The Setup
Unit 4 is an AVT(R) once-through unit. The 0600 sample round shows dissolved oxygen at the deaerator outlet running at 18 ppb against a 7 ppb target — clearly abnormal. Every other monitored parameter — pH, cation conductivity, sodium, iron, copper — reads within target. A note on the data sheet flags the DA vent valve position at roughly 40% open, worth investigating against its normal design setting.
Running the Six-Source Checklist
Using the contamination source checklist from Module 3.6, work through each possibility against what the data actually shows:
Ruled out — condenser tube leak: The signature for this source is rising cation conductivity paired with rising sodium. Both are reading clean. This doesn't fit.
Ruled out — feedwater heater tube leak: No inventory or drain level anomaly is noted, and this source doesn't typically present as an isolated oxygen spike with everything else normal. Doesn't fit the data as given.
Ruled out — makeup water treatment upset: This source shows as a conductivity or hardness trend. Conductivity is clean. Doesn't fit.
Ruled out — copper alloy corrosion: Copper reads well within target. Doesn't fit, and there's no oxygen-correlated copper rise to explain either.
Ruled out — chemical feed error: A scavenger underfeed would be the obvious first guess for elevated oxygen from a chemistry-only standpoint, but Module 3.3 specifically warned against jumping to this conclusion before checking DA mechanical performance. Worth investigating, but not yet confirmed and not the first thing to check given what else is in the data.
Best fit — air in-leakage (Module 3.3): Isolated dissolved oxygen elevation with no conductivity signal, no sodium movement, and no other contamination indicator is exactly the air in-leakage signature described in Module 3.6's checklist. The flagged DA vent valve position adds a second, independent clue pointing the same direction — Module 3.3 specifically identified a vent valve throttled away from its design setting as a classic, easily overlooked cause of degraded deaeration.
Why This Isn't Automatically a Scavenger Problem
An 18 ppb dissolved oxygen reading might tempt an operator toward the straightforward fix: increase reducing agent feed rate to compensate. But recall Module 3.3's core lesson directly: if the underlying issue is inadequate venting or another mechanical DA performance problem, the oxygen load reaching the scavenger feed point is simply higher than the system was calibrated for. Increasing chemical feed can mask the symptom temporarily, but it doesn't fix the mechanical root cause, and dosing beyond what's actually needed carries its own considerations, as discussed in Module 2.6.
Working the Investigation
- Confirm the vent valve position against design specification — is 40% open actually correct for current load, or has it drifted/been throttled?
- Check DA operating temperature and pressure against saturation conditions for current load, per Module 3.3's discussion of subcooling risk.
- Survey known air in-leakage points — condenser vacuum system performance, low-pressure joint integrity — for any recent changes.
- Only after mechanical causes are checked, consider whether reducing agent feed rate genuinely needs adjustment to handle a confirmed, otherwise-unaddressable oxygen load.
Field note: This scenario is a direct callback to the diagnostic sequencing taught across this entire track: don't reach for the chemical fix before ruling out the mechanical cause, and use the full pattern of readings — including which parameters are conspicuously normal — to narrow down a specific, testable hypothesis rather than the first plausible-sounding explanation.
Isolated Parameter Elevation
A chemistry reading abnormal on its own while related parameters remain normal, often a key clue for narrowing down a contamination source's identity.
Root Cause vs. Symptom
The underlying mechanical or process issue (root cause) versus its observable chemistry effect (symptom); addressing only the symptom, such as increasing chemical feed, can mask but not resolve the underlying problem.
Design Setpoint (Vent Valve)
The intended operating position or flow rate for a DA vent valve under given load conditions, deviation from which can degrade deaeration performance.
Elimination Diagnosis
A troubleshooting method that systematically rules out known possible causes based on which observed parameters do and do not fit each one, narrowing toward the most consistent explanation.