Module 6.1 — What "steam purity" actually means, why turbines are uniquely sensitive to it, and how purity is specified and measured.
Click any stage to see its role. Once steam leaves the boiler, there's no further opportunity to remove impurities before it reaches the turbine.
Everything covered in Tracks 1–4 concerned liquid water somewhere in the cycle. Steam is a different phase entirely, and the turbine that consumes it is a different kind of equipment than anything covered so far — precision-machined blades operating at high velocity and tight tolerances, rather than tubes carrying flowing water. Steam purity is the connecting concept between boiler water chemistry and turbine health, and it deserves dedicated treatment because the consequences of getting it wrong show up somewhere entirely different (turbine efficiency and blade condition) from where the chemistry problem originates (the boiler drum).
Steam purity refers to the concentration of dissolved and suspended impurities present in steam as it leaves the boiler — ideally as close to zero as achievable, since steam is meant to be essentially pure water vapor. Any measurable impurity in steam got there one of two ways, covered in depth in Module 6.2: mechanical carryover (physical transport of boiler water droplets into the steam) or vaporous carryover (certain compounds, notably silica, that can actually dissolve into steam itself under the right conditions, covered in Module 6.3).
Turbine blades are precision aerodynamic components, shaped to extract mechanical energy from expanding steam as efficiently as possible. Unlike a boiler tube, which primarily needs to survive corrosion, a turbine blade also needs to maintain its exact aerodynamic profile — a deposit that would be a minor corrosion concern elsewhere in the cycle can measurably degrade turbine performance simply by changing blade geometry, independent of any corrosion damage the deposit might also cause.
Steam purity is most commonly specified and monitored via cation conductivity of the steam sample (the same core measurement introduced in Module 1.1 and used throughout Tracks 1–3, applied here to a steam sample) and sodium concentration, since sodium is a reliable, easily measured indicator of overall carryover — if sodium is escaping into steam, other boiler water contaminants are very likely escaping too. Typical steam purity targets are extremely tight, often specified in the low single-digit parts-per-billion range for key impurities, reflecting how much more sensitive turbine components are to contamination than boiler tubes.
Every module in Tracks 1 and 2 that discussed TDS control, blowdown, and treatment program targets was, in part, protecting steam purity as much as it was protecting the boiler itself. A boiler running with high TDS due to inadequate blowdown (Module 2.5) isn't just risking scale and corrosion inside the boiler — it's also raising the baseline risk of carryover into steam, extending the consequences of a boiler-water-side chemistry lapse all the way to the turbine, potentially hundreds of feet away and operating under completely different physical conditions.