Preventive maintenance of chemical process automation instrument systems

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Preventive maintenance of chemical process automation instrument systems: As the degree of automation in the production processes of petrochemical enterprises increases, the monitoring and control points in these processes, as well as the design of the instrument systems, become more sophisticated. The reliability of these instrument systems is closely related to the safe operation of the production facilities. Practice has shown that implementing preventive maintenance for instrument systems in petrochemical enterprises not only reduces the failure rate of instruments and contributes to improved safety management of these systems, but it also saves on maintenance costs, bringing significant economic benefits to the enterprise. However, due to the large number and wide distribution of instrumentation devices in petrochemical plants, a single plant can have anywhere from hundreds to thousands of such devices. These devices are spread throughout the plant – on the ground and in the air, atop and beneath towers, as well as within ducts and pipelines. It is extremely difficult to carry out comprehensive daily inspections of all of them. Therefore, routine inspections of instrumentation equipment can only cover the instruments in critical areas of the installations as well as any obvious leaks or spills. Instruments in less critical areas, as well as those located in high or remote locations, may not be inspected for extended periods of time ； Moreover, for the inspection and maintenance of instrumentation equipment, it is crucial to check the integrity of its operating environment and installation conditions, ensuring protection against water, moisture, freezing, high temperatures, lightning, and corrosion. By doing so, the operating conditions of these devices can be improved, thereby extending their service life. Therefore, it is necessary to establish different regular inspection items and corresponding inspection intervals based on the specific characteristics of the instrumentation equipment, to carry out preventive maintenance, conduct comprehensive regular inspections of such equipment, and identify and address issues promptly. Preventive maintenance of equipment is different from equipment repair and fault handling. Equipment repair and fault handling fall under reactive maintenance, which involves carrying out repair activities after a failure occurs; it is passive and results in losses. In contrast, preventive maintenance entails taking necessary measures and methods to identify and eliminate defects in the equipment before failures occur, keeping it in good condition and thus avoiding disruptions to production. It is proactive and active. However, it is very difficult to carry out preventive maintenance on instrumentation equipment. For example, with dynamic and static equipment, instrument-based monitoring is used to collect and analyze operational data in order to determine whether preventive maintenance is necessary. As for instrument equipment, it is difficult to find effective ways to monitor its operating condition and issue early warnings of potential failures; by the time an alarm signal is generated by the instrument itself, the failure has already occurred. Therefore, the preventive maintenance of instrumentation equipment is primarily achieved through regular manual inspections, in combination with the alarm information on instrument failures collected by the control system. Based on the characteristics of different types of instruments, corresponding inspection standards and intervals should be established. Regular inspections should be conducted to detect any defects in the instruments at an early stage, enabling timely repairs and maintenance to keep them in good working condition. Below, taking into account the characteristics of different types of automated instruments, we will discuss some aspects of preventive maintenance for such instruments in petrochemical enterprises. 1. Preventive maintenance of regular instruments Regular instruments in petrochemical plants include temperature, pressure, flow rate, level, and other measuring instruments. With the development of electronic technology, computer technology, and automated instrumentation technology, new types of instruments continue to emerge; as a result, there is a large variety of instruments, a large number of units, and they are widely distributed. In addition to the manufacturing quality of the instrument itself, its measurement principles, and the limitations imposed by operating conditions, factors such as its operating environment, the medium being measured, the quality of installation, and the integrity of its components also have a significant impact on the instrument’s performance and service life. To improve the operating conditions of conventional instruments, keep them in good condition, and ensure their stable operation, the regular maintenance tasks for such instruments should include: inspection and treatment of the instruments’ seals and waterproofing features, checks to ensure they are protected from high temperatures and freezing, inspections of the instruments’ grounding systems, comparison of the readings from redundant instruments, removal of condensation from the pressure conduits used in instruments that work with gaseous media, and regular replacement of the isolation fluids used in these instruments. 2. Preventive maintenance of control valves: Control valves are part of the actuation mechanism in automatic control systems; they are installed on process pipelines and serve to reduce pressure and regulate flow. They help control parameters such as temperature, pressure, flow rate, and liquid level during production, ensuring that these parameters remain within their normal operating ranges. Control valves can be classified functionally into shut-off valves and control valves. In terms of their structural design, they include gate valves, butterfly valves, ball valves, single-seat valves, double-seat valves, sleeve valves, angle valves, eccentric rotary valves, multi-stage pressure-reducing valves, etc. Their actuators are equipped with a wide range of accessories, such as filter pressure reducers, positioners, solenoid valves, valve position switches, speed increase devices, hold-down valves, directional valves, check valves and other pneumatic accessories, as well as rubber components like sealing rings and diaphragms. Due to the large number of components in control valves, their frequent operation, as well as erosion and corrosion caused by the medium, issues such as packing leakage, loose components, detached connections, leaks in the air pipelines, air leakage from the diaphragm head, and broken springs can occur, resulting in the control valve failing to operate or operating incorrectly. To prevent such malfunctions from occurring, the regular maintenance tasks for control valves mainly include: (1) Regularly checking whether the control valve experiences sticking or oscillation, whether there is any medium leakage from the valve body, whether the accessories of the actuator have been exposed to high temperatures, whether the supply pressure is normal, and whether connectors such as the positioner feedback components are loose. (2) Regularly check whether there are any leaks in the control valve pneumatic system; also check whether there is any air leakage or loose connections in the spare components, spare fittings, and pneumatic diaphragms of the pneumatic system. (3) Regularly clean the dirt from the valve stem to maintain its smoothness, preventing damage to the inner rings of sealing fillers such as tetrafluoroethylene and graphite, which could impair their sealing performance and lead to fluid leakage. (4) Regularly lubricate the shaft of the feedback mechanism and other rotating parts to prevent rust, corrosion, and sticking, ensuring smooth operation. (5) Regularly drain the liquid from the main air supply tank for instrument equipment, as well as from the drain valves located at the lowest points of the instrument air supply systems, and empty the filters used for centralized air supply. This is done to prevent liquid from entering the instrument air supply. During the drainage process, it is essential to keep the air pressure within the normal range at all times. 3. Preventive maintenance of combustible/toxic gas detectors. The main characteristic of petrochemical plants is their susceptibility to leaks, as well as their flammability and explosiveness. Therefore, combustible/toxic gas detectors are installed in areas where leaks are likely to occur within the plants; these detectors send out alarm signals as soon as a leak happens, allowing for prompt action to address the issue. Hence, as safety instruments, combustible/toxic gas detectors must be kept in good working condition and operate properly at all times. However, due to the limitations of their measurement principles, flammable/toxic gas detectors have a high failure rate, low sensitivity, significant drift, poor linearity, and delayed response times, which requires regular inspection and maintenance. To promptly address any malfunctions, it is necessary to ensure an adequate supply of spare parts on one hand, and to conduct regular inspections and maintenance on the other. This mainly includes: (1) Regularly checking whether there is any zero drift in the readings of the combustible/toxic gas alarms displayed on the DCS screen. (2) Regularly check whether the displays of flammable/toxic gas alarms are in good condition, whether the wiring is properly sealed, whether the probe guards are unobstructed, whether the enclosure is properly grounded, whether the display is functioning correctly, and whether the audible and visual alarm functions properly. (3) Regularly test whether the audible and visual alarm system of the combustible/toxic gas detectors on the DCS auxiliary control panel is functioning properly. (4) Regularly calibrate combustible/toxic gas alarms; the calibration tasks include zero-point adjustment, first-level alarm testing, response time measurement, and verification of the actual concentration of the standard gas. (5) Regularly entrust a statutory metrology institution to conduct inspections in accordance with the requirements of the inspection regulations. (6) Regularly update the sensors of combustible/toxic gas detection and alarm devices, and establish and improve a log for regular replacements. 4. Preventive maintenance of control systems: As petrochemical enterprises become more automated, there are an increasing number of control systems in use, and these systems are becoming larger in scale. These include plant-wide DCS, SIS, and CCS systems, as well as various types of PLC systems. These control systems automatically regulate the operation of the controlled objects through preset control programs; they allow for both centralized and decentralized control, helping to save labor while enabling rapid adjustments and reducing operational delays. A control system is a collection of electronic components; it has a compact yet complex structure and high requirements regarding its operating environment. For example, a DCS system requires an ambient temperature of (20 ±2) °C in winter and (26 ±2) °C in summer, with a temperature change rate of less than 5 °C/h. The required relative humidity is 50% ± 10%, and the air quality in the surrounding environment must have dust levels below 0.2 mg/m3 (with particle sizes less than 10 μm). As for corrosive gases, the level of corrosion allowed is below grade G (0.03 μm/month). Control systems are of such great importance and have such high requirements regarding their operating environment. To ensure their safe operation, it is necessary to take good care of them and carry out regular maintenance: regularly check the operating status of controllers and I/O cards, the system power supply, DC power supplies, network switches, and cooling fans; regularly verify that the temperature and humidity in the server room, as well as the temperature inside the cabinets, are within acceptable ranges; regularly check the alarm functions of the operation stations to ensure that the alarm sounds work properly; regularly examine the system’s diagnostic information; regularly check the storage capacity of the operation stations’ history databases and transfer data as needed; regularly check the clock synchronization function of the control system; regularly clean the filters in the chassis; regularly back up the control system software; and regularly check the grounding of the control system. 5. Preventive maintenance of online analytical instruments. Online analytical instruments, also known as process analytical instruments, are installed directly in industrial production processes or at the site of other fluid streams, where they perform automatic and continuous measurement of the components or parameters of the fluid being analyzed. Online analytical instruments are not only used for real-time analysis in production processes, but also find wide application in the continuous monitoring of emissions from environmental pollution sources. With the increasing demands for energy conservation, pollution control and emission reduction, as well as product quality, the importance and usage volume of online analytical instruments are on the rise. The measurement principle of online analytical instruments differs from that of conventional instruments; they often rely on optical or chemical principles. These instruments are installed directly in industrial settings, and they have strict requirements regarding environmental conditions (including explosion-proof capabilities and environmental protection features). They also have strict demands on the conditions of the sample (such as temperature, pressure, flow rate, etc.). Compared to laboratory analysis instruments, they have a poorer ability to adapt to various environmental and operational conditions, their performance is less stable, and their maintenance requirements are higher than those of conventional instruments. Regular maintenance is necessary for them. Preventive maintenance of online analyzers includes: (1) Regularly inspecting the operating environment around the online analyzers, such as the condition of air conditioning, ventilation, and the presence of any medium leaks. (2) Regularly inspect and clean the pretreatment system. (3) Regularly check the operating condition of online analytical instruments. For different instruments, corresponding inspection items are established, such as reagents, calibration gases, carrier gases, and pre-treatment units; and relevant operational data is recorded, such as the measurement values of the analytical instruments and the pressure of the carrier gas. (4) Regularly maintain and calibrate analytical instruments; based on the structural principles of these instruments, determine the appropriate maintenance tasks and calibration intervals. The monthly routine maintenance tasks for environmental protection flue gas analyzers include: purging the sampling pipelines, cleaning the sampling filters, replacing worn pipe joints and peristaltic pump tubes, cleaning the gas-liquid separators and flow meters, cleaning the diaphragms of the sampling pumps, cleaning the fan filters, cleaning the optical surfaces used for detecting dust, and performing calibrations. As for COD analyzers, the monthly routine maintenance tasks include: cleaning the quantitation tubes, replacing or cleaning the pipeline joints and pipes, checking for corrosion in solenoid valves, cleaning the digestion tanks, and performing calibrations. Implementation of preventive maintenance programs: For different types of instruments, there are both general maintenance tasks and specialized maintenance procedures required due to their structural principles. When formulating preventive maintenance programs, both aspects must be taken into account and considered holistically. Based on the estimated failure cycle of the maintenance items and the importance of the instruments, different preventive maintenance schedules are set to conduct comprehensive checks on each instrument one by one. For each type of instrument, there is a corresponding preventive maintenance checklist. Maintenance personnel record the results of the inspections as well as any issues identified on this checklist, sign it to confirm their actions, and indicate the date of the inspection. For any problems identified during inspections, a “four-fix” approach—namely, “defining tasks, assigning responsible units and personnel, determining corrective measures, and setting deadlines for rectification”—is adopted to ensure follow-up corrections until they are completely resolved, thereby nipping instrument defects in the bud. To ensure the effective implementation of preventive maintenance programs, regular inspections must be conducted. For cases where the plans are not followed, or where they are followed but instrument defects remain unresolved, timely reminders and follow-ups should be provided. Since its implementation, it has yielded good results: the failure rate of instruments has decreased significantly, which not only saves on maintenance costs but also reduces the need for emergency repairs to instruments. Even during the long rainy season, no serious instrument failures have occurred. In summary, there is a wide variety of automatic instruments. The maintenance tasks for each type of instrument have their own characteristics and differ from one another. Additionally, the large number of instruments and their scattered distribution within the equipment make it quite difficult to carry out preventive maintenance comprehensively and without any omissions. The preventive maintenance plan should be easy to implement; the tasks and frequency of maintenance should be as reasonable as possible, so as to eliminate instrument defects in a timely manner while keeping the amount of maintenance work at an appropriate level. The list of items included in the maintenance plan should not be too extensive, and the maintenance intervals for these items should be adjusted appropriately so that the workload does not become excessive. Each item should be assigned to a specific maintenance worker, one who is willing to take on such tasks; otherwise, it may lead to resistance and perfunctory handling, failing to achieve the desired results. Automation instruments in petrochemical plants are susceptible to erosion and corrosion caused by process media, exposure to high-temperature media, attack from harmful substances in the air, penetration of rain and moisture, direct sunlight, etc. It is difficult to determine their service life. Premature preventive replacement leads to waste of resources and increased maintenance costs. On the other hand, waiting until a failure occurs before replacing them—especially in the case of interlock instruments and critical instruments—may result in local fluctuations or even shutdowns of the plant, thereby causing even greater production losses. Therefore, for interlocked instruments and critical instruments, it is necessary to calculate and assess their service life in order to enable regular replacement, thus avoiding excessive waste of resources while ensuring safe production.