With the advent of the 21st century, people will enter the information age characterized by the knowledge economy. Technologies such as computers, automation, microelectronics, and communication networks will continue to develop at a high speed. Automation instruments and devices as industrial automation technology tools will also It has entered an era characterized by digitization, intelligence, and networking. The degree of automation of chemical production equipment is gradually improved, and the safety and stability of chemical production will be directly affected by the stable and reliable operation of instrument automatic control devices. As the detection, control, process and other equipment of chemical instruments are more and more closely integrated, the failure phenomenon will become more and more complicated. Therefore, relevant personnel must have rich practical experience, master the correct method of judging and analyzing failures, and have timely Ability to handle failures.1. Analysis of common failures of chemical instrumentsBecause chemical production operations have the characteristics of automation, process-oriented, and fully enclosed, especially with the rapid development of science and technology, the automation level of modern enterprises has been higher, and the process operation is closely related to the detection instrument. The operator is displayed by the detection instrument Various process parameters such as temperature, material flow, container pressure, liquid level, raw material composition, etc., are used to judge whether the process production is normal and the quality of the product is qualified, and then increase or decrease the amount according to the instructions of the chemical instrument, or even Stopped production.When chemical instruments indicate abnormal phenomena such as high, low, unchanged, and unstable, they themselves include two factors that may cause these phenomena: process and instrumentation. Among them, the former correctly reflects the abnormal conditions of the process; the latter is caused by the failure of a certain link of the instrument, which causes the process parameter indication to be inconsistent with the actual situation. The two factors of craftsmanship and instrumentation are always easy to appear together, so it is difficult to immediately judge where the fault has occurred. In order to improve the ability to judge the failure of the instrument, the maintenance personnel of the instrument are not only familiar with the working principle, structure, performance and other characteristics of the instrument, but also need to be familiar with each link in the measurement system. In addition, the process flow and the characteristics of the process medium and equipment should also be understood.In short, when analyzing the causes of failures of field instruments, special attention should be paid to the changes in the characteristics of the controlled objects and control valves. These may be the reasons for the failure of the on-site chemical instrument system. The two aspects of the operating system are comprehensively considered, and after careful analysis, the cause of the failure can be judged. 2. Judgment and treatment measures of valve positioner failureThe 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. The output signal to the actuator is changed, so that the actuator takes place, and the relationship between the displacement of the valve stem and the output signal of the controller is established. Therefore, the valve positioner system uses the valve stem displacement as the measurement signal and the controller’s output as the set signal feedback control system, and the manipulated variable of the control system is the output signal of the valve positioner actuator.Three temperature transmitter failure judgment and treatment measuresWhen the thermocouple changes, an unbalanced weak electrical signal will be generated through the bridge of the temperature transmitter, and then amplified and converted into a DC4-20mA current signal or a 1～5Vd voltage signal to the working instrument. The corresponding temperature value will be displayed. The common failure phenomena mainly include: unstable output signal, no output signal, large or small output signal and the actual input signal does not match, etc. When encountering such a fault, the processing ideas are as follows: firstly, judge the working power supply to see if it is normal, and check the wiring of the instrument; secondly, judge whether the on-site temperature sensor and temperature transmitter are good or bad, and then check The input point and output point of the PLC module are normal or not to be judged.Four frequently moving parts are stuck or the connection piece falls offIn the detection element and the actuator, the frequently moving parts are jammed or the connectors fall off, which are more likely to cause failures. If the solenoid valve cannot be reversed, the spring may be stuck or damaged. Due to the deformation of the rubber in the valve body and poor lubrication, the sliding resistance of the valve core will increase. At this time, it is necessary to re-assemble it, replace the spring or sealing ring, and improve the lubrication. After long-term use of the float-type level gauge, there will be corrosion in the tube, which will cause the float to move inflexibly, resulting in inaccurate liquid level measurement. At this time, you only need to remove the float, clean it, and reinstall it. Because the valve moves frequently, the connection between the valve positioner and the cylinder position feedback rod is prone to loosening and falling off, which often causes the valve opening to be inaccurate.Five valve action failure1. Reasons for the failure of the valve to operate: no air source or insufficient pressure at the air source; failure or leakage of the actuator; no output signal of the control valve; broken or deformed air supply pipe, damaged joints and air leakage; incorrect flow direction, The valve may fall off due to excessive force; the internal parts of the valve stem and shaft valve are stuck or damaged; the valve positioner or the electric-pneumatic converter is malfunctioning; the valve core is stuck in the valve seat. When encountering this fault, the common solutions are: check the air source, repair the control valve, or replace the lubricating oil, reinstall the direction, or repair.2. The rated stroke that the valve cannot reach is caused by: there is no calibration positioner; improper stroke adjustment; the rated value of the spring actuator is too small; the position of the manual operating mechanism and the limit block is not accurate. At this time, the method of calibration, readjustment, and spring replacement is needed to eliminate the fault.3. Sluggish or slow valve action causes: packing friction or deterioration and aging; actuator piston friction is too large; bearing friction is too large; the response performance of the positioner is poor or the piston ring is worn out. At this time, you can use methods such as replacement and repair, re-adjustment of damaged components, or cleaning and grinding cylinders and pistons to eliminate the fault. Conclusion: In the chemical production process, most of the on-site chemical instrument failures are caused by the sensors and regulating valves that are in contact with the measured medium. This type of failure accounts for more than 60%. In actual production practice, if the regularity of the failure of the on-site chemical instrumentation is grasped, the cause of the failure can be quickly and accurately identified with the process, so as to eliminate the failure to ensure the stable operation of the control system and achieve the purpose of preventing problems before they occur.
Post time: 21-09-21