Corrosion Treatment by AVT (All Volatile Treatment) and CWT (Combined Water Treatment)

Corrosion Treatment by AVT (All Volatile Treatment) and CWT (Combined Water Treatment)

The purpose of water quality control (AVT and CWT) is getting proper water to minimize deposit and reduce corrosion in thermal power plant. Hence safe and reliable operation will increase and unit availability will be increase also.

Corrosion

Corrosion in water tube is influenced by the following condition:

1. Improper Dissolved Oxygen Content
2. Dissolved Mineral (impurity)
3. Improper pH (acid, pH < 7 , tend to corrosion)
4. Temperature
   

Figure 1. Basic corrosion reaction

Corrosion Reaction :

Fe + O2 + ½ H2O  →  Fe(OH)2

1. Corrosion by Oxygen

The most common form of corrosion is caused by the presence of oxygen. The most common method of removing oxygen is by the dearating feed water heater or condenser. For further  assurance to remove oxygen is by adding chemical oxygen scavenger ( Hydrazine /N2H4 ) after condensate polishing system.

N2H4 + O2   →     N2 + 2H2O   

      

2. Corrosio by pH

Ammonia or Amines used for pH control to solute mineral

3. Corrosion by Impurities

An oxygen differential cell is rarely the sole cause of metal loss under deposit. Coexisting with most oxygen differential cell are the corresponding Chloride and Sulfate concentration cell. These coexist because Chloride and Sulfate ions penetrate the deposit or crevice and concentrate. The deposit behave like a semi-permeable membrane, as the iron ions (Fe2+) leave the anodic surface, the Cl- + SO42- anions diffuse through the deposit to maintain neutrality, resulting in a build up of electrolyte under deposit. This further accelerates the corrosion already taking place because of the oxygen differential cell.

4. Corrosion by Temperature

As a general rule, each 15 0F(8 0C) increase in temperature doubles the rate of chemical reaction. Therefore, temperature increase the speed of corrosion.

              

Figure 2. Corrosion by Temperature

Magnetite ( Fe3O4 ) is normally formed in the inside of pipe.

Fe + 4H2O      →    Fe3O4 + 4H2           

Whereas Fe(OH)2 can also converted to Magnetite at the temperature above 100 0C

3Fe(OH)2     →       Fe3O4 + H2  + 2H2O               

Corrosion Treatment by Chemical Injection (AVT-CWT)

AVT (All Volatile Treatment): Conditioning of feed water and boiler water by adding volatile alkalizing chemical such as Ammonia, Oxygen scavenger(Hydrazine) etc.

     LO (Low Oxidizing) → < 5 µg/L

     Oxygen scavenger is not used, Oxygen is removed only by Condenser and Deaerator. 

CWT (Combined Water Treatment): Conditioning of feed water and boiler water by adding Ammonia & Oxygen.

Figure 3. Potential pH Diagram of Fe

qThe Chemical injection system is to inject chemicals to the plant cycle water to maintain appropriate pH value and to create “protection film” on the inside surface of the boiler heat transfer tubes to avoid corrosion.

qCWT (Combined Water Treatment) or Oxygenated Treatment (OT) is provided for the cycle water during the plant normal operation where ammonia and oxygen are injected in the cycle water. Ammonia is injected for the purpose of pH control (around 8.5 to 9.3) and is injected at the outlet of condensate polishing system. Oxygen is injected for the formation of “Hematite (Fe2O3)” which is more stable protective film, on the surface of boiler tube.

qThe AVT (LO) (All Volatile Treatment - low oxidizing) is considered during the start up, pH is controlled to around 9.3 to 9.7 by injecting ammonia to form a protective film of magnetite (Fe3O4).

qUnder AVT (LO) operation, pH is controlled to around 9.3 to 9.7 by injecting ammonia to form a protective film of magnetite (Fe3O4) under condition of degasified (deaerated) cycle water. Ammonia is injected at the outlet of condensate polishing system.

                                                                        Figure 4. Inside tube during AVT-CWT

The Chemical Injection location is the following:

The Case Study:

Question:

In the case that the conductivity (cation pass) at the boiler feed water exceeds > 0.2 μS/cm, oxygen injection is stopped under the oxygen treatment. What kind of problems will occur, if oxygen injection into boiler feed water continues in the case of the conductivity (cation pass) is higher than > 0.2 μS/cm?

Answer:

The case in which conductivity (cation pass) at the boiler feed water is higher than > 0.2 μS/cm indicates the increase of contaminants, it means the amount of anions such as chloride and sulfate ion increases. In this case, the iron oxide protective film is destroyed and corrosion will progresses. The process must back to AVT.