Control Valve Partial Stroke Testing (PST) for Safety Valves

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Practical Procedure · Safety Systems Control Valve Partial Stroke Testing (PST) for Safety Valves Applies to: Emergency shutdown (ESD) valves and safety instrumented function (SIF) final elements  |  Equipment needed: PST-capable positioner/logic solver, position feedback monitoring, coordination with process/operations Full Stroke vs. Partial Stroke — Why PST Exists Tap a tab to see the difference between a full functional test and a partial stroke test, and why PST lets you test without shutting down. Full Stroke Test Partial Stroke Test (PST) ESD Valve — Process Running 0% → 100% → 0% process fully interrupted during test ESD Valve — Process Running ...
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Zirconia Oxygen Analyzer Reading Maximum O2

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Fault Log · Field Report

Zirconia Oxygen Analyzer Reading Maximum O2 — Not the Cell, a Rusted Outlet

Plant: Cement  |  System: Zirconia oxygen analyzer, sample extraction system  |  Downtime: None

Symptom

The zirconia oxygen analyzer started reading maximum oxygen — pegged at the top of the range rather than tracking the actual process value. A reading stuck at maximum on a zirconia cell usually means the cell is seeing something close to ambient air instead of process gas, which points straight at a sample delivery problem before you even think about the analyzer itself.

Investigation

Started at the sample path, from the process side inward:

  • Probe, extractor filter, sampling line, pump — checked the full sample path from the probe through the extraction filter and sampling line, including the sample pump. Performed a leakage test across this section. All found ok.
  • Panel-side tubing and membrane filter — checked all tubing inside the panel along with the membrane filter and flow meter for any leakage point. No leaks found anywhere in the panel-side path.
  • Analyzer module itself — checked the zirconia cell operating temperature. Found stable at 800°C, which is correct and ruled out a heater or temperature control problem.

With the sample path, panel tubing, and cell temperature all checked clean, the next suspicion was that the zirconia cell itself was internally damaged — a real possibility once everything upstream checks out ok. But before jumping to cell replacement, did one more pass: checked the inlet and outlet tubing and fittings directly at the zirconia module itself, rather than just the panel tubing leading up to it.

That's where the fault was found — heavy rust on both the inlet and outlet sides of the module, especially concentrated at the outlet drain point.

Root Cause

Rust buildup at the module's inlet/outlet fittings, worst at the outlet drain point, was restricting the sample gas from flowing properly through the zirconia module. With sample flow effectively blocked, the cell was seeing something closer to ambient air rather than actual process gas — producing a maximum O2 reading that looked exactly like a failed cell, even though the cell itself was fine.

Fix

Cleaned the rust from the inlet and outlet fittings of the zirconia module, restoring proper sample flow. Reading returned to normal, tracking actual process oxygen levels.

Lesson

A maximum O2 reading on a zirconia analyzer looks like a cell failure, but a full leakage test on the sample line and panel tubing doesn't cover everything — it won't catch a restriction that's blocking flow without actually leaking. Before condemning the cell, check the fittings directly at the module itself, inlet and outlet both, especially the drain point where rust and condensate naturally collect. A flow restriction can produce the exact same symptom as a damaged cell, and it's a far cheaper fix.

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Chased a maximum O2 reading back to a flow restriction instead of the cell? Drop a comment — worth comparing notes.

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