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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Differential Pressure Flow Transmitter Calibration — Wet Leg vs Dry Leg

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Practical Procedure · Calibration

Differential Pressure Flow Transmitter Calibration — Wet Leg vs Dry Leg

Applies to: Orifice plate, venturi, and other DP-based flow measurement using a differential pressure transmitter | Equipment needed: DP calibrator/pump (or two-point pressure source), HART communicator (if applicable), reference for leg fluid density (wet leg installations)

Before You Start

  • Confirm whether this is a dry leg or wet leg installation — this changes your entire approach, so check the P&ID or physically trace the impulse lines before touching anything.
  • Isolate the transmitter from the process using the 3-valve or 5-valve manifold, following your plant's isolation procedure.
  • Note the configured range from the nameplate or configuration (e.g., 0–500 mmH2O = 4–20 mA) and the square-root extraction setting if flow is derived from DP.
DP Flow Measurement: The Orifice Plate Principle
Flow speeds up through the restriction, creating a measurable pressure drop
DP
Transmitter
High side
Low side
The orifice plate forces fluid through a smaller opening, causing velocity to increase and pressure to drop on the downstream side. The transmitter measures this pressure difference (high side minus low side) and, after square-root extraction, converts it into a flow rate. This is why the "high" and "low" tap locations — and keeping their impulse lines correctly filled (wet leg) or dry — matter so much to calibration accuracy.

Step 1 — As-Found Check

  • With both isolation valves closed and the equalizing valve open, verify the transmitter reads zero DP (this is your zero reference before doing anything else).
  • Apply DP at 0%, 25%, 50%, 75%, and 100% of range using the calibrator, and record the transmitter's actual output at each point before adjusting anything.
  • Compare against tolerance — if all points are within spec, document and skip adjustment; if not, proceed to Step 2.

Step 2 — Zero Adjustment (Critical for Wet Leg)

  • For a wet leg installation, the static head of the fill fluid in the impulse line creates a constant offset — this must be zeroed out with the process in its normal filled condition, not with the lines dry.
  • For a dry leg installation, zero can typically be done with both impulse lines at atmospheric/equal condition without needing to account for fluid head.
  • Adjust the zero setting until the transmitter reads exactly 0% output with zero differential applied under the correct leg condition.

Step 3 — Span Calibration

  • Apply DP at 100% of range using the calibrator.
  • Adjust the span setting until the transmitter outputs exactly 20 mA (or 100% of its configured digital range).
  • Re-check zero after span adjustment — the two interact, so a final zero-span-zero pass is standard practice.

Step 4 — Linearity Check

  • Re-apply DP at 25%, 50%, and 75% and confirm the output tracks correctly at each point — a transmitter can be perfect at zero and span but still non-linear in between.
  • If linearity is off but zero/span are correct, this usually points to a transmitter capsule issue rather than a simple calibration error.
Wet leg maintenance note: Wet leg fill fluid can evaporate, leak, or change density over time (especially with temperature swings), silently shifting your zero without any alarm. If a DP transmitter on a wet leg system drifts repeatedly at the same rate, check the fill fluid level and condition before assuming the transmitter itself is failing.

Common Mistakes to Avoid

  • Zeroing a wet leg transmitter with the legs empty/dry — this bakes in a permanent offset error once the process fills the legs again.
  • Forgetting the square-root extraction when checking flow values against DP calibration points — a DP calibration point doesn't map linearly to a flow value.
  • Not re-checking zero after span adjustment.
  • Ignoring impulse line blockage or partial plugging as a root cause when a transmitter reads correctly on the bench but wrong when reconnected to process.

This is a general field procedure — specifics vary by transmitter brand and plant calibration standards. Adjust to your own SOP where needed.

📘 For complete books related to these topics: • Electrical Fundamentals for Beginners — 30-Lesson Animation-Driven Training Course • Industrial Instrumentation Fundamentals — 30-Lesson Practical Training Course • Intermediate/Advanced Electrical Theory — 31-Lesson Professional Training Course • LADDER LOGIC FOR PLANT TECHNICIANS • PLC Fundamentals for Plant Technicians

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