Case Study – Zero Shift After Maintenance

Real field troubleshooting example from a refinery shutdown activity.

1️⃣ Background

During a scheduled maintenance shutdown, a DP transmitter used for level measurement in a closed vessel was removed for inspection and reinstalled.

After startup, the transmitter showed a constant +8% reading even though the vessel was empty.

2️⃣ Initial Observations

• Process confirmed empty
• Manifold valves in correct position
• Reading stable but offset
• No active alarm in transmitter

3️⃣ Diagnostic Steps Taken

1. Closed HP & LP isolation valves
2. Opened equalizing valve
3. Observed reading did NOT return to zero
4. Checked calibration – within tolerance

This confirmed the transmitter electronics were healthy.

4️⃣ Root Cause Investigation

• Impulse lines disturbed during removal
• HP leg contained trapped air pocket
• LP leg partially filled with condensate
• Unequal hydrostatic head created false DP

Even a small liquid column difference caused measurable offset.

5️⃣ Corrective Action

1. Both impulse lines fully drained
2. Vent points opened to remove air
3. Impulse routing inspected for slope
4. System re-equalized
5. Reading returned to 0%

6️⃣ Technical Lesson

• Mechanical imbalance can mimic calibration error
• Zero shift after maintenance is common
• Always verify impulse condition before trimming zero
• Never recalibrate blindly

Related Technical Pages

• Zero Shift After Shutdown – Root Causes
• Pressure Calibration – Field Procedure
• Pressure Troubleshooting – Decision Tree