Introduction
The Condensate
Polishing Unit removes 'crud' - corrosion products consisting mostly of
oxide of iron, copper or nickel, dissolved solids - mostly consisting of
sodium, chloride and silica and carbon dioxide. Condensate polishing
units are typically installed for super thermal power station with the
main objective of improving the boiler water quality. The benefits of
condensate polishing is quicker start up and as a result full load
conditions are reached early giving economic benefits. Orderly shut down
is possible in the case of condenser tube leak conditions.
Process Description
The condensate
polishers are located in the turbine hall and the exhausted resins are
hydro pneumatically transferred to the water treatment plant areas where
they are regenerated and transferred back to the polisher.
It is normal to operate the polisher initially in the
hydrogen cycle in which the cation resin is in hydrogen form and the
anion resin is in the hydroxide form. The process typically takes around
7 -8 days after which the cation resin gets converted into ammonium
form and the polisher is then operated in the ammonia cycle. Experience
has shown that the hydrogen cycle operation is almost always problem
free and produces condensate of the required quality. Boiler drum
sodium, chloride and silica increases within 2 - 3 days of operation of
the polisher in ammonium cycle.
Separation of ion exchange resin in a mixed bed is
done by backwashing the unit with water when cation resin settles at the
bottom and the light anion resin is at the top. However, the process
almost always results in presence of a small percentage of cation resins
in the anion portion and vise versa - a phenomenon called cross
contamination. On regeneration of the anion resin with alkali, the
cation resin presents in the anion portion gets converted into sodium
form and simillarly, the anion resin present in the cation portion gets
converted into chloride form.
Fig-1 shows the location of the condensate polisher in the boiler turbine circuit.
Benefits
- Improvement in the quality of condensate and "cycle" clean up.
- Reduced blow down and make up requirements
- Improvement in boiler water quality for drum type boilers
- Quick start up and as a result, full load conditions are reached early giving economics benefits.
- Orderly shutdown possible in case of condenser tube leak conditions.
- Improvement in quality of steam which results in enhanced turbine life.
- Condensate polishing units are typically used in nuclear (pressurised water reactor ) and fossil power plants.
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