Quick answer
Condensation in a kiln/preheater gas-analyzer sample line can create false or unstable O2 and CO readings by absorbing soluble gas species, blocking or diluting the sample, changing flow and damaging filters or analyzer conditioning components. Diagnose sample-line temperature, heated-line continuity, probe/filter condition, sample flow, cooler/conditioning performance, drain traps and response to calibration gas before changing kiln combustion settings.
Problem / job
O2 or CO readings drift, lag or spike without matching process evidence, especially during cool ambient conditions or after sample-system maintenance.
Evidence
Capture sample-line temperature, sample flow/pressure, analyzer response time, condensate evidence, filter condition, calibration response and nearby process indicators.
Decision
Separate sample-conditioning error from real combustion change, probe plugging, analyzer calibration drift and air leakage into the sample system.
As-found evidence capture
Before changing kiln fuel or air, compare analyzer readings with independent process evidence: flame condition, draft, secondary measurements and historical correlation. Record sample flow, pump status, heated-line temperature, conditioner/cooler temperatures and any visible liquid in traps, tubing or filter bowls.
Note whether the problem is worse during startup, cold weather, after a long unheated shutdown or after maintenance on heated hose, probe or conditioner. Condensation faults often follow a temperature-control or drainage change.
Seven-step diagnostic workflow
1. Verify calibration response
Apply the approved zero/span or calibration-gas check according to the analyzer procedure. If the analyzer itself responds correctly but process gas does not, focus on the extraction/conditioning path.
2. Check heated-line temperature end to end
Verify heater power, controller setpoint, sensor location and continuity along the entire transport path. A short unheated section, cold fitting or failed heat-trace zone can become the condensation point.
3. Inspect for liquid and blocked drains
Check condensate traps, drain pots and conditioning hardware. Liquid accumulation can restrict flow or intermittently release, causing sudden analyzer changes. Service drains using the system procedure.
4. Verify sample flow and pressure
Low or unstable sample flow increases residence time and cooling risk. Inspect probe/filter restriction, pump performance, rotameter/flow indication and leaks. Compare response time with the healthy baseline.
5. Check probe and filters
Dust plugging can reduce flow and create a cold/slow sample even when the line heater works. Inspect filters and probe condition before increasing pump suction or purge pressure.
6. Check for false air into the sample system
Loose fittings or damaged tubing can pull ambient air into a negative-pressure sample line, falsely increasing O2 and diluting CO. Pressure/leak testing should follow the analyzer supplier procedure.
7. Prove process readings after correction
Once the line is dry, heated and flowing correctly, repeat calibration and compare analyzer response with process changes. Confirm O2/CO trends are plausible and response time has returned to baseline before operators retune combustion.
Evidence-to-decision table
| Evidence | Likely direction | Next check |
|---|---|---|
| Analyzer calibrates correctly but process reading drifts | Sample path/conditioning issue | Heat, flow, filters, leaks |
| Liquid visible in trap/line | Condensation/drain problem | Heated line and drainage |
| High O2 with weak/low CO and low sample flow | Air ingress or dilution | Fittings, tubing, pump suction |
| Reading recovers when line reheats | Temperature-related condensation | Heat tracing and cold spots |
| Slow response plus high filter DP/restriction | Probe/filter plugging | Probe and filter service |
Why condensation creates misleading analyzer signals
Condensation changes both the sample composition and the way the sample moves through the conditioning train. Liquid water can collect in low points, filter bowls, cool fittings or drains, increasing resistance and slowing the arrival of fresh process gas at the analyzer. Some gas components may also be absorbed into the condensate, while a partially blocked sample path can exaggerate the effect of even a small leak. The result can be an O2 or CO value that looks smooth but no longer represents the current kiln or preheater condition.
Use response behavior as evidence. If the process changes but the analyzer reacts late, in steps, or only after a drain clears or line temperature recovers, the sampling system deserves priority over combustion adjustment. Compare the fixed analyzer with other process evidence before allowing the suspect reading to drive fuel or air changes.
Field test for a suspected cold spot or flooded section
- Record the current process reading, sample flow and response time before intervention.
- Check heated-line controller status, then verify the actual line condition at accessible points permitted by the maintenance procedure.
- Inspect traps, drains, cooler bowls and filters for liquid or moisture evidence.
- Confirm the sample pump and flow indication remain stable through the test.
- After restoring the intended heated and drained condition, repeat the same zero/span and process-response checks.
A successful repair should improve sample response without requiring operators to retune the process. If the analyzer still disagrees with process evidence after the sample line is dry, heated and flowing correctly, continue with leak testing, probe/filter checks and analyzer diagnostics rather than declaring the condensation fault solved.
Repeat-failure checks after maintenance
If condensation returns after a short period, do not keep draining the same trap without finding the thermal or drainage cause. Check whether insulation was removed, a heated hose was replaced with a different type, a fitting or valve created a cold bridge, a drain line lost its fall, or a maintenance change altered sample flow. Record the location of recurring liquid because repeated accumulation at the same point is valuable evidence of a local temperature or drainage defect.
Return-to-service acceptance criteria
Accept the analyzer system when calibration is correct, the sample path remains above its required temperature profile, sample flow is stable, drains are clear, no liquid is present and O2/CO respond plausibly to process changes with normal response time.
Common mistakes
Do not tune kiln combustion from a suspect analyzer. Do not raise sample-pump suction to overcome a blocked probe without finding the restriction. Do not ignore a short unheated fitting. Do not assume calibration proves the sample line is healthy. Finally, hot-process gas systems require proper isolation and analyzer maintenance procedures.
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