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Preface: The daily maintenance of process monitoring and control instruments is a highly important task; it is an essential part of ensuring production safety and smooth operation. Such routine maintenance reflects the preventive approach of total quality management. Instrument technicians should carry out this maintenance work diligently to ensure the proper functioning of the instruments. The key tasks for the daily maintenance of instruments include the following five items: 1. Patrol inspections 2. Regular lubrication 3. Regular sewage removal 4. Insulation and heating 5. Daily maintenance of the DCS system. 1. Patrol inspections: Instrument technicians usually have designated areas responsible for the maintenance of instruments. Based on the distribution of instruments in their respective areas, they determine the most efficient patrol routes, carrying out at least one patrol inspection per day. During these inspections, the technicians should inquire with the on-duty process operators about the operating status of the instruments. (1) Check whether the instrument indications and recordings are normal; verify whether the indications from field primary instruments (transmitters) correspond to those shown on the display and control instruments in the control room, and whether there is consistency between the regulator output indication and the valve position of the control valve, etc. (2) Check whether the power supply voltage of the instrument is within the specified range, and whether the air supply reaches the rated value. (3) Check the insulation and heat tracing condition of the instruments. (4) Check for damage and corrosion of the instrument body and connectors. (5) Check for leaks at the instruments and process interfaces. (6) Check the condition of the instruments. Note: The condition of the instruments can be checked in accordance with the \"Equipment Maintenance and Repair Procedures\" issued by the Ministry of Chemical Industry. 2 Regular lubrication: Regular lubrication is also part of the daily maintenance tasks for instrument technicians, but it is often overlooked in actual work. The interval for regular lubrication is determined based on specific circumstances; it can be either once a month or once a quarter. The instruments and components that require regular lubrication include the following: (1) Gear-driven parts of on-site indication devices such as elliptical gear flow meters and rotameters; (2) Drive parts of pneumatic long-stroke actuators; (3) Rotating parts of pneumatic cam-deflecting valves; (4) Rotating parts of pneumatic shut-off ball valves; (5) Rotating parts of pneumatic butterfly valves; (6) Felts on the elliptical gaskets of control valves; (7) Door hinges of protective and insulated enclosures. In addition, the double-headed bolts used to secure ring chambers, as well as the exposed threads, and the bolts and threads used to fix instruments, control valves, etc. in harsh environments, should have their exposed parts coated with lead grease (graphite powder mixed with butter) to prevent the threads from rusting and causing difficulties in installation and removal. 3 Regular waste discharge: Regular waste discharge involves two main tasks – one is the actual discharge of waste, and the other is performing cleaning operations on a regular basis. This work should be carried out according to local conditions; not all process monitoring instruments require regular drainage. (1) Contamination: Contamination mainly affects instruments such as differential pressure transmitters, pressure transmitters, and float level gauges. Since the measuring medium contains dust, oil residues, and tiny particles that deposit within the pressure guiding tubes (or in the pressure sampling valves), this has a direct or indirect impact on the measurement accuracy. The discharge cycle can be determined by the instrument technician based on practical experience. The following precautions should be observed for regular waste discharge: ① Before discharging waste, it is necessary to contact the process engineers and obtain their permission before proceeding ； ②Before draining the sewage from a flow, pressure, or level control system, it should first be switched from automatic to manual mode to ensure that the opening degree of the control valve remains unchanged ； ③For differential pressure transmitters, close the positive and negative pressure valves of the three-valve assembly before draining ； ④Place a container under the drain valve, and slowly open the drain valves for the positive and negative pressure pressure pipes to allow the material and waste to enter the container, preventing them from being discharged directly into the sewer. Otherwise, it not only pollutes the environment but also leads to waste ； ⑤Due to the poor quality of the valve, the drain valve fails to close properly after being opened and closed several times. As a temporary solution, a blind flange is installed to prevent leakage at the drain valve, thereby avoiding any impact on accuracy ； ⑥Open the pressure tapping valves for positive and negative pressures on the three-valve assembly; loosen the drain (vent) screw on the differential pressure transmitter body to drain fluid, and tighten the screw once drainage is complete ； ⑦Observe the on-site indicator instruments until the output is normal; if it is a control system, switch it from manual to automatic mode. (2) Purging: Purging is a method that uses air blowing or fluid injection to prevent the medium being measured from coming into direct contact with the instrument components or measurement pipelines, thereby ensuring that the measuring instrument can perform accurate measurements. Air blowing involves continuously and quantitatively injecting gas into the object being measured through the measurement pipelines. Flushing involves continuously and quantitatively injecting liquid into the object under measurement through a measuring pipeline. 4 Check insulation and heating for instruments. Checking the insulation and heating of instruments is one of the tasks in the daily maintenance work of instrument technicians. It plays a role in saving energy, preventing instruments from being damaged by freezing, and ensuring the proper operation of the instrument measurement systems; it is therefore an important aspect of instrument maintenance that cannot be ignored. This work is highly regional and seasonal in nature. In winter, during their routine inspections, instrument technicians should check the insulation of instruments. They need to examine the insulation of instruments installed on process equipment and pipelines, such as gear flow meters, electromagnetic flow meters, vortex flow meters, turbine flow meters, mass flow meters, flanged differential pressure transmitters, float level gauges, and control valves, to determine whether the insulation material has fallen off or been wetted by rain, which could render it ineffective. When individual instruments require thermal insulation and heating, it is necessary to check the condition of the heating system and address any issues promptly. It is also necessary to check the insulation of the pressure difference transmitters and pressure transmitters’ pressure guiding pipelines, as well as the insulation of the insulation boxes. Since the material inside the pressure guiding tubes of differential pressure transmitters and pressure transmitters is at rest, heating is sometimes required in addition to insulation; the types of heating include electric heating and steam heating. For electric heating, parameters such as the power supply voltage and temperature setting values should be carefully checked. The temperature setting should be determined based on the properties of the medium being measured; it is advisable to keep the medium from vaporizing or crystallizing (freezing) in order to ensure the proper operation of the instruments. Steam tracing is the most common form of tracing in chemical plants. For steam tracing, due to the large temperature fluctuations in winter, with differences of around 20°C, instrument technicians should adjust the flow rate of the tracing steam according to these temperature changes. The steam flow rate can be determined by observing the venting condition of the heat-tracing steam pipe trap; continuous venting from the trap indicates an excessive steam flow rate, while no venting for an extended period suggests a too low steam flow rate. The steam flow rate adjustment margin is quite large, as steam tracing is used to prevent the material in the pressure guiding pipe from freezing. It should be noted that a larger amount of heating steam is not necessarily better. Some instrument technicians, in an effort to save time, increase the amount of heating steam and fail to reduce its flow even when the weather gets warmer; this leads to unnecessary waste of energy, increased consumption, and sometimes even measurement errors. Since chemical materials have different freezing and boiling points, excessive insulation and heating for those with relatively low boiling points can lead to vaporization; this results in a two-phase mixture of vapor and liquid inside the pressure gauge tube, causing output fluctuations. Therefore, it is essential to adjust the amount of heating steam in accordance with the weather conditions during winter. 5 Maintenance of DCS systems: As computer technology is increasingly widely applied and developed in the industrial sector, DCS systems have become increasingly important in the production process. If any component in the system fails, it can result in the loss of some of the system’s functions or cause failures in the control system; in severe cases, this may lead to a shutdown of production. Therefore, DCS system maintenance personnel must perform routine maintenance and preventive maintenance to ensure the normal operation of the system. (1) Daily maintenance of the DCS system: ① Check the operation of the air conditioning equipment to ensure that the room temperature variation remains within ±5°C, thereby preventing condensation on the system equipment due to sudden changes in temperature and humidity. ②Check the UPS power supply status and load. ③Check the ventilation and cooling conditions inside the control cabinet, with a focus on the operation of the exhaust fans. If an increase in noise is observed, the fan bearings should be lubricated promptly ； And clean the filter screen promptly. ④Check the operation status of the power supply, all cards, and communication components inside the control cabinet; when the indicator lights show abnormal behavior, the faults should be resolved promptly. ⑤Check the operating condition of auxiliary components such as terminal wiring, safety barriers, and relays, and address any issues found promptly. ⑥The filters at each operator station and control station should be cleaned regularly. ⑦Try to minimize the interference of electromagnetic fields on the system; avoid moving operator stations, displays, etc. while they are in use, and be careful not to pull or damage the equipment’s connection cables and communication cables. ⑧Ensure proper backup of the files in the control subdirectories, as well as keep records of system data such as the PID parameters for each control loop and the forward and reverse actions of the regulators. ⑨Check whether the hardware such as the operator station, monitors, mouse, keyboard, etc. is in good condition, and ensure that real-time monitoring is functioning properly. ⑩Check the fault diagnosis screen to see if there are any fault warnings. ⑪Check the remaining space on the storage medium for historical data; if the space is insufficient, back up the historical data to an external storage device in a timely manner. After the system is powered on, the communication connectors must not come into contact with conductive objects such as cabinets. The redundant communication cables and connectors should also not be in contact with each other, to prevent damage to the communication network card. (2) Preventive maintenance: Preventive maintenance should be carried out once a year during major overhauls, in order to monitor the system’s operating condition and eliminate potential faults. During the major overhaul, thorough maintenance of the DCS system should be carried out, including: ① Power shutdown for maintenance of the operation stations and control stations. This includes dust cleaning of components such as the inside of the industrial computer, the control station chassis, the I/O card cage, and the power supply box. ②UPS power supply maintenance and inspection of system power supply circuits. ③Inspection and maintenance of the grounding system. This includes terminal inspection and ground resistance testing. ④On-site equipment maintenance. For specific procedures, refer to the relevant equipment manual. After the major repair, the person in charge of system maintenance must confirm that all conditions are met before powering on the system, and must strictly follow the power-on procedures. (3) Common fault maintenance ① The DCS system has rich self-diagnosis functions. Based on the alarms, it is possible to locate the fault point directly, and the results of repairs can also be verified by resolving the alarms. ② Poor contact at the communication connectors can cause communication issues; once such poor contact is identified, tools can be used to reseat the connectors ； Damaged communication lines should be replaced promptly. ③If the fault light of a card is flashing or all the data on that card are zero, possible reasons include incorrect configuration information, the card being in standby mode with the redundant terminal connections not made, a fault with the card itself, or the absence of configuration information for that slot. ④When an abnormality or alarm occurs in a certain production state, we can first locate the instrument that reflects this state, and then follow the direction in which the signal is transmitted to use instruments to check the accuracy of the signal one by one, until the source of the fault is identified.