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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Common Pressure Measurement Faults
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Part 2, Chapter 6 — Common Pressure Measurement Faults
Part 2, Chapter 6 · Process Instrumentation Mastery
Common Pressure Measurement Faults
A frozen reading, a fixed offset, a slow creep, and a sudden permanent shift are four genuinely different symptoms — and Part 2 already explained the mechanism behind every one of them.
Interactive · Match the Symptom to the Fault
Every fault in this chapter connects back to a mechanism already covered in this Part: trapped gas and plugging (Chapter 5), zero elevation (Chapter 4), and the general calibration discipline running throughout. Distinguishing symptom PATTERNS, not just possible causes, is the actual troubleshooting skill.
Symptom Pattern
Likely Fault
Completely frozen, unresponsive reading
Plugged impulse line or fully plugged snubber (Chapter 5)
Erratic or offset, varies with process conditions
Trapped gas in a liquid-filled line (Chapter 5)
Fixed, stable, consistent offset from commissioning
Improperly set zero elevation/suppression (Chapter 4)
Slow, gradual creep over months
Ordinary zero/calibration drift (Chapter 4)
Sudden shift immediately after a pressure transient
Overpressure damage to the sensing element
Why the Timing and Pattern Matter More Than the Symptom Alone
"The reading is wrong" describes almost every fault in this chapter — genuinely useless on its own. WHEN the error appeared (immediately after commissioning vs. gradually over months vs. suddenly after an event) and HOW it behaves (frozen vs. erratic vs. a stable fixed offset) narrow the diagnosis dramatically, exactly the same discipline as Part 1's temperature fault chapter and Fault Finder's Field Book's universal troubleshooting sequence.
Overpressure: A Discrete Event, Not a Gradual Process
Exceeding a transmitter's rated range — from a water hammer event, a blocked-in line heating up, or a process upset — can permanently damage the sensing element, shifting its zero and degrading accuracy afterward. This is genuinely distinct from every other fault in this chapter: it's a single event causing lasting damage, not an installation issue or gradual drift, and the timing correlation with a known transient is the diagnostic giveaway.
Field Practice
A genuine field test to confirm a plugged impulse line (vs. a calibration or electronics fault): with the manifold in isolate mode, open the equalizing valve and vent briefly, then close everything and re-open the block valve slowly. A healthy, unplugged line will show the transmitter respond promptly as pressure re-establishes. A plugged line will show little or no response, or a very slow, sluggish return — confirming the blockage is between the process and the transmitter, not inside the transmitter itself.
A genuine leak-check procedure for a suspected fitting leak: apply a soap-and-water solution (or a commercial leak-detection fluid) to suspect fittings while the line is pressurized. Bubbling confirms a leak at that specific point — this is faster and safer than incrementally tightening fittings on a live line and hoping the symptom goes away.
Before condemning a transmitter as "overpressure damaged," check whether anything genuinely capable of an overpressure event happened recently — a valve lineup change, a pump start, a blocked-in line left in the sun. If nothing like that occurred, the zero shift you're seeing is more likely ordinary drift (Part 5) than overpressure damage, and the fix is recalibration, not replacement.
Check Your Understanding
Why does a completely frozen, unresponsive reading point specifically toward a plugged impulse line or snubber, rather than a calibration issue?
Answer: A calibration or drift issue still allows the transmitter to respond to genuine process changes, just with an error in the reported value — a complete lack of response to known process changes means the transmitter is physically isolated from the actual pressure, which is what a full blockage does.
Why is a fixed, stable offset present since commissioning a different diagnosis than a slowly drifting reading?
Answer: A stable offset present from the very start points to something in the original configuration (like an uncorrected zero elevation) — a slow drift developing over time instead points to gradual physical or electronic changes in the instrument itself, a genuinely different root cause requiring a different fix.
CHAPTER RECAP
Every pressure fault in this chapter connects back to a mechanism already covered earlier in Part 2
Timing (when the error appeared) and pattern (frozen, erratic, fixed offset, sudden shift) narrow diagnosis dramatically
A frozen reading signals physical isolation from the process (plugging); a stable offset since commissioning signals a configuration issue (zero elevation)
Overpressure damage is a discrete event causing lasting sensor damage, distinct from gradual drift or installation issues
✓ Part 2 Complete — Pressure Measurement Next up: Part 3 — Flow Measurement
Fault Log · Field Report Schenck Weigh Feeder — Zero and Span Drift, Two Different Root Causes Plant: Cement | System: Schenck weigh feeder, belt scale, hopper load cells | Downtime: None Symptom A Schenck weigh feeder started showing both zero drift and span drift at the same time. Two separate accuracy problems showing up together made this a two-part investigation from the start — zero and span errors don't always share the same root cause, even when they appear together. Investigation — Zero Drift Started with the load cell loop and mechanical side of the belt scale: Load cell loop — checked the full loop, cleaned the load cell area. Idlers/rollers — checked the rollers for wear or misalignment. Belt tracking — with the belt empty, noticed it was drifting to one side. Zero was being tared in that empty, drifted position. Once material loaded onto the belt, the belt tracked back closer to center — meaning the zero reference had been tak...
Practical Procedure · Calibration Load Cell Calibration for Weigh Feeders — Dead-Load, Test-Weight, and Why Shunt Calibration Isn't Enough Applies to: Belt weigh feeders, weigh hoppers with load cell-based measurement | Equipment needed: Certified test weights or a dead-load/material reference, shunt calibration resistor (if supported), digital indicator/weigh controller access Before You Start Confirm the feeder/hopper is mechanically free — check idlers, belt tension, and load cell mounting for anything that could add friction or false load before assuming a load cell problem. Check load cell cabling and junction box connections for corrosion or loose terminations, especially in dusty or humid plant environments. Note whether the system uses a single load cell or multiple load cells summed together — multi-cell systems need each cell checked individually, not just the combined output. Method 1 — Shunt Calibration Shunt calibration simulates a known load...
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...
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