With the advent of the 21st century, the automation of chemical production equipment has been gradually improved, and the safety and stability of chemical production will be directly affected by the stable and reliable operation of the instrument automatic control device. As the combination of chemical instrumentation detection, control, and process equipment becomes more and more tight, the phenomenon of failure will become more and more complicated. Therefore, relevant personnel must have rich practical experience, master the method of correct judgment and analysis of faults, and have timely The ability to handle failures.
Because the chemical production operation has the characteristics of automation, process, and full closure, especially with the rapid development of science and technology, the automation level of modern enterprises has been higher, the process operation is closely related to the instrumentation, and the operator displays it through the instrumentation. Temperature, material flow, container pressure, liquid level, raw material composition and other process parameters to determine whether the process is normal and whether the quality of the product is qualified, and then according to the instructions of the chemical instrument to increase or decrease, or even Parking is discontinued.
When chemical instruments indicate abnormal phenomena such as high, low, non-change, and instability, they themselves include both process and instrumentation factors that may cause these phenomena. Among them, the former correctly reflects the abnormal process of the process; the latter is caused by the failure of a certain part of the instrument and the process parameter indication is inconsistent with the actual. Both process and instrumentation are always easy to come together, making it difficult to immediately determine where the failure is occurring. In order to improve the ability to judge the fault of the instrument, the instrument maintenance personnel need to be familiar with each link in the measurement system in addition to the familiarity with the working principle, structure and performance of the instrument. In addition, the process flow and the characteristics of the process media and equipment should be known.
In short, when analyzing the cause of the failure of the field instrument, pay special attention to the changes of the measured control object and the characteristics of the control valve. These may be the cause of the failure of the on-site chemical instrument system. Therefore, it is necessary to go from the field instrument system and process. The two aspects of the operating system are considered comprehensively. After careful analysis, the cause of the failure is judged.
The valve positioner is the main accessory of the control valve. It uses the displacement signal of the valve stem as the input feedback measurement signal, and the output signal of the controller is used as the setting signal to compare the two. When there is deviation, it is correct. The output signal to the actuator is changed, so that the actuator acts and establishes a corresponding relationship between the stem displacement and the controller output signal. Therefore, the valve positioner system uses the valve stem displacement as the measurement signal, and the output of the controller as the feedback control system of the set signal, and the manipulated variable of the control system is the output signal of the valve positioner actuator.
When the thermocouple changes, it will generate an unbalanced weak electric signal through the bridge of the temperature transmitter, and then be amplified to convert it into a DC4-20mA current signal or a 1~5Vd voltage signal to the working instrument, and the working instrument will be It will display the corresponding temperature value. The common fault phenomena are: the output signal is unstable, there is no output signal, the output signal is large or small, and the actual input signal does not match. In the case of such a fault, the processing ideas are as follows: First, judge the working power supply to see if it is normal, and check the wiring of the instrument; secondly, judge the good or bad of the on-site temperature sensor and temperature transmitter, and then The PLC module input point and output point are judged whether it is normal or not.
Among the detecting elements and the executing elements, frequently-moving parts are stuck or the connecting parts are detached, which are more likely to cause malfunction. If the solenoid valve does not change direction, it may be that the spring is stuck or damaged. Due to rubber deformation and poor lubrication in the valve body, the sliding resistance of the valve core will increase. At this point, reassemble it, replace the spring or seal, and improve lubrication.
After the float type liquid level gauge is used for a long time, there will be rust in the tube, which will cause the float to move inflexibly, which will result in inaccurate liquid level measurement. At this time, it is only necessary to remove the float and clean it, and then reinstall it.
Because the valve moves frequently, the connection between the valve positioner and the cylinder position feedback lever is prone to loosening and falling off, which often causes the valve opening to be inaccurate.
1, the valve does not move
Cause: There is no pressure from the gas source or the gas source; the actuator is faulty or leaking; the control valve has no output signal; the gas supply pipe is broken, deformed, the joint is damaged and leaks; the flow direction is incorrect, due to excessive force The valve is detached; the internals of the valve stem and the shaft valve are stuck and damaged; the valve positioner or the electro-pneumatic converter is faulty; the valve core is stuck in the valve seat. In the event of this failure, the common solution is to check the air supply, repair the control valve, or replace the oil, re-install the correct direction, or repair.
2. The rated stroke that the valve cannot reach
Cause: The locator is not calibrated; the stroke adjustment is improper; the rating of the spring actuator is too small; the position of the manual operating mechanism and the limit block is not accurate. In this case, the fault should be eliminated by calibration, re-adjustment, and spring replacement.
3, the valve is slow or slow
Cause: The friction of the packing is large or deteriorated; the friction of the actuator piston is too large; the friction of the bearing is too large; the response of the positioner is poor or the piston ring is worn. At this time, replacement repair can be used to re-adjust the damaged device or clean the grinding cylinder and piston to eliminate the fault.
In the chemical production process, the cause of failure of most on-site chemical instruments is that on the sensors and regulating valves that are in contact with the measured medium, such faults account for more than 60%. In the actual production practice, if you master the regularity of the failure of the on-site chemical instrument, you can cooperate with the process to quickly and accurately identify the cause of the failure, thus eliminating the fault, to ensure the stable operation of the control system, to prevent the accident. .