Fault symptoms and troubleshooting of common instruments and meters

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Principles for analyzing faults in field instruments: 1. First, before analyzing faults in field instruments, it is necessary to have a thorough understanding of the production process, manufacturing methods, and conditions related to the instrument system in question. It is also important to know who designed the instrument system, what the design objectives were, as well as the structure, characteristics, performance, and parameter requirements of the system. 2. Before analyzing and checking for faults in the on-site instrument system, it is necessary to obtain information from the operators on site regarding the production load and changes in the parameters of the raw materials. The recording curves of the faulty instruments should also be examined, and a comprehensive analysis conducted to determine the cause of the instrument failure. 3. If the instrument’s recorded curve is a straight line (a line with no changes at all is called a straight line), or if the curve, which was originally fluctuating, suddenly becomes a straight line ； The fault is likely in the instrumentation system. Since most of the current recording instruments are DCS computer systems, they have high sensitivity, allowing for a very accurate detection of changes in parameters. At this point, the process parameters can be manually adjusted to observe the changes in the curve. If there is no change, it is almost certain that there is a problem with the instrumentation system ； If there are normal variations, it can be generally concluded that there is no major issue with the instrumentation system. 4. When the process parameters are changed, if it is observed that the recorded curve experiences a sudden change or jumps to its maximum or minimum value, the fault is often related to the instrumentation system. 5. Before the fault occurred, the instrument recording curve showed normal behavior; once fluctuations appeared, the curve became irregular or made it difficult to control the system, to the point where even manual operation was ineffective. In such cases, the fault may be caused by the process control system. 6. When it is found that the instruments displayed by the DCS are abnormal, one can go to the site to check the readings of the same physical instruments; if there is a large difference between them, it is likely that there is a fault in the instrument system. In summary, when analyzing the causes of faults in field instruments, special attention should be paid to changes in the characteristics of the controlled process and control valves, as these can all be factors leading to failures in the field instrument system. Therefore, we need to conduct a comprehensive and careful analysis from both the perspective of the on-site instrumentation system and the process control system in order to identify the cause. Steps for fault analysis of flow control systems “Solutions for troubleshooting common issues with instruments and meters…” 1. Common faults of electromagnetic flowmeters and their analysis: Sequence Number, Fault Symptoms, Causes, Solutions. 1. Abnormal operation of the meter after it has been in use for some time: Check whether the power supply is functioning properly. First, examine the external condition of the flowmeter; be careful not to disassemble it blindly. Check for leaks in the pipes and address them if necessary. Ensure that the pipes are not at a partial fill level and take action accordingly. Check for bubbles inside the pipes and deal with them if present. Verify whether the signal cable is damaged and replace it if needed. Check whether there is an open circuit in the output signal from the converter. 2. Decreased or suddenly dropping signal strength: Poor insulation between the electrodes, or short circuits; dirt accumulation on the inner walls of the measuring tube, which should be cleaned and wiped off. The lining of the measuring tube may be damaged and needs to be replaced. The signal socket may be corroded and should be cleaned or replaced. 3. The output is 4 mA when the fluid in the measuring tube is stationary, but the current decreases or becomes zero when there is flow (a fault during calibration): The flow direction of the flowmeter during installation might be opposite to the actual flow direction. Check the flow direction indicator on the flowmeter, adjust its installation direction, or swap the X and Y wires inside the converter. In split-type flowmeters, one of the pairs of wires A&B or X&Y might be connected incorrectly. Carefully check the excitation wires and signal wires, and correct any wiring errors. 4. The flowmeter only outputs 4 mA current (this usually occurs due to incorrect wiring in split-type flowmeters): The fluid being measured isn’t actually flowing. Take appropriate measures to ensure proper flow of the fluid inside the measuring tube. The excitation section of the converter might be in test mode; insert the switch between pins 2 and 3 or 3 and 4 (for normal operation). In split-type flowmeters, the excitation wires and signal wires between the sensor and the converter might be connected incorrectly. Correct the wiring; if there are any issues related to broken wires or short circuits, contact the company for assistance. In the event of a sensor failure, contact the company. In case of a fault in the excitation circuit of the converter, contact the company as well. The 5 output signals are unstable (conductivity issues mainly occur during the calibration phase). If the conductivity of the medium is below the lower limit of 5 us/cm, it is not possible to measure it accurately using an electromagnetic flowmeter. The medium may contain solid particles or slurry. Insufficient connection to liquid or poor grounding can also cause problems. Insulated pipelines must be equipped with grounding rings ； The flow meter must be reliably connected to the liquid through a grounding wire ； Separate grounding can improve the interference resistance of the flow meter. External electromagnetic interference 1: Stay away from sources of interference ； 2. Stray currents are primarily controlled through proper grounding; it is best to use separate grounding, avoiding sharing the same grounding system with other motors, electrical devices, or instruments. 3. The signal wires of split-type instruments should have enhanced shielding; they can be enclosed in iron tubes, or the shielding wire should be grounded at the other end. The flow meter’s measurement pipeline contains a large number of bubbles; therefore, the pipeline or valves need to be inspected, and care should be taken to prevent air from entering the fluid inlet ； An exhaust valve can be installed at an appropriate location to remove air from the measurement section ； Install the flow meter in a location where gas does not tend to accumulate ； For pipelines with open discharge, the location downstream of the flow meter can be raised appropriately. If the fluid flow is fluctuating or pulsating, the flow meter should be placed as far away as possible from the source of these fluctuations; if necessary, a flow stabilizing device should be installed at an appropriate point in the pipeline to help keep the fluid flow stable. If liquid is being added upstream, it may not have mixed evenly by the time it reaches the flow meter. The power supply wires for the flow meter and its output signal wires should run parallel to each other, or they can be placed inside the same conduit. The signal output wires should avoid running alongside the AC power wires, and in particular, it is important to avoid using the same wire for both purposes. 6. When the output signal indicates over-full scale, it means that the actual flow rate is greater than the full-scale flow rate of the flow meter (out of range). Carefully analyze the piping system; do not rely solely on empirical values ； Sometimes the pump’s rated flow rate cannot be used as the sole basis for determining the flow rate ； Reduce the flow rate appropriately to observe the meter’s output. When the usual flow rate exceeds the meter’s full scale, it is necessary to contact the company to arrange for the range to be adjusted. For split-type flow meters, correct any errors in the wiring of the excitation and signal lines. Ensure that the meter’s measurement tube is always filled with fluid; if the insulation of the piping is poor, a grounding ring must be installed ； The flow meter body must be reliably connected to the liquid through a grounding wire ； Separate grounding can prevent interference caused by the ground wire. For converter malfunctions, check the signal amplification and output sections; contact the company in such cases. In case of sensor malfunctions, also contact the company. If the measurement results do not match the actual flow rate, it may be due to issues with the instrumentation setup; rectify any problems related to flow rate measurement and its compatibility with downstream instruments or computers. If there is leakage (or slight short-circuiting) in the output signal wires, this can result in an artificially low indicated flow rate; the signal wires must be replaced. To determine whether the empirically derived “actual flow rate” is reliable, carefully analyze both the flow rate and pipeline conditions; if possible, seek a comparative reference point, and contact the company when necessary. Improper installation of sensors—such as insufficiently long straight pipe sections, incomplete fluid filling, presence of air bubbles, or violent liquid pulsations—must be corrected by installing sensors strictly according to the instructions. Clean the sensor surfaces from scale buildup while being careful not to damage the lining. Address any leaks or branch lines in process pipelines by eliminating valve leaks or unintended flow diversion. If sensor insulation deteriorates, contact the company.

2. Common faults and analysis of vortex flow meters
| No. | Fault phenomenon | Cause | Solution |
|----|------------------|-------|----------|
| 1 | Output signal present despite no flow after power-on | Poor input shielding or grounding, leading to electromagnetic interference | Improve shielding and grounding to eliminate electromagnetic interference |
| | | Instrument located near high-voltage equipment or sources of high-frequency pulse interference | Relocate the instrument away from such sources and implement isolation measures; enhance power supply filtering |
| | | Excessive pipeline vibration | Implement vibration-damping measures and strengthen signal filtering; reduce amplifier sensitivity |
| | | Excessively high converter sensitivity | Lower the sensitivity setting and raise the trigger level |
| 2 | No output signal after power-on and flow initiation | Power supply malfunction | Inspect the power supply and grounding |
| | | Broken input signal wires | Examine signal wires and connection terminals |
| | | Malfunction in one stage of the amplifier | Check operating points and inspect components |
| | | Damaged sensing elements | Inspect sensing elements and their leads |
| | | No flow or excessively low flow rate | Check valves; increase flow rate or reduce pipe diameter |
| | | Pipeline blockage or sensor jamming | Clear the pipeline and clean the sensor |
| | | Scale buildup on the vortex generator | Clean the vortex generator |
| 3 | Irregular and unstable output signals; strong electrical interference | Insufficient shielding and grounding | Strengthen shielding and grounding measures |
| | | Sensor contamination or moisture exposure, resulting in reduced sensitivity | Clean or replace the sensor; increase amplifier gain |
| | | Excessively high sensor sensitivity | Reduce amplifier gain and raise the trigger level |
| | | Damaged sensor or poor lead connections | Inspect sensor and leads |
| | | Presence of two-phase flow or pulsating flow | Improve process management to eliminate such phenomena |
| | | Influence of pipeline vibration | Implement vibration-damping measures |
| | | Unstable process conditions | Adjust the installation location |
| | | Misaligned sensor installation or protruding gasket into pipe interior | Verify installation status and correct the inner diameter of the gasket |
| | | Disturbances caused by upstream/downstream valves | Extend straight pipe sections or install flow straighteners |
| | | Fluid does not fully fill the pipe | Change both the installation site and method for the flow sensor |
| | | Obstructions wrapped around the vortex generator | Remove such obstructions |
| | | Cavitation occurring | Decrease flow velocity and increase internal pipe pressure |
| 4 | Large measurement errors | Insufficient length of straight pipe section | Extend the straight pipe section or install a flow straightener |
| | | Zero drift or incorrect full-scale adjustment in analog conversion circuit | Recalibrate zero point and scale markings |
| | | Excessive fluctuations in supply voltage | Inspect the power supply |
| | | Instrument overdue for calibration | Submit it for calibration promptly |
| | | Significant difference between sensor and pipe inner diameters | Measure pipe inner diameter and adjust instrument coefficient accordingly |
| | | Misaligned installation or protruding gasket into pipe interior | Realign installation and modify gasket dimensions |
| | | Sensor contamination or physical damage | Clean or replace the sensor |
| | | Presence of two-phase flow or pulsating flow | Eliminate such flow patterns |
| | | Pipeline leaks | Fix any leaks |
| 5 | Leakage at measurement pipe | Excessively high internal pipe pressure | Adjust pipe pressure and relocate the instrument |
| | | Incorrect selection of nominal pressure rating | Choose a sensor with a higher nominal pressure rating |
| | | Damaged seals | Replace damaged seals |
| | | Sensor corrosion | Apply anti-corrosion and protective measures |
| 6 | Abnormal whistling noise from sensor | Excessively high flow velocity causing intense vibration | Adjust flow rate or switch to an instrument with larger bore size |
| | | Cavitation occurring | Regulate flow rate and increase liquid pressure |
| | | Loose vortex generator | Tighten the vortex generator securely |
| 7 | Flow totalizer fails to operate | Inflexible or jammed counter mechanism | Clean counter gears or replace the counter unit |
| | | Broken counter coil | Rewind the coil or substitute with identical spare parts |
| | | Circuit faults in coefficient-setting or programmer components | Repair relevant component circuits or replace faulty elements |
| | | Circuit faults in display board pre-amplifier components | Repair relevant component circuits or replace faulty elements |
| | | No output from vortex transmitter | Inspect or replace the transmitter unit |

3. Common faults and analysis of rotameter flow meters
| No. | Fault phenomenon | Cause | Solution |
|----|------------------|-------|----------|
| 1 | Slight pointer oscillation | Usually caused by medium fluctuations; can be mitigated by adding damping | Apply additional damping measures |
| | Moderate pointer oscillation | Typically due to varying medium flow states; can be addressed via pressure or flow stabilizers, or by increasing air damping in the rotameter | Install stabilizers or augment air damping |
| | Severe pointer oscillation | Primarily caused by medium pulsations, unstable air pressure, or mismatches between user-specified operating parameters (pressure, temperature, flow rate) and actual rotameter conditions | Carefully verify all operating parameters against actual conditions | Adjust the relevant parameters. If the pointer stops at a certain position too quickly when the valve is opened, this can cause the stopper to deform and lock the rotor; therefore, the valve should be opened slowly. The rotor guide rod and the stop ring must be aligned properly. Remove the instrument and take out the deformed stopper to reshape it. 3. Large measurement errors may occur if the installation is not proper; for vertically installed rotameters, it is essential to maintain a vertical orientation, with an inclination of no more than 20 degrees ； For horizontally installed rotameters, it is necessary to keep them level with an inclination of no more than 20 degrees; there should be no ferromagnetic objects within 100 mm around the rotameter. The installation location should be far away from valve transitions, pump outlets, and bends in the process pipelines. It is required to maintain a front straight section of 5D and a rear straight section of 250 mm. Since the density of liquid media can change significantly, the updated density of the medium can be used in formulas to calculate an error correction factor; the flow rate measured by the flow meter is then multiplied by this factor to obtain the actual flow rate. Gas media are greatly affected by temperature and pressure, so temperature and pressure compensation is employed to determine the true flow rate. Prolonged use, pipeline vibrations, and other factors can cause the moving parts of rotameter devices, such as the magnetic steel, pointer, counterweight, and rotating magnetic steel, to become loose, resulting in significant errors. You can first verify by manually pushing the pointer. First, place the pointer at the RP position and check whether the output is 4 mA and whether the flow display shows 0%. Then verify sequentially according to the scale. If any discrepancies are found, the position of the component can be adjusted. It is generally required that professionals make adjustments; otherwise, the position will be lost and it will be necessary to return to the manufacturer for correction. 4. No current output: First, check whether the wiring is correct. With proper wiring, if the LCD displays something but there is no output, it is likely that the output transistor is damaged and the circuit board needs to be replaced. If the calibration values are lost, data recovery procedures can be used; if this does not work, set the values indicated in password 2000 first, and then those indicated in password 4011. The method involves using a finger to adjust the pointer and set the values from RP to 100%. 5. There is no on-site indication to check whether the wiring is correct; ensure proper wiring. Check whether the power supply is set correctly and adjust it if necessary. For liquid crystal modules, check for poor contacts and reinstall them. In the case of a multi-wire power supply system, check whether terminals 12 and 13 are connected to an ammeter or are short-circuited; rewire accordingly. 6. If the overall LCD display shows 0 or the full scale value, check the scale and zero-point parameters set in the 2000 password. The VALUE for ZERO must be less than the VALUE for SPAN; the two values cannot be equal. Check whether sampling data is being received; move the pointer manually to observe any changes in the sampling values. If there are no changes, it is usually due to a fault in the sampling circuit, and the circuit board needs to be replaced. 7. If the alarms are incorrect, check that the deviation setting value d is not too large; adjust this value. In the FUN function, verify that the logical functions are working properly. HA-A represents the upper limit logic. LA-A indicates lower-limit positive logic; check the set value of the alarm in SU and reset it. If the LCD display shows the correct value, no action is required; check the external power supply as well as whether its negative terminal is connected to the negative terminal that powers the instrument, and adjust the power connection. In the event of a circuit board failure, replace the circuit board. 8. Incorrect cumulative pulse output: check whether the alarm setting for the channel selected for cumulative pulse output is set to zero, and reset the parameters. In the event of a circuit board failure, replace the circuit board. 4. Common faults and analyses of ultrasonic flowmeters: Serial number, fault phenomenon, cause of the fault, and method of handling. 1. No characters displayed, no backlight: fuse blown. Determine the voltage level, check whether the load is short-circuited or poorly connected, and replace the fuse. The 220V AC voltage is not connected; the power supply has been turned off due to a protection mechanism. Turn the power back on – there is no flow display, but the serial bar graph shows normal values. 1. Pump stopped ； 2. Valve closed ； 3. Connected to other pipes, resulting in local static water; 1. Start the pump ； 2. Open the valve ； 3. Adjust the valve to change the local pressure. If the flow rate inside the pipeline is below the minimum flow rate cutoff value, check this value and set it correctly. If the current flow rate was used as the zero point due to an incorrect operation, reset that zero point setting. There is no flow rate display, no bar graph display, and the status symbol “S” does not disappear; the actual installation distance of the sensor differs significantly from the distance shown on the instrument. Verify the instrument input parameters and the actual installation distance of the sensors. Enter the correct parameters and carry out the installation. The signal is too weak to be detected. 1. Adjust the transducer and clean any dirt in the acoustic path where the signal is weak. 2. Remove gas from the pipeline and adjust the transducer in the acoustic path with a weak signal. 3. Modify the outer diameter of the pipeline. 4 Large flow deviation due to incorrect parameter input, such as pipe diameter, wall thickness, correction factors, etc. Enter the parameters correctly; severe scaling inside the pipe has reduced its inner diameter. 5. Large fluctuations in the flow rate indication indicate a high amount of gas present in the liquid. 1. Repair air leakage points in the pipeline network system ； 2. Install an exhaust valve on the pipeline ； 3. Install at a different point. (Detection methods: (a) Large fluctuations in A1 and A2 values; (b) Presence of gas noises in the pipeline.) When the content of suspended solids in the liquid is too high, a Doppler ultrasonic flow meter should be used. In such cases, the Doppler ultrasonic flow meter should be installed at a different location. Severe scaling in the pipes can prevent the transmission of sound waves ； Adjust the sensor insertion depth or remove pipe scale ； Scaling on the acoustic wedge surface of the sensor ； Remove the sensor, clean the acoustic wedge surface, and reinstall it. The sensor was not installed correctly; install it properly ； Sensor damaged; replace the sensor ； The sensor cable is not properly connected to the converter; reconnect it ； Converter malfunctions: Replace the converter circuit board.
6. Discrepancies between instantaneous and cumulative flow rates: The main unit is malfunctioning; replace the main unit.
5. Common malfunctions of differential pressure flow meters and their analysis:
| No. | Symptom | Cause | Solution |
|-----|---------|-------|----------|
| 1 | Indicator reads zero or shows minimal movement | Balance valve not fully closed or leaking | Close the balance valve; repair or replace it. |
| | High- and low-pressure valves at the base of the throttling device are not opened | Open these valves. |
| | Valves or pipelines between the throttling device and the differential pressure gauge are blocked | Flush the pipelines; repair or replace the valves. |
| | Steam in the pressure guiding pipes has not fully condensed | Wait until condensation is complete before operating the gauge. |
| | Gaskets between the throttling device and process pipelines are not airtight | Tighten bolts or replace the gaskets. |
| | Internal faults in the differential pressure gauge | Inspect and repair it. |
| 2 | Indicator reads a negative value | High- and low-pressure pipelines are connected reversely | Check and reconnect them correctly. |
| | Signal lines are connected reversely | Check and reconnect them correctly. |
| | Severe leakage or rupture in high-pressure pipelines | Replace damaged parts or pipelines. |
| 3 | Indicator reads a lower-than-expected value | Leaks in high-pressure pipelines | Locate and eliminate leaks. |
| | Balance valve is not airtight or not fully closed | Check, close, or repair the valve. |
| | Air remains trapped in high-pressure pipelines | Purge all air from the pipelines. |
| | Zero point of the differential pressure gauge or secondary instrument is off | Inspect and adjust it. |
| | Throttling device and differential pressure gauge are incompatible with design specifications | Replace the differential pressure gauge to meet design requirements. |
| 4 | Indicator reads a higher-than-expected value | Leaks in low-pressure pipelines | Locate and eliminate leaks. |
| | Air accumulates in low-pressure pipelines | Purge all air from these pipelines. |
| | Pressure of steam, etc., is below design values | Make corrections based on actual density values. |
| | Zero point drift in the differential pressure gauge | Inspect and adjust it. |
| | Throttling device and differential pressure gauge are incompatible with design specifications | Replace the differential pressure gauge as per design requirements. |
| 5 | Indicator reading exceeds scale limits | Actual flow rate exceeds design limits | Use a differential pressure gauge with an appropriate range. |
| | Severe leakage in low-pressure pipelines | Eliminate the leaks. |
| | Broken signal lines | Inspect and repair them. |
| 6 | Indicator responds sluggishly to changes in flow rate | Blockages in connecting pipelines or valves | Flush pipelines and unclog valves. |
| | Internal faults in the differential pressure gauge | Inspect and rectify them. |
| 7 | Indicator fluctuates significantly | Flow parameters themselves fluctuate greatly | Partially close high- and low-pressure valves. |
| | Pressure measuring elements are highly sensitive to parameter fluctuations | Adjust damping settings appropriately. |
| 8 | Indicator remains unchanged | Anti-freeze measures have failed; hydraulic fluid in the differential pressure gauge and pressure guiding pipes has frozen | Enhance anti-freeze measures. |
| | High- and low-pressure valves are not opened | Open these valves. |

6. Common malfunctions of mass flow meters and their analysis:
| No. | Symptom | Cause | Solution |
|-----|---------|-------|----------|
| 1 | Instantaneous flow rate always shows maximum value | Transmission signal cable is broken or sensor is damaged | Replace the cable or sensor. |
| 2 | Converter displays nothing | Power supply issue or blown fuse | Check power supply; replace the fuse. |
| 3 | No AC voltage but DC voltage present | Measurement tube is blocked | Unclog the measurement tube. |
| | Excessive installation stress | Reinstall the meter. |
| 4 | Zero point drift | Valve leakage | Eliminate leaks. |
| | Incorrect calibration coefficient of the flow meter | Check and correct it. |
| | Insufficient damping | Check and rectify it. |
| | Two-phase flow present | Eliminate two-phase flow. |
| | Sensor junction box is damp | Inspect and repair it. |
| | Wiring faults | Check wiring connections. |
| | Grounding issues | Check grounding connections. |
| | Excessive installation stress | Reinstall the meter. |
| | Electromagnetic interference present | Improve shielding to eliminate interference. |
| 5 | Fluctuations in displayed and output values | Low damping | Check and increase damping. |
| | Unstable drive amplifier | Inspect the drive amplifier. |
| | Unstable density readings | Check density calibration coefficient. |
| | Wiring errors | Check wiring connections. |
| | Grounding issues | Check grounding connections. |
| | Vibration interference | Eliminate vibration interference. |
| | Sensor pipelines are blocked or fouled | Clean pipelines and sensors. |
| | Two-phase flow present | Eliminate two-phase flow. |
| 6 | Incorrect mass flow meter readings | Incorrect flow calibration coefficient | Check and correct it. |
| | Incorrect flow units | Check flow units. |
| | Incorrect zero point | Adjust zero point. |
| | Incorrect flow meter configuration | Reconfigure it. |
| | Incorrect density calibration coefficient | Check and correct it. |
| | Wiring/grounding issues | Check wiring and grounding. |
| | Two-phase flow present | Eliminate two-phase flow. |
| 7 | Incorrect density readings | Incorrect density calibration coefficient | Check and correct it. |
| | Wiring/grounding issues | Check wiring and grounding. |
| | Two-phase flow, slug flow, or vibration interference | Eliminate these issues. |
| 8 | Power present but no output | Power supply issue | Check power supply between different sensor terminals. |
| 9 | Zero point is stable but cannot be reset to zero | Installation problem | Reinstall the meter. |
| | Significant differences between fluid temperature/density and calibration water | Increase or decrease zero-adjustment resistance. |
| | Measurement tube of sensor is blocked | Unclog the measurement tube. |