| HRSGs and boilers
Figure 6. IAPWS corrosion product decay map
Table 2. Achievable total Fe (& Cu) levels for various plant types/ optimised chemistries, steady/ base loaded plants
Feedwater OT AVT AVT (mixed)
The superimposed plot (lower left corner) shows decay profile for a well operated CCGT/HRSG plant, AVT (O) chemistry, dry air preservation of boiler. Plants should aim to be in the green area. Source IAPWS white paper, Jan 2025.
of magnetite from the surface of equipment (Barry Dooley and Derek Lister, Flow- accelerated corrosion in steam generating plants, PowerPlant Chemistry, 2018, 20(4)). FAC is still occurring at a high rate in fossil, combined-cycle and nuclear plants worldwide. In HRSGs it remains the leading cause of failure/damage. FAC has been researched for over 50 years, and scientifically all the major influences are well recognised. However, the application of this science and understanding to generating plants has not been entirely satisfactory. Major failures are still occurring at locations that are basically the same as they were in the 1980s and 1990s. However, it is now clear that FAC is heavily controlled and influenced by the cycle chemistry, and that RCCS have a major influence on the damage/failure mechanism.
Each of the following RCCS contribute to FAC mechanisms, all have been observed in hundreds of fossil, HRSG and ACC case studies, but, again, it’s important to note that each is covered in detail by an IAPWS Technical
Guidance Document and can be avoided: ● Incorrect feedwater, condensate and LP chemistries - Thinking AVT(O) or increased levels of oxygen will address all FAC and not separating types
- Continued use of reducing treatments - AVT(R) (even new HRSGs)
- pH too low and not adjusted through iron monitoring
● Non-monitored feedwater corrosion products ● Lack of on-line instrumentation ● Not challenging the status quo
- Using same chemistry since commissioning - Not using latest international guidance - Use of proprietary chemicals such as FFS
Role of IAPWS guidance and deposition and decay maps Each of the three failure/damage mechanisms discussed in the preceding section (UDC/ HD, PTZ failures, FAC) has been shown to be controlled and influenced by the cycle chemistry. The related RCCS for each mechanism is, as noted, covered by an IAPWS Technical Guidance Document. Table 1 ranked the RCCS and shows that two of the most important relate to corrosion products and internal deposits. A brief overview of the relevant IAPWS TGDs is presented here:
• Corrosion product monitoring The IAPWS TGD on corrosion products provides information on the locations and optimum processes to monitor total iron for steady loaded plants. Table 2 shows the achievable levels that should be maintained with optimum cycle chemistry. Continuously
Figure 7. IAPWS deposition map for HRSG HP evaporators. Source: IAPWS TGD TGD7-16(2016), with data from > 150 units. Original: Dooley/ Weiss, PPChem 2010 (with data from 45 units)
operating at higher levels than these is considered an RCCS. A recent IAPWS white paper explains that for cycling, frequently and fast started plants the key iron monitoring feature is how quickly the iron level reaches/ decays to the achievable levels during a start. To
HRSG evaporators/drums
Units with air-cooled condensers (ACC) ACC outlet
Post condensate filter
Cogeneration/industrial plants Condensate return
Source IAPWS
gauge this the decay profile generated by proxy iron measurement (turbidity) can be compared with the IAPWS decay map, see Figure 6. Decay profiles in the green area indicate optimum cycle chemistry during operation and shutdown. Those in the red area indicate an RCCS and that improvement is needed to bring the profile into the green area.
• Internal HRSG HP evaporator deposits Removing tube samples and metallurgically analysing the internal deposits is key to avoiding UDC. The relevant IAPWS TGD provides information on sampling tubes and analysis. Each analysis can be compared with the IAPWS deposit map, see Figure 7. If the deposit levels are in the green area then the plant is operating with good chemistry and is not likely to experience UDC. However, if the deposit levels are in the red area then this is an RCCS and the plant is not operating with good chemistry. This most often means the evaporator should be cleaned to prevent UDC.
Getting down to the basics In summary: RCCS relate to the very basics of power plant chemistry and thus plant reliability and performance and must be identified. Individual RCCS are generally not a problem but more than two or three RCCS means the plant has or will have a problem in the (near) future. It is essential to recognise the negative impacts
of RCCS and develop action plans to correct them in a timely manner to prevent future failure/ damage. As noted above, the most important RCCS are related to: corrosion products; deposits; instrumentation; and reluctance to change the status quo.
As this article emphasises, it is good practice to apply the IAPWS Technical Guidance Documents. Examples have also been provided of how IAPWS deposit and decay maps can be used to determine how good a plant’s cycle chemistry is and the likelihood of experiencing cycle chemistry- influenced failures (FAC, UDC/HD, PTZ).
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