How to analyze on-site instrument failures

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Analysis of the causes of faults in on-site instruments and measures to prevent them: Improving the reliability of automatic control instruments and ensuring the long-term stable operation of measurement and control systems is the foundation for maintaining stable production. It is therefore crucial to reduce the failure rate of measuring instruments and control systems. How can the failure rate of instruments be reduced? Years of maintenance have shown that when instruments operate in a favorable environment and the product quality is satisfactory, their failure rates are generally very low. In harsh environments, even if the product quality is excellent, its failure rate remains high. Below are some insights regarding the classification of common faults in field instruments, their causes, and the preventive and corrective measures to take. I. Classification of field instrument failures Looking at the common failures of field instruments over the years, these can be roughly divided into three categories based on their causes: environmental factors, human factors, and inherent factors of the instruments themselves. Environmental factors can be further divided into sealing, vibration, corrosion, electromagnetic interference, high temperature, and non-human-induced damage ； Human factors can be further divided into improper use, improper design and installation, intentional damage, and inadequate maintenance ； Internal factors refer to quality issues with the instrument itself. Among them, environmental factors are the most common cause of failures in on-site instruments, followed by human factors, and then factors related to the instruments themselves. Among environmental factors, failures caused by poor sealing are the most common. II. Characteristics of faults in on-site instruments and corresponding countermeasures 1. Environmental factors (1) Faults caused by poor sealing The instrument enclosures come in various protection ratings, such as IP44 and IP54; they are generally installed in areas with low dust levels indoors ； IP65 and IP67 are generally installed on-site. This type of failure can be categorized into three main types: One is the use of instruments with a lower level of protection in outdoor or humid environments, which may allow rainwater or other liquids, dust, and humid gases to enter the instruments, leading to poor contact within the instruments, faults in the electrical components, or inadequate lubrication of the mechanical parts ； The second category is caused by poor sealing of the cables leading to the field instruments ； The third category involves poor sealing of the instrument panel, caused by issues such as the gasket not being installed properly, not being installed at all, or the gasket becoming worn out. Our company is located by the sea, where humidity and salt levels are high; malfunctions resulting from poor sealing are among the most common issues affecting the instruments on site. For field instruments installed outdoors, special attention should be paid to their protection rating. Instruments with a low protection rating should preferably not be installed in outdoor areas or in locations prone to leaks; protective measures such as the use of instrument enclosures should be adopted instead. There are several reasons for poor sealing at the cable entry point; one of them is that a sealing joint has not been installed on the cable ； One issue is that the instrument cable connector does not match the sealing joint – there are types such as PG13.5, 1/2NPT, M20*1.5, etc., which may appear similar but have incorrect threads, resulting in poor sealing; in other cases, the sealing sleeve is not tightened properly; yet in still other cases, the inner diameter of the rubber ring used for sealing inside the connector does not match the outer diameter of the cable. In response to this situation, when selecting components it is important to ensure that the size of the connectors matches that of the cables; installation and maintenance should not be done carelessly – proper installation procedures must be followed and the connectors tightened properly ； When reliable sealing is not possible due to special circumstances, silicone or epoxy can be used to fill the joint. Poor sealing of the instrument cover at the site is also one of the causes of failures. Maintenance personnel may fail to tighten the instrument cover properly, or they might not install seals, or the quality of those seals may be poor; all of these factors can lead to water or other corrosive substances entering the instrument, thereby causing failures. In response to this situation, first install the sealing washer in place; if the sealing washer is damaged or missing, it should be replaced promptly. Then tighten the gauge cover, and if necessary, silicone or epoxy can be poured around the cover, or sealing tape can be used for wrapping it. In outdoor, humid environments, wrapping the instruments in plastic bags is also a method of sealing them. We often see instruments covered with plastic bags at the site; although it doesn’t look very elegant, it remains a simple and effective way to reduce the incidence of malfunctions in those instruments. (2) Non-human-induced damage: Failures caused by abnormal conditions in the production process that result in damage to the on-site instrumentation components. For example, forcing the valve to open with a cold melt ； The control valve is stuck by foreign objects ； Pressure instruments in ultra-high pressure applications ； The instrument was damaged as a result of external impact ； Lightning causes damage to instruments ； Abnormal fluctuations in the power supply cause abnormalities in the instruments. Due to its suddenness and unpredictability, it is difficult to prevent and control; however, protective measures can be incorporated into the design, along with cooperation from operators during use and maintenance, to reduce and avoid such failures. (3) Vibration: A certain proportion of failures in field instruments are also caused by vibrations in the surrounding environment. Common issues include loose instrument mounting screws or clamps, poor contact of the instrument, poor soldering, vibrations that change the set values of the switches, and vibrations that cause instability in measurement or control. Appropriate countermeasures can be taken, such as tightening the screws with anti-loosening spring washers, adding rubber pads and shock absorbers, moving the instruments away from sources of vibration, and strengthening inspections to eliminate potential problems at an early stage. (4) Corrosion: Acids and bases with strong corrosive properties can cause damage to instruments; one type of corrosion affects the instrument’s casing, while another affects the measuring elements that come into contact with the materials. This problem is not very severe in our company. However, the salty air near the sea severely corroded the contacts of the relay and the switch contacts, increasing the resistance when they were closed and causing certain amounts of malfunction. (5) Electromagnetic interference: One type is interference with the output signal; instruments with millivolt signals and frequency signals are particularly susceptible to such interference. The other type is interference with the signals inside the instrument itself. The approach taken is to avoid interference sources as much as possible ； Use a shielding shell ； Shield grounding ； Use shielded cable ； Shielded cable should be used for the instrument cable during installation. (6) Failures caused by high temperatures and changes in operating conditions: Excessively high or low ambient temperatures or temperatures of the medium can cause instruments to malfunction or even get damaged. Similarly, changes in operating conditions can cause instruments to fail to meet production requirements; for example, replacing instruments with low precision with those of higher precision will not satisfy the production needs ； Rotary flow meters were originally used to measure clean, low-viscosity liquid media; they are now used for measuring suspensions or liquids containing oily impurities, and it is difficult for them to meet the requirements of production processes ； Diaphragm pressure transmitters designed for measurements at normal temperatures, when used to measure high-temperature media, can easily be damaged. Take measures to pay attention during the selection process; when the process changes, replace the instruments accordingly in a timely manner ； In addition, corresponding protective measures should be taken; for example, cooling measures can be employed when the ambient temperature is high. 2. Human factors: (1) Instrument failures caused by human error, such as accidentally damaging the diaphragm of a level gauge while working on a hot well, damaging instruments due to the handling of goods, causing failures when water used for cleaning equipment splashes onto the instruments, damaging cables through cutting and welding operations, damaging instruments as a result of high currents passing through them during welding, and severe disruption to instruments caused by electromagnetic interference from flaw detection processes. Such faults are primarily prevented by taking proper precautions during operations. (2) Failures caused by improper maintenance: Failure due to water ingress resulting from the maintenance personnel not tightening the instrument cover, poor contact caused by loose wiring, and failures resulting from illegal operations. In response to this situation, it is necessary to strengthen the sense of responsibility among maintenance personnel, foster a spirit of dedication to their work, and at the same time they should study professional knowledge thoroughly and operate in strict accordance with technical procedures. (3) Failures caused by improper installation: Unreasonable or non-standard installation methods result in the instrument not functioning properly or being unable to operate properly over a long period of time. If a rotameter that needs to be installed vertically does not have the proper levelness, it may experience measurement errors or fail to function properly ； Support is required but not installed; installation in an inappropriate environment causes instrument errors, etc. In this regard, improvements should be made based on the actual situation. 3. Failures caused by issues with the instrument itself. This type of situation generally refers to failures resulting from the substandard quality of the instrument. If this occurs frequently, it is necessary to choose a different product and opt for one of high quality. III. Faults of off-site instruments: Control fluctuations caused by process abnormalities, which are difficult to adjust ； Material blockage causes difficulties in valve feeding ； Interlocked shutdown of heat transfer furnaces, pelletizers, or other equipment due to external conditions ； Alarms caused by process abnormalities or suspected instrument failures such as measuring instruments exceeding their range. In this regard, it is necessary to make a distinction and take timely measures to minimize losses.