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 DescriptionThe 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.