In high‑pressure steam cycles, even trace minerals can kneecap efficiency. Plants are leaning on condensate polishers—mixed‑bed ion exchangers plus filters—to hold feedwater to ultra‑pure standards and ride through condenser leaks.
In high‑pressure steam cycles, including heat recovery steam generators (HRSGs), the margin for error is measured in parts per billion. Industry guidelines (ASME PTC 19.11, ABMA, EPRI) target virtually zero hardness and silica; recommended feedwater hardness is under 0.1 mg/L as CaCO₃, and often under 0.05 mg/L in practice (erunwas.com) (erunwas.com), while silica limits hover below 20 ppb—and often below 10 ppb for steam‑turbine HRSGs (ebrary.net). Achieving those numbers requires systematically purifying returned condensate, and that is where a dedicated condensate polisher (CP) becomes critical. As Power magazine and water.co.id note, the CP—essentially a mixed‑bed ion exchanger plus filter—removes the last traces of dissolved salts and particulates that would otherwise compromise the boiler and turbine.
It is a small safeguard with outsized stakes. One analysis tied rigorous hardness control via ion exchange to roughly an 80% cut in scaling incidents, directly improving efficiency (erunwas.com). Conversely, omitting polishing—or monitoring it poorly—invites minor leaks to cascade into tube failure or forced shutdowns. Even a single condenser tube leak can inject enough salt to trigger rapid damage (Power Engineering) (ChemAqua).
Ion exchange and filtration mechanism
A CP is built on ion exchange chemistry: hydrogen‑form cation resin and hydroxide‑form anion resin swap contaminant ions for H⁺ and OH⁻, recombining as H₂O. In practice, the cation bed removes sodium, calcium and magnesium (hardness), and even ammonium (NH₄⁺) when ammonia is used for pH control (Hungerford & Terry) (water.co.id). The anion bed removes chloride, sulfate and silica (silicates) (Hungerford & Terry) (water.co.id).
As one technical summary puts it, “the cation resin will exchange ammonium ions for sodium and hardness, while the anion will exchange hydroxide ions for … chlorides, sulfates and silica” (Hungerford & Terry). Many modern systems employ mixed‑bed designs that combine resins in one vessel. Alongside resin exchange, condensate polishers include filtration stages that trap corrosion particles and oxides, removing both dissolved ions and suspended solids—something chemical treatment or simple softening cannot replicate (Veolia Water Technologies) (Ion Exchange).
The net effect is straightforward: leaked salts are immobilized on resin and only water is released back into the cycle. Selecting and maintaining high‑quality ion‑exchange resin is therefore central to CP performance.
Contaminant load during condenser leaks
Leaks or ingress typically deliver a specific “salt signature”—for water‑cooled condensers that is often sodium and chloride, with calcium and magnesium hardness. The CP captures those ions while online, keeping feedwater composition nearly pure even as resin load rises (Hungerford & Terry).
Against, say, a 5 ppm Ca/Mg intrusion, the unit reduces hardness to essentially zero—orders of magnitude below typical feed limits—preventing scale formation. CPs also strip out trace silica; some designs, especially precoat units, target sub‑ppb silica to protect turbine blades (Power magazine) (water.co.id). In summary, the polisher converts any incoming contaminant ion into a benign proton/hydroxide exchange, releasing only ultra‑pure water.
Performance outcomes and operating flexibility
The payback is measurable. An Indonesian combined‑cycle plant reported that installing a condensate polisher dramatically reduced silica and corrosion‑product levels—yielding a 3% improvement in boiler efficiency, a 60% reduction in turbine maintenance needs, and about IDR 2.5 billion (~USD 170k) in annual savings (water.co.id). More broadly, polishers allow plants to continue operating with minor condenser leaks or air ingress that otherwise would force outages (Power Engineering).
High‑performance chemistry regimes—AVT (all‑volatile treatment) and OT (oxygenated treatment)—rely on polished feedwater; maintaining tight targets such as cation conductivity (CACE) below 0.2 μS/cm and sodium under 2 ppb is not feasible without a CP (Chemical Engineering). Plants also gain an economic edge: purified condensate can be recycled instead of wasted, reducing makeup water costs and blowdown, and ensuring compliance with stringent ASME/ABMA limits (Xylem) (erunwas.com). The counterfactual is blunt: even a single condenser tube breach can inject enough salts to cause rapid damage if polishing and monitoring lapse (Power Engineering) (ChemAqua), while rigorous hardness control via ion exchange has been shown to cut scaling incidents by about 80% (erunwas.com).
Monitoring feedwater quality (key points)
Maintaining purity requires vigilant sampling and on‑line monitoring. Best practice following ASME/ASTM guidance calls for continuous or frequent checks of conductivity, pH, oxygen and specific ions at strategic points in the HRSG cycle.
Condensate Pump Discharge (CPD). Located just downstream of the condenser, CPD is the primary alarm point for water‑cooled systems (Power Engineering). Plants commonly monitor CPD conductivity or cation conductivity (CACE) continuously—recommended under 0.2 μS/cm (Chemical Engineering). Chloride or sodium analyzers are often used to detect ingress; brief tube leaks spike sodium. Rising dissolved oxygen (DO) at CPD—beyond about 10–20 ppb—can also indicate air leaks (Power Engineering). Analytics are installed at CPD or just downstream of the polisher to catch compromises quickly (Power Engineering) (Chemical Engineering). Periodic grab samples at the CPD/polisher outlet for silica (SiO₂) help verify whether any is being carried into the LP drum (Power Engineering).
Post‑Polisher (polished condensate). Where a CP is installed, its effluent is sampled to verify resin performance. Targets are essentially zero: ideally no residual chloride, hardness or silica. Continuous monitors are less common here, but frequent conductivity checks—near ultrapure water values—and lab assays for Na, Cl, Si, TOC, etc., confirm effectiveness. Any appreciable conductivity or sodium at the polisher outlet indicates resin exhaustion or breakthrough and triggers regeneration or resin replacement.
Deaerator (DA) outlet. After polishing, condensate is deaerated. DO is monitored here, typically under 10 ppb; an upsurge suggests DA malfunction or air ingress (Power Engineering). In OT regimes, residual oxygen is controlled by design.
Feedwater/Economizer inlet. The final feedwater checkpoint—often at the economizer inlet or drum—must meet boiler specifications, including very low total iron, silica around or under ~0.02 ppm, and pH roughly 9.8–10.0 (Chemical Engineering) (Power Engineering). Cation conductivity and/or resistivity meters, plus periodic silica analyses, confirm that any contaminant breakthrough has not reached the boiler. Conventional boiler‑cycling sample loops (boiler drum water blowdown) play a similar role in spotting impurity accumulation.
Additional monitoring for air‑cooled systems. In air‑cooled condensers, leak risk shifts, so corrosion surveillance is emphasized. Particulate monitors for iron oxide in the condensate/feedwater line can provide early warning of flow‑accelerated corrosion products and are often more insightful than dissolved iron analysis (Power Engineering). Meanwhile, pH—commonly controlled with ammonia in feedwater conditioning—is tracked alongside specific conductivity and online pH (or ammonia dosing control) (Chemical Engineering).
Standards, targets and outcomes
The through line is consistent: a CP keeps boilers operating at intended ultra‑low impurity levels by ion‑exchanging leaked minerals and filtering particulates (Hungerford & Terry) (Power magazine). Rigorous monitoring at the condensate pump, polisher outlet, deaerator and feedwater inlet—with targets like CACE ≤ 0.2 μS/cm, Na under 2 ppb, and DO under 10–20 ppb—catches excursions early (Chemical Engineering) (Power Engineering).
The field data are emphatic: plants with polishers routinely hit vendor purity specs, extend equipment life, and translate small feedwater gains into multi‑percent efficiency improvements and substantial cost savings (water.co.id) (erunwas.com).
Sources: Xylem/ASME recommendations (xylem.com) (erunwas.com); vendor literature (watertechnologies.com) (ionexchangeglobal.com); and power‑plant case studies and monitoring guidance (water.co.id) (erunwas.com) (power-eng.com) (chemengonline.com).
