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At the current stage, China Petrochemical’s main refining and chemical processing units are required to undergo maintenance cycles every four years. Factors such as heavy renovation tasks during these maintenance periods, limited time available for repairs, and the use of multiple subcontractors have become bottlenecks that affect the quality of such maintenance work. Here, by analyzing the frequency of instrument failures that occur on a daily basis and the resulting operational risks for the equipment, emphasis is placed on formulating preventive maintenance strategies during major instrument overhauls. Meters are essential for the stable and reliable operation of a device. To ensure the reliable operation of instrumentation equipment and extend its service life, petrochemical plants have established various regulations related to the maintenance of such equipment. Most of these regulations are based on experience, or on preventive measures taken in response to faults that have occurred over time. At the current stage, China Petrochemical’s main refining and chemical processing units are required to undergo maintenance cycles every four years. Factors such as heavy renovation tasks during these maintenance periods, limited time available for repairs, and the use of multiple subcontractors have become bottlenecks that affect the quality of such maintenance work. With the gradual adoption of smart instruments equipped with fault self-diagnosis functions, a new technical approach has become available for predictive maintenance of instrument failures. In particular, the widespread use of DCS fault self-diagnosis technology has significantly reduced unplanned shutdowns caused by DCS failures. However, most of the field instruments or intermediate circuits do not possess such fault self-diagnosis capabilities. Statistics show that auxiliary instruments, which often go unnoticed in daily operations, are actually the main cause of equipment shutdown incidents. By analyzing the frequency of instrument failures that occur on a regular basis and the resulting risks to equipment operation, scientific and effective preventive maintenance strategies can be implemented for various instruments during major equipment overhauls. 1. Types of instrument failures: At present, Sinopec’s various enterprises generally use written work orders; it is practically impossible to manually keep track of all the various minor instrument failures in the dozens or even hundreds of units within these enterprises. It is worth promoting the fact that some large petrochemical companies adopted advanced electronic work order systems several years ago, which makes it possible to systematically count and analyze the types and frequency of instrument failures. Figures 1 to 5 show the instrument failure data of a large enterprise, compiled using the electronic work order system, from January 2014 to July 2015 (Note: The data excludes non-failure-related work orders, such as those related to renovation projects and inspections that showed no issues). The statistical data in Figure 1 show that the instruments with the most failures, in order, are: primary instruments and their accessories, control valves, operation stations, circuit components, and analysis instruments. Figures 2 to 5 further illustrate the composition of each part in Figure 1 and their failure rates. As can be seen from Figure 2, the inspection and adjustment of primary meters are the main tasks in daily meter troubleshooting, accounting for 56% of all troubleshooting activities. In petrochemical plants, the frequency of this task is greatly influenced by operating conditions, as well as operational habits — sometimes operators are accustomed to relying on the inspection results provided by instrument technicians to make accurate judgments. The main tasks involved in inspection and adjustment include checking whether the transmitter is at zero, injecting flushing oil and draining it. Such tasks do not pose a significant threat to the safe operation of the equipment. However, patching and sealing leaks in the pressure transfer pipes or heating pipes, which rank second among these tasks, requires attention; leaks in these pipes can lead to the release of H2S, a substance commonly present in refineries, and in severe cases, such leaks can result in casualties ； Although a heat tracing pipe leak poses no major danger on its own, the need to weld it requires the use of fire, which creates serious operational safety risks for the equipment that is in use. The operating performance of the control valve directly affects the smooth operation of the plant. As can be seen from Figure 1, failures of control valves account for about 1/5 of the total failures among instruments, and failures of the accessories associated with these control valves constitute the largest proportion within that figure. Some studies indicate that failures of the accessories related to control valves in hydrocracking units in refineries account for 33.6% of the total failures of such control valves. If the statistical data are extended to all the units in petrochemical plants, especially those cylinder gate valves used for oil storage and transportation, the figure becomes much higher. As can be seen from Figure 3, feedback failures account for 45% of the total failures of control valves. Since most replies do not participate in interlocking, reply failures pose little threat to the stable operation of the device. Valve body component failures are the second-largest cause of control valve failures. During major maintenance campaigns, petrochemical companies give special attention to the maintenance of control valves. However, there are few ways to reduce the failure rate of these valves through maintenance, as for existing valves, it is difficult to ensure that they will not experience problems again in the next cycle, unless the valve itself is modified or the process is altered. Based on the analysis of accidents reported by Sinopec’s various companies over the years, incidents caused by problems with the valve itself that led to train derailments are also very rare. Attention should be paid to valve accessories; failures of positioners and solenoid valves account for 25% of all control valve failures, and positioner failures often lead to fluctuations in the system performance or even shutdown of the equipment ； Solenoids have been the main cause of accidental shutdowns in the plants operated by Sinopec’s various companies in recent years. Failures of solenoids can directly close or open the interlock valves, leading to accidental shutdowns; sometimes solenoids can also prevent the interlocks from functioning, resulting in even greater hazards. In addition to inherent problems, failures of positioners and solenoid valves have also been found in practice to be related to factors such as water entering the wiring box and dirty instrument air. The pressure relief valve with filtering function is likely a component that has not yet received sufficient attention in most refining and petrochemical companies. Although the number of shutdowns caused directly by failures in this valve is much lower than that caused by failures in positioners or solenoid valves, it is certain that a significant portion of the failures in positioners and solenoid valves are due to the inability of the air supply filter to function properly, resulting in contaminated air. DCS failures rank third among all instrument failures (Figure 1); the main causes of DCS failures are issues with the host and peripherals, followed by problems with cards. Failures in the host and peripherals have minimal impact on the operation of the device, and rarely cause fluctuations or shutdowns. Due to the redundant configuration, card-related failures also seldom lead to shutdowns. What requires attention are the problems related to the circuit components, which account for the smallest proportion – only 7% – yet they pose a high risk of causing shutdowns. As can be seen from Figure 3, the three main causes of circuit component failures are terminal and wiring issues, power supply problems, and issues with intermediate components such as alarm setters, safety barriers, relays, fuses, and buttons; these are all “secondary” instruments or accessories that tend to be overlooked. In summary, the main factors causing the device to trip or experience fluctuations are the positioner, solenoid valve, pressure reducing valve, and circuit components. From the perspective of construction safety, the corrosion control of instrument pipelines should also be a key focus. 2. Preventive maintenance measures: The preventive maintenance in refining enterprises is divided into routine maintenance and overhauls during shutdowns. Daily preventive maintenance is carried out through regular maintenance schedules. At present, various refining and chemical companies carry out preventive maintenance work to varying degrees on the instruments on site or on DCS systems, depending on the specific characteristics of each company. However, for positioners, solenoid valves, and pressure relief valves that are prone to causing accidental disengagement or fluctuations, preventive maintenance is difficult to carry out on a daily basis; therefore, it needs to be scheduled during major overhauls. As for reducing instrument failures in the circuit system, some indirect improvements can be made on a daily basis. 2.1 Management of air quality in DCS cabinets. Managing the air quality in DCS cabinets is an important measure for extending the lifespan of components within the circuit. At present, many refining and petrochemical companies are carrying out renovations to consolidate the small control rooms that were previously located next to the various units, turning them into centralized control rooms (CCR). Meanwhile, the on-site cabinet rooms have also been expanded by merging the previous small control rooms. However, a common issue in the cabinet areas on site is poor sealing, which allows corrosive gases from the outside to easily penetrate. This leads to accelerated oxidation and corrosion of accessories such as the safety barriers, relays, and push-button switches used in DCS systems; in practice, blackening of the terminals is often observed. According to the classification of air quality levels in ANSI/ISA-71.04-1985, indoor air quality can be divided into four categories, as shown in Table 1: Classification of Indoor Air Quality Levels. A petrochemical company conducted corrosion testing on exposure coupons for 86 cabinets at its facility; the results showed that 62 of them fell under Grade G1 corrosion, accounting for 72% of the total ； There are 13 G2 corrosion grades, accounting for 15% of the total ； There are 11 G3 corrosion grades, accounting for 13% of the total ； 0 GX corrosion grades. The cabinet areas in the G2 and G3 corrosion severity zones mainly include sulfur units, wastewater treatment units, coking units, power plant units, flare units, and circulating water systems. There are relatively high levels of corrosive substances or dust in the air surrounding these units, which leads to a higher frequency of damage to the indoor instrumentation and control equipment. The test results are consistent with practical experience. Regarding measures to improve air quality, installing indoor air purification and recirculation units in cabinet rooms is an excellent measure for older installations where it’s not possible to maintain a slightly positive pressure environment. At the same time, it’s essential to ensure proper sealing of the cabinet rooms—particularly at cable entry points, doors, windows, partition walls, ceilings, and beneath raised floors. Additionally, the air conditioner should be operated with the fresh air mode used as little as possible, otherwise the effectiveness of the air purifier will be significantly reduced. In the cabinet room adjacent to a company’s coking unit, there was once a set of components in the emergency shutdown system that had a high failure rate; a thin layer of coke dust could often be seen on the movable floor in that room. It was found that this dust was brought in by the fresh air from the air conditioning system. The problem was resolved after the operating mode of the air conditioning system was changed and an air purifier was installed. 2.2 Preventive maintenance strategies for control valves and their accessories: The valve body components of control valves rank second in terms of frequency of failure. Nevertheless, no petrochemical company currently conducts thorough disassembly and maintenance of all valves during major overhauls. The basis for valve maintenance is usually the defects that have been identified, taking into account valves in vulnerable or critical locations. Valves located at vulnerable and critical parts of petrochemical plants are generally defined as those used in high-temperature oil environments, fuel gas (oil) valves, and valves situated in areas subject to frequent operation and easy erosion (such as wear, coking, and polymerization) ； Cylinder actuators for applications with high temperatures such as heating furnaces (regenerators), and self-acting N2-sealing valves for tank tops. To address the issue of worn packing, the packing of control valves should also be replaced prophylactically on a regular basis. If the valve can be diagnosed for faults using valve diagnostic software, the scope of maintenance should be determined based on the diagnostic results. Valve diagnosis enables a comprehensive assessment of the valve’s condition, including issues such as sticking of internal components and gaskets. Currently, valves equipped with intelligent positioners from some foreign manufacturers allow for online fault diagnosis. For solenoid valves, special attention should be paid to checking the terminal contacts and coil insulation. For the two five-way solenoid valves, air path switching block, unit shutdown solenoid valve, and electro-hydraulic converter, it is necessary to consider disassembling, cleaning, and inspecting them during each maintenance cycle. For some particularly important solenoid valves, it should be considered to replace them entirely after each maintenance cycle or after several cycles. For pressure reducing valves equipped with Plexiglas covers or those having plastic adjustment handles, a gradual replacement process should be carried out to prevent a loss of air pressure in the control valve due to sudden breakage of the filter cover or adjustment handle during operation. Special attention must also be paid to inspecting the filter elements in some domestically produced brands of pressure reducing valves; if any non-metallic filter elements are found, they must all be replaced. Non-metallic filter elements tend to crumble easily, which can have a detrimental effect on the positioner. In light of this, for newly purchased control valves, it is essential to ensure the proper selection of filter-pressure reducing valves from the outset. For mechanical positioners equipped with constant-throttling orifices, during maintenance it is necessary to clear these orifices (such as in HEP positioners) or purge them (such as in KOSO positioners), as well as clean the nozzle dampers. 2.3 Preventive maintenance of power supplies: The power supplies for instruments mainly include 24V (DC) switch-mode regulated power supplies, UPS units, as well as power cards built into various system devices such as DCS systems. Due to the presence of high-power heating elements and electrolytic capacitors in these power supplies, their lifespan is relatively shorter compared to that of other instrument devices. Some voltage stabilizers are equipped with fans; while helping to dissipate heat, these fans also tend to draw in dust, which increases the heating of the internal components and thus shortens their lifespan. To this end, the power supply should be inspected and maintained during the shutdown for maintenance, with the approach combining dust removal and complete replacement. To achieve good dust removal results, the 24V (DC) regulated power supply module and the power switch should be opened for dust cleaning ； As for the unit power supply of critical equipment, its complete replacement should be scheduled on a periodic basis. Another issue related to power supply is that for interlocked instrumentation equipment powered by a single 220V (AC) power source, it must be ensured that the associated process unit does not shut down in the event of a power failure on this line. Although in petrochemical plants, most instrument devices are powered by a dual 220V (AC) supply or by direct current power, some instruments such as local unit controllers, analyzers, and ray level gauges are typically powered by a single 220V (AC) supply. These devices need to be identified; those for which a failure of the single power supply could cause the plant to shut down must be modified. To ensure comprehensive and thorough identification, power switching tests should be conducted during major maintenance, as shown in Figure 6. First, disconnect the GPS output power supply QF4 on the electrical side; the instrument control system operates normally, so power supply can be restored ； Turn off the switch QF3 on the UPS output side; the instrument control system operates normally, and power supply is restored ； Turn off power switch QF4 on the output side of the GPS, and simultaneously turn off power switch QF1 on the input side of the UPS. The instrument control system operates normally; then restore power supply. 2.4 Maintenance of circuit components: For the maintenance of circuit components, it is recommended to replace aged components on a regular basis, such as the 220V (AC) contact relays used for electrical isolation, fuses, and buttons located on the field and control room auxiliary consoles. These components can generally be replaced every 3 maintenance cycles, while alarm setters and safety barriers should be replaced no more than every 4 maintenance cycles ； Second, conduct a thorough inspection of the wiring terminals; in particular, attention must be paid to the switch buttons on the auxiliary control panel, the emergency stop buttons on site, the wiring boxes, and the wiring terminals of various components of the control valves. For each instrument on site, it is necessary to check at least the terminals related to its interlock functions. Additionally, the terminals at the interface between the electrical and instrumentation disciplines are often overlooked. If the division of responsibilities between these two disciplines is not clearly defined, inspections of these terminals may be neglected, thereby creating potential hazards. 2.5 Preventive maintenance of DCS (SIS, CCS, etc.) Generally, a check-up of the DCS is required after each maintenance cycle. Such checks include using specialized software to assess system performance and back up software, removing viruses, as well as cleaning dust from components and replacing hardware based on its lifespan. The main hardware components include CPU backup batteries, filters, various fans, power supply cards, and fuses. Table 2 lists the recommended replacement periods for the hardware of some major system manufacturers. 2.6 Corrosion maintenance of pressure Introduction pipes and heat tracing pipes beneath the insulation layer: In petroleum refining and chemical processing plants, when the measuring elements of instruments come into direct contact with corrosive media, it accelerates the corrosive damage to those instruments. There are many methods available to resist the \"internal corrosion\" of instruments, such as using improved instrument materials or filling them with isolating media. However, the corrosion issue beneath the insulation of gauge pressure pipes or heat tracing pipes has not received sufficient attention. Especially in winter, the frequent leaks in these heat tracing pipes greatly increase the risk to the safe operation of the facility, while external leaks from gauge pressure pipes can sometimes pose a threat to human life. To prevent the instrument pressure tubing from corroding, it is necessary to apply an anti-corrosion coating followed by a insulation layer to stop rainwater from penetrating the insulation. Insulation is the final step in construction, and it is often not completed until the maintenance of the installation is finished. Little attention is paid to the technical skills of those responsible for insulation work or to the quality of this work; moreover, the insulation layer cannot protect against rainwater, so corrosion occurs again after a short time. In this regard, the practices of Nippon Export Oil Refining Company can serve as a model; that company maintains a fixed team of insulation technicians throughout the year to carry out ongoing maintenance of the insulation systems across the facility. According to published reports, the quality of insulation for instruments at that company is extremely high. 2.7 Preventive maintenance of other instruments: Double-flange diaphragm boxes used in high-temperature environments may develop bulging and other issues due to prolonged exposure to high temperatures. When the diaphragm box bulges, the transmitter reaches a critical state of damage, and this condition cannot be detected in time by operators during operation; therefore, it is necessary to carry out preventive inspections and repairs during maintenance periods. Additionally, the insert-type flow meter probes installed on the fresh water, wastewater, and circulating water pipelines within the refinery should also be removed for cleaning; these probes are prone to the growth of algae or scaling, which can affect the long-term operation of the instruments. 2.8 Instrument calibration: The maintenance of Sinopec’s instruments is carried out in accordance with the requirements of the \"Petroleum and Chemical Equipment Maintenance and Repair Regulations\". During each maintenance cycle, each instrument must be inspected and calibrated. However, no Sinopec company has managed to do this yet. Over the years, the question of whether instruments should be thoroughly calibrated during maintenance periods, and how to carry out such calibration, has puzzled the instrument specialists at various companies. Based on the distribution of daily failures, transmitter failures account for only 3% of all failures in primary meters (Figure 2). Calibration provides very limited opportunities to detect meter failures, and the labor costs involved are completely disproportionate to the number of issues identified. To this end, it is necessary to establish more scientific and effective verification principles. Apart from the **instrumentations that are required to undergo mandatory calibration, the calibration of other instruments should be adjusted. For the sake of ensuring their effectiveness, it is recommended that the following instruments be calibrated: 1) Joint calibration of all control valve circuits (valves that have been repaired should also be individually calibrated with a positioner) ； 2) The unit probes, unit housing vibration switches, surge tanks, pressure switches, and alarm setpoints should be calibrated ； 3) For pipe-type flow meters that can be removed and calibrated using technical methods, as well as the thermal resistance (thermocouples) of the shaft bearings newly installed in the units, calibration should be carried out ； 4) Intelligent transmitters (including intelligent double-flange transmitters) should have their parameters compared in order to identify any differences between the values set in control systems such as DCS and the actual values, without the need for actual pressure calibration. 3. Conclusion: To enhance the scientific nature and effectiveness of preventive maintenance of instruments, it is necessary to base such efforts on accurate judgments derived from the analysis of instrument failures. At present, the various companies under Sinopec have begun to use a unified, more advanced EAM equipment management system. It is believed that through several years of collecting and analyzing fault data, the relationship between faults and maintenance or replacement cycles will become clearer, which will play a positive role in guiding the preventive maintenance work related to instruments in Sinopec’s companies.