Analysis and Handling of 10 Common Faults in Classic Level Gauges

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Malfunction of the float level gauge in the carbon removal tower for water treatment
1. Symptoms of the malfunction: The liquid level indicator in Carbon Removal Tower No. 4 suddenly showed a decrease. When the instrument indicated a level of 60%, the glass plate level gauge used in the process indicated 80%. This gauge operates in automatic mode; due to the apparent drop in the liquid level, the control valve kept closing, resulting in a continuous rise in the actual liquid level within the system.
2. Fault analysis: (1) As the control valve was adjusted, the change in the liquid level exhibited good linearity, indicating that the control valve was functioning normally. (2) Check the process glass level gauge; if the reading matches the actual liquid level, it indicates that the process system is relatively stable. (3) Check the electrical components on the instrument panel for any damage. (Heart) Check the buoy connectors and the condition of the inner buoy. 3. Fault handling: (1) Check the electrical components on the measurement board; there is no obvious burning or damage. (2) When tested with water, it was found that the zero point and range changed significantly with each watering test; it was determined that there was a problem with the inner float. The inner float was removed for inspection, and it was found to have multiple corrosion defects that were causing leakage and water ingress. (3) Replace the internal float and re-check; it is normal. 2. Malfunction of the float level gauge for the coking seal oil tank
1. Symptoms of the malfunction: During normal regulation of the liquid level in the seal oil tank, the float level gauge consistently indicated a value of 83% without any change. Since this gauge has an adjustment function for the liquid level, its erroneous indication led to a failure in the regulation process by the control valve, thereby causing significant fluctuations in production. 2. Fault analysis: (1) When the float level gauge transmitter is operating normally, the medium in the float is paraffin oil; paraffin oil remains in a liquid state at temperatures between 90°C and 100°C, while it begins to solidify at around 50°C. Therefore, this float transmitter is kept heated throughout the year. If the heat tracing fails and the temperature drops, the paraffin oil inside the float will solidify on its own, causing the float to get stuck. In such a situation, even if the actual liquid level changes, the float level gauge will be unable to detect this change, and its output value will remain unchanged. (2) Go to the site to inspect the float level transmitter: close the primary valve and open the vent valve. Medium cannot be discharged due to the adoption of condensate recovery measures; it is suspected that debris is blocking the air release valve. After that, the air release valve was removed, and it was found that the process medium had condensed inside the cylinder and could not be discharged. At this time, the trace heating was checked. The temperature of the inlet pipe was quite high, at about 100°C, while the temperature of the return pipe was around 30°C (the ambient temperature on site at the time was approximately 10°C). It could thus be determined that the malfunction was caused by the condensation of the process medium due to inadequate trace heating. (3) The tracing system of this gauge was inspected. Since the inlet pipe of the gauge was very hot while the return water pipe had a low temperature, there was a suspicion that the steam trap was clogged. After disassembly for inspection, no problems were found. Heating was re-applied, but the return water pipe remained cold; the problem was not resolved. Recalling, this heating air inlet pipe had leaks on multiple occasions during winter, but such issues did not occur after the repairs were carried out. It is suspected that the heating gasket of the float level gauge has become clogged at the air inlet due to repeated compression. (4) The inlet trace heating valve was turned off, and the plug of the float level gauge’s trace heating wire was opened. It was observed that steam appeared intermittently; thus, it was concluded that the fault was caused by a blockage in the inlet gasket, which led to the self-solidification of wax oil and subsequent malfunction of the indicator. 3. Fault handling: Open the heating air inlet connector of the float level gauge, repair the gasket, then activate the heating system – the level indication returns to normal. 4. Take measures: Regularly inspect the tracing heating for this pipe. If any leaks or blockages in the tracing heating are detected, promptly rectify the issues. 3. No indication of liquid level in the steam condensate recovery tank. 1. Fault symptom: There is no indication of the liquid level in the steam condensate recovery tank. 2. Causes of the malfunction: (1) Malfunction of the LT-8701-1 transmitter; (2) The process flow was not established, resulting in no liquid level in the tank. 3. Handling method: After on-site inspection, it was confirmed that the issue was caused by a fault in the float level gauge. The drain valve for the float was opened, but no condensate flowed out. Upon removing the floats and inspecting them, it was found that due to the tank not having been cleaned for a long time, a large amount of debris had filled the floats, preventing the float level gauge from functioning properly. After cleaning the debris and recalibrating the gauge, it resumed normal operation. 4. Large indication deviation of the float level gauge 1. Fault symptom: The float level gauge shows a large deviation in its indication, and the issue persists even after performing calibration at two points. 2. Fault cause analysis: In May 2007, staff working with the styrene process reported that the readings from LT-3005 were too high. After using a 375 communicator to perform a two-point calibration, the results were still not satisfactory. Upon analysis, it was determined that over time, the stiffness of the float’s torsion tube changed, and conventional calibration methods were insufficient to eliminate the errors. It was necessary to recalibrate its dry coupling point so that the torsion tube could operate within the normal range; only after that could a two-point calibration be performed, allowing the instrument to function properly. After verification using the above method, the instrument returned to normal. 3. Preventive measures: It is necessary to thoroughly understand the instrument manuals in order to be able to make more accurate judgments and address issues more effectively when problems arise with the instruments on site. 5. The indication on the electric float level gauge remains unchanged. 1. Fault symptoms: When the liquid level is being regulated normally, the electric float level gauge LT653 consistently indicates 100% without any change. Since the liquid level in this tank is adjustable, its erroneous indication causes great difficulty for the control valve LV653 in regulating the liquid level. Process operators cannot use this table for automatic adjustment; they can only control it manually. 2. Fault analysis and handling: When the float transmitter LT653 is operating normally, the medium inside the float remains relatively stationary, which causes various impurities to settle within it. Over time, this sludge can trap the float, and as a result, even if the liquid level changes, the float remains stuck, leading to an unchanged output value for the liquid level. Upon arriving at the site, we first closed the root stop valve, opened the drain valve, and washed the inner walls of the float bowl with water. After cleaning, the meter was put into use and it operates normally. 3. Preventive measures: Regularly clean the buoys and floats, and periodically check the operation of the buoy amplifiers. 6. The electric float level gauge indicates a low level. 1. Fault symptoms: The electric float level gauge LT101 is used to measure the level of ethylene in the copolymer monomer tank; the glass plate level gauge shows that the tank is full, whereas the level transmitter indicates around 40%. 2. Fault analysis and handling: Inspection of the various components of the level transmitter revealed no abnormalities. When checking for emissions in both the gas and liquid phases, ethylene was detected. The float was opened, and the torque arm as well as the float itself showed no issues. It was only after removing the float that it was discovered to have a sand hole; the float was hollow, and after filling it with hexene its weight increased, resulting in a lower measured liquid level. 3. Measures: Weld the float back in place, adjust it properly, and the liquid level will return to normal. 7. Inaccurate readings from intelligent float transmitters 1. Fault symptoms: The ZTD intelligent float transmitters in use on site provide inaccurate readings; the zero point and range settings are incorrect, making it impossible to use them. 2. Treatment method: (I) “Mark the dry coupling point”. The operation steps are as follows: (1) Hang the tube; (2) Slide the handle into the locked position to expose the access hole; (3) Insert a concave socket wrench 10 mm deep, passing through the access hole to reach the torque tube shaft clamp, clamp the nut, and tighten the clamp nut. (4) Slide the handle into the released position to cover the access hole. (5) Perform operations through the 275 (375) interface: Basic settings – Sensor calibration – Mark the dry coupling point – Mark the zero point according to the instructions provided by the HART communicator. (II) Use the 275(375) interface to view the buoy parameter information and make corrections to any errors. (III) Use the 275(375) interface to view the density of the measured medium and make corrections for any errors. (IV) PV bias reset. (5) Calibrate the transmitter for zero and range. 8. The output linearity of the float level transmitter is poor. 1. Fault phenomenon: The output of the LT322 lacks linearity. 2. Fault analysis and handling: The LT322 is a float level transmitter, and its output lacks good linearity. Check whether there is any jamming in the movable parts; confirm that none is present. Checked whether the upper limit of the overload protection spring blade range has come loose; confirmed it is not the case. Check for any contact between the float and the exterior of the measurement chamber, and confirm that there is none. The installation position of the measuring lever and the shaft seal frame was checked, and it was found to be incorrect. After reinstalling them in the proper positions, the system operated normally. 3. Preventive measures: Instrument technicians must be careful during installation and emergency repairs, avoiding carelessness and reckless actions. 9. In the float level gauge, the instrument does not change as the liquid level drops. 1. Fault phenomenon: The LT-3106 level suddenly drops to 60%, and the instrument no longer responds. 2. Fault cause analysis: The gauge normally displays the liquid level at 80%–90%. After the level drops to around 50% and then rises to 60%, the gauge’s display does not increase in line with the actual liquid level. Possible causes include the following: 1. A fault in the transmitter’s conversion unit; 2. A problem with the signal output; 3. Reasons for measuring child float buoys. 3. Fault handling: Based on observations and the phenomena reported by the process team, the likelihood of faults occurring in the transmitter or transmission system is very low; therefore, inspection of the float should be carried out first. The liquid in the float was drained as required; the upper valve cover was opened to separate the float from the transmitter, and the float was removed. It was found that sticky substances had adhered to the float, so it was cleaned before being reinstalled. The instrument was put into use, and its readings were compared with those in the control room based on the actual liquid level; the instrument displayed normal readings. 4. Preventive measures: The float should be inspected and drained regularly. 10. Vapor-liquid mixing causes fluctuations in liquid level. 1. Fault symptom: Fluctuations in the liquid level of the isopentane recovery tank. 2. Cause of the fault: Most of the isopentane required for the reaction is obtained through recovery; inaccurate liquid levels in the recovery tank directly affect the progress of the reaction. Moreover, the material recovered from the degassing chamber and then condensed still contains a large amount of reaction gases, which leads to pressure fluctuations inside the tank. These gases can also enter the float of the level gauge, causing fluctuations in the liquid level and resulting in inaccurate measurements. 3. Handling method: Close the upstream and downstream shut-off valves of the level gauge, open the discharge valve at the bottom of the float, release the air, then close the discharge valve. Open the shut-off valves gradually to allow the recovered liquid to fill the float, thereby restoring stable level measurement readings. (Source: Instrument Circle)