Displacer level transmitters are widely used in industrial applications such as oil tanks, chemical vessels, and storage tanks because of their stable structure and reliable measurement. However, during long-term operation, users may sometimes encounter a situation where the indicated level is significantly higher than the actual liquid level.
This issue may be caused by mechanical drift, impulse line blockage, or signal transmission faults. The following real field cases illustrate the most common causes and troubleshooting methods.
1. Case 1: Torsion Tube Stiffness Change
Problem Description
Operators reported that the DLC3000 displacer level transmitter displayed a higher level than the actual process level.
Inspection
A two-point level calibration was performed using a 375 handheld communicator, but the problem remained unchanged.
After further analysis, it was suspected that the stiffness of the torsion tube had changed.
Displacer level transmitters measure liquid level based on buoyancy acting on the displacer, which produces a small angular displacement in the torsion tube. Over long periods of operation, the mechanical characteristics of the torsion tube may change slightly, leading to measurement drift.
Solution
The dry coupling point was recalibrated using the following procedure:
Push aside the slider below the transmitter head to expose the locking hole containing the hex nut that locks the torsion rod.
Place the displacer at the lowest liquid level position (maximum weight condition), insert a socket wrench into the locking hole, and tighten the nut. Then return the slider to its original position.
Enter the transmitter menu:
On line → Basic Setup → Sensor Calibrate → Mark Dry CouplingAfter marking the dry coupling point, the instrument reading should return close to zero.
If deviation remains, check PV Setup → Level Offset to ensure the offset value is zero.
Finally perform Two-Point Calibration to restore normal operation.
Maintenance Summary
This transmitter had been in service for more than 8 years, so slight mechanical changes in the torsion tube were expected. Recalibrating the dry coupling point restored the transmitter to its normal operating range.
2. Case 2: Impulse Line Blockage
Problem Description
Operators reported that the glass level gauge showed the tank level at zero, but the displacer level transmitter still indicated approximately 40% level.
Inspection
Process personnel confirmed that the tank level was indeed very low.
Reviewing the DCS historical trend showed that the transmitter level had remained stable at about 40% since 08:50 without change.
During on-site inspection, a blockage was found in the positive side impulse line.
When the impulse line becomes blocked, the liquid inside the chamber surrounding the displacer cannot reflect the real tank level. As a result, the displacer position does not change with the process level.
Solution
The impulse line was drained and flushed until it became fully clear.
After restoring normal operation, the transmitter reading returned to normal.
Maintenance Summary
This failure occurred due to insufficient routine maintenance. If regular drainage and inspection procedures are properly followed, such problems can usually be prevented.
3. Case 3: Safety Barrier Failure
Problem Description
Both the glass level gauge and displacer transmitter indicated that the liquid level was near zero, but the DCS system displayed approximately 20% level.
Inspection
Current measurements were taken before and after the safety barrier in the control cabinet.
Results:
Current before the safety barrier: 4.5 mA
Current after the safety barrier: 7.2 mA
The DCS reading corresponded to the 7.2 mA signal, indicating that the safety barrier was introducing an error.
Solution
The safety barrier was replaced.
After replacement, the DCS display returned to normal.
Maintenance Summary
Based on experience, it is rare for two independent level instruments to fail simultaneously. Therefore, when both the glass gauge and transmitter indicate normal values while the DCS reading is incorrect, the problem is usually located in the signal transmission path, such as:
Safety barriers
Signal isolators
DCS input cards
Comparing current values before and after these devices is an effective way to locate the fault.
Conclusion
A displacer level transmitter showing higher-than-actual level readings is usually caused by one of the following factors:
Mechanical drift in the torsion tube
Impulse line blockage
Faults in signal transmission components such as safety barriers
Proper troubleshooting should include checking:
Mechanical calibration of the transmitter
Condition of impulse lines and drainage systems
Signal current in the control loop
Regular maintenance and inspection can effectively prevent most of these issues and ensure accurate level measurement.