August 15, 2008

Condensate polishers add operating reliability and flexibility

Colleen M. Layman and Lisa L. Bennett, Bechtel Power Corp.

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 (Figure 1).

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.

Another deep-bed condensate polisher option, which has been gaining popularity in recent years because of its reduced capital cost, is off-site regeneration. In this case, a service company removes the resin from the site, regenerates the resin at its facility, returns it to the site, and reinstalls the resin. However, costs increase with distance from the service company, as transportation costs are a major factor in the price of this service. Off-site regeneration does eliminate concerns with chemical storage and handling and wastewater neutralization and disposal.

The final option is to rent the polishers. Rental polishers are frequently used during commissioning and start-up, when the plant doesn’t have a permanent condensate polisher. Pressure and flow limitations on rental units normally restrict their use to partial-flow side-stream applications recirculating treated effluent to the condenser hotwell.

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