In high‑pressure HRSGs (heat recovery steam generators), chemistry is destiny. Parts‑per‑billion impurities can shave 5% off a 30‑MW turbine’s output and, in extreme cases, precipitate catastrophic failures.
A 30‑MW steam turbine lost more than 5% of its capacity to boiler‑water carryover deposits, according to an industry handbook (www.watertechnologies.com). The stakes are higher than megawatts: feedwater line corrosion has caused catastrophic failures, including the 1986 Surry plant elbow rupture that killed four workers and created losses in the tens of millions (www.powermag.com).
It’s why experts call water chemistry the “lifeblood” of steam plants (www.powermag.com). For modern HRSGs, that lifeblood is ultra‑pure makeup water paired with a coordinated all‑volatile treatment, or AVT (volatile ammonia and an oxygen scavenger), designed to suppress flow‑accelerated corrosion (FAC, thinning of steel from high‑velocity corrosive water) and keep deposits out of the turbine.
International guidance—from IAPWS (International Association for the Properties of Water and Steam), EPRI (Electric Power Research Institute), and ASME—underpins the program. In practice, makeup water is produced to around 18 MΩ‑cm resistivity (≈0.055 µS/cm) with hardness and silica in the single‑digit ppb range. With proper treatment and no process leaks, makeup water will normally contain <2 µg/kg Na and have cation conductivity <0.2 µS/cm (www.power-eng.com). ASME/EPRI recommendations cite conductivity <0.1 µS/cm, hardness <0.2 mg/L as CaCO₃, and silica <10–20 ppb (ebrary.net).
Guideline limits and Indonesian context
Indonesia’s PP 82/2001 regulates water pollution control for blowdown discharge, while Permen ESDM 10/2021 requires boiler makeup systems adequate to sustain operation (es.scribd.com). Local codes do not prescribe exact feedwater chemistries; plants generally adopt international best practices—e.g., IAPWS (2010) and EPRI (2013/2019)—for limits on conductivity, pH, oxygen, and impurities at key cycle locations (citations below). Any effluent must meet Ministry of Environment water‑quality limits before discharge.
Makeup water plant design and targets
Pretreatment starts with filtration and activated carbon to remove particulates and organics, then hardness removal by water softeners or lime‑soda precipitation to <0.1–0.2 mg/L as CaCO₃ (ebrary.net). Chlorine is stripped or reduced to ppb levels. Reverse osmosis (often two‑pass) cuts TDS to very low levels, and a final polish—mixed‑bed ion exchange or EDI (electrodeionization)—brings water to ultra‑pure standards.
Pretreatment filtration can include sediment/carbon steps; many plants specify cartridge elements, making a case for cartridge filters before RO. Activated carbon stages align with activated carbon media, while hardness control maps to a water softener. Two‑pass RO is often configured with industrial systems such as brackish-water RO, and polishing mirrors either EDI stacks or a mixed‑bed exchanger. Where condensate is recovered, a condensate polisher helps keep metals and ions low.
Quality targets mirror OEM and IAPWS/EPRI expectations:
- Resistivity/Conductivity: On‑line monitors target ≥10 MΩ·cm (18.2 MΩ·cm ideal). In practice, residual conductivity is <0.1–0.2 µS/cm at 25 °C (ebrary.net; www.power-eng.com), implying virtually no ionic contamination (e.g., no more than ~0.05 mg/L NaCl equivalent).
- Silica: Ideally <10 µg/L; for once‑through HRSGs, <10 ppb in feed (ebrary.net).
- Hardness (Ca/Mg): Essentially zero (<0.2 mg/L as CaCO₃) (ebrary.net).
- TOC: Ideally <300 ppb.
- Other ions: <20–50 ppb; one operator notes Na⁺ <2 µg/kg in purified makeup (www.power-eng.com).
Industry tables cite storage‑tank conductivity <0.1 µS/cm, silica <20 ppb (with <10 ppb for once‑through units), and hardness <0.2 mg/L (ebrary.net). Plants often deploy integrated membrane systems end‑to‑end and specify analyzers as part of water treatment ancillaries to maintain these targets.
Deaeration and oxygen control (AVT chemistry)
Physical deaeration sets the baseline. A steam‑jet or spray deaerator removes ~95–99% of dissolved O₂, down to tens of µg/L. Residual oxygen and air inleakage are addressed chemically via AVT: a volatile oxygen scavenger (redox agent) with ammonia. Historically hydrazine (N₂H₄) was standard; modern alternatives include carbohydrazide and diethylhydroxylamine (DEHA), with films and polymer‑based scavengers also noted.
The design objective is feedwater DO ≲10 µg/kg at the condensate pump discharge (fliphtml5.com). IAPWS guidance notes that with low oxygen (≈10 µg/kg or less) plus ammonia, the feedwater attains a reducing environment protecting copper and steels (fliphtml5.com). In operation, deaerators are commonly run at ~1–3 bar with tight venting and oxygen analyzers at the feed pump outlet verifying the <10 µg/kg target. A dosing scheme might maintain hydrazine of 20–100 ppb in boiler water. Chemical feeds are typically metered with a dosing pump at the condensate polisher outlet or feed pump discharge for full‑train distribution.
Implementation details matter: (a) injection location governs distribution; (b) control can be by ORP or DO; (c) metallurgy dictates AVT variants—ammonia is corrosive to copper, so mixed‑metallurgy systems typically run AVT(R) (reducing), while all‑ferrous systems may choose AVT(O) (allowing slight O₂) (fliphtml5.com); and (d) many scavengers decompose to ammonia, contributing to alkalinity.
Alkalinity and pH management (ammonia dosing)
AVT relies on ammonia (or neutral volatile amines) to raise pH and minimize corrosion. Typical feedwater (ambient) pH targets are ≈9.2–9.8 (fliphtml5.com), corresponding to generated steam pH ~8.8 (at 1000 °C). Injection is usually at the condensate pump discharge or polisher outlet.
Recent guidance (EPRI 2013; ASME) has elevated recommended pH: for all‑ferrous HRSGs on AVT, 9.6–10.0 (condensate basis) is now common (www.powermag.com). Higher pH improves magnetite stability and reduces FAC (www.power-eng.com) but greatly increases free ammonia: raising condensate pH from 9.6 to 10.0 increases ammonia from ~2.3 mg‑N/L to ~11.8 mg‑N/L—a 5× jump (www.powermag.com). Mixed Cu/Fe systems or AVT(R) regimes often select pH ~9.2–9.5 to prevent copper stress corrosion (fliphtml5.com; www.power-eng.com), while all‑ferrous circuits frequently run 9.6–9.8 (www.powermag.com).
Control leans on pH and cation conductivity (CACE, conductivity after cation exchange) to infer ammonia. IAPWS suggests feedwater (economizer inlet) pH ~9.2–9.8 and condensate pH ~8.6–9.8 for AVT(O) (fliphtml5.com). At pH 10.0, free ammonia in condensate can be ~11.8 mg/L (www.powermag.com), which can overload condensate‑polisher resins.
Where neutral amines are specified, their function aligns with neutralizing amines in boiler cycles, but dosing still hinges on the same CACE and pH feedback loops.
Cycle monitoring and blowdown control
Monitoring is rigorous. Nominal targets (IAPWS/EPRI): dissolved oxygen <10 µg/kg at feed pump discharge (fliphtml5.com), feedwater pH ~9.2–9.8 and drum water pH ~9.0–9.6 (fliphtml5.com), and cation conductivity <0.2 µS/cm at feed (www.power-eng.com; fliphtml5.com). In boiler water, keep CACE <1.5 µS/cm (fliphtml5.com). High ammonia elevates raw conductivity, so “after cation exchange” readings provide the uncontaminated signal.
Sodium and silica are held low: Na⁺ in drum water <2 µg/kg with similarly low silica. For saturated steam, IAPWS suggests Na⁺ <2 µg/kg and SiO₂ <10–20 µg/kg (fliphtml5.com). By design, steam‑generator makeup is so pure that Na⁺ is usually <2 ppb (www.power-eng.com). Metals: under AVT reducing conditions, Fe <5 µg/kg and Cu <2 µg/kg are common targets; under AVT(O) in all‑ferrous circuits, Fe <2 µg/kg (fliphtml5.com). Turbidity into feed is typically <1 NTU.
Regular lab work (ICP‑MS/AAS for ions and metals, TOC analyzers, pH and conductivity meters) supplements online data. Action levels are often set at ~2× normal as Level 1, with defined responses (e.g., resin regeneration or chemical cleaning) if exceeded (fliphtml5.com). IAPWS recommends keeping long‑term accumulations (annual total Fe or Na) below limits to avoid unscheduled cleaning (fliphtml5.com).
Blowdown is timed or conductivity‑controlled to purge solids, typically holding boiler TDS ~500–1000 mg/L (pressure‑dependent) or at 5–8× concentration. HRSGs typically include multiple blowdown valves on the economizer or steam drum. Blowdown is treated (neutralized) to meet environmental limits.
When resin work is needed, sourcing ion exchange resins built for condensate polishing and demineralizers helps maintain capacity; resin management is often tracked alongside analyzer trends. Supporting parts are stocked via water-treatment parts and consumables.
Chemical handling and metallurgy considerations
AVT chemistries are volatile and non‑depositing. Phosphates, filming amines, or other solid additions are not part of this regime; they risk under‑deposit corrosion and carryover. The core program is ammonia and a volatile oxygen scavenger. Hydrazine (N₂H₄) remains effective but is toxic (IARC Group 2B carcinogen), so many plants prefer organics like carbohydrazide and DEHA. Chemical feed systems (tanks, pumps, mixers) must meet safety regs (PPK3) and prevent free hydrazine venting.
Ammonia analyzers on condensate and ORP probes in feedwater provide dosing checks. High‑pH ammonia levels—up to ~10–12 mg/L NH₃‑N at pH 10—may overload condensate polishers; compensating by increased blowdown or larger polisher capacity may be required (www.powermag.com). Operators also watch for ammonia‑related corrosion (e.g., ammonia “grooving” of stainless steel). Research notes very high ammonia concentrations (>~9.3 pH) can cause localized attack on 300‑series stainless alloys (www.power-eng.com). Regular inspection—borescopes or eddy‑current tests on areas like superheater bends—is prudent when running high‑pH AVT.
Where oxygen scavenging is specified explicitly, the chemistry aligns with oxygen scavengers in boiler service. Dosing hardware and interlocks are typically integrated with the plant’s boiler chemical skids for traceable feeds.
Performance outcomes and operational trends
Well‑executed chemistry delivers measurable benefits: reduced outages and longer component life. Minimizing impurities—e.g., Fe to <2–5 µg/kg and silica to single‑digit ppb—avoids boiler tube shrinkage and turbine fouling. One study noted strict chemistry control (Ni‑free alloys, all‑volatile chemistry) allowed feedwater iron to drop below 2 ppb (fliphtml5.com). Even sub‑10 µg/kg variations in Fe or NH₃ can markedly affect FAC rates (www.power-eng.com).
Efficiency dynamics are straightforward: clean steam preserves heat rate; deposits increase flow resistance and thermal resistance. One turbine was forced offline after only 3 months due to rapid deposit buildup (www.watertechnologies.com). A maintenance‑driven chemistry program can save millions in deferred capital (less frequent cleaning/retubing) and maximize generation (5% output gain per turbine in the cited example, same source).
Trends include a shift toward higher pH/less‑reducing regimes where metallurgy allows. EPRI’s 2013 update (and a 2019 BestPractices report) recommend condensate pH 9.6–10.0 for all‑ferrous HRSGs to combat FAC (www.powermag.com). If copper exists, copper‑passivating amines or a return to slight reduction may be needed. Plants are adopting continuous online analyzers and advanced chemistry modeling for finer control. Environmental and health regulations are driving replacement of hydrazine with benign organics, and any blowdown containing scavenger by‑products (nitrates, etc.) must meet local effluent standards.
Program checklist and action levels
Recommended practice consolidates into a few data‑backed points. Makeup water is produced via multistep purification—filters + softener + 2‑pass RO + EDI/mixed‑bed—to achieve ≤0.1 µS/cm and sub‑ppb contaminants (ebrary.net; www.power-eng.com). Plants often build this using RO trains like brackish-water RO ahead of either EDI or a demineralizer.
Deaeration is operated to minimize oxygen—plants commonly run units at ~1–3 bar with tight venting—but summary guidance also notes operation under vacuum can reduce O₂ to <20 ppb; a volatile oxygen scavenger (hydrazine or equivalent) then drives DO <10 µg/kg (fliphtml5.com).
Chemical dosing maintains feedwater pH ~9.2–9.8, adjusted for metallurgy. Ammonia is added at the condensate pump to meet setpoints. Scavenger is dosed concurrently (e.g., 20–100 ppb hydrazine in boiler water or equivalent organics). Non‑volatile chemicals (phosphates, sodium hydroxide) are not employed under normal operation.
Monitoring includes loop analyzers for conductivity (normal and CACE), pH, ORP/DO, plus periodic lab checks (silica, Fe, Na, Cu) per IAPWS/EPRI tables (fliphtml5.com; fliphtml5.com). Action limits are typically ~2× normal. Sampling is structured at the drum, feed‑pump discharge, and steam lines. Resin capacity and analyzer reliability are maintained with spares from water-treatment consumables.
Action plans keep blowdown sufficiently frequent to prevent conductivity or alkalinity drift. Inconclusive readings or excursions trigger investigation and cleaning. In the case of condenser leaks or contamination, IAPWS protocols (e.g., shutting valves, chemical flushes) apply (fliphtml5.com; fliphtml5.com). Documentation covers treatment runs, chemical usage, and trend records to ensure compliance with Indonesian and global standards.
In numerical terms: achieving <10 ppb silica and <2 µg/kg Fe in feed avoids costly turbine blade fouling; maintaining feed conductivity <0.2 µS/cm correlates with “turbine‑grade” water (www.power-eng.com; fliphtml5.com). Given that just a few ppb change in impurities can translate into percent‑level output differences, the investment pays back in uptime, efficiency, and equipment life (www.watertechnologies.com).
Sources and reference guidance
Authoritative guidance from IAPWS/EPRI, industry handbooks, and case studies inform the numerical targets and practices above. Key sources include: IAPWS (2010) “Volatile Treatments for the Steam–Water Circuits of Fossil and Combined Cycle/HRSG Plants” (fliphtml5.com; fliphtml5.com); Colleen Scholl & Rob Swanekamp, “Operating HRSGs with Elevated Feedwater pH,” Power (Nov 2019) (www.powermag.com); Brad Buecker’s Power‑Engineering analyses on protecting steam turbines and HRSG corrosion control (www.power-eng.com; www.power-eng.com); Veolia Water Technologies’ Water Handbook chapter on turbine deposition (www.watertechnologies.com); and POWER Magazine’s “Water Chemistry: Power Plant Life and Death” (www.powermag.com). Indonesian regulations: PP No. 82/2001 (effluent standards) and Permen ESDM No. 10/2021 (boiler water systems) (es.scribd.com).
