[Useful Tips Sharing] Instruments should be “prevented from failing” rather than “repaired”; A compiled handbook on preventive maintenance for instrumentation and control

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This post was last edited by Xiao Fu fu on 2024-6-19 at 14:07. Thoughts before reading: Why is it necessary to carry out preventive work in instrumentation and control? What does preventive work in instrumentation and control include? What are the key points of preventive work in instrumentation and control? How to carry out preventive work in instrumentation and control? At present, many people’s approach to preventive work in instrumentation and control still remains: I am responsible for the instruments, and you are responsible for the electrical systems ； The equipment is operated by me, while you’re responsible for repairs – that’s an outdated way of thinking. These ideas are directly the reason why there are only fire-fighting teams at the production sites nowadays, with no maintenance teams ； Only individual combat, no teamwork ； An excessive reliance on high-speed equipment and other aspects of the current production situation. We know that 70% of equipment failures are caused by inadequate lubrication and cleaning; therefore, lean production places great emphasis on comprehensive equipment maintenance involving all employees. Such comprehensive maintenance refers to autonomous preservation activities in which all staff participate. The purpose of doing this is to foster a management philosophy in which everyone takes responsibility for the maintenance and care of their own equipment, becoming operators who are familiar with it and capable of handling any abnormalities. Referencing the maintenance procedures for instrumentation and control systems, and drawing on the enterprise’s experience in instrumentation and control management, this document compiles key aspects and essentials such as regular maintenance, seasonal maintenance, preventive repairs, and specialized inspections for these systems. It is intended to assist those working with instrumentation in optimizing the operation and maintenance of such systems, reducing the amount of time required for shutdowns for maintenance, improving professional management standards, lowering the incidence of equipment failures, and ensuring the reliable operation of instrumentation and control systems. I. Terms and Definitions 1. Preventive maintenance refers to the activities of testing, maintaining, repairing, and upgrading equipment in accordance with predetermined plans or relevant guidelines, in order to prevent equipment damage, ensure its safe operation, and improve its operational reliability. It includes regular maintenance, seasonal maintenance, predictive maintenance, and special inspection maintenance. 2. Regular maintenance: Maintenance of instruments and control systems is carried out on a regular basis in accordance with relevant regulatory standards, as well as the procedures for maintaining such instruments and control systems. Regular maintenance mainly includes regular inspections, periodic checks, routine tests, regular lubrication, and regular cleaning. 3. Predictive maintenance is a maintenance strategy that involves evaluating the condition of instruments and control systems as well as conducting risk assessments, in order to determine the most appropriate time for repairs and the most efficient repair methods. This approach ensures safe and reliable operation while keeping maintenance costs reasonable. Status detection includes methods such as online monitoring and offline testing. 4. Special inspection and maintenance: In addition to regular maintenance, preventive repairs, and seasonal maintenance, it refers to preventive repair work aimed at addressing specific defects or faults in the instrumentation and control systems. 5. Online monitoring involves the continuous and real-time assessment of various health conditions of instruments and control systems, as well as data related to their operating environment. 6. Offline testing makes use of specialized offline testing equipment to assess the condition of instruments and control systems while they are in operation. This type of testing involves short-term inspections, which is different from continuous online monitoring over extended periods of time. 7. Ying, Yi, Ke – “Ying” represents a mandatory requirement that must be fulfilled ； “\"It is advisable\" represents a recommended requirement; companies are advised to implement it at an appropriate time once the conditions are met ； ““Can” is a selective requirement; enterprises decide whether to adopt it based on actual circumstances. 8. Periodic inspection: Methods of periodic inspection include verification, calibration, validation, and comparison. Verification refers to the activity of determining, through experiments and in accordance with **metrological verification regulations**, whether the error of an instrument’s indicated value meets the required standards. Calibration refers to the process of determining the indicated value of a instrument through experiments, in accordance with relevant calibration standards. Typically, comparison measurements are carried out with standard instruments of higher accuracy to determine the relative error of the instrument, thereby obtaining a correction value for its indicated value. Verification refers to a method of establishing traceability for value transfer when there are no relevant testing procedures or calibration specifications, by employing methods developed by the enterprise itself. Comparison refers to the process of comparing readings using measuring instruments or methods with a defined accuracy level under specified conditions. 9. The maintenance cycle specified in the standard periodic maintenance procedures for petrochemical equipment is generally 3 to 5 years, or it may coincide with the maintenance of the facility itself. 10. Level 3 verification is aimed at eliminating potential risks such as loose wiring connections, poor contacts, or incorrect wiring that can lead to power supply failures, unit shutdowns, and device stops. It involves strengthening the inspection and verification of the correctness, integrity, and stability of instrument circuit wiring, through a verification process in which the operators, quality reviewers, and technical managers are all involved. II. Regular maintenance: During the production and operation of the equipment, in order to improve the reliability of instruments and control systems and reduce sudden failures, it is necessary to create conditions for carrying out regular maintenance work on a planned basis. Based on factors such as equipment condition and regional environment, the periodic maintenance schedule is determined within the baseline cycles specified in the equipment maintenance and repair regulations for the petrochemical industry. 1. Regular inspections: (1) Regularly verify/calibrate fixed combustible and toxic gas detectors, measuring instruments used in trade settlements, online environmental monitoring instruments for nationally (provincially) regulated pollution sources (such as CEMS, COD, VOCs, ammonia nitrogen, etc.), as well as energy and material measurement instruments, in accordance with the requirements of **measurement standards. (2) Pressure (differential pressure) transmitters (switches) in interlock circuits, control circuits, and measurement circuits related to product quality, flow meters with bypass lines, control valves with bypass lines, on-line analysis instruments, external float level gauges, temperature instruments with sleeves, etc., shall be calibrated in accordance with the maintenance procedures for instruments and control systems as well as the operation guidelines established by the enterprise itself; the calibration interval shall not exceed 1.5 times the standard interval. (3) For interlock and control instrument circuits, as well as measurement circuits related to product quality such as radar level gauges, float level gauges, flow meters without auxiliary lines, and control valves, which cannot be calibrated during operation, online comparison is carried out; the execution cycle shall not exceed 1.5 times the baseline cycle. 2. Regularly check the zero points of fixed combustible and toxic gas alarms, the clock synchronization of the control system, as well as the load on the controllers and communication systems; this should be done at least once a month. 3. Regular tests of the auxiliary control panel screens and audio systems should be conducted at least once a month. Hot standby switching of DCS servers, and ventilation tests for fixed flammable and toxic gas detectors shall be carried out at least once every six months. 4. Regular lubrication should be applied to instruments such as high-temperature float level gauges, control valves with oil filling ports, long-stroke actuators, actuators equipped with oil mist lubricators, and scraper flow meters, at least once every six months. 5. Regular cleaning: Clean the air conditioning filters in the cabinet room and the online analysis booth, as well as the cooling fins of the outdoor units, at least once every six months. Clean the filter screens and fans in the control system cabinet at least once every six months. Clean the host computers such as engineer stations, servers, and operation stations at least once a year. Clean the lenses of optical measuring instruments at least once every six months. III. Predictive Maintenance Predictive maintenance relies on the status monitoring of instruments and control systems as well as intelligent inspections; it analyzes the results of equipment condition monitoring to identify defects, guides the repair processes, and determines appropriate maintenance intervals. Through the monitoring of the status of instruments and control systems, the detection results are analyzed and evaluated, and any abnormalities identified are incorporated into the equipment defect management process. The condition monitoring platform should possess automatic analysis functions such as defect detection and fault diagnosis, which can directly guide enterprises in carrying out inspection and maintenance work. Establish a unified status monitoring platform to manage the equipment status monitoring data from various enterprises in a centralized manner. 1. Online monitoring: (1) The control system self-diagnosis and alarm monitoring function automatically collects system alarm events such as controller alarms, card and channel status alarms, communication alarms, grounding alarms, power module alarms, and fan alarms from the self-diagnosis data of control systems like DCS, SIS, CCS, and PLC, and transfers them to the status monitoring platform. These alarms can be sent to managers and maintenance personnel in a hierarchical manner via text messages, enabling timely handling of any abnormalities in the control systems. (2) Fixed combustible and toxic gas alarm monitoring transmits the zero drift and fault alarm signals from fixed gas alarms to the status monitoring platform, and can deliver them in a hierarchical manner to managers and maintenance personnel via text messages. (3) Interlock operation status monitoring introduces the operation status signals of interlock circuits into the status monitoring platform, enabling real-time supervision of their operation status; these signals can also be sent to managers in a hierarchical manner via text messages. (4) Interlock instrument alarm monitoring: In the safety instrument system, logic for detecting deviations, open circuits, and short circuits in redundant interlock instruments is established; these alarm signals are transmitted to the status monitoring platform, and can be sent to managers and maintenance personnel in a hierarchical manner via text messages. (5) Monitoring of control valve actuation time: For control valves that require a specific actuation time and are equipped with valve feedback devices, it is advisable to monitor their actuation time within the control system and set up timeout alarms; such alarms should be transmitted to the status monitoring platform. (6) Instrument power status monitoring: For AC power supplies such as 110VAC and 220VAC, as well as DC power supplies such as 100VDC and 24VDC, voltage transformers and current transformers should be installed on the feed side of the main air switch in the instrument power cabinet. The current and voltage values are then sent to the DCS in the form of standard signals ranging from 4mA to 20mA, and from there to the status monitoring platform. Connect the fault alarm of the 24VDC DC switching power supply for the instrument to the DCS, and transmit it to the status monitoring platform. (7) Temperature and humidity monitoring in the cabinet room: A temperature and humidity meter with a 4mA–20mA standard signal output should be installed in the cabinet room. The temperature and humidity signals are fed into the DCS system as well as transmitted to the status monitoring platform, with alarms set up for abnormal conditions. The temperature in the cabinet rooms and engineer’s rooms is 20°C ± 2°C in winter and 26°C ± 2°C in summer, with a relative humidity of 40% to 60%. (8) The instrument heating monitoring system collects the temperature of the transmitter diaphragm box using protocols such as HART or fieldbus, or monitors the heating condition of the instruments by adding thermal resistors, etc.; it then transmits this data to the status monitoring platform and sets up alarms. (9) The intelligent instrument management system enables online configuration, debugging, calibration, status monitoring, and alarm management of HART or fieldbus instruments such as intelligent transmitters, flowmeters, and level gauges. Valve management software is used to control intelligent valve positioners, and functions such as automatic testing, fault diagnosis, diagnosis of valve operation characteristics, and friction analysis are employed to enable monitoring of the status of control valves and predict maintenance needs. 2. Offline inspection: (1) Infrared temperature detection should be carried out at least once a month using infrared imaging to inspect the electronic equipment located in cabinets such as power supply cabinets, system cabinets, terminal cabinets, safety barrier cabinets, and network cabinets. For solenoid valves such as those operating at 220VAC and 100VDC that have reached 80% of their designed service life, infrared temperature testing should be conducted at least once per quarter. (2) For the measurement of ground resistance, a portable ground loop resistance tester is used; the ground resistance of the working ground busbars and the protective ground busbars in the control system cabinets should be checked at least once a year, with the value not exceeding 4Ω. (3) For the instrument air system, which serves as the power source for pneumatic actuators, it is advisable to use a portable dew point detector to conduct offline measurements of the dew point of the instrument air. (4) Use a HART handheld terminal or debugging software for monitoring the status of mass flow meters; once per quarter, check operational parameters such as the basic zero point, drive gain, detection coil voltage, and vibration frequency of those mass flow meters used for trade transfers as well as for measuring energy and materials. Analyze their stability by comparing these values with historical trends. 3. Status assessment involves collecting information on the status of instruments and control systems through online monitoring and offline testing, including parameters such as intensity, value magnitude, and trends in changes. Various methods are employed for status assessment and analysis; by combining the results of analyses conducted on similar devices, a comprehensive judgment is made to identify any abnormalities in the equipment, which are then recorded in defect management systems ； At the same time, a database and a condition monitoring platform for equipment condition assessment are established. 4. Service life management (1) Principles for determining service life: To ensure both the reliability and economic viability of equipment, and by taking into account hierarchical equipment management, as well as factors such as equipment selection guidelines and manuals, mean time between failures (MTBF), service life requirements specified in technical documents, and results of equipment reliability assessments, the recommended service life for different types of equipment is determined. (2) Service life assessment: A dynamic evaluation mechanism for the service life of instruments and control systems is established. By referring to the recommended service life and taking into account factors such as the equipment’s operating conditions, operating environment, condition monitoring, and historical failure statistics, the performance of these instruments and control systems is assessed. This serves as a basis for deciding whether to upgrade them in advance or allow them to continue operating beyond their intended service life, and it also guides the implementation of preventive maintenance tasks for such instruments and control systems. When the operating conditions of the instrumentation and control systems are poor, or when they deviate from the designed process conditions and operating ranges for an extended period, the impact of operating conditions such as temperature, pressure, and corrosion on the equipment’s service life should be taken into consideration during evaluation. When instruments and control systems operate in harsh environments such as high temperature and humidity, lightning, dirt, and strong radiation, or when the operating conditions exceed the permitted range over a long period of time, the impact of these operating conditions on the equipment’s service life should be taken into consideration during evaluation. Based on the scope of impact in the event of a failure in the instrumentation and control systems, the service life of instruments and control systems for critical and important circuits is recommended. Among them, the critical circuits are those interlock circuits and important control circuits that can lead to unplanned shutdowns of the equipment, or to safety, environmental, or quality-related incidents ； Important circuits are the interlock circuits that can cause unplanned shutdown of the unit, and the main control circuits. The recommended service life is specified in point 7. IV. Seasonal Maintenance Seasonal maintenance tasks mainly include protection against lightning, rain and moisture, high temperatures, wind and floods, freezing and condensation, small animals, as well as dust control. 1. Lightning protection is implemented by determining the lightning protection rating for instruments based on the data regarding the number of thunderstorm days in the area where the enterprise is located, and by carrying out targeted lightning protection measures in light of actual lightning strike incidents. Conduct lightning protection inspections before the thunderstorm season. Develop lightning protection emergency response plans based on the specific conditions of the instrumentation and control systems, and enhance drills. 2. Check the temperature and humidity in the instrument cabinet room to prevent rain and moisture; also check the operation of the air conditioning and dehumidifiers in the cabinet room and the analysis booth. Check the waterproofing of the ceiling in the cabinet room, the functionality of the roof drainage outlets, and the sealing of the cable entrances in the cabinet room. Check for water ingress in the cable trench; the cables inside must not be submerged in water. Check the sealing of the field control cabinet, field instrument junction boxes, and wiring boxes to prevent moisture from entering. Check that the cable protection tubes, explosion-proof flexible hoses, and gland seals are in good condition. Check that the rain cover of the pneumatic actuator is in good condition. Check the seal of the rotating shaft of the top-mounted piston actuator. 3. Heat protection: During hot seasons, the frequency of infrared imaging inspections of the control system should be increased. During the hot seasons, the frequency of infrared temperature measurement of solenoid valves should be increased. For instruments and control systems located in areas exposed to high temperatures and direct sunlight, measures to protect them against high temperatures (or to ensure their tolerance to such temperatures) should be considered. Check whether the temperature control in the instrument cabinet room and the analysis booth meets the requirements ； Check the cabinet fans and ensure that the exhaust fans in the analysis booth are operating properly. Enhanced inspection of positive pressure ventilation and cyclone cooling systems in on-site cabinets (control boxes) should be carried out. Conduct a thorough inspection and maintenance of the air conditioning systems in the cabinet rooms and analysis booths before the hot season. During the high-temperature season, inspect the electrical control components and various mechanical parts of the air conditioning equipment in the cabinet room and analysis hut at least once a month, and adjust the operating conditions of the refrigeration system ； Inspect the entire air conditioning system (including air inlets and outlets, air control valves, heating, humidification, cooling, ventilation systems, drainage pipes, etc.) to ensure proper operation of the equipment. When there is a backup machine, switching should be performed once a month. 4. Before typhoons and the flood season, inspect the stability of instrument cable trays and covers, high-altitude instruments and their accessories, as well as instrument pipelines to prevent wind and flood damage ； Under abnormal weather conditions, increase the frequency of inspections on field instruments and instrument boxes located at elevated places and windy areas. Check whether the doors and windows of the cabinet room, the doors of the on-site control cabinets, and the doors of the instrument panels are tightly closed. Check the readiness of emergency supplies and the condition of flood control and drainage equipment. Formulate emergency response plans for typhoons and floods, and organize drills. 5. Antifreeze and anti-condensation measures: Develop work plans and emergency response strategies for antifreeze and anti-condensation before their onset. To reduce the labor intensity of filling the isolation fluid, it is advisable to install an instrumented isolation fluid filling system. Before the onset of the anti-freezing and anti-condensation period, carry out the following tasks: complete the centralized drainage of condensate from instrument air and gas media prone to carrying liquid. Inspect the instrument trace heating system. Adjust the heat tracing system in a timely manner according to weather changes and the actual conditions of the process medium. Inspect the isolation fluid system and perform centralized filling. During the anti-freezing and anti-condensation period, carry out the following tasks: strengthen inspections for anti-freezing and anti-condensation measures, ensure the proper operation of the instrument heating systems, and eliminate any leaks or spills. Check whether the instrument enclosure is in good condition and whether the door of the instrument box can be closed tightly. Check the insulation condition of instrument pressure piping and trace heating lines, especially whether the insulation of instrument pipes located at heights or in drafty areas is intact and effective. Check the operation of the steam traps in the instrument heating pipelines, as well as whether the return water in these heating pipelines is functioning properly. Check whether the gauge isolation fluid system and the flushing oil system are operating properly. Fill the heavy medium circuit with isolation fluid before extreme weather conditions. Based on the dew point measurement of the instrument air, discharge air from the end of the instrument air pipeline and keep records. 6. Seal cable entry points and other holes and openings in small animal-proof cabinet rooms, analysis huts, etc. The doors and windows between cabinets should be in good condition and tightly closed. A removable mouse barrier should be installed at the entrance to the instrument cabinet room. Appropriately place a sufficient number of rodent control devices along the corners of the walls or under the floor in the cabinet room, and inspect them regularly. 7. The key areas for dust prevention efforts are regions prone to sandy and dusty weather, as well as installations related to thermoelectric power generation, coking, and coal chemical industries where dust is present in the environment. Take appropriate preventive measures before dust storms arrive. Regularly clean the cabinet room and engineer’s room. The doors and windows of the cabinet room should be tightly closed, and the inlet ports of the cabinet room, the bottom of the instrument control cabinets, and the on-site wiring boxes should be properly sealed. V. Special Inspection and Maintenance 1. Failure Analysis of Circuit Boards: Given the high degree of integration of electronic components, which makes fault analysis difficult, failure analysis is conducted on the faulty components to identify the root causes of the problems. Targeted measures are then taken to prevent such faults from occurring again. (1) Generally, it is required that the faulty electronic components of the instruments and control systems, or certain circuit boards selected prior to maintenance, be sent to the manufacturer or a testing facility with the capability for failure analysis, so that a failure analysis report can be prepared. (2) Visual inspection: Using a magnifying glass and a 2D imaging device, check the appearance of the faulty devices or modules for signs of wear, to determine whether the screen printing is clear and accurate; examine the device surface for any signs of abrasion, and check the pins for oxidation, detachment, or looseness. Also, verify that the soldering is adequate. (3) Electrical performance testing: Measures voltage, current, overshoot, undershoot, etc., on the circuit board where the faulty device is located. (4) Fault location determination: If the defective component is a chip, methods such as X-RAY inspection, ultrasonic micro-scanning analysis, and anatomical analysis can be employed to locate the fault ； If it is the modular circuit that has failed, anatomical analysis can be carried out first to locate the fault point. 2. Terminal tightening inspection: The scope of inspection includes the terminal connections of instruments, such as those on field instruments, in intermediate junction boxes, and in cabinet connections. Specific implementation requirements: Prepare a terminal tightening plan by referring to the inspection records from the previous cycle and daily inspection records. Prepare the appropriate tools and materials based on the actual needs on site. Pre-set torque screwdrivers are recommended for tightening; the torque required to tighten the terminals should be set quantitatively, and the torque setting of the screwdriver must match the size of the terminal screws. The torque parameters are provided in Attachment 2. To ensure proper terminal tightening, three levels of verification are employed. The on-site pipe markings are unclear and should be replaced. During normal operation, an infrared imager can be used to check whether the power terminals of the control system’s power supply, high-power devices, solenoid valves, and other important instruments are loose. Keep proper records of terminal tightening, noting the numbers of terminals that are significantly loose or oxidized, to use as a basis for formulating the terminal tightening plan for the next cycle. 3. To ensure both safety and reliability, the interlock system should adopt a three-out-of-two configuration in order to improve single-point interlocks. The emergency stop button should use a mechanical multi-contact switch to achieve redundancy. Analog instruments are recommended for interlocks, and systems with redundantly configured interlock instruments should be equipped with deviation alarms. Redundant solenoid valve assemblies should be used for critical interlock solenoid valves. When the tank root valve is an interlocked shut-off valve or a control valve, the maintenance unit should carry out regular maintenance work offline, and the production unit should create the necessary conditions for this. During routine inspections, the SOE records of the interlock system are checked, focusing on the operation status of the SOE software and any abnormal alarms. The logical diagrams, wiring diagrams, and other related drawings, documents, and records of the interlock system are complete and accurate. Interlocked instruments (including safety barriers and relays) should be equipped with clear warning labels, and terminals within wiring boxes and cabinets should also be labeled. 4. Instrument power supply: Redundancy testing of the power supply: Before putting the instrument control system into operation, it is necessary to conduct tests on the dual-circuit power supply system together with the electrical engineering team. This involves checking whether the equipment and connections related to the dual-circuit power supply are correct, and verifying that all functions of the control system remain operational in the event that one of the power supply circuits fails. Check the specifications, capacity of the circuit breakers and fuses in the power supply circuit, as well as their compatibility at different levels, to ensure that the load requirements are met. High-quality shorting clips should be used for the 24VDC parallel power supply terminals and circuit breakers; self-made shorting wires should not be employed. When using a redundant configuration for 24VDC power modules, it is advisable to provide a reverse polarity protection device for each switching power supply separately. When replacing a DC switching power supply online, the effect of reverse charging of the capacitors inside the supply must be taken into account; it is necessary to check the integrity of the anti-backflow devices before making the replacement. Instrument power supplies with fans should not be used. 5. Control System: The control system should promptly apply patches to address vulnerabilities in the operating system. The compatibility of system patches must be verified through testing by the control system manufacturer. The control system should be equipped with an allowlist or antivirus software and updated regularly. The compatibility of the allowlist or antivirus software and virus database must be verified by the control system manufacturer. The unused USB ports on the human-machine interfaces such as the engineer station and operation station should be physically blocked. Weak passwords are strictly prohibited for operating permissions of control system application software and for logging into the operating system. The backup of control system software should be carried out separately for each device and system; at least two backup media copies should be created, indicating the name of the device or system as well as the backup date, with these copies stored in a separate location. Based on the backup, GHOST disks can be used for offline storage in the hosts of the operation station and engineer station. Establish a comprehensive emergency response plan for system virus infections, organize drills, and disconnect from the information network immediately once the system is infected by a virus. In control systems equipped with a status transition switch on the controller, it is strictly prohibited to set the switch to the off state during system operation. Technical expertise should be mobilized to carry out the configuration of the control system, as well as factory acceptance and on-site acceptance, in order to ensure the quality of the control system project. The use of communication signals for interlocking is strictly prohibited. The control signals that the SIS system outputs to the DCS should not be transmitted via communication methods. 6. Control valves and accessories: Filters and pressure reducers should be made of metal to ensure reliability. For solenoid valves, it is advisable to use those with 24VDC power supply and low power consumption (≤4W), made of stainless steel; 220VAC power supply should not be used in order to achieve redundant power supply. To prevent sticking of the electro-hydraulic converter or electro-hydraulic servo valve, it must be installed only after the oil quality has been verified to be satisfactory. Regularly inspect the instrument air system for leaks using soapy water or other leak-detection liquids. Stainless steel pipes are recommended for instrument air ducts, and the hardness, ellipticity, and surface finish of these ducts should be compatible with the selection of ferrule connectors. During inspections, if it is found that the rain cover of the actuator is damaged or missing, it should be addressed promptly, and it should also be checked whether water has entered the actuator. High-frequency actuation valves use packing that meets the requirements of ISO 15848 CLASS B or higher for the measurement, testing, and qualification of escape emissions from industrial valves. Newly installed or repaired interlock on-off valves must undergo leakage tests in strict accordance with the standard GB/T4213 for Pneumatic Control Valves. 7. Cable trays and instrument trays should not be installed above high-temperature process pipelines and equipment, or below process pipelines and equipment containing corrosive liquids. It is indeed impossible to avoid taking protective measures to prevent the cables from being damaged. During the construction acceptance of instrument junction boxes, the welding quality of the box supports should be checked to ensure they are firmly secured. 8. Inspection of instrument pressure lead pipes: The inspection scope for instrument pressure lead pipes should cover all pressure-taking pipelines and pipe-fitting components downstream of the primary valve, and it is also necessary to assist the equipment team in inspecting the primary valve and the small connections upstream of it. The pressure tapping pipes for instruments should be made of stainless steel, except in those areas where corrosion by chloride ions makes the use of stainless steel pipes inappropriate. Threaded connections should not be used for instrument pressure taps. For pressure tap pipes for which online monitoring is not possible, the rate of wall thickness corrosion at the container or pipeline where the pressure is taken can be used as a reference to predict the thinning of the pressure tap pipe’s wall thickness. Inspect the external anti-corrosion coating and corrosion thinning of the pressure transfer tubes made of carbon steel. 9. Instrument cables: Inspect the power supply circuits for instruments; pay special attention to analyzing cables of different specifications used within the same power supply circuit, and make immediate corrections if they do not meet the load requirements. Regularly conduct insulation testing on the cables of critical or important circuits. 10. When installing a Coriolis mass flow meter, it is necessary to ensure that the pipes upstream and downstream of the meter are concentric. To ensure accurate measurement by the mass flow meter, there should be straight pipe sections of over 500 mm upstream and downstream. There should be reliable fixed supports at both ends of the flow meter, with a margin to eliminate stress. When installing the flow meter, it is necessary to ensure that the distance between the instrument flange and the pipeline mating flange is equal to the thickness of the gasket. When tightening the flanges, a torque wrench should be used to ensure even distribution of force on the bolts. Before introducing the medium after the mass flow meter is installed, the empty-tube drive gain and zero point should be checked to ensure that the values are within the normal range ； After the flow meter is filled with the medium, check the drive gain and zero point to ensure that the values are within the normal range. For mass flow meters used to measure easily vaporizable gases, a \"flag\" installation is recommended, with the fluid flowing from bottom to top. When measuring volatile liquids such as liquid hydrocarbons and propylene, the pressure at the outlet of the mass flow meter should be higher than the saturated vapor pressure of the liquid being measured. In mass flow meters used for measurement, when the actual pressure is higher than the pressure at which they were calibrated, the output signal from the pressure transmitter should be fed into the mass flow meter to achieve dynamic pressure compensation. 11. Differential pressure steam flow meter: For differential pressure steam flow meters, intelligent flow transmitters or flow computers that are capable of performing dynamic steam density compensation should be used. When a production unit is equipped with a steam generation unit and the steam inlet and outlet of the unit need to share the same pipeline, a two-way throttling device is recommended for steam metering, along with two intelligent flow transmitters, and dynamic density compensation is applied to the two measurement circuits. VI. Delay Management: When the instrumentation and control systems cannot undergo preventive maintenance as planned, the equipment management department should take the lead in organizing a technical assessment. Changes must be processed if the following conditions are met. The planned execution time can be delayed by up to 1 year, but preventive actions must be completed within 1.5 benchmark cycles: no abnormalities were found during inspections that could threaten the safe operation of the instrumentation and control systems. The self-diagnosis information of the instrumentation and control system shows a normal status. Offline testing shows that the instrument and control system are in normal condition. There are no familial defects that could endanger the safe operation of the instrumentation and control systems. Since the last maintenance, it has not been subjected to any severe adverse operating conditions. Technical evaluations should adhere to the principle of \"safety first, prevention foremost,\" and be conducted by taking into account a range of factors such as the condition monitoring of instruments and control systems, their technical status, service life, previous maintenance, cleaning, and calibration activities, the operating environment of the equipment, and any incidents of failure. The evaluation results should include whether an extension is possible, the safety measures during the extension period, and the duration of the extension. VII. Service Life of Instruments and Control Systems (Recommendations)
**Objects** | **Equipment Type** | **Critical Circuits (years)** | **Important Circuits (years)**
---|---|---|---
Instrument control systems (indoor) | DCS, SIS, CCS, PLC (including FGS) | 15 | 15
Instrument control systems (on-site) | On-site PLCs, CCS, microcontrollers, control units in turbines and special valve hydraulic stations, etc. | 12 | 12
Field instrument transmitters | — | 12 | 15
High-pressure thermocouples | — | 8 | 12
Wear-resistant thermocouples | — | 4 | 8
Ordinary thermocouples/thermoresistors (including turbine temperature probes) | — | 10 | 15
Level instruments | — | 10 | 15
Flow meters | — | 10 | 15
Switch-type instruments | — | 8 | 12
Control valves (including valve bodies, internal components, valve stems, actuators) | — | 15 | 20
Gate valves (including valve bodies, internal components, valve stems, actuators) | — | 15 | 20
Solenoid valves (220VAC) | — | 5 | 10
Solenoid valves (24VDC) | — | 10 | 15
Servo valves | — | 4 | 8
Feedback devices | — | 8 | 12
Positioners | — | 8 | 12
Pneumatic components (including filters, pressure reducers, speed increase devices, etc.) | — | 5 | 10
Turbine vibration and displacement probes along with pre-amplifiers | — | 10 | 15
Turbine electro-hydraulic converters | — | 10 | 15
On-line analysis instruments | — | 10 | 15
Fixed-type combustible and toxic gas detectors | — | 8 | 10
Other field wiring boxes/cabinets | — | 10 | 15
Air conditioners | — | 8 | 8

In addition to the above, preventive measures should be taken, and enterprises need to enhance internal and external training to understand common equipment failure symptoms and repair methods ； Secondly, it is necessary to make everyone aware of the importance of preventive measures and establish corresponding assessment and management systems ； Finally, hold more related activities to help instrumentation and control personnel develop the good habit of consciously maintaining the equipment.