The quiet retrofit cutting outages and water use in turbine halls

There’s a simple reason large generators are ditching open circuits for closed ones: the tiny cooling passages inside stator windings and oil coolers don’t forgive scale or rust. Industry literature says only ultra‑pure demineralized water—on the order of 0.1–0.3 μS/cm (microSiemens per centimeter), that is ≥3–10 MΩ·cm (megaohm‑centimeter)—is acceptable for the job (ResearchGate) (pdfcoffee.com).

The modern setup is closed recirculation of that ultra‑pure water on the machine side, continuously polished, and thermally coupled—without mixing—through a dedicated plate or shell‑and‑tube heat exchanger to the plant’s main cooling water. Generator stator systems “must be operated with demineralized water, which also eliminates concerns regarding corrosion” (ResearchGate). In practice, the stator loop contains only deionized water, continuously passed through mixed‑bed polisher resins (Power).

The payoff is not just thermal efficiency. Closed‑loop designs require less make‑up water and minimal water treatment compared to tower systems (Alfa Laval), and can eliminate effluent so plants avoid discharge permits and testing burdens (WaterTechOnline).

Ultra‑pure DM loop, zero mixing

All piping and components on the demineralized (DM) loop—water stripped of dissolved ions to minimize conductivity—are chosen to avoid shedding ions; large generator circuits are specified “free of unalloyed steel” (ResearchGate). Plants target very high resistivity and low dissolved gases, with operating conductivity often <1.0 μS/cm and sometimes <0.1 μS/cm, to protect insulation and bearings (pdfcoffee.com).

Make‑up water is supplied via dedicated demineralization—commonly ion exchange or reverse osmosis (RO) (Power). Plants often implement a demineralizer for primary ion removal before the loop.

Continuous polishing is standard. Loop piping sends a slipstream through mixed‑bed resins to catch any ionic contaminants, with resin beds sometimes replaced early when solids clog them, even before exhaustion (Power). Many operators specify a compact mixed‑bed polisher and maintain a stock of ion‑exchange resins to sustain resistivity.

When RO is the chosen make‑up technology, brackish sources are common; a packaged brackish‑water RO unit can help reach the sub‑μS/cm conductivity window before final polishing.

Hydraulics, flow, and expansion control

The coolant flow is set by auxiliary heat load and allowable temperature rise (ΔT, the change in temperature). Large generators can dissipate roughly 1–5 MW in stator and bearings. One 660 MW unit circulates about 116 m³/h (≈32 L/s) of stator cooling water, typically at a 10–20 °C ΔT (Scribd). Vendors note flows can be “large – up to thousands of cubic meters per hour,” with inlet temperatures ~50–80 °C and outlet ≤40 °C (VR Shell & Tube).

Two parallel pumps—one duty, one standby—are standard to meet design flow at a few bar of pressure. An expansion tank, often nitrogen‑blanketed, absorbs thermal expansion. Pressure gauges, flow meters, and temperature sensors track loop health, with alarms on conductivity and ΔT recommended (Power).

Corrosion, oxygen, and deposit control

Even pure water can corrode if oxygen sneaks in. Field evidence shows that 10–20% of water‑cooled generators see initial copper oxide (Cu₂O/CuO) deposits in stator coils, which can plug narrow channels only a few millimeters wide (ResearchGate). Plants counter with tight seals or N₂ covers to keep O₂ low and with slight alkalinity to passivate copper surfaces (Power).

Where dosing is used, oxygen scavengers are added in low concentrations; a dedicated oxygen‑scavenger program can be paired with conductivity monitoring to avoid overshoot. Slight alkalinity is commonly maintained with a neutralizing amine to stabilize pH (for reference on interpreting pH vs. conductivity data in these loops see ResearchGate).

Filtration helps, too. Plants insert fine traps on the return line to catch particulates before they reach stator bars; a high‑purity cartridge filter in a low‑shedding housing limits fiber release. For materials compatibility at pharmaceutical‑grade cleanliness, 316L stainless steel housings are often selected.

Inhibitors like molybdate or nitrite are sometimes used in closed loops, though careful operators prefer minimal chemistry to avoid raising conductivity (Power). Where justified by metallurgy, plants standardize on closed‑loop inhibitors and meter them precisely with a dosing pump.

Heat exchanger interface with main cooling water

A dedicated heat exchanger couples the pure loop to the main plant cooling water circuit with two isolated circuits—no mixing—to prevent contamination. Plate‑and‑frame exchangers are often chosen for compactness and efficiency; they occupy roughly 20% of the footprint of equivalent shell‑and‑tube units and can be uprated later by adding plates (Alfa Laval).

Closed‑loop sides tend to stay clean—“no fouling on closed loop side”—and plate designs minimize hold‑up volume (Alfa Laval). Where plant water carries silt or biofouling, operators protect exchanger plates with a main‑side automatic screen filter.

By thermal design, the exchanger must reject the full heat load. Removing 2–5 MW with a ~10 °C approach (the temperature “approach” is the small difference between outlet and supply streams used in sizing) can drive several hundred m³/h on the main side, which typically has river or tower water at ~25 °C available at high flow. Designers use the log‑mean temperature difference (LMTD) method to size area, often selecting stainless or coated tubing on the main side for durability (VR Shell & Tube).

The DM side runs pressurized with moderate pressure drop—on the order of a few 0.1’s of bar—to keep flow turbulent and heat transfer high. Tube‑side metallurgy is conservative (often copper or 316SS) so that, in a worst‑case leak, ions do not flood the closed loop (Alfa Laval).

Operational and regulatory outcomes

A properly designed closed loop shows very low make‑up—typically tens of liters per day for small losses—versus thousands of cubic meters for an open once‑through system. In some applications, closed designs can eliminate makeup and discharge entirely, avoiding sampling and testing (WaterTechOnline). That aligns with analyses showing >95% reduction in makeup water for closed‑loop solutions (Alfa Laval).

Reliability follows. Because the closed loop is clean and self‑contained, scale and biofilm are minimal; only the main‑water side of the exchanger needs periodic cleaning. Continuous high purity virtually eliminates the flow restrictions that used to force turbine derates (ResearchGate). A case study reports that an open‑loop, river‑water system caused frequent clogging and shutdowns, while switching to a closed heat‑exchanger loop eliminated those failures (ResearchGate).

In Indonesia, as elsewhere, environmental rules encourage recycling and minimal discharge. Closed recirculation means essentially zero effluent from stator and oil‑cooling loops—removing the need for thermal discharge permits or wastewater testing specific to these loops (WaterTechOnline).

Design checklist and setpoints

Data‑driven best practice is clear: maintain >1 MΩ·cm DM water (conductivity <1–2 μS/cm) and continuously polish the loop (ResearchGate) (Power). Plants often backstop that with a final polisher such as a mixed‑bed resin unit on a small recirculation slipstream.

Include oxygen control and slight alkalinity; an oxygen scavenger and careful pH management via a neutralizing amine are common choices, balanced against the need to keep conductivity low (Power). Use corrosion‑resistant exchanger materials and size the cooler for up to ~5 °C approach with extra capacity (Alfa Laval).

Provide redundant pumps and instrumentation alarms on conductivity and ΔT. Consider a main‑side debris guard with an automatic screen to protect plate exchangers. In aggregate, these specifications support >95% reductions in makeup water and materially lower outage risk (Alfa Laval).

For plants expanding DM capacity or replacing aged trains, a packaged demineralizer or a brackish‑water RO for make‑up can integrate directly with the loop’s polisher to hold conductivity near the 0.1–0.3 μS/cm tier cited in generator practice (pdfcoffee.com) (ResearchGate).

Sources across peer‑reviewed and industry publications underpin this design approach, including stator water chemistry, closed‑loop treatment, and heat‑exchanger configuration (Power) (Power) (VR Shell & Tube) (Alfa Laval) (ResearchGate) (ResearchGate) (WaterTechOnline).