1. Process Background Introduction In chemical production, electric control valve in the industrial control system is widely used. Automated control can reduce the labor intensity of workers, improve control accuracy and reduce the overall product cost. The typical automatic control principle is: The detection unit transforms the data detected in the process system into an analog or digital signal and sends it to the integrated arithmetic unit. The integrated arithmetic unit compares the received signal with the control command (given signal) Then the results of the operation sent to the executive unit, the executive unit automatically based on the signal control / regulation of the process system. In the current practical application, most of the execution units are based on the control valve as the terminal actuator, so almost all the faults in the circuit will eventually be reflected in the control valve, and in many failures, the control valve fluctuations compare Common, but the worst deal. As the regulating valve is the terminal actuator, so the control valve fluctuations must be combined with the entire control loop analysis to find out the real cause of fluctuations in regulating valve in order to fundamentally eliminate the control valve fluctuations. 2, the reasons for analysis and processing A complete automatic control system involves a number of independent units. In general, there are many reasons leading to fluctuations in the control valve, some of the fluctuations of the control valve itself, the other is the signal fluctuations sent to the control valve cause fluctuations in the control valve, the reason for the signal fluctuations from almost every unit, it is possible produce. 2.1 Detection unit output data signal fluctuation detection unit is obtained from the process system test data instrument unit, the test instrument is divided into two types: direct measurement, such as temperature probe, buoyancy level gauge, etc .; the other is indirect measurement, such as Optical thermometer, radar level meter. From our use and handling of the situation, the detection unit to detect fluctuations in the data are mostly direct measurement instruments. There are two main reasons for the data fluctuations: First, the system itself caused by signal fluctuations. Another case is the failure of the instrument itself, such as buoyancy buoy level gauge, the principle is a buoy in the outer sleeve is not in the medium, according to the level of the medium different buoyancy buoyancy is also different, The output torque varies to produce different output signals. When the medium contains more particulate impurities, because the gap between the float and the outer sleeve is small, if the media into the inside, it is possible to float stuck in the buoyancy buoyancy can still overcome the jamming friction When the force, the float level gauge output signal is prone to fluctuations, the signal sent to the regulating valve is also volatile, causing the regulator valve fluctuations. This situation is to deal with the media to avoid impurities into the middle of the buoy and sleeve gap, such as adding filter and so on. 2.2 integrated computing unit caused by fluctuations in the output signal fluctuations For the integrated computing unit in the large-scale automation control system, the main use of DCS distributed control, all of the signal acquisition, integrated computing, signal output is completed in the DCS system, the final system If the refinement of the operation point of view, signal acquisition, signal output is mainly DCS card at work. The actual instrument circuit shown in Figure 2. Transmitter to detect the data through the DCS system on the card for integration to the CPU computing, and then the results output to the execution unit card, the implementation of the unit according to the signal received on the process system regulation / control. So, in this circuit, if the card fails, it will cause the corresponding control valve problems. This phenomenon is caused by the system hardware failure, under the conditions of the circumstances, the replacement of the corresponding hardware can solve the problem. The other is not the reason for the hardware, for example: steam boilers in the steam water temperature regulating valve, dropped to a large automatic control, a separate adjustment of the control valve, the performance is satisfactory, and later in control Room PID parameters to re-set the original temperature fluctuation range from ± 5 ℃ ± 10 ℃, the control valve fluctuations significantly reduced. This situation is due to the unreasonable setting of the parameters, which may occur in the liquid level or pressure control of some small containers. Therefore, the system PID parameters set unreasonable caused by the control valve fluctuations, generally only need to modify the corresponding system parameters on it. 2.3 Execution unit caused by the control valve fluctuations Execution unit receives the signal output by the integrated computing system, and then converted to pressure signals, or direct current signal to control the action of the control valve to achieve the purpose of control / regulation. To pneumatic control valve, for example; in the implementation of the unit, generally divided into three areas: electrical converters, locators, actuators. In the electrical converter section, the main cause of fluctuations are mainly nozzle / baffle dirty and poor connection two. In the instrument air is not clean, such as meter air, oil, dust, etc., will gradually accumulate in the nozzle / baffle, a long time, the accumulation of too much dirt, it will affect the nozzle exhaust, leading to electrical converter output fluctuation. This happens, only need to promptly clean the nozzle / baffle can be solved. The second is the connection is not strong, sent from the main control of digital / analog signals, through the electrical converter can be converted to positioner or actuator acceptable 20-100Kpa standard signal, when the electrical converter terminal contact is not good or not When docked firmly, even a slight vibration may cause the output of the converter to fluctuate, causing the regulator to fluctuate. Locators, the main cause of fluctuations caused by the structure. The main function of the positioner is to improve the positioning accuracy and speed up the response. If the positioner fails, it may cause many problems. Specific fluctuations in terms of the following possible reasons. One is the same as the electrical converter, because the instrument air is not clean, the nozzle / baffle structure will cause the output of the positioner to fluctuate because the nozzle / baffle is too dirty. For slide valve controlled "O" ring seals Aging, wear and tear can also cause output fluctuations that can cause the regulator to fluctuate. Therefore, it is also acceptable to clean the nozzle / baffle only if the nozzle / baffle is too dirty, and to replace the "O" ring if the spool "O" ring is worn or aged. In addition, the locator feedback loosening may also cause the locator output fluctuations, such as my factory boiler feed valve 100FV106, because the feedback cam loosening screws, resulting in loose feedback, causing the regulator to fluctuate. When the feedback of the positioner loosens, the valve position change of the control valve can not form the negative feedback to the positioner, the signal and the output do not correspond, the control valve action lags behind, and the signal requirement conflicts with the control valve lag, causing the control valve to fluctuate. Therefore, for loose feedback, you need to fasten the loose parts. There is also a situation in the search for the cause of the failure can easily be overlooked, that is, locator exhaust passage is blocked, when the locator action, the amplifier or spool valve may need to exhaust, if the locator exhaust passage is not clear, it There will be fluctuations in the regulation valve. In the actuator part, in addition to the above reasons for signal fluctuations, fluctuations in itself there are two reasons. Common is the implementing agencies bad gas chamber, there leak, string of gas caused by fluctuations. For diaphragm actuators, if the diaphragm is damaged, the diaphragm will leak when the air chamber is pressurized and the positioner will need to constantly adjust the output to replenish the instrument air to the chamber. At this point, the regulator may fluctuate . For the cylinder piston actuator, if the piston can not be sealed, the gas chamber airstrips, the positioner also need to constantly adjust the output, the same prone to fluctuations. Therefore, the leak, string of gas, the need to replace the relevant components to eliminate leakage, string of gas. Another reason is that the effective output of the implementing agencies is not enough, under certain conditions, can not overcome the imbalance force of the spool and packing friction, valve fluctuations, this failure can generally increase the air pressure or spring Preload, but also by the appropriate loose packing to reduce the friction to eliminate. 2.4 The middle part of the valve caused by fluctuations in the control loop from the point of view, in addition to the main part of the control valve caused by fluctuations in the middle of some links also caused the possibility of fluctuations in control valves. Connection between the various units are connected by shielding the signal cable or wiring, if the middle of a place where the connection is not strong, it may cause signal fluctuations. Therefore, the connection between the various major links can not be ignored, in the entire instrumentation control loop, any connection problems will cause the valve can not operate normally. In particular, each terminal, be sure to connect a solid and reliable.
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