Continuing the series on All Volatile Treatment, we now turn to AVT(O), the oxidizing variant of this type of conditioning.
We saw that the presence of copper metallurgy in the system requires reducing conditions to be reached in order to protect the copper while also providing adequate protection to carbon steel. It is a compromise to obtain the best possible performance for both metallurgies.
Since there is no copper in the system, the reducing environment is no longer necessary. There is no metal with electrochemical behavior opposite to iron, competing for the conditions of the environment. So, what is best in this case?
The answer came mainly from the 1990s onward. Instead of scavenging the residual oxygen, a small amount of dissolved oxygen is left in the water, with no additional reducing agent dosing. The resulting program, developed by EPRI as an offshoot of OT (Oxygenated Treatment) for drum units, was named All-Volatile Treatment Oxidizing, AVT(O).
In an environment with a low, controlled residual of dissolved oxygen, carbon steel forms a double oxide layer, magnetite at the base and a more external layer of ferric oxide. This layer has a characteristic deep red color and is denser and more adherent than the magnetite alone formed in a purely reducing environment. It is this film that justifies keeping oxygen in the circuit instead of eliminating it.
What matters is which film forms, how stable it is, how much iron can leave that film into the water, and how susceptible the system becomes to Flow-Accelerated Corrosion (FAC). EPRI points out that FAC is favored by conditions such as low pH and low dissolved oxygen, whereas raising pH and properly controlling oxygen can significantly reduce its rate.
The central difference relative to AVT(R) is that the reducing agent is not dosed under any condition, neither during normal operation nor during shutdown or layup of the equipment. Any presence of copper in the feedwater system rules out this approach; AVT(O) is reserved for cycles where copper alloys exist, at most, in the condenser.
In the absence of copper, a higher pH can be used. The ranges recommended by EPRI for fully ferrous systems under AVT went from approximately 9.2-9.6 in the 2006 guidelines to 9.6-10.0 in the 2013 guidelines, with the aim of reducing the solubility of iron oxides and mitigating FAC.
The residual dissolved oxygen allowed in AVT(O) is usually in a relatively low range, on the order of 10 to 20 ppb in combined cycle plants with good control of the dissolved oxygen residual, resulting in a slightly positive ORP in the feedwater.
An important distinction must be made: AVT(O) does not mean "injecting oxygen". In AVT(O), oxygen is normally not injected deliberately. The oxygen existing in the cycle, mainly associated with air ingress and deaeration control, is kept within a range that allows a slightly oxidizing condition.
In OT (Oxygenated Treatment), on the other hand, oxygen is deliberately added to the feedwater to reach a specific concentration, typically between 30 and 150 ppb.
The monitoring parameters are pH, dissolved oxygen and ORP, with total iron in the feedwater as the result indicator. The ORP should be close to 0 mV, and may be slightly negative or positive.
Air contamination in the condenser becomes a critical variable. Lower oxygen levels do not passivate the entire surface, and excess oxygen shifts the regime toward localized corrosion instead of a uniform protective film. Condenser tightness and continuous monitoring of dissolved oxygen gain a weight they did not have in AVT(R).
Converting AVT(R) programs to AVT(O), when that option exists, is not just a matter of changing the chemical program. It is a surface repassivation process, in which the Fe₃O₄ film formed under a reducing regime is replaced by the double film characteristic of the oxidizing regime, and this takes time.
In conclusion, AVT(R) is used when copper and iron share the same cycle. AVT(O) is the solution when that sharing does not exist and keeping the environment reducing no longer makes sense. A well-controlled, slightly oxidizing regime offers more robust protection and can operate over a wide pH range. Neither variant is superior in absolute terms; each responds to the metallurgy of the cycle in which it is used.
In the next post of this series we will discuss the precautions to take when opting for AVT and the most frequent mistakes.
If your plant has a fully ferrous cycle and you have not yet evaluated migration to AVT(O), or you have questions about dissolved oxygen control in your case, talk to us for a technical assessment.
References
- Power Engineering. The importance of accurate dissolved oxygen condensate/feedwater monitoring. Available at: power-eng.com.
- Power Engineering. Re-emphasizing HRSG feedwater chemistry evolution. Available at: power-eng.com.
- ScienceDirect Topics. Oxygenated Treatment - an overview. Available at: sciencedirect.com.