5. Mollier diagram with regions of chemical
and corrosion effects (turbine).
LP
inlet
6. Mollier Diagram with LP Turbine Steam Expansion
Line and Behavior of Chemical Impurities in Relation
to Condensation and Corrosion
7. Heater drain and feedwater piping
Mechanism -Flow accelerated
corrosion (FCA) of carbon steel.
Chemistry influence- attack by
reducing feedwater conditions and
high velocities, excessive hydrazine
with zero oxygen ,low pH.
8. Condensate and feedwater systems
Mechanism – carbon steel and
copper alloy corrosion and product
transport.
Chemistry influence- low pH, acid
constitutes, excess carbon dioxide
and oxygen present in condensate
,alternating oxidizing and reducing
conditions, excess hydrzine
solubilizing magnetite.
11. Condenser and heater tubes
Mechanism – admiralty brass stress
corrosion cracking.
Chemistry influence- excessive
ammonia /chloride present in steam
synergistic with residual stress at
tubesheets and in u bends.
12. Feedwater heater tubes
Mechanism – copper/nickel
exfoliation.
Chemistry influence- excessive
oxygen on shutdown combined with
thermal cycling and thermal stresses.
20. Recommendations to Reduce
Deposition
Deposition in turbines is mainly due to
carry-over of impurities from the boiler.
The amount of carry-over can be reduced
by lowering the feedwater purity limits,
improved boiler level control, operation at
lower pressures, and elimination of organic
feedwater treatment chemicals which could
cause foaming.
21. Exfoliation
Exfoliation of deposits from
superheater and reheater tubing can
cause significant damage to turbine
blade surfaces and contributes to iron
and copper transport to the turbine.
22. Optimization of cycle chemistry
Optimization of cycle chemistry is the easiest
method for reducing impurity transport and
deposition.
The optimal cycle chemistry will result in
reduced corrosion and minimized impurity
transport.
This is especially important if copper alloys are
present in the system because the optimal pH
for copper alloys and ferrous materials are not
the same and the incorrect pH can result in
high levels of iron or copper transport.
24. Boiler Water treatment
The primary purposes of boiler water treatment are
to ensure that
(i) the steam has minimum impurities to protect the
turbine
(ii) that the treatment can neutralize any contaminant
ingress to prevent concentration and resultant boiler
tube failures.
There are current five choices for boiler water
treatment for drum cycles:
• Equilibrium phosphate treatment (EPT)
• Phosphate treatment (PT)
• All-volatile treatment (AVT)
• Caustic treatment (CT)
• Oxygenated treatment (OT)
