Condensate polishers are resin-based ion exchange systems that are commonly used in power plant condensate systems to remove dissolved contaminants (chlorides and silica) and suspended contaminants (iron or copper oxide particulates). Polishers maintain cycle purity and efficiency by controlling the effects of corrosion transport, condenser cooling water leaks, and condenser air in-leakage. Polishers do not make a system immune to chemistry issues, but they do lessen the impact of chemistry problems and often allow a plant to continue operating with a minor condenser tube leak or air in-leakage problem that might otherwise require an immediate outage.
In addition, condensate polishers are suitable for operation with high-performance chemistry programs such as all-volatile treatment (AVT) and oxygenated treatment (OT), they may accelerate plant start-up by minimizing chemistry holds, and they may allow a more orderly plant shutdown in the case of a significant water contamination.
Two basic types of condensate polishers are commercially available: deep- or mixed-bed and precoat condensate polishers. The two types have very different designs and are intended to address site-specific water quality issues found in the power industry.
Option 1: Deep-bed condensate polishers
A deep-bed polisher is typically employed in applications requiring very pure condensate, where it’s vital to remove every trace of contamination. This polisher contains a mixture of cation and anion resin beads in a bed about 3 to 4 feet deep. The resin mixture can consist of varying cation-to-anion resin ratios, depending on the amount of dissolved contaminants expected, with 2-to-1 and 1-to-1 resin ratios being the most common.
-Due to the large volume of resin present, the ion exchange capacity of this type of polisher is high; however, its filtering capability is limited. Large quantities of total suspended solids can cause plugging and high pressure drop issues in deep-bed condensate polishers. Filtered particles may also cause fouling of the resin, particularly the anion resin beads, and loss of polisher efficiency
Deep-bed condensate polishers are favored in applications where high levels of total dissolved solids (TDS) are present (such as those using sea or brackish waters), where the plant must continue operation with small condenser leaks, and at plants that have minor air in-leakage control problems.
Nuclear power plants normally require deep-bed condensate polishers because their steam systems are sensitive to even minimal levels of contamination. The extremely low cation conductivity limits (<0.15 µS/cm) associated with operation on OT chemistry also call for deep-bed condensate polishers in supercritical and ultrasupercritical plants. Also, EPRI’s latest AVT guidelines recommend condensate polishers on most high-pressure units, advocating that they maintain steam cation conductivity of <0.15 µS/cm.
When the ion exchange capability of a deep-bed condensate polisher is exhausted, the resin beads are cleaned and regenerated with sulfuric or hydrochloric acid and sodium hydroxide. The regeneration process may be external to the service vessels in an on-site regeneration station, in-situ in the service vessels, or off-site at a subcontractor’s regeneration facility, depending on site-specific issues.
On-site external regeneration is the most common technique used today for deep-bed condensate polishers. The resin is sluiced out of the service vessel using water or water and air before being transferred to an on-site regeneration station. The cation and anion resin beads are separated into different regeneration vessels, dosed with dilute acid and caustic to restore the resin’s ion exchange capability, rinsed to remove the residual chemicals, and then recombined.
Resin can also be regenerated in-situ by segregating the cation and anion resins right in the vessels, although the resulting level of polisher performance is usually lower due to cross-contamination of the resins. For instance, externally regenerated polishers typically have a sodium leakage of 3 µg/l or less, whereas an in-situ regenerated polisher may have a sodium leakage as high as 10 µg/l.
Option 2: Precoat condensate polishers
Precoat condensate polishers use a thin coat of powdered ion exchange resin applied to specially designed retaining elements called septa to treat the condensate stream. To create the powdered resin mixture, resin beads are ground to about 200 mesh and mixed with a fibrous fill media. The powdered resin mixture is blended with water to create slurry that is applied to the septa to form a ⅛- to ¼-inch precoat layer
This thin coat of resin will remove some dissolved contaminants from the condensate, but its capacity is quite limited due to the small quantity of resin used. However, the elements do extremely well at filtering suspended particles from the condensate. Once the precoat condensate polisher is loaded with suspended solids or the ion exchange capacity is exhausted, the spent material is backwashed off the septa to waste and is replaced with a fresh layer.
This filtration approach is relatively inexpensive but quite effective when the suspended solids loading in the condensate is low. These filters are also able to remove oil and other insoluble organics from the condensate stream in addition to particulates. Because precoat filters must be recoated each time the differential pressure limit is reached, high particulate loadings can make this type of filter expensive. Effectiveness of the filter can also vary depending on the success of the precoat application.
Filter early and often
A condensate polisher can also be invaluable on combined-cycle plants where the steam turbine original equipment manufacturer (OEM) includes a warranty limit (commonly around 0.2 µS/cm) for the cation conductivity of the low-pressure steam. The cation conductivity of this stream is typically 0.5 to 0.6 µS/cm due to carbon dioxide absorption in the water, which makes the OEM-imposed limit very challenging to meet. A condensate polisher will remove carbon dioxide in the form of carbonates and bicarbonates from the cycle.
Combined cycles used as cycling units have special water treatment challenges: They must balance the EPRI- and OEM-recommended chemistry action levels against guaranteed start-up times. A condensate polisher can be used on these units to reduce the level of contaminants in the steam/water cycle and ensure that chemistry limits do not play a major role in delaying unit start-up times.
Also, any steam plant with a drum pressure greater than 2,400 psia will easily vaporize boiler water contaminants such as sodium and chloride and carry them over to the steam turbine system, rather than leaving them in the boiler drum. This tendency will limit the effectiveness of blowdown as an easy chemistry control practice on drum units and lower the amount of condenser in-leakage that can be tolerated without rapidly increasing blowdown or shutting down the unit. Once-through boiler designs with main steam throttle pressures in excess of 3,600 psia must also use a polisher to maintain system cleanliness.
Air-cooled condensers (ACC) also present unique challenges for the water treatment plant designer. An ACC features a very large surface area to efficiently condense turbine exhaust steam, but the surface area can also upset the steam/water cycle chemistry balance. New ACCs are difficult to thoroughly clean and tend to contribute high levels of contaminates into the condensate during initial commissioning.
Condensate filters are usually included to remove oil and suspended solids from the condensate before the polisher to prevent the loss of efficiency and bed fouling problems. Filters are also recommended when high levels of iron are expected in the condensate return of an export steam system.
Finally, reclaimed wastewater is an attractive option for power plant cooling water systems in regions with limited freshwater for power plant use. The most commonly encountered reclaimed or graywater source is municipal sewage that is treated via a secondary and/or tertiary process. Though the TDS content of graywater is not as high as that of brackish or seawater, the risk of contaminating the steam/water cycle remains significant. A polisher provides protection to allow the unit to initiate a safe and orderly shutdown in case of a leak.
“This article for , Colleen M.Layman is manager of water treatment engineering”