Oxygenated treatment

Oxygenated treatment 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 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." This forms a thicker protective layer of hematite on top of the magnetite. This is a denser, flatter film so that there is less resistance to water flow compared to AVT. Also, OT reduces the risk of flow-accelerated corrosion.
When OT is used, conductivity after cation exchange at the economiser inlet must be maintained below 0.15μS/cm this can be achieved by the use of a full-flow condensate polisher.

Comparison of AVT to OT

Characteristic	All-Volatile Treatment	All-Volatile Treatment	Oxygenated Treatment	Oxygenated Treatment
Feedwater system piping	ferrus or mixed metallurgy	all-ferrous metallurgy	all-ferrous metallurgy	all-ferrous metallurgy
Dissolved oxygen level	< 10 ppb	1 to 10 ppb	30-50 ppb, 30-150	30-50 ppb, 30-150
Chemicals added	a reducing agent, ammonia to raise pH	ammonia to raise pH	an oxidizing agent	an oxidizing agent, ammonia to raise pH
pH	9.0-9.3	9.2-9.6	9.2-9.6	8.0-8.5, 9.0-9.4
Top layer composition	magnetite on steel piping, cuprous oxide on copper piping	hematite forms on top of the porous magnetite	ferric oxide hydrate or hematite forms over the porous magnetite	ferric oxide hydrate or hematite forms over the porous magnetite
Advantages	Can be used with mixed metallurgy piping	More protection against FAC than AVT, minimizes orifice fouling	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 are used.	A deaerator is required.	Air leakage is more serious. Two-phase FAC can be a concern.	Condensate polishers are required.