| HRSGs and boilers
Drum carryover ● Amount not known ● Not measured (every six months) since commissioning
● Too high (above IAPWS guidance) ● Incorrect measurement (Na in steam) ● Sampling systems not adequate
Inadequate on-line alarmed
instrumentation ● Inadequate level of instrumentation (compared to IAPWS guidance document)
● Some missing (most common: no Na steam monitoring)
● Using grab samples to control plant ● Instruments not maintained or calibrated frequently
Not challenging the status quo ● Same chemistry since commissioning –
Figure 2. Hydrogen damage in fossil plants. Requires heavy deposits and contaminants. How and why are they allowed? incorrect or outdated guidelines
● Continuing to use wrong chemistry - phosphate treatment (CPT (congruent phosphate treatment) instead of PT (phosphate treatment))
- reducing agents in combined cycle/HRSGs and in the feedwater of all-ferrous fossil plants
- AVT(O) (all-volatile treatment, oxidising) vs AVT(R) (all-volatile treatment, reducing) or both incorrectly chosen and used
● No chemistry manual for plant, or manual assembled by chemical supplier rather than having power plant company guidance to follow
● No questioning or knowledge of proprietary chemical additions
● Incorrect addition point for chemicals (most often reducing agent)
● Lower end of pH ranges is often bad
Table 1. Instances of RCCS (in 276 plants worldwide). The numbers are the percentages of plants with the RCCS
RCCS category Corrosion products
HRSG HP evaporator and fossil boiler water wall deposition
Non-optimal chemical cleaning
Contaminant ingress (without remedial actions)
Drum carryover High level of air in-leakage
Lack of protection during shutdown
Inadequate on-line alarm instrumentation
Not challenging the status quo
In 155 fossil fuelled plants In 121 combined cycle/HRSGs 92
86 55 17 13 70 37 70 80 83 59 4 5 78 10 50 80 73
The RCCS concept in practice: controlling damage mechanisms, dealing with UDC/HD, PTZ failures and FAC
Under-deposit corrosion (UDC)/hydrogen damage (HD). Hydrogen damage is the most frequently occurring UDC mechanism; it requires a combination of heavy internal deposits and acidic contamination. The first step leading to damage is when excessive corrosion products (total iron) are transferred from the feedwater or lower pressure sections of the plant and deposit in the boiler/ HRSG. The second step involves an acidic contaminant concentrating beneath the heavy internal deposits. This acidic environment changes the magnetite growth process usually producing a thick multilaminated oxide layer that grows at a rapid rate. Embrittlement of the tube material follows. Examples are shown in Figures 1 and 2.
Each of the following RCCS contribute to the hydrogen damage mechanism and it’s important to note that each has been observed in hundreds of case studies, is covered in detail by an IAPWS Technical Guidance Document (freely available from the IAPWS website, www.
IAPWS.org), and can be avoided: ● Excessive corrosion products (total iron) & outside IAPWS decay map green area (see p13). Levels exceed international and “local” guideline values.
● Non-monitored feedwater corrosion products or using Millipore filters
● Measuring only soluble corrosion products (which represent only about <5% of total)
● Excessive internal deposits on HP evaporator & boiler water wall surfaces, and no samples taken for IAPWS deposition map analysis (see p13)
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