and
Figure 5. Examples of two-phase FAC in LP evaporator tubing Source: Dooley & Lister, PPChem 2018, 20(4), 184.
● Delayed/postponed chemical cleaning (management decision?)
● Repetitive contamination above action/ shutdown levels
● Incorrect feedwater chemistry choice, AVT(R) or AVT(O)
● No operational/management support for unit shutdown on contaminant event
● Lack of, reduced or out of service on-line continuous instrumentation
● Use of only grab samples to control chemistry ● Use of film forming substances
Phase transition zone (PTZ) failures. The PTZ in the low pressure steam turbine is the primary location of the three most important cycle chemistry-related failure mechanisms in any LP steam turbine: pitting; corrosion fatigue; and stress corrosion cracking.
The local steam environment determines whether these damage mechanisms occur on blade and disk surfaces. The phase transition
12 | March 2026 |
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can be avoided: ● Repeated contamination above action level ● Contaminant ingress above shutdown limit ● Continued attemperation during contamination ● Lack of/reduced level of, on-line continuous
instrumentation (with no steam monitoring most common)
● No knowledge of carryover from drums ● No LP steam turbine shutdown protection with DHA (and/or FFS)
● High level of air in-leakage ● No challenge to status quo (eg, operating with
original chemistry/changing chemistry, but keeping part of the old chemistry)
Flow-accelerated corrosion. Flow-accelerated corrosion ccurs in both HRSG and fossil plants (see Figures 4 and 5) due to the dissolution
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