13 buoy level gauge maintenance cases to help you quickly resolve on-site faults

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The typical faults of float level gauges manifest in seven aspects: no level display or a minimum level display, a maximum level display, an abnormally high level display, an abnormally low level display, fluctuating level readings, slow response to level changes, and no change in the level display. Changhui Instruments has compiled 13 repair examples for float level gauge faults to share with instrument maintenance technicians. Level gauges: yunrun.com.cn/product/ 1. The float level gauge does not display the level or only shows the minimum value; there is no indication of the level in the steam condenser. When draining the float level gauge, only a small amount of condensed water flowed out, indicating that the float is blocked. After removing the outer float and opening it, it was found that dirt and debris had almost filled the float. After cleaning and recalibrating, the instrument returned to normal. In this case, the blockage was caused by process-related factors; the container’s process did not involve regular cleaning and removal of impurities, which led to the accumulation of a large amount of debris and dirt in the condenser, thereby blocking the float. 2. The float level gauge shows the maximum level of the sulfur unit’s stripping tower; the glass level gauge indicates 60%, while the DCS shows 100%. Upon on-site inspection, the glass level gauge was found to be normal. A sewage inspection revealed that there was blockage caused by debris. The stopwatch was used to clean the debris inside the float, and after restarting it, the liquid level displayed normally. In this case, the dirt inside the cylinder caused the float to get stuck at 100%, resulting in the maximum output current from the float. The most effective way to check whether the float is stuck is to drain waste. Close the sampling valve connecting the float to the equipment, then open the drain valve to allow waste to be discharged. If the instrument reading returns to zero, it can be concluded that the float is not stuck; if the reading remains at 100%, then the float is indeed stuck. When measuring the level of media that tends to crystallize and cause blockages, it is first necessary to determine whether there is a blockage issue with the local level gauge. Only after confirming that the local level gauge is functioning properly should the instrument itself be inspected, in order to avoid unnecessary steps. 3. The liquid level indicated by the float level gauge is too high; the process team reported that the DLC3000 float level gauge shows a higher level than actual. Calibrating the level at two points using the HART475 handheld device did not bring any improvement. After analysis, it is believed that the stiffness of the buoy’s torsion tube may change. To recalibrate the dry coupling point, follow these steps: ① Push the slider below the dial to expose the hex nut of the locking torque rod inside the locking hole. Place the float at its lowest liquid level position (i.e., the position where the float is heaviest), use an adjustable wrench to reach into the locking hole and tighten the nut, then push the slider back to its original position. ②After entering the online menu, select Basic Setup → Sensor Calibrate → Mark Dry Coupling. The dry coupling point calibration is complete; the instrument reading should be essentially at zero. If the deviation is large, go to PV Setup to check whether the Level Offset has returned to zero; if not, repeat the process. ③Enter Two Point calibration to perform two-point level calibration, and the instrument will return to normal. This gauge has been in use for over 8 years, so it is not surprising that the stiffness of the torsion tube has changed. The purpose of calibrating the dry coupling point is to ensure that the torsion tube operates within the normal range. The operator reported that the glass level gauge in the oil tank indicated a level of zero, but the float level gauge showed around 40%. Indeed, the oil level is very low according to the process implementation. By checking the DCS historical curves, it was observed that the float level remained at around 40% at 8:50 without any changes; an on-site inspection for sewage discharge revealed blockages in the sampling pipes on the positive and negative sides. Continued discharge was made until the positive pressure side became unobstructed, and the instruments showed normal readings once they were put into operation. This incident is caused by inadequate operation and maintenance; if the waste discharge procedures had been properly implemented and monitored, this fault could have been avoided. Both the glass level gauge and the float level gauge indicate that the liquid level is near zero, but the DCS shows a liquid level of 20%. The current was measured before and after the safety barrier in the cabinet; the current before the safety barrier was 4.5 mA, while the current after it was 7.2 mA. This value matches what is displayed on the DCS, indicating that there is a fault with the safety barrier. After replacing the safety barrier, the DCS display returned to normal. Based on experience, it is rare for both instruments to fail at the same time; one should trust the readings of the glass level gauge and the float level gauge. The higher reading shown by the DCS is likely due to an elevated current signal, and the current can be measured to determine the source of the problem. If the input and output currents of the safety barrier are measured to be inconsistent, it can be determined that there is a problem with the safety barrier. If the input and output currents of the safety barrier are the same, the cause should be investigated in the DCS card. 4. The liquid level indicated by the float level gauge is low; this is due to the low level reading in the carbon tower control system, as well as control failures that result in an increase in the process liquid level. The indicator of the Guanliao glass level gauge showed 80%, while the instrument displayed only 60%. The transmitter circuit was found to be functioning properly; the float level gauge was removed for calibration, and it was discovered that the float had corroded and leaked, allowing water to enter. After replacing the float and recalibrating, the liquid level display and control returned to normal. The gauge reading was low, sending an erroneous signal to the control system; this caused the control valve to keep closing, resulting in a continuous rise in the liquid level within the decarbonization tower. Calibration was performed using water, and it was found that the zero point and range changed significantly with each calibration. It was suspected that there was a problem with the float; upon inspection, it was discovered that the float had corroded and water had entered it. The newly installed float level gauge outputs a current that gradually decreases as the liquid level rises. This fault occurs due to internal leakage in the float; new gauges do not experience internal leakage, except in cases of quality issues. The operator checked the settings and found that the output signal was set to \"reaction\". Set the output signal to \"direct action\"; the instrument’s output current will vary in line with the liquid level changes. Smart transmitters have numerous setup options, and the menus are mostly in English; even a slight mistake can lead to incorrect settings or missing configurations. For newly installed or replaced instruments, after they are installed on site, the settings should be checked again using a handheld controller; it is advisable for two people to carry out this task, as this reduces the likelihood of errors. http://yunrun.com.cn/upload/201807/20/201807201636170437.png 5. Fluctuations in the liquid level display of the float level gauge: The liquid level in the boiler’s drum experiences smaller fluctuations at low load levels, while larger fluctuations occur at high load levels. It was observed that the steam supply pressure of the process remained basically stable. After inspection, the float and transmitter were found to be in good condition; a closer check revealed that the liquid-phase sampling valve was not fully open. After fully opening the liquid phase valve, the liquid level stabilizes. The opening degree of the liquid-phase valve in the level gauge is low, resulting in less water flowing into the level gauge ； The vapor valve is fully open, and the steam reheats the water inside the level gauge, causing the volume to expand and the level to appear higher than it actually is. After the boiler reduces its load, the steam pressure rises and the water level in the level gauge drops; this cycle repeats itself, resulting in fluctuations. As per regulations, the valves for the gas and liquid phases at the boiler water level must be fully open; this is also a key aspect of safety inspections ； Any inadequacy in the procedures will cause unnecessary difficulties in the maintenance work. Level fluctuations in the liquid level control system of the ammonia separator. The level transmitter and related circuits were checked, and no abnormalities were found; it was decided to carry out sewage discharge. After discharging waste water and flushing the buoy, the liquid level control is normal. When the machine is in operation, the compressor releases a lot of oil, which accumulates in the float. At low temperatures, this oil turns into sludge, preventing the float from moving freely and causing delays in the liquid level signal. As a result, the regulator is unable to adjust the liquid level in a timely manner, leading to incorrect adjustments and large fluctuations in the liquid level. Original address: yunrun.com.cn/tech/2093.html The process indicates that the level of LT205 fluctuates greatly. After discharging waste, ensure that the float sampling tube is unobstructed. Insert an iron nail from the lower vent, touch the float and move it up and down; the gauge shows a change, but it is very small. When the float is removed to reset the zero point and range, the output current remains essentially unchanged. Upon further inspection, it was found that the fixing bolts of the torsion tube were loose, and the position of the torsion tube had shifted. Readjust the position of the torsion tube and tighten the bolts, after which the instrument returned to normal. During calibration, it was found that the output current changed very little, leading to suspicion that there is a problem with the torsion tube. Under normal conditions, the torsion tube has an initial torque that keeps the float drive linkage in a suspended position, allowing it to detect subtle changes in the buoyant force acting on the float with sensitivity. When the fixing bolts of the torsion tube become loose, under the action of torque the initial torque of the torsion tube is zero, and no change in the displacement of the float linkage can be detected; as a result, the output current remains essentially unchanged during calibration. 6. The level change of the float level gauge is sluggish; although there are changes in the glass level gauge of the mother liquor tank, the remotely displayed level remains unchanged for a long time. It was judged that the sampling tube was likely blocked; an on-site inspection revealed that the liquid phase tube was not unobstructed. After closing the sampling valve and clearing the sampling pipeline, the instrument readings returned to normal. When the liquid-phase sampling tube is blocked, the liquid in the mother liquor tank cannot flow into the outer float, so the liquid level inside the outer float does not change, which in turn results in no change in the output current of the transmitter. The liquid level of a certain condenser tower does not change. The power supply and transmitter are both functioning normally. On-site wastewater discharge measurements showed a slight change, and inspection after disassembly revealed that the float was stuck. The level gauge was reinstalled during the parking for maintenance, and this fault has not occurred again within a year. This type of fault has occurred several times; initially, it was found that the verticality of the cylinder was not up to standard, but acceptance was carried out without any rework, which led to subsequent problems. Reinstallation resolved the remaining issues. 7. The liquid level displayed by the float level gauge does not change. The LT-106 level of the gasoline separation tank at a certain plant shows 25% with no change. It is estimated that too much dirt accumulated at the bottom of the tank; after it was cleaned and the system was put back into operation, the same problem occurred again after a week. When sewage discharge cleaning is resumed and the system is put back into operation, the value still shows at 25%; calibrating it with water does not bring about any change, but using a wire to move the float causes the gauge reading to change. After repeatedly rinsing the inside of the float with water, it was adjusted and the instruments resumed normal operation. The liquid level is shown as 25% with no change; in reality, the inside of the float is too dirty, causing it to stick to the wall and not move in response to changes in liquid level. This example illustrates that regular drainage is an important condition for ensuring the proper operation of float level gauges.