Examples of fault repair in complex control systems

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This post was last edited by yunrun on 2016-2-27 at 21:22. An automatic control system that consists of multiple loops, with two or more transmitters, two or more controllers, or two or more control valves, is referred to as a complex control system. Of course, the analysis, design, parameter tuning, and commissioning of such systems are also correspondingly more complex than those of simple control systems. Commonly used complex control systems include: cascade control, proportional control, ratio control, feedforward control, selector control, and range control systems. You can learn the methods and approaches for troubleshooting faults in complex control systems by referring to these fault repair examples compiled by the engineers from Changhui Instruments. 1. Maintenance example of a cascade control system: A cascade control system has two closed loops. The main controller and the auxiliary controller are connected in series; the output of the main controller (a single-loop PID regulator) serves as the setpoint for the auxiliary controller (an externally supplied regulator). The system controls the operation of the control valve through the output of the auxiliary controller, thereby achieving fixed-value control of the main variable, as shown in Figure 1. Therefore, in a cascade control system: the main loop is a constant-value control system, while the secondary loop is a follow-up system. http://yunrun.com.cn/News/UploadFiles_5183/201602/2016021718375215.jpg Analysis of the fault instance: ① In a certain petrochemical plant, there was a cascade control system for level and flow (with the level serving as the primary loop and flow as the secondary loop, using a DCS system); the quality of control was poor. Fault inspection and analysis: The reasons for the fault included the following: improper setting of the controller’s control parameters, the control object not operating under normal conditions, significant lag in the response of the control object, and non-linear behavior of the control valves. Re-tune the controller parameters, re-calibrate the control valve, and check the test lag time. Fault handling: Comprehensive adjustment tests revealed that the delay time in level control was too long, preventing cascaded adjustment from being implemented. ②The butyl octyl alcohol unit’s LIC_0406 and FIC_0411 form a cascade control system that regulates the output from the 400A reactor. The secondary loop of this control system can be set to automatic mode, but cascade control cannot be enabled; when cascade control is attempted, significant fluctuations occur. Fault inspection and analysis: Tests revealed that the issue stems from improper setting of the PID parameters for cascade control, and these parameters were adjusted based on the actual operational conditions of the process, which is why cascade control cannot be used. Fault handling: The cascade control is activated only after the PID parameters have been readjusted. ③In a cascade control system, the main controller is the steam pressure controller, whose output is normal; this output, along with the flow signal from the pressure pipeline, is fed to the external setpoint of the flow secondary controller via a differential adder. It was found that the external setpoint dropped suddenly, but the output of the main controller remained normal. Fault inspection and analysis: In this system, the output of the main controller and the external setpoint signal from the secondary controller are combined using a differential adder with feedforward functionality, and the resulting value is used as the external setpoint for the secondary controller. The inspection revealed that the fault was caused by poor contact of the fuse in the differential adder, which led to a power loss; as a result, its output became zero, causing the external setpoint for the secondary controller to drop. Fault resolution: Replace the fuse to ensure good contact, and the fault is resolved. ④It is a level-flow cascade control system; the cascade regulation exhibits large fluctuations and fails to meet the control requirements. Fault inspection and analysis: It was determined that the cause of the fault was improper setting of the control parameters for the secondary controller – its integral action was too strong, making it difficult to control the loop. Cascade control systems have high requirements for the main parameters, allowing no errors to exist. A proportional-integral control law is generally used for the main controller; when the controlled object exhibits significant lag, an appropriate differential action can be introduced. However, strict requirements are not imposed on the secondary parameters; since these parameters change in response to the output of the main controller, a proportional control law is generally sufficient for them, with integral action being introduced if necessary. Fault handling: Reset the secondary parameters. 2. Maintenance example of a ratio control system: During the adjustment of the syngas in the vaporization furnace, it was found that the furnace temperature remained normal and the quality of the gas was satisfactory only when the gasoline-to-oil ratio was at a low set value of 10%. Fault inspection and analysis: In the process of cracking residue oil in the vaporization furnace, the amount of steam added is controlled by a ratio system (steam/oil ratio). As the gasoline-to-oil ratio decreases, the amount of steam also decreases; however, since the furnace temperature remained normal and the quality of the gas was good, it indicates that the actual amount of steam did not decrease. First, check whether the loop signal is normal; measurements show that there are no issues with signal transmission. The primary differential transmitter was also verified, and it was found to be in good condition. The valves of the steam pipes that lead to the vaporizer were checked again, and it was found that the emergency steam isolation valve used for shutdown was in the open position; steam entered the vaporizer through this valve. Fault handling: Close the emergency steam manual valve, and the gasoline ratio gradually returns to normal. Then the shut-off valve was checked; it turned out that a blockage in the air circuit of the solenoid valve had caused it to be in the wrong position. After replacing the solenoid valve, everything returned to normal. 3. Maintenance example of a uniform control system: During operation of a certain cascade uniform control system shown in Figure 2, it was found that the main parameter, namely the liquid level, remained stable at a constant value, while the secondary parameters fluctuated significantly, causing considerable interference to subsequent processes. http://yunrun.com.cn/News/UploadFiles_5183/201602/2016021718383430.jpg Figure 2: Cascade uniform control system. Fault analysis: A uniform control system typically takes into account both liquid level and flow rate as parameters; through uniform adjustment, it ensures that these two conflicting parameters remain within allowable ranges – in other words, both parameters should change gradually during the adjustment process, rather than remaining constant at a fixed value. The occurrence of the above issues is clearly due to incorrect approaches and methods for tuning the controller parameters. The parameters of the level controller and flow controller should be tuned following these steps: ① Set the proportionality of the level controller to an appropriate empirical value, then adjust the proportionality of the flow controller gradually, while observing the adjustment process until a slow periodic decay occurs. ②Fix the proportionality of the flow controller at the set value, adjust the proportionality of the level controller from low to high, observe and record the curves, in order to achieve a slower rate of periodic decay. ③Depending on the conditions of the process, an integral action is appropriately added to the level controller in order to eliminate the residual error caused by disturbances. ④Observe the adjustment process and fine-tune the controller parameters until both the liquid level and flow rate exhibit a slower periodic decay. 4. Selection of a control system maintenance case: In the energy-saving control system of a certain ammonia synthesis plant, the automatic control system for synthetic off-gases suddenly triggered a high-pressure alarm while in automatic operation. This system is a selective control system composed of the pressure control system PIC for the synthesis process and the gas composition control system AIC for synthetic off-gases. http://yunrun.com.cn/News/UploadFiles_5183/201602/2016021718390217.jpg Figure 3: Automatic control system for synthetic off-gas. Fault analysis: When this system is operated in automatic mode, the component transmitter AT measures the total amount of inert gases CH4 and Ar in the circulating gas. The AIC controller is used to maintain a constant level of these inert gases within the synthesis system, thereby minimizing losses due to the release of synthetic gas and achieving energy savings. When the pressure in the synthesis system exceeds the rated value, the pressure controller PTC will increase its output continuously based on the signal detected by the pressure transmitter PT, and through the PIS selector, it will take over from the AIC controller to regulate the pressure at a set level, thereby preventing overpressure in the synthesis system. When a high system pressure alarm is triggered, manual operation using a handwheel should be carried out immediately at the site. First, it is necessary to confirm that the pressure transmitter, alarm device, etc., are not faulty; thereafter, check whether the output of the pressure controller replaces the output value of the component controller. If the pressure controller is working properly and its output value has reached the normal replacement value without the component controller being replaced by the selector, it is determined that there is a fault with the selector; if it is a fault with the pressure controller, this controller should be switched to manual mode immediately, and the output value should be increased step by step to control the pressure in the synthesis system. Source: http://bbs.hcbbs.com/?274320