Float-drum level gauge working principle and structure
Principle and Structure
The float-drum level gauge consists of three parts: detection, conversion, and transmission. The detection part is composed of the float and the linkage. The conversion part includes the lever, torque tube assembly, and sensor. The transmission part includes the CPU, A/D, D/A and an LCD display.
The float is immersed in the liquid inside the outer float chamber and is rigidly connected to the torque tube system. Changes in the liquid level or interface position inside the outer chamber change the buoyant force on the immersed float, which in turn changes the torque tube twist angle.
When the liquid level rises, the buoyant force on the float increases, the torque applied to the torque tube decreases, and the twist angle becomes smaller; the opposite occurs when the level falls. The change in twist angle is transmitted to the sensor rigidly connected to the torque tube. The sensor output voltage changes accordingly and is amplified and converted to a 4-20 mA current output.
Fault Diagnosis and Handling
(1) No level display or display shows minimum
This fault refers to the process liquid being at a normal level while the instrument shows no reading, shows the minimum value, or even a negative value.
Perform a drain/flush to check whether the sampling valve or sampling piping has scale accumulation or blockage. Blockages can be cleared by cleaning or blowing; severely blocked or leaking sampling valves must be replaced. Inspect the interior of the outer float chamber: a cracked float or material adhering to the float can cause the level reading to be low or negative.
If the transmitter connection circuit is open, power supply abnormal, or the transmitter amplifier/display board is damaged, the transmitter may show no display or reduced output current, causing display and output mismatch. Replacing a circuit board requires reconfiguring parameters. If the transmitter has no current output, check wiring connections and whether the LCD head shows a display. If there is a display but no output current, the output transistor may be damaged; replacing the board can confirm this. EEPROM corruption can cause loss of instrument calibration data and also result in no current output.
(2) Level displays maximum
Check in the sequence of mechanical first, then electrical. Corrosion, crystallization, or deposit buildup from the process medium; large density changes in the medium; the float being stuck; float detachment; or incorrect vertical installation can all cause the level to display maximum. Mechanical parts that contact the process medium directly tend to have higher failure rates than electrical parts.
If the float is stuck, remove and clean the adhered material. If the float has detached, reinstall the float and perform calibration. If the process medium density has changed significantly or the medium temperature exceeds the design value, recalculate and recalibrate the instrument to the new range. Once mechanical issues are ruled out, check the transmitter power, zero and span settings, and whether zero drift or offset exists. Verify span settings and measure transmitter output current to evaluate the transmitter and any safety barrier.
(3) Level reads high or low
If the level reading has an offset, use the handheld communicator to check whether the transmitter parameters are correct. Many display deviations are related to the measured medium: large differences between actual medium density and the design/set value will cause inaccurate readings.
Some media such as certain gases or gasoline may contain sulfur or other substances that crystallize or form lumps on the float suspension rod, causing measurement errors. Signal wiring issues can cause the DCS to display levels higher or lower than the local indicator. Often this is due to reduced insulation resistance to ground from water ingress at terminal blocks or junction boxes, or short circuits between signal lines. Signal shunting can make the DCS read lower than the field instrument; ground current interference can make the DCS read higher. These issues often appear during rainy seasons or after cleaning. Ensure terminal boxes and junction boxes are properly sealed to prevent water ingress; temporary measures include wrapping with plastic or sealing with explosion-proof putty.
(4) Level display fluctuates
Observe the historical trend of the measured level to determine the nature of the fluctuation. Slow fluctuations may result from process fluctuations or mechanical faults of the float. The float immersed in the medium exhibits inertia and damping, so changes are not usually instantaneous.
Large or sudden fluctuations are often due to circuit or signal wiring problems, such as loose transmitter connections. Measure the resistance of wire sections to identify issues, and check whether the instrument is subject to electromagnetic interference.
If the process level frequently fluctuates, increase damping time and filtering to compensate. For large process fluctuations, consider installing a stilling tube. Understand the properties of the measured medium: for example, a condenser measured with a float may show frequent fluctuation if the refrigerant contains many bubbles, which cause float movement. When diagnosing and handling faults, consider both instrument and process factors.
Design or selection errors, incorrect float buoyancy calculation, poor installation position, measured medium properties differing from design values, or excessive pressure and flow fluctuations in the process can all cause level display instability. Check the transmitter measurement loop for unstable output current and ensure terminal voltages are stable. Verify wiring and grounding for intermittent connections. Use a handheld communicator to force the transmitter to output fixed currents (4 mA or 20 mA) to determine whether the transmitter or safety barrier is faulty and then address accordingly. Mechanical faults, such as unstable torque tube behavior or damaged float hooks, can lead to unstable output current and poor linearity; mechanical faults require disassembly and inspection.
(5) Slow response to level changes
If the instrument display changes more slowly than the actual process level, perform a drain/flush and inspect sampling valves and piping for blockages. Slow response is often caused by material adhering to the float or friction between the float and outer sleeve. Use periodic steam blowing, or add tracing to the outer sleeve to mitigate this.
Blocked gas or liquid sampling lines or sampling valves, especially blocked gas-phase lines, can cause pressure imbalance between the measurement chamber and the vessel top, creating trapped pressure above the float that slows float movement and delays the displayed response. If the sampling valve is opened too little, the level change will be slow and deviate from the actual level; this is more pronounced when gas-phase lines are blocked.
(6) No change in level display
If the process level is changing but the instrument display remains constant for a long time and the DCS trend is a straight line, use drain/flush to locate the issue. While draining, tapping the outer float chamber may free a stuck float and restore normal operation. If the mechanical parts are fine, inspect the transmitter circuitry: display or amplifier board faults can be diagnosed by swapping with spare boards. Replacing circuit boards requires re-entering parameters and performing linear adjustment.
Blocked gas or liquid sampling lines or sampling valves with insufficient opening will cause the measured level to remain unchanged for long periods, producing a straight trend line. Liquid-phase sampling lines are especially prone to blockage from pipeline impurities, and longer lines increase blockage probability. Blocked gas-phase lines or narrowly opened sampling valves cause slow level changes and display deviation.
Media that easily crystallize, or temperature and pressure changes that induce crystallization, can cause deposits on the float, torque tube, or hook, leading to no change in level display. These faults typically require disassembly and repair. Mitigating measures include insulating the outer float chamber to reduce external temperature effects and prevent internal crystallization, or using steam or hot-air purging to reduce deposits. If crystallization persists despite these measures, consider alternative measurement methods.
Maintenance Examples
(1) No display or minimum
Example 1: Condenser level had no display.
Fault inspection: After performing a drain/flush, only a small amount of condensate flowed out, indicating float blockage.
Fault handling: After removing the outer float chamber, the float was almost completely filled with debris. Cleaning and recalibration restored the instrument to normal operation.
[Maintenance summary] This case was a process-related blockage caused by failure to perform routine cleaning and debris removal, resulting in heavy deposits that blocked the float.
(2) Level displays maximum
Example 2: Sulfur unit stripper tower level showed 60% on the glass gauge but 100% on the DCS.
Fault inspection: The glass gauge on site was normal. A drain/flush revealed debris blockage.
Fault handling: With the unit shut down, debris was removed from inside the float. After restarting, the level reading returned to normal.
[Maintenance summary] Debris inside the chamber had jammed the float at the 100% position, causing maximum output current. The most effective way to check whether the float is jammed is to drain: close the sampling valve connecting the float to the equipment, open the drain valve, and if the instrument returns to zero, the float is not jammed; if it remains at 100%, the float is likely jammed. For media prone to crystallization or blockage, first verify the local gauge is not blocked before troubleshooting the transmitter.
(3) Level reads high
Example 3: Process reported the DLC3000 float gauge reading high.
Fault inspection: Two-point calibration with a 475 handheld communicator showed no improvement. Analysis suggested the torque tube rigidity may have changed.
Fault handling: Recalibrate the dry coupling point using the following steps:
- Open the slider below the indicator head to expose the hex nut that locks the torque rod. Position the float at the lowest level (heaviest state). Use a socket wrench through the lock hole to tighten the nut, then return the slider to its original position.
- Enter the online menu and select Basic Setup → Sensor Calibrate → Mark Dry Coupling. After marking the dry coupling point, the instrument display should be essentially at zero. If deviation remains large, check Level Offset under PV Setup and repeat if necessary.
- Perform Two-Point calibration to complete the level calibration. The instrument returned to normal.
[Maintenance summary] The instrument had been in service for more than eight years, so a change in torque tube rigidity is plausible. Marking the dry coupling point ensures the torque tube operates within the intended range. For newly installed or replaced instruments, verify settings on site with a handheld communicator, ideally with two people to reduce errors.
(4) Level fluctuation
Example 4: Boiler steam drum level showed small fluctuations at low load and large fluctuations at high load.
Fault inspection: Steam supply pressure was generally stable. The float and transmitter checked fine, but the liquid-phase sampling valve was not fully open.
Fault handling: Fully opening the liquid-phase valve stabilized the level.
[Maintenance summary] With the liquid-phase valve partially closed, the liquid flow into the level gauge was limited while the gas-phase valve remained fully open, causing steam to reheat the water in the gauge and produce apparent high level. When boiler load decreased, steam pressure rose and the gauge water level dropped, producing repeated fluctuations. According to procedures, the gas and liquid valves for boiler water-level measurement must be fully open; improper operation leads to avoidable maintenance issues.
Example 5: Ammonia separator level control system showed level fluctuation.
Fault inspection: The level transmitter and related wiring showed no anomalies, so a drain/flush was performed.
Fault handling: After draining and cleaning the float, level control returned to normal.
[Maintenance summary] During startup, compressors carried oil that accumulated in the float and formed sludge at low temperatures, making the float sluggish and causing delayed level signals and large fluctuations.
Example 6: LT205 reported large level fluctuations.
Fault inspection: After draining to confirm the float sampling line was clear, a metal rod was inserted through the lower drain to touch and move the float; the indicator changed only slightly.
Fault handling: The float was removed, zero and span adjusted, but output current hardly changed. Further inspection found the torque tube fixing bolts were loose and the torque tube had shifted. Restoring the torque tube position and tightening bolts returned the instrument to normal.
[Maintenance summary] During calibration, minimal output change suggested a torque tube issue. Normally, the torque tube has initial torsion to keep the float linkage in a free state to sense small buoyancy changes. When the torque tube fixing bolts are loose, the initial torsion becomes zero and the linkage movement is not detected, resulting in negligible output change during calibration.
(7) Slow response to level changes
Example 7: The local glass gauge on the mother liquor tank showed changes, but the remote transmitted level remained unchanged for a long time.
Fault inspection: A blocked sampling line was suspected; on-site inspection confirmed the liquid-phase line was obstructed.
Fault handling: Close the sampling valve, clear the sampling line, and the instrument display returned to normal.
[Maintenance summary] If the liquid-phase sampling line is blocked, the outer float chamber cannot be replenished with liquid, so its level does not change and the transmitter output remains unchanged.
Example 8–35: A condenser level remained unchanged.
Fault inspection: Power and transmitter checks were normal. On-site draining produced slight display change; disassembly revealed the float had been repeatedly stuck.
Fault handling: During shutdown maintenance, the level gauge was reinstalled correctly. The issue did not recur for a year.
[Maintenance summary] This fault had recurred several times. The initial installation did not meet verticality requirements and was accepted without rework, leaving a latent defect. Reinstallation resolved the problem.
(8) Level display stuck at a fixed value
Example 8: A gasoline separation tank LT-106 displayed 25% and did not change.
Fault inspection: Suspecting excessive debris accumulation at the tank bottom, cleaning was performed. The problem recurred after one week. After another drain and clean, the display remained at 25% and water calibration produced no change; probing the float with a wire produced display variation.
Fault handling: Repeatedly flushing the float interior with water and recalibrating restored normal operation.
[Maintenance summary] The float interior was so dirty that the float adhered to the wall and did not move with level changes. This case highlights the importance of routine drain/flush maintenance to ensure normal operation of float-drum level gauges.
