(29b) Problems Arising out of Impurities in Steam Turbines

A long time research has been run on steam turbines for 22 months.In this way, many points found.Different impurities are transported into the steam turbine from the Preboiler cycle in the feedwater and the boiler as a total ( mechanical plus

vaporous ) carryover . Their major sources include condenser leak , make up water , and improperly operated condense polishers . The turbine environment is controlled through a control of impurity ingress and various feed water , boiler water , and steam chemistry limits . The corrosiveness of the steam turbine environment is caused by one or more of following :

1) Concentration of impurities from low part per billion levels in steam to percent levels on surfaces and the formation of concentrated aqueous solutions (concentration by deposition or evaporation of moisture)

2) Insufficient pH control (in both acid and alkaline regions)

3) High-velocity , high-turbulence , and low-pH moisture

The steam impurities that are of most concern include chlorides , sulfates , fluorides , carbonates , hydroxides , organic and inorganic acids , oxygen , and CO2 .Their behavior in turbine steam and deposits is well documented .

There are strong synergistic effects and interactions with metal oxides .

In addition to corrosion during operation , turbines can corrode during :

1-Manufacture (machining fluids and lubricants )

2- Storage ( airborne impurities and preservatives)

3- Erection (airborne impurities ,preservatives,and cleaning fluids )

4-Nondestructive testing (cleaning and testing fluids )

5- Lay up (deposits plus wet air)

Many of the above substances may contain high concentrations of sulfur and chlorine , which could form acids upon decomposition . Decomposition of typical organic for example , carbon tetra-chloride(CCl4) , occurs at about 150 C . there fore , the composition of all of the above substances should be controlled and most of them should be removed before operation .