Oxygenated treatment
From Wikipedia, the free encyclopedia
Oxygenated treatment (OT) is a technique used to reduce corrosion in a boiler and its associated feedwater system in flow-through boilers.
Process
With oxygenated treatment, oxygen is injected into the feedwater to keep the oxygen level between 30 and 50 ppb. OT programs are most commonly used in supercritical (i.e. >3250psi) power boilers. The ability to change an existing sub-critical boiler over to an OT program is very limited. "Common injection points are just after the condensate polisher and again at the deaerator outlet."[1] This forms a thicker protective layer of hematite (Fe2O3) on top of the magnetite. This is a denser, flatter film (vs. the undulation scale with OT) so that there is less resistance to water flow compared to AVT.[2] Also, OT reduces the risk of flow-accelerated corrosion.[3]
When OT is used, conductivity after cation exchange (CACE) at the economiser inlet must be maintained below 0.15μS/cm [4] this can be achieved by the use of a full-flow condensate polisher.[5]
Comparison of AVT to OT
| Characteristic | All-Volatile Treatment (Reducing) | All-Volatile Treatment (Oxidizing) | Oxygenated Treatment (Neutral Water Treatment) | Oxygenated Treatment (Combined Water Treatment) |
|---|---|---|---|---|
| Feedwater system piping | ferrus or mixed metallurgy (e.g. copper feedwater train) | all-ferrous metallurgy | all-ferrous metallurgy | all-ferrous metallurgy |
| Dissolved oxygen level | < 10 ppb | 1 to 10 ppb | 30-50 ppb (drum), 30-150 (supercritical) | 30-50 ppb (drum), 30-150 (supercritical) |
| Chemicals added | a reducing agent (such as hydrazine), ammonia to raise pH | ammonia to raise pH | an oxidizing agent (such as hydrogen peroxide or oxygen) | an oxidizing agent, ammonia to raise pH |
| pH[6] | 9.0-9.3 | 9.2-9.6 | 9.2-9.6 | 8.0-8.5 (once-through), 9.0-9.4 (drum) |
| Top layer composition | magnetite (Fe3O4) on steel piping, cuprous oxide (Cu2O) on copper piping | hematite (Fe2O3) forms on top of the porous magnetite (Fe3O4)[7] | ferric oxide hydrate (FeOOH) or hematite (Fe2O3) forms over the porous magnetite | ferric oxide hydrate (FeOOH) or hematite (Fe2O3) forms over the porous magnetite |
| Advantages | Can be used with mixed metallurgy piping | More protection against FAC than AVT(R), minimizes orifice fouling [8] | Less flow resistance, lower dissolved feedwater iron concentrations, FeOOH film is more stable, reduced boiler cleaning frequency | - |
| Disadvantages | Increased risk of FAC, a deaerator is required, more frequent chemical cleaning is required, hazardous chemicals (hydrazine) are used. | A deaerator is required. | Air leakage is more serious. Two-phase FAC can be a concern. | Condensate polishers are required. |
See also
References
External links
Wikiwand - on
Seamless Wikipedia browsing. On steroids.