In modern HRSGs (heat recovery steam generators), a few parts per billion of the wrong ions can erode blades, clog nozzles, and slash output. Plants are fighting back with high‑efficiency drum internals and online steam quality monitoring designed to hold steam total solids to 10–30 ppb.
Power producers have learned the hard way that impurities in boiler steam are not a housekeeping issue — they are a reliability risk. Even tiny carryover erodes blades, clogs nozzles, and causes control‑valve sticking, which can reduce efficiency and capacity by orders of magnitude (www.watertechnologies.com).
Real losses have been documented: a 30 MW turbine losing more than 5% of its output due to deposits (www.watertechnologies.com), and deposition on blades producing up to 5% efficiency loss and 20% capacity loss (www.watertechnologies.com). In extreme cases, slug carryover or corrosive carryover can trigger turbine overspeed or blade failure — past incidents have caused fatalities (www.power-eng.com).
Because of this, modern turbines specify ultra‑pure steam. Industry guidance cites steam total‑solids levels of only 10–30 ppb to avoid deposits (www.watertechnologies.com). In practice at high pressure, chemistry specs require Na and Cl kept below 2 ppb and silica at 0.01–0.02 ppm (10–20 ppb) in the steam (www.power-eng.com, www.watertechnologies.com). Attemperating (bypass spray) water — the spray used to control steam temperature — must be equally pure, because any impurity there goes directly to the turbine (www.watertechnologies.com).
Failure modes and chemistry limits
Silica and sodium in steam crystallize on blades as temperature drops. Below roughly 20 ppb silica, deposit formation accelerates markedly, and even less than 10 ppb may cause fouling over time (www.watertechnologies.com, www.power-eng.com). Solid particles (such as oxides from corrosion) cause blade erosion. Contaminated attemperation water has forced turbines offline within months (www.watertechnologies.com).
Typical HRSG steam‑phase limits cite Na ≤2 ppb, SiO₂ ≤10 ppb, and Cl⁻/SO₄²⁻ ≤2 ppb (www.power-eng.com). Condensate Na is often held at less than 1–2 ppb (www.power-eng.com), while achieving these levels usually requires feedwater at or below 0.1 μS/cm (microSiemens per centimeter), Na ≤2 ppb, and silica ≤10 ppb (www.power-eng.com). Manufacturers routinely require degassed cation conductivity (a conductivity measured after passing the sample through a hydrogen‑form cation exchanger to strip out ammonia and CO2) at 0.1–0.2 μS to meet turbine guarantees (www.power-eng.com, www.power-eng.com).
Plants commonly polish returning condensate to maintain those low sodium levels, and a condensate polisher becomes a frontline defense when chemistry upsets occur.
Steam drum internals: two‑stage separation
Inside the drum, the physics are unforgiving. With as many as 15–20 lb of water circulating per lb of steam generated, at least 99.97% of that water must be removed to ensure the desired steam purity (www.watertechnologies.com).
The first line of defense is primary cyclonic separation. Steam/water mixture enters large swirl chambers or cyclones whose tangential inlets throw heavier droplets outward by centrifugal force, where they impinge on walls and drain down (www.watertechnologies.com). One design uses vertical cyclones — “corrugated plate pyramids” — as primary separators (pdfcoffee.com), with multiple rows or alternating “handed” cyclones to ensure balanced flow and water discharge that does not cause any water level instability (pdfcoffee.com).
Secondary scrubbers — also termed demisters or steam purifiers — then strip the remaining fine mist. After the cyclones, steam is directed through dense coalescer screens or corrugated‑plate packs where micron‑scale droplets impinge, coalesce, and drain away (www.watertechnologies.com, pdfcoffee.com). One specification calls for a primary steam scrubber (corrugated plate plus perforated base) followed by secondary pad scrubbers and a final perforated distribution plate (pdfcoffee.com, pdfcoffee.com). Advanced internals go further: a patented double‑cylinder cyclone with a finish swirler and cap traps ultra‑fine mist (patents.google.com).
Done right, this two‑stage scheme leaves essentially dry steam. After separation, carryover solids are often in the low ppb range — even 0.001–0.01 ppm total solids in the best systems (www.watertechnologies.com). High‑efficiency demister pads can remove about 99.99% of droplets ≥8 μm, leaving approximately 0.1 gallon of water per million scf of steam (bbs.hcbbs.com). In practice, modern drums routinely achieve steam with total dissolved solids (TDS) far below 0.01 ppm, meeting turbine purity specs (www.watertechnologies.com, bbs.hcbbs.com). Equipment vendors often guarantee maximum carryover below 0.03% by mass, and internal devices are selected to meet or exceed that.
Quantitatively, good separation translates to ppm/ppb‑level carryover: even in high‑pressure boilers one can routinely limit steam TDS to 10–30 ppb (www.watertechnologies.com).
Real‑time steam‑side monitoring
Given such tight bands, real‑time monitoring is essential. Continuous analyzers provide early warning of upsets — condenser leaks, chemical misfeeds, resin exhaustion — so operators can prevent turbine fouling. Modern combined‑cycle plants install analyzers at feedwater/economizer inlet, condensate pump discharge, drum blowdown, and in main/reheat steam lines. Typical online targets include feedwater conductivity ≤0.1 μS/cm, Na ≤2 ppb, silica ≤10 ppb (www.power-eng.com); condensate Na ideally under 1–2 ppb (www.power-eng.com). In steam, plants follow turbine‑maker limits: degassed cation conductivity ≤0.1–0.2 μS, Na about 1–2 ppb or less, SiO₂ about 5–10 ppb or less, with Cl⁻/SO₄²⁻ about 2 ppb or less (www.power-eng.com, www.power-eng.com).
Instrumentation spans conductivity and cation‑conductivity meters on feedwater, condensate, and steam (after degassing) — degassed conductivity is often trended with a setpoint near 0.1 μS (www.power-eng.com). Ion‑selective sodium (Na⁺) analyzers provide direct ppb readings, with ≈0.1 ppb detection sensitivity reported for continuous steam checks (www.watertechnologies.com, www.watertechnologies.com). Online silica analyzers — typically colorimetric — measure SiO₂ at sub‑ppb to low‑ppb levels (www.powermag.com), while new ion‑chromatography units can report Cl⁻/SO₄²⁻ at about 0.2 ppb if needed (www.power-eng.com). Trend analysis is simple: alarms or limits are set at known thresholds — for instance, silica alarms at roughly 10 ppb on a demineralizer effluent (www.powermag.com).
Monitoring also keeps chemical programs honest. Because misfeeds are a known upset source, plants often pair analyzers with precise chemical metering; on the hardware side, a dosing pump can be part of keeping neutralizing amine or oxygen scavenger additions stable.
Case data: silica warning buys time
One combined‑cycle plant installed four online silica units at critical points — RO permeate and anion effluent — to get ahead of resin breakthrough (www.powermag.com). The RO point is a natural place to monitor, and many power stations rely on brackish‑water RO for makeup purification.
The same plant also watched mixed‑bed effluent and condensate (www.powermag.com). Mixed‑bed polishers — including mixed‑bed deionizers — are typically the last ion‑exchange step, and the anion stage sits within a complete ion‑exchange system upstream. Monitoring silica gave an 8–15 minute warning of resin exhaustion — much faster than conductivity — and when silica at one point exceeded about 5–10 ppb, staff regenerated the resin before any significant carryover (www.powermag.com, www.powermag.com). As a result, 45‑year‑old boilers ran with exceptional reliability — only two tube leaks in one year and less than 116 total hours offline across two units (www.powermag.com, www.powermag.com).
To hold those low silica numbers continuously, plants often backstop resin with membrane and polishing stages. Continuous deionization via EDI and classical demineralizers are routinely paired with upstream membranes. Where surface water is the source, pretreatment like ultrafiltration helps stabilize RO feed quality before downstream ion exchange.
Industry trend: analytics first
Spending on real‑time purity monitoring is rising. A market report notes the power sector is the dominant driver of online silica analyzers, in part due to stringent national standards. China’s largest utility group now mandates ≤20 ppb silica in ultra‑supercritical boilers (pmarketresearch.com), and EU/US guidelines emphasize continuous monitoring as well. In practice, plants obeying such limits routinely keep silica in steam below 0.01–0.02 ppm and sodium under 1 ppb (www.power-eng.com, pmarketresearch.com) — achievable only with active chemistry control and monitoring.
Across this control stack, ancillary equipment matters. Sampling panels, filters, and metering components — the nuts and bolts found in supporting equipment for water treatment — underpin analyzer uptime and data quality.
Chemistry programs for HRSGs are maintained with careful dosing: a neutralizing amine can control pH, and oxygen scavengers drive dissolved O₂ to very low levels; dosing accuracy helps avoid the “chemical misfeeds” that monitoring is designed to catch.
Sources and references
Authoritative industry and technical sources provide the data above, including Veolia’s Water Handbook chapters on steam purity, separation, and measurement (www.watertechnologies.com, www.watertechnologies.com); vessel/HRSG design manuals on drum separators (www.watertechnologies.com, pdfcoffee.com); field‑case reports and monitoring best practices (www.watertechnologies.com, www.powermag.com); and market/regulatory analyses of steam purity requirements (www.power-eng.com, pmarketresearch.com). Specific data on separation efficiency and monitoring performance are from these sources, including demister removal efficiency at about 99.99% for droplets ≥8 μm, equating to roughly 0.1 gallon of water per million scf of steam (bbs.hcbbs.com) and patented cyclone geometries (patents.google.com).
