Analysis of Fault Examples in Single-Loop Feedback Control Systems

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The article \"Steps and Methods for Troubleshooting Faults in Single-Loop Feedback Control Systems\" describes the procedures and methods for handling faults. In this article, Changhui Instruments has compiled 16 real-world fault examples, using the analysis of these examples to enhance the practical skills of instrument technicians. Changhui Instruments http%3a//www%2eyunrun%2ecom%2cn 1. Automatic level control system for air separation units: The level measurement is carried out using differential pressure transmitters, DCS systems, and pneumatic diaphragm control valves to form the control mechanism. At the beginning of operation, the automatic level control led to significant fluctuations in the level, which did not meet the production requirements. Fault diagnosis and analysis: At the start of operation, the lack of equilibrium in the system’s cooling capacity, along with uneven temperature distribution within the level gauge tubes, resulted in a mixture of gas and liquid inside those tubes; this caused large fluctuations in the measured values. The introduction of automatic control further exacerbated these fluctuations. Fault handling: First, switch to manual mode for control, and address the level difference issues by making appropriate adjustments. After draining the positive and negative pressure pipes from the tower wall until frosting occurs, then start the instrument. After it has been manually stabilized for a while, switch to automatic mode. After the above treatments, the process requirements can be basically met, resulting in minimal fluctuations. 2. There is a simple control system; both the controller’s deviation and its output are normal. However, even when there is a deviation, the PID regulator continues to operate according to its usual control logic, and the parameter being controlled does not return to its set value; sometimes it even acts in the opposite direction. Fault diagnosis and analysis: Based on the symptoms of the fault, there are no issues with the measurement part, the control part, or signal transmission in the system. The fault may lie in the control valve itself (such as issues with the valve stem movement, clogging of the diaphragm head, or faults in the valve positioner), or in the signal lines that transmit signals from the PID regulator to the valve positioner. It was checked that the circuit from the PID regulator’s output signal to the control valve was in good condition. Upon inspecting the diaphragm head of the control valve, it was found that the air pathway at the diaphragm head connection was blocked by sand particles, which prevented the controller from functioning properly. Fault resolution: After clearing the blockage, the system returned to normal. 3. There is a simple control system in which the medium-pressure steam pressure is controlled through PV108 venting. One day, the operator noticed that the pressure was dropping; the valve position output by the PID regulator was correct, but the deviation could not be eliminated. Fault inspection and analysis: Based on the symptoms of the fault, there is no problem with the signal transmitted from the PID regulator to the safety device; the issue might lie in the signal transmitted from the safety device to the valve positioner or the control valve. First, check the valve positioner and control valve; both are fine. Then use a multimeter to measure the output signal of the safety device, but no output was detected. Fault resolution: Replacing the output fuse restored normal operation of the system. 4. A certain device features a simple control loop (DCS control system, PID controller); after switching to automatic mode, the controller fails to operate. Fault inspection and analysis: The reasons for the controller’s failure to operate may be issues with the DCS configuration or incorrect self-tuning parameters of the controller. The DCS configuration was checked and found to be correct, but the P, I, and D parameters of the controller were found to be incorrect. Fault handling: After the PID parameters are properly set, normal operation is restored. http://www.yunrun.com.cn/News/UploadFiles_5183/201511/2015111714330696.jpg Artificial intelligence regulator YR-GAD series: In single-loop configurations of the Yokogawa DCS system, an OOP fault alarm is generated. Fault analysis: OOP stands for open circuit at the input; this could be due to a broken transmitter cable, or the transmitter’s input signal being below 4 mA or above 20 mA. 6. Analysis of OOP faults in single-loop systems of Yokogawa DCS systems ； OOP stands for open output, which may be caused by loose wiring. 7. There is a single-loop control system that, even after prolonged tuning when set to automatic mode, fails to operate stably. When a disturbance is added to the set value, there is a tendency for the system to return to its original state, and both the adjustment period and the magnitude of the adjustments are normal; yet stability cannot be achieved. Fault analysis: Based on the symptoms of the fault, it can be seen that the tuning parameters of this system are appropriate, so the reason for the instability does not lie in the parameters. Fault handling: Re-analyze the circuit to correctly set the forward and reverse actions of the PID regulator, so that the system operates in negative feedback mode, and it functions properly after being put into use. 8. There is a control loop that has been in use since the system was put into operation years ago; under unchanged operating conditions, its control performance has been deteriorating. Fault inspection and analysis: The fact that it could function properly initially indicates that the parameters were normal, while the gradual decline in performance suggests that some changing conditions are affecting its control functionality. When inspecting the control valve, it was found that the valve spool was severely eroded. Fault handling: The valve functioned normally after repair. 9. In a control loop of the DCS system, it was functioning normally before; however, after the AO channel failed, replacing it did not allow for the control of the valve opening at the site. Fault inspection and analysis: Since the fault occurred as a result of the channel change, the analysis started from that point where the change was made. Because the AO card channel has been changed and reinstalled, the control point cannot find the original channel, so it cannot be controlled. Fault resolution: After reinstalling the corresponding control point, normal control can be restored. 10. The liquid level control system for the boiler’s drum was functioning normally before, but suddenly experienced significant fluctuations; it was switched to manual control. Fault inspection and analysis: The symptoms indicate that the process operating conditions have changed from those under which the system was originally used, so it is necessary to reset the parameters. Fault handling: By increasing control intensity and overcoming interference fluctuations, normal operation was restored. 11. The liquid level control of a certain tower is a single-loop control system. During load adjustment, it was found that the liquid level was unstable with large fluctuations. Fault inspection and analysis: It was discovered that the opening degree of the liquid level control valve LV0405 was only 7%, and the valve position kept changing. Control valves are prone to surge at low opening degrees, and the output from the positioner was unstable; as a result, the control valve operated unstably. Fault handling: Opening the control valve to 15% restored normal operation, but an excessive opening could not maintain the liquid level; only by using the manual valves to set limits before and after use was proper control ensured. 12. In a single-loop control system for liquid level in a petrochemical plant, due to reduced load, the opening degree of valve LV402 was only 5%, which resulted in vibration and unstable liquid level. Fault analysis: Control valves are prone to vibration when operated at low opening degrees; such vibration affects the valve actuator, leading to unstable output and thus unstable automatic control. Fault handling: Slightly reduce the opening of the front or rear shut-off valve to limit the flow rate; when the valve opening reaches 15%, the vibration disappears, the valve positioner stabilizes, and automatic operation proceeds normally. 13. A simple level control system (DCS system with a PID controller, and a gas-actuated control valve) cannot be put into automatic control mode. Fault inspection and analysis: The main reasons for the inability to switch to automatic mode are improper controller parameters, problems with the controlled process, or issues with the control valve. After checking these potential fault points, it was found that the control valve had high internal leakage; even at a small opening degree, a large amount of flow would occur. Fault resolution: The issue was resolved after treating the internal components of the control valve. 14. An example of a fault in the steam pressure control system: a) The pressure reading on the steam pipeline displayed by the paperless recorder suddenly dropped to zero, and the safety valve activated; this was due to a problem with the instrument. When a fault occurs between the pressure tap pipe and the recording instrument, the opening of the control valve changes suddenly, causing the steam pressure to rise sharply, while the paperless recorder shows no response. In such cases, it is possible to switch to manual control of the valve first, and then address the fault. b. If the pressure reading on the steam pipeline gauge on the paperless recorder does not exceed the set value, the safety valve will activate. The instrumentation technician can check the relevant instruments; if the temperatures at all points are normal, it indicates that the safety valve has not been adjusted properly ； If the temperature values at various points increase, it indicates that the pressure readings are lower than the actual pressure. c. Although the pressure fluctuations are large, they occur slowly; generally, the cause should be sought in the process itself. d. If the pressure fluctuations are in a state of rapid vibration, the causes should be sought in parameter tuning and the instruments themselves. e. Changes in load, feeding reflux, temperature, etc., as well as improper operation, can all cause changes in the internal pressure of the equipment; the causes must be sought in the process operations. f. One should be aware of the normal pressure fluctuations for each instrument, determine whether they represent abnormal or normal conditions, and be able to make judgments by referring to other process parameters. 15. An example of a fault in the level control system: a) The level value displayed on the light column regulator moves towards the maximum or minimum value; it is advisable to check the gauge first. If the primary meter is normal, it is a fault of the secondary meter. If the readings of instruments 1 and 2 are consistent, check the manual control valve to see if there is any change in the liquid level. Changes are usually due to process reasons, while no changes are generally indicative of instrument problems. b. If the reading of the instrument with negative migration moves toward maximum, a leak on the negative pressure side should be suspected. In instruments where the gas pressure is directly applied to the negative pressure side, the indicated value tends to reach its minimum; this suggests that the liquid level in the collection tank on the negative pressure side has risen too high. c. A high frequency of fluctuations in the recorded values is usually caused by improper parameter setting or oscillations in the primary instrument. If the fluctuations are slow, it is generally due to operating conditions. d. If it is suspected that the instrument is giving false level readings, the system can generally be switched to manual mode, and process and instrumentation personnel can jointly use a calibrated pressure gauge to measure the gas phase pressure for analysis. 16. An example of a temperature system failure. A certain device has a heavy oil temperature control system; the heavy oil passes through a heat exchanger and is heated by steam. By adjusting the opening degree of the steam control valve, the temperature of the heavy oil changes slowly. However, when automatic control is applied, significant fluctuations in temperature occur. Fault analysis: Since the temperature of the heavy oil does not change significantly even when the steam flow is adjusted, it indicates that there is some delay in the detection system. After checking the thermocouple measurement system and finding no issues, it is likely that there is a problem with the heat transfer system. To make full use of the latent heat of steam, medium-pressure steam must be condensed into water and then discharged regularly through a drain valve. Steam and heavy oil exchange heat through a heat exchanger, and this heat exchange process takes some time. The temperature of the medium-pressure steam is 280°C, while that of the heavy oil after heating is 150°C. As the temperature of the heating steam gradually decreases from 280°C and approaches the temperature of 150°C of the heavy oil after heat exchange, the heat exchange almost reaches a state of equilibrium (with a slight temperature difference due to thermal resistance). At this point, the heating steam has not yet completely condensed into a liquid; it still occupies space within the heat exchanger. Even if the control valve is opened wider, no new steam can flow in, or only a small amount can enter. As a result, the temperature of the steam used for heat exchange does not reach the designed value of 280°C (even though the temperature of the incoming steam is 280°C); instead, it varies between 280°C and the temperature at which the steam condenses into water. Since the steam temperature actually used for heat exchange is lower than the designed value, the heat exchange time increases, resulting in a delay in temperature measurement. A large measurement delay leads to system instability. Fault handling: For this system, the PID parameters of the intelligent temperature controller were tuned by increasing the derivative term, adding an appropriate amount of integral term, and raising the proportional term; the values set were P=50%, Ti=5 min, and Td≈1.5 min, with fairly satisfactory results.