In combined‑cycle power, even tiny droplets in the steam can slash output by 20% and trigger dangerous overspeed events. Plants are fighting back with high‑efficiency HRSG drum internals, online purity analytics, and last‑stage turbine designs hardened for wet steam.
Steam purity is not a nice‑to‑have in combined‑cycle plants; it’s the difference between full load and a forced derate. Industry guidance documents report measured losses of 5% efficiency and 20% capacity in turbines hit by boiler carryover deposits (watertechnologies.com). Contaminated steam can foul stop/governor valves, stick mechanisms, and lead to turbine overspeed with potentially catastrophic outcomes (watertechnologies.com). Put bluntly, “even small amounts of solids” erode blades and block nozzles, driving sharp drops in output and availability (watertechnologies.com) (watertechnologies.com).
That’s why vendors specify turbine‑inlet steam chemistry in parts per billion (ppb). One industry summary cites limits of ≤2 ppb sodium and chloride, ≤10 ppb silica and sulfate, and cation conductivity <~0.1 μS/cm (microsiemens per centimeter) (power-eng.com). Veolia’s Water Handbook puts it plainly: “precise control of steam purity is critical” (watertechnologies.com), while IAPWS (International Association for the Properties of Water and Steam) sets cycle chemistry guidance so the turbine sees only ultra‑pure steam, even under cycling, with explicit attention to organics and carbon dioxide (iapws.org) (power-eng.com).
The performance stakes are direct and costly. Veolia documents cases where contaminated steam drove up to 20% loss of turbine capacity and frequent unplanned shutdowns (watertechnologies.com). “Loss of production may result,” the handbook notes, and the impact “overshadows all other considerations” (watertechnologies.com). Plants therefore invest heavily in monitoring and purification because a single condenser leak or chemistry upset can spike impurities and threaten blades and valves.
That investment typically starts with ultra‑pure makeup water. Power‑sector reverse osmosis (RO) platforms such as sea‑water RO are widely deployed for industrial and power plant duty, often preceded by pretreatment steps like ultrafiltration. To push purity into the ppb realm demanded by turbine specs, demineralization technologies — for example EDI (continuous electro‑deionization) and mixed‑bed ion exchange that provides less than 20 ppb silica and very low TDS — are standard options in high‑reliability makeup trains.
Steam purity limits and reliability economics
Superheated steam turbines are particularly vulnerable to solids and droplets that survive separation and superheating, with documented cases of deposit‑driven 5% efficiency and 20% capacity losses (watertechnologies.com). Manufacturers routinely verify that commissioning steam meets ≤2 ppb sodium and chloride, ≤10 ppb silica and sulfate, and cation conductivity <~0.1 μS/cm before sign‑off (power-eng.com). IAPWS power cycle chemistry guidance codifies ppb‑level limits to protect blades and reheaters (iapws.org).
In practice this amounts to “an essentially zero‑tolerance” stance on contaminants: any carryover shortens service life and forces derates (power-eng.com). Chemistry programs frequently extend into the condensate path; polishing steps such as a condensate polisher target trace ions after heat exchange cooling to keep return quality aligned with turbine‑side purity goals.
Steam drum internals (separators and demisters)
Modern HRSG (heat recovery steam generator) drums are built to almost eliminate liquid carryover. Water‑steam mixtures from the evaporator enter a large cylindrical vessel where a sequence of mechanical devices removes droplets before the steam outlet: cyclonic separators or swirl vanes spin out larger droplets by inertia; a gravity settling zone leverages drum volume; and high‑efficiency demisters — mesh pads or vane packs — trap fine mist. Some designs add steam scrubbers (wash sprays) to absorb particulates or vaporous silica.
The numbers are revealing: industrial vane separators used in HRSG drums can remove 99.9% of droplets ≥8–10 μm (royalguardstrainers.com). By contrast, gravity alone without a demister only captures droplets >100–300 μm (scribd.com). Mesh demisters then finish the job by coalescing sub‑10 μm mist, reducing remaining moisture to just ppm‑level solids in the steam.
Design studies confirm the importance of sizing, internal baffles, and element placement. A steam drum design paper notes the drum “must be sized properly to separate steam by gravity and [fitted with] auxiliary internals, such as a demister, … to filter the steam,” with CFD (computational fluid dynamics) showing how poor placement can create high velocities or turbulence that drag droplets into the outlet (koreascience.or.kr). In well‑designed HRSG drums, two‑stage separation (cyclonic + demister) routinely delivers steam with turbidity on the order of ppb solids, meeting turbine purity specs (koreascience.or.kr) (royalguardstrainers.com). The takeaway matches HRSG chemistry guidance: achievable steam purity depends on the design of steam drum internals as much as on chemistry (ebrary.net).
Online steam quality monitoring
Given the risks, continuous purity monitoring has become baseline practice. Industry consensus (IAPWS, EPRI and others) treats on‑line steam monitoring as a reliability tool because “steam purity can have a major impact on turbine performance,” and on‑line analysis is standard to catch off‑spec conditions in real time (researchgate.net). Typical measures include:
• Silica and sodium analyzers to measure tiny concentrations that slip into steam via carryover or leaks, providing early warning of feedwater upsets or condenser leaks.
• Conductivity probes (cation conductivity on the steam or downstream condensate) to track total ionic impurities, sometimes reported as “steam purity” in μS/cm; newer instruments condense a portion of the sample (analyzer technologies exist that isolate 100% of the sample steam】) for real‑time measurement.
• pH and dissolved oxygen in condensate return to track secondary amines or oxygen ingress — signals of chemistry breaches that could propagate into the HRSG.
In the context of Indonesian operations, major utilities (PLN, IPPs) align with these global trends: advanced combined‑cycle units incorporate on‑line silica/sodium monitors on high‑pressure steam to safeguard turbine health. PT Hyprowira notes that on‑line silica and sodium analyzers “help control steam quality in geothermal plants” — the same principles apply to HRSGs (hyprowira.com).
Continuous analytics catch transient excursions that manual sampling would miss; one report cites an off‑nominal silica spike that was only detected because an on‑line analyzer was in place (researchgate.net). Early detection in steam turbines can prevent weeks of lost production or costly repairs (watertechnologies.com) (researchgate.net). IAPWS guidance implicitly assumes modern combined‑cycle plants will use continuous monitoring to enforce steam purity limits (iapws.org).
Operators act fast on alarms. When a conductivity or sodium spike is observed, they can immediately raise blowdown or scrub the steam (add wash water) to protect the superheater. Chemistry control relies on precise dosing and conditioning, where tools such as an accurate chemical dosing pump, a neutralizing amine for pH control, and oxygen scavengers for dissolved O₂ are aligned with the analyzer feedback loop.
Final-stage turbine design against wet steam
Even with ideal purity, moisture tends to form in low‑pressure (LP) stages, making the last rows most erosion‑prone. Vendors counter with hardened materials, drainage, and aerodynamics. Siemens reports that in its latest LP turbines the large final‑stage blades have inlet (leading) edges “flame or laser hardened … to prevent droplet erosion,” with corrosion‑resistant alloys/coatings used on impact zones (doczz.net).
Stationary vanes (diaphragms) in the last stage are built hollow with drain slots or channels; Siemens notes these “last stationary blades…are designed as hollow blades…to remove moisture from the blade surface” (doczz.net). Three‑dimensional blade shaping optimizes reaction near the hub to minimize condensation — increasing local flow speed helps “flash off” moisture and preserve performance (doczz.net). Material selection trends toward high‑chrome stainless or nickel alloys, paired with polished surfaces, to resist erosion/corrosion under wet conditions.
Veolia highlights that LP‑stage erosion accelerates “when water condenses,” especially if the turbine runs below its design steam inlet temperature or under deep part‑load (watertechnologies.com). Avoiding off‑design operation reduces droplet formation; when wet steam is unavoidable, hardened leading edges, internal drains, and aerodynamic shaping collectively minimize degradation. Field experience shows turbines with such features exhibit dramatically lower last‑stage replacement rates after long partial‑load runs than older designs.
Chemistry control as an operating backbone
All of this relies on keeping contamination out of the cycle. Plants pair their online analyzers with robust make‑up and return treatment. A resin‑based demineralization step — e.g., a demineralizer consisting of cation and anion exchangers (both strong/weak types) — remains a cornerstone of water prep, complemented by the RO/EDI polishers already noted. Ancillary systems, spares and instruments housed under supporting equipment for water treatment keep the analytics‑and‑chemistry loop responsive to the transient upsets that IAPWS warns about (iapws.org).
The common thread across these measures is the same one Veolia repeats: “precise control of steam purity is critical” (watertechnologies.com). With turbine‑spec limits set at ≤2 ppb sodium and chloride, ≤10 ppb silica and sulfate, and cation conductivity <~0.1 μS/cm (power-eng.com), the HRSG drum internals, continuous analyzers, and last‑stage blade engineering form a single reliability system — one designed to catch impurities before they become 5% efficiency and 20% capacity losses, valve sticking, or overspeed events (watertechnologies.com).
