5 Secrets to Daily Maintenance and Repair of Major Instruments

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The key tasks in the daily maintenance of instruments include the following: ① Periodic inspections； ②Regular lubrication ； ③Regular sewage discharge ； ④Thermal insulation and heating ； ⑤Daily maintenance of the DCS system. 1. Generally, instrument technicians on patrol have their own designated areas responsible for the maintenance of instruments. Based on the distribution of instruments in these areas, they determine the optimal patrol route. They must conduct at least one patrol per day. During such patrols, the instrument technicians should consult with the on-duty process operators regarding the operating conditions of the instruments. ①Check whether the instrument indications are normal, whether the records are accurate, whether the indications of the field primary instruments (transmitters) are consistent with those of the instruments in the control room and the control instruments, and whether the output indication of the regulator is consistent with the valve position of the control valve. ② Check whether the instrument power supply voltage is within the specified range and whether the air supply has reached the rated value. ③Check the insulation and heat tracing condition of the instruments. ④Inspect the instrument body and connectors for damage and corrosion. ⑤Check for leaks at the instruments and process interfaces. ⑥ Check the condition of the gauge. 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 frequency of 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 transmission components of the on-site indication parts such as elliptical gear flow meters and rotameters ； ② Transmission components of pneumatic long-stroke actuators ； ③ Pneumatic cam flexure valve rotating components ； ④ Rotating components of pneumatic cut-off ball valve ； ⑤ Pneumatic butterfly valve rotating components ； ⑥ Felt gasket on the oval gland of the control valve ； ⑦ Door hinges for protective boxes and insulated boxes. In addition, the double-headed bolts used to secure ring chambers, as well as the exposed threads, and those 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 rusting of the threads and to avoid difficulties in disassembly and assembly. 3. Regular sewage discharge: There are mainly two tasks involved in regular sewage discharge – one is the discharge of sewage itself, and the other is carrying out cleaning operations on a regular basis. This work should be adapted to local conditions; not all process monitoring instruments require regular drainage. (1) Contamination mainly affects instruments such as differential pressure transmitters, pressure transmitters, and float level gauges. Since the measuring medium contains dust, oil residues, tiny particles, etc., which deposit inside the pressure guiding tubes (or within the pressure sampling valves), this has a direct or indirect impact on the measurement accuracy. The sewage 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 discharge can only take place with their approval ； 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 beneath the drain valve, and slowly open the drain valves for the positive and negative pressure pressure conduits to allow the material and waste to flow into the container, preventing them from being discharged directly into the sewer. Otherwise, this 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 used to prevent leakage at the drain valve, thereby avoiding any impact on accuracy ； ⑥Open the pressure-taking valves for positive and negative pressures on the three-valve assembly; loosen the drain (vent) screw on the differential pressure transmitter body to drain waste 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 carry out 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. Inspection of insulation and heating for instruments: Inspecting the insulation and heating of instruments is one of the tasks involved 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 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 condition of instruments installed on process equipment and pipelines, such as elliptical gear flowmeters, electromagnetic flowmeters, vortex flowmeters, turbine flowmeters, mass flowmeters, flanged differential pressure transmitters, float level gauges, and control valves, to ensure that the insulation is in good condition. They should also check whether the insulation material has fallen off or been wet 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, heat tracing is sometimes required in addition to insulation; heat tracing can be electric or steam-based. 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 enterprises. 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 exhaust condition of the steam trap in the heat tracing steam pipe; continuous exhaust from the trap indicates an excessive steam flow rate, while no exhaust for an extended period suggests a too low steam flow rate. The steam flow regulation margin is quite large, as steam tracing is used to prevent the material in the pressure guide tube 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, if the heat preservation and heating for materials with relatively low boiling points are too intense, vaporization will occur; a two-phase mixture of vapor and liquid will form inside the pressure gauge tube, leading to output fluctuations. Therefore, it is essential to adjust the amount of heating steam in accordance with the weather conditions during winter. 5. Daily maintenance of DCS systems: With the increasingly widespread use and development of computer technology in the industrial sector, DCS systems have become increasingly important in the production process. Any issue in any part of the system can result in the failure of some of its functions or cause problems with the control system; in severe cases, it can lead to the shutdown of production. Therefore, DCS system maintenance personnel must carry out regular maintenance and preventive upkeep to ensure the proper operation of the system. (1) Daily maintenance of the DCS system: Check the operation of the air conditioning equipment to ensure that room temperature variations remain within plus or minus 5 degrees, thereby preventing condensation on the system’s equipment due to sudden changes in temperature and humidity. Check the UPS power supply status and load. Check 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. Check the operation status of the power supply, all cards, and communication components within the control cabinet; whenever the indicator lights show abnormal behavior, the faults should be resolved promptly. Check the operating condition of auxiliary components such as terminal wiring, safety barriers, and relays, and address any issues found promptly. The filters at each operator station and control station should be cleaned regularly. Try 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 cables connecting the equipment and the communication cables. Ensure 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. Check 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. Check the fault diagnosis screen to see if there are any fault alerts. Check the remaining space on the storage medium for historical data; if the space is insufficient, the historical data should be backed up 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 outage 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 chassis, and the power supply cabinet. ② UPS power supply maintenance and inspection of system power supply circuits. ③Grounding system maintenance. This includes terminal inspection and ground resistance testing. ④ On-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 the 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 cables 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 within the card itself, or the absence of configuration information for that slot. ④When a certain production status becomes abnormal or an alarm is triggered, we can first locate the instrument that reflects this status, 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.