1. Normal Operating Principle 💡

• Low-pressure side (L):Taps vapor-phase pressure from the tank’s upper space (approximately atmospheric pressure for open-top tanks).
• High-pressure side (H):Taps bottom pressure = Vapor-phase pressure + Hydrostatic liquid column pressure.
• Transmitter measured differential pressure: ΔP=PH​−PL​
• With stable medium density & unobstructed impulse piping and proper vapor pressure compensation: ΔP≈ρgh
• Differential pressure varies synchronously with liquid level height.

Key logic: The DP value directly reflects liquid level via liquid column gravity.

2. Consequences of Clogged Impulse Lines ⚠️

Case A: High-pressure side impulse line clogged

• High-side pressure is trapped/locked and cannot transmit real-time process pressure.
• Indication sticks/lags and fails to follow actual liquid level fluctuation.
• Under steady vapor pressure: Indication reads low when level rises; reads high when level drops.
• Under fluctuating vapor pressure: Erratic reading swing & distorted trending may occur.
• Severe clogging leads to false level reading & wrong operational judgment.

Case B: Low-pressure side impulse line clogged

• Abnormal vapor pressure feedback to low-pressure chamber, causing sluggish & laggy indication (worse under fluctuating headspace pressure).
• When vapor pressure rises: Level reading tends to be falsely high; when vapor pressure falls: Reading tends to be falsely low.
• Deviation range depends on tank type & actual process conditions.

Case C: Both sides fully / partially clogged

• Fixed stuck readings, erratic trending with large deviation against field actual level.
• Distorted historical curve, highest risk of misleading operator decisions.

Warning: Once impulse line clogs, differential pressure can no longer reflect real liquid level changes.

3. Root Causes for Severe Measurement Drift 📌

Differential pressure level measurement relies on instant pressure transmission via impulse piping; clogging breaks pressure transfer and generates false DP readings, potentially triggering false level indication, abnormal trending & spurious process alarms.

Four Common Clogging Causes

1. Crystalline precipitation & sediment buildup inside piping
2. Rust particles & solid contaminants from process tank
3. Viscous medium adhesion & fouling on pipe inner wall
4. Medium solidification under low ambient temperature (caused by failed heat tracing)

4. On-site Remedial Actions & Preventive Measures ✅

Field Troubleshooting Steps

1. Cross-check: Compare DCS trending, local sight glass gauge and actual tank level for fault confirmation.
2. Inspect heat tracing, thermal insulation and root valve status.
3. Isolate equipment per procedure → Depressurize → Confirm medium hazard before purging/blowdown.
4. Prohibit hot work / piping disassembly under pressurized condition.
5. Calibrate transmitter after pipeline unclogging if required.

Preventive Optimization

• Impulse piping design: Shorten pipe run, minimize elbow fittings & eliminate dead legs.
• Regular routine blowdown & inspection for fouling-prone service; maintain heat tracing system periodically.