A mixed‑bed ion‑exchange “polisher” on the condensate loop routinely drives sodium to single‑digit ppb and cation conductivity to ≤0.15 µS/cm — a buffer that can save boilers and turbines when condenser leaks hit.
Power producers have learned this the hard way: skip polishing, pay later. In one documented case, a small condenser leak into a 1,250 psi utility boiler — without polishing — led to severe tube under‑deposit corrosion and ultimately a complete boiler tube replacement, dwarfing any short‑term savings from running bare www.power-eng.com.
That is why plants install a condensate polishing unit (CPU — a mixed‑bed ion‑exchange system) on the condensate/feedwater loop: fossil plants “employ condensate polishers to remove ionic and suspended impurities … to provide high‑purity feedwater,” improving availability, reliability, and performance pdfcoffee.com. Put simply, a condensate polisher is the buffer between a condenser leak and an expensive outage.
When the condensate inlet is “far worse than the specified standard” — think start‑up upsets or tube leaks — a functioning polisher has kept its outlet “within the indexes,” fully meeting design purity at Qinshan Nuclear Power Plant ntrl.ntis.gov.
Condensate polishing performance targets
EPRI emphasizes that properly operated polishers produce water purer than guideline demands, with targets on the order of sodium <2–3 ppb, cation conductivity <0.15 µS/cm, and silica <10 ppb pdfcoffee.com pdfcoffee.com. For high‑pressure HRSGs running AVT (all‑volatile treatment), EPRI recommends polishing with extremely strict steam‑side cation conductivity limits below 0.15 µS/cm (≈6.7 MΩ·cm) for OT chemistries (oxygenated treatment) www.powermag.com.
The contrast with unpolished water is stark. Even prefiltered boiler feedwater may carry trace hardness up to 0.2–0.3 mg/L as CaCO₃ (roughly 10–15 ppb Ca) and iron up to 0.03–0.05 mg/L watertechnologyreport.wordpress.com. Polisher effluent, by comparison, often reaches essentially demineralized quality as H⁺ and OH⁻ recombine into H₂O watertechnologyreport.wordpress.com www.power-eng.com. Plants with polishers routinely see feedwater conductivities an order of magnitude lower than those without, reducing corrosion rates and blowdown needs www.powermag.com.
Mixed‑bed ion exchange mechanism
A CPU is a deep‑bed mixed‑bed ion‑exchange system: strong‑acid cation resin (in H⁺ or NH₄⁺ form) swaps H⁺/NH₄⁺ for hardness and alkali cations; strong‑base anion resin (in OH⁻ form) swaps OH⁻ for anions. The net effect: all salts (Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻, and even dissolved silica as HSiO₃⁻) are replaced by H⁺ and OH⁻, which recombine into water www.power-eng.com watertechnologyreport.wordpress.com. As Buecker summarizes: “Cations such as sodium, magnesium, and calcium are exchanged for hydrogen ions (H⁺). Anions are exchanged for hydroxyl ions (OH⁻). The reaction of H⁺ and OH⁻ produces water.” www.power-eng.com
When seawater or brackish cooling water leaks across a condenser, the polisher traps Na⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻ and silica before they reach the boiler, acting as a filter for fine iron/copper oxides too — deep beds serve as depth filters so “crud” penetrates the resin rather than coating a surface watertechnologyreport.wordpress.com www.power-eng.com. Low‑pressure systems sometimes deploy cation‑only polishers for iron/copper and hardness capture, but typical HRSG pressures call for mixed‑bed units.
Capacity is finite. End‑of‑service is signaled by ion breakthrough or conductivity rise; EPRI action levels are tight, around 4–6 ppb Na or >0.15 µS/cm cation conductivity pdfcoffee.com. In practice, leakage can be just a few ppb sodium — Eskom targets 0.5 ppb Na pdfcoffee.com. When exhausted, the bed is regenerated with dilute H₂SO₄ and NaOH or replaced with fresh ion‑exchange resin. Precoat (powdered‑resin) polishers use a thin slurry layer for short runs and need frequent replacement once capacity is reached watertechnologyreport.wordpress.com.
Overall, the polisher eliminates dissolved mineral contamination from makeup, leaks, or accidental introduction. ChemTreat guidance underscores that a tube leak injects Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻ and silica into condensate; the polisher “provides a buffer against contaminant ingress” www.chemengonline.com. Without that buffer, operators dump or blow down condensate; with it, plants can continue despite minor leaks www.powermag.com www.power-eng.com. EPRI notes that polishers “produce a product water of far higher purity than that of feedwater demanded by the guideline values,” especially for sodium and conductivity pdfcoffee.com.
Sampling nodes and online limits
Monitoring spans multiple “nodes” along the condensate/feedwater loop to guarantee purity. Recommended sample points include makeup treatment, condensate pump discharge (CPD — hotwell), condensate polisher outlet (CPO), economizer inlet (EI — combined feedwater), boiler water, and main and reheat steam www.chemengonline.com pdfcoffee.com.
Conductivity/resistivity: plants measure specific conductivity and cation conductivity (an acidified measure that suppresses weak base signals) online at several points; resistivity (inverse of conductivity) is also used. Policies often require cold‑grade conductivity alarm <0.2 µS/cm or resistivity >5 MΩ·cm ahead of the boiler; EPRI AVT guidance for supercritical units is even tighter at <0.15 µS/cm cation conductivity for feedwater/steam www.powermag.com. A dedicated feedwater cation conductivity meter (often called “drum level gauge” probe) sits at the boiler inlet or drum. Polisher‑equipped units routinely hold CPD and EI conductivity to <0.1 µS/cm (30 °C basis) www.chemengonline.com.
Condensate polisher effluent checks
CPO is tracked continuously for conductivity and via periodic lab assays for silica and sodium. Typical targets: ≤0.1 µS/cm, silica ≤10 ppb, sodium ≤2 ppb www.chemengonline.com. A rise above ~0.1 µS or 10 ppb silica at the makeup treatment polisher triggers corrective action www.chemengonline.com.
Condensate pump discharge surveillance
Near the hotwell, CPD conductivity/pH sensors detect saline ingress or organics from the condenser. Even small spikes (µS/cm, ppm TDS) signal trouble. Many plants aim for CPD conductivity <0.2 µS and silica <0.1 ppm; water‑cooled condensers also deploy sodium or chloride analyzers inline, or an equivalent degassed conductivity detector. A tube leak shows up quickly as a few ppb Na/Cl in the CPD sample www.chemengonline.com www.chemengonline.com. A properly sized ion‑exchange polisher then scrubs the ions, though major events can still force blowdown or repair.
Feedwater, drum, and steam quality
At the economizer inlet, operators verify pH and cation conductivity (near zero) and often analyze dissolved oxygen, silica, and ions to confirm the entire treatment train is functioning. Typical feedwater total conductivity is held below ~0.1 µS.
In drum units, boiler water is sampled continuously for pH, dissolved oxygen (via a Doyle cell or equivalent), and conductivity — especially “acid conductivity” for anion content. Blowdown tests track residual alkalinity, silica, phosphate (if used), and TOC. ASME‑style guidance used in Indonesia’s SNI 7268:2009 calls for boiler water silica <0.25 ppm, iron <0.05 ppm, and hardness effectively zero; feedwater must meet these before entering the boiler, or automatic blowdown follows watertechnologyreport.wordpress.com pdfcoffee.com.
Steam sampling (saturated before reheater) checks sodium and conductivity. Typical turbine inlet sodium spec is ~3 ppb for high‑pressure steam, with more sensitive plants pushing 0.5–1 ppb pdfcoffee.com.
Standards and compliance linkage
These monitoring practices align with national and industry norms. Indonesia’s SNI 7268:2009 mandates treated feedwater hardness ≤1 mg/L as CaCO₃ and dissolved oxygen ≈0 for typical low‑pressure boilers (Table 1), analogous to ASME/Consensus guidance; monitoring ensures compliance, and polishers hold residual ions to single‑digit ppb pdfcoffee.com pdfcoffee.com.
What the data and case studies show
Across sources — EPRI guidelines and technical reports pdfcoffee.com pdfcoffee.com, power‑industry textbooks and blogs watertechnologyreport.wordpress.com www.chemengonline.com, and standards such as Indonesia’s SNI 7268 pdfcoffee.com — the through‑line is consistent: a condensate polishing step removes leak‑borne minerals, drives sodium into the 0.5–3 ppb band, holds silica to ≤10 ppb, and keeps cation conductivity at or under 0.15 µS/cm. Inline citations in this article point to the detailed recommendations, case histories, and parameter limits at the source links.
