Steam Purity Troubleshooting

The Setup

Unit 5, a high-pressure drum boiler, has shown a 1.8% heat rate degradation over six months along with a slight increase in HP turbine vibration. A full review of steam purity trending shows cation conductivity and sodium — the general mechanical carryover indicators from Module 6.2 — essentially unchanged from six months ago. Steam silica, by contrast, has roughly tripled, climbing from 9 ppb to 28 ppb, tracking a corresponding rise in boiler water silica. The plant's demineralizer resin bed is noted as approaching scheduled change-out.

Working the Diagnosis Using Module 6.2's Framework

Tracing the Root Cause Further Upstream

The demineralizer resin bed approaching change-out is the strongest lead. Recall from Module 1.3 that demineralization is the makeup water treatment step responsible for silica removal, and from Module 6.3 that a demin system nearing exhaustion can let silica through even while other parameters look acceptable — precisely because silica is often one of the first species to break through as resin capacity declines, before more common ions like sodium show comparable breakthrough. This would explain why sodium and general conductivity remain normal while silica specifically climbs: the demin system isn't failing broadly, it's specifically losing its silica removal margin as the resin approaches end of life.

Connecting to the Turbine Findings

Applying Module 6.4's location logic: silica-driven deposition is expected in early, high-pressure turbine stages, precisely matching the HP turbine vibration trend noted in this scenario (rather than a low-pressure or wet-stage vibration signature, which would point more toward copper or moisture-concentrated salts). Applying Module 6.5's consequence framework: a hard, glassy silica deposit altering blade aerodynamic profile is a well-matched explanation for measurable heat rate degradation, and if that deposit isn't forming with perfect uniformity across all blades in the affected stages, it's also a plausible driver of the vibration increase.

Verifying Before Committing to the Fix

1. Confirm the steam sample system's isokinetic performance (Module 6.6) hasn't drifted, ruling out a sampling artifact before committing resources to a demin resin change-out and eventual turbine inspection.
2. Review demin resin capacity/breakthrough data directly, not just its scheduled age, to confirm silica breakthrough is the actual mechanism rather than assumed from timing alone.
3. Schedule demin resin change-out and monitor boiler water and steam silica for recovery afterward.
4. Plan HP turbine inspection at the next available outage opportunity to confirm and address any silica deposit already accumulated, since correcting the chemistry doesn't remove deposits already formed — the same lesson from Module 5.7's cooling water scenario applies here.