10 Key Methods for Diagnosing Failures in Chemical Process Instruments and 25 Repair Tips

Thread Content

I. 10 Major Methods for Diagnosing Chemical Instrument Failures 01 The investigation method: By examining the symptoms of the failure as well as its development process, the causes of the failure can be analyzed and determined. 02 Visual inspection method: Without using any testing instruments, faults are identified by relying on human senses (eyes, ears, nose, hands). 03 Cut-off method: Disconnect the suspected part from the entire device or the individual circuit to see if the fault disappears, thereby determining where the fault lies. 04 Short-circuit method: Temporarily short-circuit the circuit stage or component suspected of having a fault, and observe whether there is any change in the fault condition to determine the location of the fault. 05 Replacement method: Determine where the fault lies by replacing certain components or circuit boards. 06 Division method: During the process of identifying faults, the circuit and electrical components are divided into several sections to determine the cause of the fault. 07 Human interference method: When a person is in a chaotic electromagnetic field (including those generated by alternating current power grids), a weak low-frequency electromotive force is induced (ranging from several tens to several hundred microvolts). When a person touches certain circuits of the instruments, those circuits respond, and this principle can be used to simply identify the faulty parts in the circuits. 08 Voltage method: The voltage method involves using a multimeter (or other voltmeter) set to an appropriate range to measure the suspected area. There are two types of measurements: AC voltage and DC voltage. 09 Current method: The current method is divided into direct measurement and indirect measurement. Direct measurement involves disconnecting the circuit and connecting an ammeter in series to measure the current value; by comparing this value with the one obtained when the instrument is in normal operation, a fault can be identified. Indirect measurement involves keeping the circuit closed, measuring the voltage drop across the resistor, and calculating an approximate current value based on the resistance value; it is commonly used for measuring the current in transistor components. 10 Resistance method: The resistance inspection method involves using the resistance setting on a multimeter, without applying power, to check whether the input and output resistances of the entire circuit as well as certain sub-circuits of the instrument are normal, whether capacitors have blown or are leaking current, and whether inductors and transformers have any breaks or short circuits. II. 25 Maintenance Experiences for Chemical Instrumentation 01 Crystal Formation Issues: Ammonium salt crystals tend to form in the instruments related to the hydrogenation desulfurization tower in existing installations, as well as in the instruments associated with the sulfur-containing gases. Treatment method: Steam is used for heating to melt the ammonium salts, thereby allowing the instruments to function properly. However, the fundamental solution is to address the issue at the process level by minimizing the formation of ammonium salt crystals. 02 Instrument out of power: When it is found that the instruments on site are not functioning properly due to a lack of power, the following aspects should be checked: (1) Water may have entered the wiring box of the instruments on site, as well as the instrument panels and wire conduits, which is causing the instruments to fail to operate correctly ； (2) Poor wiring; check all wiring from the control room cabinet to the site ； (3) The safety barrier or isolation barrier is damaged ； (4) There is a problem with the card holder or the card channel ； (5) A short circuit between the conductor and the shield wire in the signal line causes voltage attenuation, resulting in the gauge on site running out of power. The solution is to disconnect the shield wire from the ground wire. 03 Thermocouple faults Common thermocouple faults (low readings, low values, instability) – Guidelines for diagnosis: Low reading (low thermoelectromotive force): (1) Short circuit in the thermoelectric electrodes ； (2) Compensation wire short circuit ； (3) Dust accumulation on the thermocouple terminals causes a short circuit ； (4) The compensation wire and thermocouple are connected with reversed polarity ； (5) Degradation of thermocouple thermoelectrodes ； (6) The compensation wire is not compatible with the thermocouple ； (7) The installation position or insertion depth of the thermocouple does not meet the requirements ； (8) The thermocouple temperature compensation does not meet the requirements ； (9) The thermocouple is not compatible with the display instrument. The displayed value is too high: (1) The thermocouple is not compatible with the display instrument ； (2) The compensation wire is not compatible with the thermocouple ； (3) Interference from DC signals. The output of the thermocouple is unstable: (1) Poor contact between the thermocouple terminals and the electrodes ； (2) Insulation damage in the thermocouple measurement circuit causes intermittent short circuits and grounding ； (3) The thermocouple is not securely installed or there is external vibration ； (4) Whether the thermoelectric electrode is broken or not. 04 Electrical converter: Common faults of electrical converters: (1) Poor linearity: Improper fit between the nozzle and the baffle, the baffle not sealing properly, or damage to the baffle nozzle ； (2) Large backlash: mechanical friction, slight sticking of the moving coil ； (3) The measurement range cannot be achieved; after multiple adjustments, it is due to demagnetization of the permanent magnet ； (4) No input signal, maximum output, or output that does not return to zero: clogged nozzle, damaged throttle orifice seal ring, amplifier malfunction ； (5) If the air supply varies by less than 10% but the output still deviates from the specified range, it is due to an oversized throttle hole or improper alignment of the baffle nozzles. 05 Four-way valve: Analyze the common faults of the four-way valve and the corresponding solutions: (1) If the 380V power supply to the four-way valve is cut off, the motor will not operate; in this case, check the wiring and restore power supply ； (2) The 9V battery is dead; replace the battery and re-calibrate the four-way valve ； (3) The valve is not closed or opened properly; power off and reset the opening and closing limit positions ； (4) The four-way valve is not functioning properly for tower switching; check whether the switch for switching between the four-way valve and the isolation valves of towers A/B is set to remote mode, and whether the position switch is working correctly. 06 Level Instruments: For instruments used to measure liquid levels, since it is not convenient to remove the float during operation due to high pressures, the 375 device can be used to adjust the float on-site (even in high-pressure situations). It is first necessary to ensure the accuracy of the glass gauge and the stability of the production process. To calibrate the level or boundary, one can use the PVTRIM ZERO function of the 375 device to directly adjust the float. If the float is at too high a level, simply set PVTRIM ZERO to the corresponding position on the on-site glass gauge; the same principle applies in reverse. In most cases, no two-point calibration is required ； Regarding the issue of poor operation in the on-site glass plate process, the first thing to check is whether the upper and lower ball valves are in the correct position; generally, turning them 3-4 turns after closing them is sufficient, without needing to open them fully. To avoid blocking the liquid and mistakenly thinking that the pressure measurement points at the top and bottom are blocked, thus leading to disassembly. If the medium is too dirty and has blocked the glass plate, it is necessary to test the operation of the upper and lower valves; close the upper and lower manual valves in sequence to allow air to pass through and any debris to be removed. If flow still doesn’t occur, then there is likely a blockage. The glass panel needs to be removed. For level gauges, especially those that use capillary pressure sensing, if there is a sudden abnormality after normal operation for an extended period of time, do not alter the gauge’s range arbitrarily. First, check that the instrument’s zero point is functioning properly. In winter, poor heating effects may cause the pressure sensing points to freeze; if the problem persists even after thorough heating with steam, it could be that accumulated dirt is causing pressure fluctuations and unstable differential pressures. In such cases, the fitting needs to be disassembled to check for any foreign objects inside. Be sure to release pressure fully and allow it to condense before proceeding with further handling. 07 Quick-close valves for chain operations: With valves equipped with solenoids, it might happen that the valve does not function when powered. First, check whether the relay inside the control cabinet is delivering 24 volts of power; then go to the site to measure if there is voltage present. The presence of voltage indicates that there is no problem with the circuit. If the valve has a handwheel, it’s easier to determine the issue – simply try operating it manually. If it can be turned manually, then the problem lies in the solenoid’s air circuit; check whether the air flow in that circuit is unobstructed and whether there are any impurities present. Open the solenoid and clean it accordingly. If the valve doesn’t have a handwheel, there are two ways to determine whether it is truly stuck. First, de-energize the solenoid and then try to open the valve manually (as is the case with actuators used in coking, hydrogenation, and flue gas control processes, where manual operation is only possible after the power is cut off). Check whether there is air supply coming from the valve. If there is no output, it’s possible the valve is indeed stuck. Then, disassemble the valve to check if there are any foreign objects stuck in it. In the case of internal valve leakage ； Ball valves and disc valves can experience internal leakage if they are not closed properly or fully closed; in such cases, phase adjustment is required. If that doesn’t work, it may be due to severe wear of the ball valve core and seat, or damaged sealing gaskets. Control valves may have an excessively high zero point resulting in poor sealing, or foreign objects may be stuck between the valve core and the valve seat. 08 Temperature instruments: Thermocouples (resistors) often show abnormally low temperatures or fluctuations. First, check whether the wires in the cabinet are secure; if there is a low-current leakage or short circuit in the cabinet components, it can cause instability in voltage and resistance, which in turn affects the temperature readings. Use a thermocouple (resistor) signal generator to read the values, and see if they match those displayed on the DCS screen. Then send the values from the generator to the DCS and check whether the values on the DCS screen correspond to those sent. If there is a discrepancy, investigate the problem with the cabinet components and replace them with new ones. When replacing the components, make sure to verify whether interlocks related to that temperature point are in place; removing the components carelessly could result in excessively high temperature readings and thus shutdown of the system (for example, a shutdown occurs when the pressure of the gas tank exceeds 95 degrees) ； If it is necessary to check the issues on site, it is possible that the wiring inside the thermocouple is loose, or that water has entered, or that the wiring terminals are not properly connected to the edges, resulting in grounding or excessive resistance and thus temperature fluctuations. In such cases, cleaning up and securing the wiring terminals will resolve the problem. If there is a short circuit in the instrument wires, it is necessary to check whether the instrument wiring box or the junction boxes at the bends have suffered short circuits due to wear or moisture from rain; in such cases, wrapping the faulty area with insulating tape will suffice. It is also possible that corrosion at the wiring connections has occurred due to long-term use or the presence of gases, in which case the connections need to be cut off and reconnected. 09 Pressure instruments: In some cases, the data transmitted by transmitters indoors and outdoors do not match. First, check whether the range set in the DCS configuration is correct; this is because all such devices have had their ranges modified before being connected to the server, and since no thorough reset was performed prior to initialization, the modified ranges may differ from those indicated on the actual instruments. Then check the wiring; it’s possible that there is a ground fault in the wiring. (For example, in the case of the differential pressure of the hydrogenation PDT3109 reactor, prolonged rainfall may cause the wiring to become grounded, as this gauge sends the pressure data from the DCS to the field display unit before it reaches the transmitter, and any abnormal readings at the field level directly affect the readings on the transmitter.) As for rotor flowmeters, it is common for them to stay fixed at a certain value, usually because some debris such as stones or welding slag gets stuck in the float. The solution is to disconnect the rotor flowmeter from the system, take it apart, clean it, and then reinstall it. 11 Mass flow meters: With mass flow meters, it is common for empty tubes or two-phase backflow to occur, resulting in no reading or inaccurate readings. The solution is to reduce the size of the back valve or the control valve behind the mass flow meter, so that the tube is filled and normal readings can be obtained. 12. If an alarm occurs during the fuel dispensing process and fueling cannot proceed, the solution is to first check the actual situation on site, turn off the alarm, and then press start to continue fueling ； Wait until the hair treatment is complete, then proceed according to the instructions. The common method for dealing with static electricity alarms is to check whether the static clamps are in good contact, and to wipe the static clamps and the metal plates to ensure proper contact ； The general approach to dealing with an oil spill alarm is to carefully clean the oil spill alarm sensor. 13 Shutdown of the loader batch controller: When the loader batch controller shuts down, first contact the process engineers and ask them to stop loading operations at the two cranes where this issue is occurring. Next, power off the faulty batch controller, and then turn it back on to restore normal operation. 14 The feedback from the programmable valve is incorrect. The reasons for incorrect feedback from the PSA programmable valve are: (1) The feedback probe is damaged ； (2) The reply cable is broken or the connection is poor ； (3) The reply probe is not installed properly ； (4) Short circuit in the power supply fuse ； (5) Card component burned out ； The reasons why the programmable valve does not operate properly or remains stationary are: (1) Insufficient air pressure ； (2) Air leakage at the air duct or diaphragm head ； (3) The solenoid coil is burned out ； (4) Blockage in the solenoid valve air supply distributor ； (5) Card component burned out ； (6) Short circuit in power supply fuse ； (7) The programmable valve is stuck. 15 Inconsistent DCS displays: If it is found that the values displayed on the DCS are multiples of the values shown on the field instruments, it may be due to a mismatch between the range of the field instruments and that of the DCS. The solution is to log in as an engineer at the operator station, adjust the range of the DCS to the correct value, or modify the field instruments accordingly. 16 Servo level gauge malfunction: If the servo level gauge is not functioning correctly, it is necessary to first check whether the gauge is working properly. Common issues include \"reset failure\" or \"motor operation timeout\"; static interference may also be a cause. The operating status of the level gauge can be viewed on the debugging interface. Once the problem is resolved, reset the level gauge. After resetting, measure the level again until the correct value is obtained. If there is a discrepancy between the measured value and the actual value, \"level adjustment\" can be performed. 17 Large fluctuations or deviations in radar level gauges: If the level indicated by a radar level gauge fluctuates greatly, shows no level reading, or is inaccurate, the following aspects should be checked: (1) Whether the model of the radar matches the design requirements; radar level gauges for crude oil tanks require waveguides, while those for other types of tanks do not ； (2) Is the installation proper? The liquid level measured by the radar is supposed to be horizontal; adding a horizontal reflector can resolve the issue of fluctuations in the liquid level ； (3) If multiple measurements show deviations from the actual liquid level, the reference height can be set via the command line. If the parameters of the tank area No. 18 cannot be transmitted to the indoor system, or if the liquid level or temperature in that tank cannot be sent to the indoor system, the following aspects should be checked: (1) Verify whether the output signal from the level gauge is normal; generally, it should provide a voltage output of 2.5V ； (2) Check whether the tank pre-processor displays properly; if there is an abnormal display, the circuit board can be replaced ； (3) Check whether the wiring from the junction box to the indoor circuits is normal ； Finally, check whether the MOXA card on the host server is working properly; it usually has a 5V voltage output. During the operation of the 19-system, replacement of the controller and cards is necessary. During normal operation, the controller and cards need to be replaced: card replacement is usually required when a channel fails; when replacing a card, it is important to check whether there is a redundant card available – in the DCS system, 148R and 152 are redundant, while the others are not. For cards with redundancy, check whether the other card is operating; a green light indicates normal operation. If everything is fine, the faulty card can be removed and replaced with a new one. For cards without redundancy, check if there are any interconnected points; if such points exist, it is necessary to coordinate with the technical team to remove them before the card can be taken out and replaced. If the main controller fails during operation, it will switch to another one. First, check that the main controller’s dial switch is set to ON; after replacing it, also set it to ON so that the controller can switch automatically in case another one fails. 20. The soft keyboard on the operation station is not functional; the software disk on the operator station is unusable or some of its buttons do not work properly. In such cases, there is no need to restart the machine, nor is it necessary to remove the software disk from the connection port to the main unit. All you need to do is exit the operator station and re-enter it. If the 21 system indicates that the relay valve or pump is not open or activated, try logging in to the main control unit first. If there is an output, check whether the fuse of the solenoid valve corresponding to that channel, or the relay associated with it, is functioning properly. If the valve does operate, then the issue lies on-site; if it’s the electrical components that are causing the problem, then that’s the source of the issue. If the relay does not send any output, check whether the fuse has been burned out. If the fuse is intact, then the relay might be damaged. After the pump starts, if no feedback is received, first check the corresponding channel in the cabinet by using a shorting wire to see whether there is normal feedback. If feedback is present, then the issue lies with the electrical components or there is a fault in the circuit from the electrical components to the DCS. 22 How to make changes from the system or cabinet. If, for safety reasons, it is required that the solenoid valve shut off when power is cut off, but you currently cause it to shut off while power is still supplied, then to make changes from the system or cabinet: in the configuration, simply invert the signal at the interlock output DO point – this will not affect the operation of the system. When making changes from the cabinet, simply reverse the normally open or normally closed state of the relay. 23 ESD system error: In the ESD system, some actuators (such as quick-opening dampers, flue dampers, etc.) may end up in an incorrect state when the interlock conditions are met or removed. This can be caused by the following reasons: (1) At the engineer station ELOPII, that particular point is in a forced state. To resolve this, open the ELOPII software, locate the corresponding program (indicated by the blue small triangle symbol), right-click on that blue triangle, select the ONLINE-TEST option to enter online mode, then find that point (it must be a DO or DI point; it cannot be an intermediate variable point). Hold down the left mouse button to draw a small yellow box with two columns, and double-click to move the black square to the upper column, thereby releasing the forced state ； (2) In the cabinet, the fuse at the corresponding point is blown; the solution is to replace the fuse ； (3) In the cabinet, in the case of a fault with the corresponding relay, the solution is to replace it with a relay of the appropriate model ； (4) In the cabinet, the corresponding knife switch on the terminal block is not closed. Additionally, when dealing with instruments related to the ESD system (such as those associated with the coking instrument panel), special care must be taken. Relevant process personnel should be informed in advance before addressing any issues, and the interlocks related to the instrument that needs repair should be disabled. Repair work should only be carried out after confirming that the interlocks have been disabled, in order to avoid unnecessary cascade shutdowns. 24 Issues with the SNETB network card: When the Macsv system servers or workstations are running normally, data exchange takes place over Network A (segments 128 or 130). Over time, the system will automatically disable Network B card. At this point, the system status diagram will show a particular machine or all machines in network B in red, but they can still be pinged. Solution: Desktop – My Computer – right-click – Manage – click on \"Device Manager\" – in the right-hand area, select \"Network adapters\" – Intel Pro100 (or Dlink) – right-click and choose \"Properties\" – Power Management – remove the checkmark from \"Allow the computer to turn off this device to save power\", then restart the computer to restore normal operation. 25. Inaccurate valve position output after putting a PID loop in automatic mode: When process engineers report that the valve position output for a certain PID loop is incorrect after it is set to automatic mode, it may be due to the reverse action setting in the PID parameters being incorrect. The solution is to log in as an engineer at the operator station and modify the reverse action setting on the PID panel.