Causes of failures in instrumentation and control systems

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When there is a fault in the instrumentation and control system, what the operator may report to the instrumentation technician is something like “The instruments are broken” or “The automatic control system is not working.” The manifestations of instrument failures are diverse, such as no display, the control valve not responding, fluctuations in the display, and readings showing the maximum or minimum values. The reasons for failures in the instrumentation and control systems are summarized as follows. (1) Failure includes early failure and normal failure. Early failure refers to the occurrence of malfunctions in instruments shortly after they are put into use; this is usually due to hidden defects in the product’s design and manufacturing process. Although electronic components have undergone aging tests or inspections, it is still possible that some defects go unnoticed, and these components are the ones that cause the failures. Both electronic components and mechanical parts have a service life; over time, they undergo aging and wear. Damage that occurs when their service life has expired is considered normal failure. Instrument maintenance often involves checking and repairing failed electronic components and mechanical parts. (2) Poor contact is directly related to connections; this fault is mostly caused by the oxidation and corrosion of contacts. Poor contact in switch and relay contacts is usually resulting from electric sparks, while the oxidation of plugs and the deformation of socket springs can also lead to poor contact and overheating. Poor processing of the connections in multi-layer printed circuit boards, as well as the vias in these boards, can also lead to contact issues. (3) Corrosion, leakage, and blockage of instrument measuring elements are often caused by contact with process media that are high in temperature and pressure, corrosive, or prone to crystallization; pressure transfer pipes, valves, flanges, and pipe fittings are susceptible to leakage or blockage issues ； The thermometer protective sleeves, sensors, and transmitters are all installed in harsh field conditions, making them susceptible to corrosion and damage. (4) Weld joints: Theoretically, weld joints are very reliable, but solder joints can also be damaged; an increase in the resistance of the weld joints hinders the flow of current. Weak or detached solder joints can both cause failures, and solder joint defects are usually resulting from defective manufacturing processes. Solder joint failures are relatively hidden and do not become apparent until a long time has passed, sometimes even years. High-power components have high operating currents, which generate heat; this heat can gradually degrade the properties of the solder joints and lead to failures. The connections between the instrument fixtures and the process equipment and pipelines are made using electrical or gas welding; stress changes over time due to prolonged use, as well as corrosion caused by the process media, can all lead to cracks and subsequent leaks. (5) Mechanical wear: Both actuators and control valves contain mechanical components. Moving parts are more prone to damage than electronic components. Bearing wear, dried lubricant, gear damage, and broken plastic components can all cause failures. (6) Thermal electronic components cannot withstand high temperatures; they may fail as soon as they overheat. Overheating caused by excessive currents resulting from short circuits can quickly damage electronic components; even normal heat can gradually degrade the performance of filter electrolytic capacitors. Heat can reduce the properties of insulating materials, leading to failures, and overcurrent-induced heating also often causes failures. (7) Excessively high or low supply voltage and surge supply voltage can also affect the instruments or systems, and may even damage them. Too high a voltage can cause the voltage regulator to overheat, and the electrolytic capacitors can be damaged due to reaching or exceeding their voltage limits. Electronic components with fixed voltage requirements can also be damaged by overvoltage. Lightning surges entering the power supply can cause damage to certain circuits in the instruments or control systems in mild cases, while in severe cases it may lead to extensive damage. Power surges can cause the mains voltage to rise suddenly, resulting in damage to instruments or control systems. Power surges may sometimes be caused by the power supply itself, but in most cases they are caused by surges introduced by lightning, with the consequence being damage to the electronic components of the instruments. (8) Human error caused the instrument to fall from a height accidentally ； Wiring errors during installation and maintenance, reversed polarity of signal wires, incorrect use of compensation wires, and mixed connection of signal wires for transmitters and display instruments ； Condensate is washed away during the cleaning of the level transmitter ； Randomly adjusting the adjustable components of transmitters or instruments without a purpose ； Damage caused by random disassembly due to unfamiliarity with the structure of a certain instrument ； The positions of the manual and automatic switches are incorrect. Practice has shown that human errors occur from time to time, and misoperations can cause system failures, which is very dangerous. (9) Many failures of instruments are caused by interference; therefore, great attention should be paid to interference issues during maintenance, especially in situations where frequency converters are used. Overcoming or eliminating interference is rather tricky, and it may take a lot of effort and time to address the issue of interference.