Turbine Deposition — Mechanisms & Locations

Steam Doesn't Deposit Everything at Once

Modules 6.2 and 6.3 explained how contaminants get into steam. This module explains what happens next: as steam travels through the turbine, expanding and cooling across many stages, different contaminants deposit at different points, driven by different physical triggers. Understanding deposition location is diagnostically useful — where a deposit is found often points directly back toward which contaminant and which upstream mechanism caused it.

High-Pressure Stage Deposition — Silica

Recall from Module 6.3 that silica solubility in steam drops as pressure and temperature fall. This means silica, carried into the turbine via vaporous carryover at the boiler's high pressure and temperature, becomes progressively less soluble as steam expands through the early, high-pressure turbine stages — and it deposits early, often in the first several stages, precisely where that pressure/temperature drop first becomes significant. This is why silica deposition is characteristically an early-stage (HP turbine section) phenomenon rather than something found predominantly in later stages.

The Wilson Line and Moisture Formation

As steam continues expanding through mid and low-pressure turbine stages, it eventually reaches a point — the Wilson line — where it transitions from superheated (dry) steam into the wet steam region, meaning fine moisture droplets begin forming within the steam flow. This transition point matters enormously for deposition, because dissolved and suspended contaminants that were relatively evenly distributed throughout the vapor can now concentrate within these newly formed moisture droplets, similar in principle to how evaporative concentration works in a cooling tower (Module 5.1) — the liquid phase gets a disproportionate share of whatever solids are present.

Copper Redeposition — Revisiting Module 3.5

Module 3.5 introduced copper transport: copper alloy components elsewhere in the feedwater train corrode at a low background rate even under well-controlled AVT(R) chemistry, releasing dissolved/particulate copper that travels forward through the cycle. Copper's solubility behavior in steam changes with the same expansion conditions driving moisture formation, and copper commonly redeposits in mid-to-late turbine stages, contributing to the aerodynamic and mechanical performance issues covered in Module 6.5. This is the direct payoff of the copper transport concept introduced back in Track 3 — the turbine is very often where transported copper actually ends up.

Using Deposit Location as a Diagnostic Clue

• Deposits concentrated in early, high-pressure stages point toward silica and vaporous carryover (Module 6.3) as the likely primary contributor.
• Deposits concentrated in mid-to-late, wetter stages point toward copper redeposition (Module 3.5) or moisture-concentrated salts as more likely contributors.
• A turbine inspection finding deposits across a broad range of stages, rather than concentrated at one location, suggests a more general mechanical carryover problem (Module 6.2) affecting overall steam purity rather than one specific vaporous or transport mechanism.