|Water Steam Cycle Chemistry
Maintaining correct water steam cycle chemistry is crucial to safe, efficient and economic operation of power stations. Problems such as corrosion and scale in the water steam cycle can be extremely costly. That is why correct water treatment and chemistry are crucial.
1. Chemical treatment programs for protection of feed water and condensate systems
pH control, scale control, corrosion control, dispersants, oxygen scavengers. Various basic substances are used or pH control: ammonia, phosphate, and NaOH are most common. low TOC water is crucial as organics decompose at high T to CO2 which is acidic and hence corrosive.
2. Water Steam Cycle Sampling and Analysis
Conductivity, cation conductivity, dissolved oxygen, degassed cation conductivity, pH, Silica, Iron, Phosphate, Sodium, Hydrogen, Copper are some of the most important parameters to watch in the water steam cycle. Sampling containers from suppliers such as ABB, Swan, and Eurosistemi collect all the sampling lines in one container and are designed for online monitoring as well as grab samples. Parameters such as Total Iron and Phosphate are ususally analysed for manually using a spectrophotometer.
3. Tube Failure Analysis
Nalco wrote the book on tube failure analysis for which there are many mechanisms.
4. Scale and Corrosion Problems
Scale is caused by soluble substances coming out of solution due to evaporation or concentration mechanisms. Silica scane is another kind: when silica is high in the feed water it can volatilize at high T and carry over with the steam and deposit on the turbine blades.
5. Condensate Polishing
This is usually needed for supercritical boilers where water purity is more strict.
6. Condenser Tube Design
SS, Titanium, Copper alloys. Chloride levels in the main cooling water are important for the design. If Chloride is too high, SS tubes will corrode. Fouling, scale, Biofilm are other things to watch on the condenser tubes.
7. Phosphate Hideout
This phenomenon occures when boiler loads change and hence temperatures in boiler drums change. This can results in a change in solubility of phosphate and hence phosphate can go into or come out of solution and result in pH swings.
8. Precommissioning Chemical Cleaning of Boilers
Acid clean, caustic boil out, detergent clean, and steam blow out. Acids such as HCl, HF, Citric acid are common in acid cleans. Corrosion inhibitors are used in conjunction with the acid to protect the metal when the acid reaches the metal surface. Passivators such has hydrazine are another category of chemical that is sometimes used as part of the pre-operational cleaning.
9. Boiler Lay Up Procedures
Wet, Dry, nitrogen Blanket: depends on the length of time the boiler will be laid up. Careful not to dry a boiler that has not been cleaned with detergent as this may result in caking of oil deposits.
10. Research Institutes: EPRI and VGB
11. Turbine Chemical Cleaning: certain surfactants are used for turbine cleanings when they get fouled.
12. Everything starts with good demin water.... The conductivity, Silica and TOC must be low... and there can be no condenser tube leaks...
13. The Schikorr Reaction: an important reaction in water steam cycles. At high temperature and pressure, water reacts with Iron to form magnetite and hydrogen:
3 Fe + 4 H2O ---> Fe3O4 + 4 H2
Magentite protects ferrous metallurgy against further corrosive attack by Oxygen.
Click here for an excellent article on Water Steam Cycle Chemistry from Jonas Inc.
Compiled by Rami E. Kremesti M.Sc.
Last updated: 15-5-2015