Inside the chemistry that keeps HRSGs clean: ppb‑level control, AVT, and a zero‑tolerance policy on silica

In modern combined‑cycle plants, a handful of parts per billion can decide HRSG reliability. Ultra‑pure demineralized makeup water—think resistivity in megaohm·cm and conductivity down near ~0.1–0.3 µS/cm at 25 °C—sets the baseline. Everything else is the discipline to hold it there with coordinated chemistry, metering, and monitoring.

Indonesia’s SNI 7268:2009 for high‑pressure service calls it plainly: feedwater conductivity ≤0.3 µS/cm (i.e., >3.3 MΩ·cm) above ~75 bar, and silica in once‑through HRSGs at ≤0.02 mg/L (20 ppb) (pdfcoffee.com; id.scribd.com). Those limits mirror international cycle‑chemistry guidance and the operating reality that even modest excursions—1 ppm dissolved solids or 100 ppb silica—can cut heat‑transfer efficiency and trigger premature cleanings (power-eng.com; diversey.co.id).

Plants typically run 3–5X cycles of concentration with blowdown around ~2–5%, keeping dissolved solids from approaching saturation. That puts makeup TDS in the single‑digit mg/L range, with “de‑gassed” ion conductivity often ≤0.1 µS/cm, feedwater iron well below 5 ppb, silica <20 ppb, and conductivity <0.2 µS/cm—numbers aligned with strict cycle chemistry practice (power-eng.com; pdfcoffee.com).

Ultra‑pure makeup specifications

The target is clear: megaohm‑class water feeding the economizer. In practice, that means multistage demineralization capable of delivering conductivity near ~0.1–0.3 µS/cm at 25 °C, silica at ≤0.02 mg/L in once‑through service, and ultra‑low metals. SNI 7268:2009 sets feedwater conductivity ≤0.3 µS/cm above ~75 bar and anchors silica at ≤0.02–0.04 mg/L depending on drum usage (pdfcoffee.com; id.scribd.com).

Many power facilities install high‑recovery membrane trains; an advanced gas‑turbine HRSG cited in industry literature runs RO plus mixed‑bed polishing to consistently exceed 10 MΩ·cm resistivity (≈0.1 µS/cm) (power-eng.com). Where membranes are central to the design, standardized skids such as brackish‑water RO and integrated membrane systems help keep pre‑boiler conductivity and silica under tight control.

Pretreatment and polishing train

The typical sequence is coarse media filtration → softening (if hardness is present) → reverse osmosis (to remove >90–95% of ions) → mixed‑bed polishers (cation–anion exchange) as needed, with electro‑deionization as a finishing step. Plants monitor RO permeate continuously; conductivity is held to <1 µS/cm, and pre‑RO silica to <0.1 mg/L so the RO and polishers can deliver an order of magnitude lower at the final point (power-eng.com).

On the ion‑exchange side, many facilities standardize on demineralizers and mixed‑bed polishers to drive silica and ionic contaminants to sub‑ppb territory, while EDI offers continuous ultra‑pure production without chemical regeneration. When hardness control is needed upstream, softeners and appropriate ion‑exchange resins prevent scale‑forming Ca/Mg slip.

(Footnote: in practical terms, a well‑designed feedwater plant will achieve ≥90% recovery and produce only a few percent waste reject; e.g., a 40,000 m³/day plant might blowdown 2–4% by volume with TDS ≤1,000 mg/L per GR82/2001 permitting.)

Any lapse shows up as deposits—sodium phosphate or silicate scale—that can raise fuel cost ~0.1–0.3% per %flow drift, a sensitivity corroborated by industry reports warning that even 1 ppm dissolved solids or 100 ppb silica can degrade heat transfer and force earlier cleanings (diversey.co.id; power-eng.com).

Feedwater conditioning and deaeration

Before the economizer, plants track conductivity, pH, and dissolved oxygen (DO) continuously. A low‑level mechanical deaerator is used only if copper alloys are present; for an all‑ferrous HRSG, the deaerator vent may be throttled to retain ppm‑level oxygen for AVT(O) while still driving residual oxygen very low. As a rule of thumb, feedwater O₂ after deaeration or polishing is held <20 ppb, with EPRI AVT(O) specifications citing condensate‑pump discharge DO ≤20 ppb and economizer inlet 5–10 ppb (power-eng.com).

Instrumentation typically includes a DO analyzer at the condensate pump and conductivity/degassed‑conductivity monitors after the polisher and before the feed pump, with blowdown adjusted to cap ionic impurities. Plants keep condensate cation conductivity at ≲0.2 µS/cm to prevent anion carryover (power-eng.com). Where condensate return needs cleanup, a condensate polisher removes transport metals and ammonia carryover after the condenser.

AVT chemistry program (ammonia, oxygen)

The boiler circuit runs all‑volatile treatment—AVT (volatile chemicals only)—with ammonia (or volatile amines) for [pH control and an oxygen “scavenger.”](https://beta.co.id/en/blog/inside-the-hrsg-feedwater-playbook-scavenge-oxygen-raise-ph-cut-corrosion) Under AVT, ammonia sets a mildly alkaline environment that promotes a protective magnetite film on steel surfaces; IAPWS notes ammonia as the common alkalizing agent, with amines also employed (scribd.com). Plants dose to maintain feedwater around ~9.0–9.4 pH at 25 °C, and—per EPRI targets for an FFLP HRSG—reach an LP economizer inlet pH of 9.6–10.0 (power-eng.com).

Typical ammonia targets are ~5–15 ppb NH₃ in the hottest LP economizer feed and ~30–50 ppb NH₃ at the economizer inlet to counteract flash‑off and lock in that 9.6–10.0 pH band (power-eng.com). Plants often meter with precision dosing pumps, pairing a neutralizing amine program with oxygen control via oxygen scavengers.

Under classic reducing AVT (AVT(R)), hydrazine follows mechanical deaeration to remove the ~7 ppb residual O₂, with legacy designs adding ~20–100 ppb hydrazine to treat “any” DO as harmful (power-eng.com). But the industry pivoted after high‑profile flow‑accelerated corrosion (FAC) failures—e.g., the 1986 Surry NPP pipe rupture—toward AVT(O) or oxygenated treatment; all‑ferrous HRSGs generally avoid deep reduction today (power-eng.com; power-eng.com).

In today’s AVT(O), a small controlled oxygen presence—sometimes by deliberate addition—favors formation of a tenacious FeOOH layer and strongly suppresses single‑phase corrosion (power-eng.com; scribd.com). If a scavenger is used, it is typically injected at the condensate pump outlet to bring DO down to <10–20 ppb (power-eng.com).

Instrumentation and trending controls

Real‑time control hinges on precise chemical metering and online analytics. Plants trend feedwater pH (online), oxygen/ORP or DO at feedpump discharge, and conductivity (direct and degassed) to spot excursions—a rising conductivity often flags condenser leaks or blowdown mis‑set. Action levels kick in when iron or copper exceed ~2–5 ppb, prompting chemistry or blowdown adjustments; some sites add online iron analyzers to keep Fe at a few ppb.

Routine lab work checks hardness (ensuring zero Ca/Mg), chloride, sulfate, silica, and metals in feed and boiler water. Simple condensate Br⁻ or Cl⁻ tests can catch cooling‑tower bleed or condenser leaks early. In HRSGs, iron coolant injection—low‑level Fe₃O₄ colloid dosing to saturate the loop—has been used to control FAC, coordinated with AVT to avoid overshooting iron ppm.

Standards, limits, and legal compliance

SNI 7268:2009 codifies stringent HRSG feedwater standards: DO ≤0.007 mg/L (7 ppb) and total Fe/Cu in the ~0.01–0.03 mg/L (10–30 ppb) range depending on pressure (Wood Chemistry Lab SNI table 3 shows Fe max 0.03 mg/L up to 10 MPa, 0.01 mg/L beyond 15 MPa) (pdfcoffee.com; pdfcoffee.com). Silica is likewise capped at ≤0.02–0.04 mg/L depending on drum service (pdfcoffee.com; id.scribd.com). These targets are in line with international practice (British JIS/BS recommendations for Fe, Cu on the order of 10 ppb in high‑voltage feedwater), and with EPRI guidance for AVT(O) (e.g., pH 9.6–10.0, DO ≤20 ppb) (power-eng.com).

Waste handling and chemical safety are regulated. Blowdown and ion‑exchange regenerants must meet Government Regulation 82/2001 water‑quality standards. Chemicals designated B3—including hydrazine—fall under MOEF Regulation 9/2024; producers of B3 waste must manage, treat, and hand off to licensed facilities (hhp.co.id). Many plants have eliminated hydrazine due to toxicity; if used, it is fed via closed‑loop injection with continuous leak detection, and any spent resin or washwater goes to a licensed B3 waste facility (hhp.co.id). Ammonia is tracked for steam‑cycle carryover and typically removed post‑condenser via polishing to avoid nitrate release.

Compliance is verified by routine sampling per SNI 7268 (e.g., conductivity, pH, hardness, Fe, Cu, Na, SO₄, Cl, SiO₂ of feed and boiler water) (pdfcoffee.com; pdfcoffee.com). Routine upkeep of the pre‑boiler plant—filters, polishers, and chemical metering—often relies on standard spares; operators commonly stock consumables for ion exchange and membranes alongside plant chemicals, though specific inventories vary by site.

Measured outcomes in service

When AVT(O) holds feedwater effective pH near ≈9.8 and O₂ ~10 ppb, plants report feedwater iron ≤2 ppb and steam‑cycle copper below detection—roughly 10× lower metal transport than under older AVT(R) regimes (scribd.com). Turbine blade deposits from chlorides/sulfates are essentially eliminated, keeping stage efficiency within design; each 10 ppb Fe in high‑pressure steam can drop turbine efficiency by ~0.01–0.02% (power-eng.com).

Case histories also point to reliability gains: shifting from phosphate programs to AVT in high‑pressure units reduced forced outages from water chemistry by >70% over five years. For HRSG operators, the message is consistent—[ultra‑pure makeup plus optimized AVT chemistry](https://beta.co.id/en/blog/inside-the-ultra-pure-water-playbook-keeping-high-pressure-hrsgs-alive) is a core enabler of high availability and long tube life (power-eng.com; power-eng.com).

Sources and citations

IAPWS Technical Guidance (2015) defines AVT and target ranges, including AVT(O) practice and volatile alkalizing agents (“commonly ammonia”) (scribd.com; scribd.com). EPRI targets summarized by Buecker provide practical limits like pH 9.6–10.0 and DO ≤20 ppb (power-eng.com). SNI 7268:2009 (Indonesia) prescribes feedwater DO ≤0.007 mg/L, Fe ~10–30 ppb and silica limits to ≤0.02–0.04 mg/L—see the cited tables for pressure‑tiered maxima (pdfcoffee.com; pdfcoffee.com; id.scribd.com). Regulatory obligations on B3 wastes are summarized from MOEF Reg. 9/2024 (hhp.co.id).

Product integration notes

Where plants seek standardized hardware, the research above maps directly to proven categories: reverse osmosis for primary demineralization, demineralizers and mixed‑bed polishers to push silica and TDS to ppb levels, and EDI for continuous ultra‑pure production. For hardness control and resin management, softeners and ion‑exchange resins align with the “softening (if hard feed)” requirement. In chemistry dosing, dosing pumps support precise pH and oxygen control via neutralizing amines and oxygen scavengers. On the return side, a condensate polisher supports the low‑metal, low‑ammonia condensate targets described above.

Inline citations: These statements are supported by published guidelines, industry reports, and standards as cited above; full source details follow. (power-eng.com; pdfcoffee.com; scribd.com; scribd.com; power-eng.com; power-eng.com; pdfcoffee.com; id.scribd.com; power-eng.com; hhp.co.id)