Pressure Troubleshooting – Decision Tree Master Guide

Most pressure problems in plants are mechanical — not electronic. This guide follows a structured decision tree used by experienced field technicians.

STEP 1 – Identify the Symptom

• Reading unstable?
• Zero not returning?
• Reading higher or lower than expected?
• No signal (4 mA or 0 mA)?
• Mismatch between field and DCS?

CASE A – Reading Unstable

1. Check impulse line vibration
2. Inspect for loose fittings
3. Verify manifold equalizing valve closed
4. Check for flashing or two-phase flow
5. Confirm stable process pressure

If instability follows ambient temperature change → suspect impulse lines.

CASE B – Zero Not Returning to 0%

1. Close HP & LP isolation valves
2. Open equalizing valve
3. Check if reading becomes zero

• If YES → impulse imbalance
• If NO → calibration required

Refer: Zero Shift After Shutdown

CASE C – Reading Too High or Too Low

1. Compare with local pressure gauge
2. Check impulse line plugging
3. Inspect manifold valve position
4. Verify DCS scaling (LRV/URV)
5. Check square root extraction (if flow)

CASE D – No Signal (4 mA or 0 mA)

• Verify loop power (24V DC)
• Check wiring continuity
• Inspect terminals for loose connection
• Confirm transmitter not in fail-safe mode
• Check for blown fuse in marshalling panel

CASE E – Field OK, DCS Wrong

1. Measure mA at transmitter terminals
2. Measure mA at marshalling panel
3. Check analog input scaling
4. Verify engineering units in DCS

Mechanical vs Electronic Rule

• 80% problems → impulse lines
• 10% → manifold handling
• 5% → wiring issues
• 5% → actual transmitter failure

Always inspect mechanical installation before replacing transmitter.

Golden Field Rules

• Never calibrate before equalizing
• Never open isolation valves quickly
• Always verify gauge vs transmitter
• Document valve positions before adjustment
• Think mechanically first