Faults and Maintenance Techniques for Automatic Control Systems in Chemical Instrumentation

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This post was last edited by sanchangyb on 2020-6-3 at 16:58. Authors: Guan Xiaoyong, Wei Bencheng, Meng Xianxia; Organization: Dezhou Shihua Chemical Co., Ltd. [Abstract] It summarizes the common faults of temperature, pressure, level, and flow control instruments in chemical production as well as the methods for diagnosing these faults. It also discusses the role of computer-based diagnostic techniques and redundancy technologies in the maintenance of automated instruments used in the chemical industry. In recent years, with the rapid development of control technology, the chemical industry has been continuously upgraded, resulting in an increasing degree of automation. To meet the needs of industrial production and development, chemical process control systems are also constantly being upgraded and improved. A complete automatic control system for chemical processes includes various control instruments and devices, such as temperature control instruments, pressure control instruments, level control instruments, flow control instruments, on-line analysis instruments, alarms for toxic, harmful, and flammable substances, and control valves. 1 Common faults in automatic control systems for chemical process instruments 1.1 Faults in temperature control instruments In chemical production, temperature is one of the key parameters; therefore, its measurement is particularly important. Common temperature instruments in chemical production include Sanchang thermocouples, thermal resistors, and temperature transmitters. Temperature control instruments often experience the following two types of failures: ① System failures. The internal components of the system consume power, and errors in display occur when the temperature control instrument fails. ②Measure the fault. That is, errors that occur on the measuring element or in the measuring circuit. In chemical production, if production processes experience fluctuations due to incorrect readings from temperature measurement instruments, the shock caused by sudden changes in material flow can damage the measuring elements, resulting in the system indicating a connection loss or an error. When the above error message appears, it can be diagnosed using the following methods: ① Use the location information displayed by the control system to locate the wiring terminals of the temperature control instrument in the control cabinet. Disconnect the wires connected to the temperature control instrument and use a multimeter for preliminary measurements. Compare these measurement values with the information provided by the system; if the values fluctuate significantly or do not match the actual values, it can be inferred that the fault lies in the temperature sensing element or the measurement circuit. ②If the temperature control instrument exhibits oscillations and periodic changes, it is necessary to consider the possibility of interference from power cables or frequency converters located near its transmission lines. ③If the on-site measurement value shown by the temperature control instrument is higher than the actual value, it is necessary to check whether the transmission lines of the temperature control instrument are appropriate, or whether the display instrument and the measuring instrument are compatible with each other. 1. 2 Pressure control instrument failures: In chemical production, especially in the manufacturing of hazardous chemicals, conditions such as high temperature, high pressure, and the presence of toxic or harmful substances are common. Pressure is often an important parameter for monitoring system operation. Pressure control systems, represented by intelligent pressure transmitters, are widely used in the chemical industry. The use of Sanchang intelligent pressure transmitters greatly facilitates operators’ control over the production process. Common faults in pressure control systems include on-site pressure faults and system operation faults. When a fault occurs, it is necessary to determine whether it is an on-site fault or a system fault, and to check whether the on-site pressure reading matches the system’s display. When the system displays errors while the on-site instruments show normal readings, a system failure should be considered, and attempting to restart the system may help. A common fault of field smart transmitters is that the output display shows zero; in such cases, it is possible to check whether the power cable is connected reversely or whether the power voltage meets the 24 V DC requirement, or it could also be due to a damaged electronic circuit board. 1. Failures of 3 level control instruments. Level control instruments are an important part of chemical process automation control systems; common types include double-flange level gauges, radar level gauges, and ultrasonic level gauges. (1) When the reading displayed by the liquid level control instrument reaches its maximum or minimum value, on-site verification is required to check whether the actual liquid level as indicated by the on-site gauge and the remote transmitted data are within the normal range of variation. Level adjustment is carried out manually on-site. If the level does not change in response to manual adjustments, it indicates that there is a fault in the level control system. If the data changes but no linear relationship or pattern can be observed, then there is a fault with the on-site level control instruments; the issue can be addressed based on the measurement methods and types of instruments installed on-site. (2) When the reading displayed by the liquid level control instrument does not change and no disconnection is indicated, it is necessary to consider whether the instrument is damaged. In the case of commonly used double-flange level gauges, if the valves at the base of the upper and lower flanges of the gauge are closed, the fixing bolts and diaphragm are removed, leaving everything exposed to the atmosphere, yet the level reading remains unchanged, then two possibilities should be considered: ① The circuit board in the transduction section of the level gauge is damaged; in such a case, trying to replace the circuit board may help. ② If replacing the circuit board does not resolve the issue, it is possible that both the diaphragms or capillaries on the high-pressure and low-pressure sides are damaged; in this situation, the gauge needs to be replaced and its parameters reset. (3) When using a radar level gauge to measure the level of a substance, if the level at the site remains constant but there are disturbances on the surface of the substance being measured, the measurement values will fluctuate irregularly. In such cases, it is necessary to check whether the signal transmission is being interfered with. If the disturbances on the substance’s surface are reducing the signal strength, maintenance personnel should activate the near-field suppression function and increase the output damping appropriately in order to reduce these interferences. 1. 4 Flow control instrument failures. Generally, failures that occur as a result of sudden changes in the system’s flow parameters are considered to be failures of the flow control system. If the reading displayed by the flow control instrument is at its minimum value or shows no value at all, it is necessary to go to the site to inspect the instrument. If there are no abnormalities in the process equipment on site, it can be determined that the control instrument is faulty. Taking a differential pressure flow meter as an example, when the indicated value is zero or changes very little, the following 3 situations need to be considered: ① The balance valve is not fully closed or there is a leak; in this case, the balance valve can be closed and then repaired or replaced; ② The high and low pressure valves at the root of the throttling device are not open; these valves should be opened; ③ The valves and pipelines between the throttling device and the differential pressure gauge are blocked; in such cases, the pipelines should be flushed, and the valves repaired or replaced; ④ The steam pressure conduit has not been completely condensed, and the condensate has not filled the pressure sensing tube; the meter can be used once complete condensation occurs; ⑤ The gasket between the Thricom throttling device and the process pipeline is not tight; the bolts must be tightened or the gasket replaced. When there are large fluctuations in flow rate and the displayed values lack stability, it is necessary to strengthen on-site management by appropriately reducing the valves located before and after the flow control instrument, in order to minimize measurement errors caused by excessive fluctuations in the flow rate parameters. If reducing the valve openings does not work, it is necessary to check whether the pressure sensing element is too sensitive to such fluctuations; in such cases, the damping effect can be adjusted accordingly to achieve precise control of the flow rate. 2 Technologies to Prevent Control System Failures 2.1 Computer Diagnosis Techniques CENTUM CS 3000 (abbreviated as CS 3000) is a DCS product suitable for the control of large and medium-sized petrochemical processes. HIS is the operation station of the CS 3000 system; it uses Windows NT/2000/XP/Vista operating systems to carry out real-time monitoring and process control of the system. In the CS 3000 system, if a fault occurs in the control system, an alarm is issued promptly and a record of the fault is made, providing technical data support for identifying the source of the fault and resolving it. In the CS 3000 system, alarms are divided into three categories: process alarms, system alarms, and operational guidance. Each type of alarm has its own dedicated display window. When a fault occurs in the instrument control system, instrument maintenance personnel can use the alarm information provided by HIS to diagnose the issue and take appropriate actions to resolve it, thereby restoring normal operation of the system as quickly as possible. This improves the efficiency of instrument maintenance work and ensures that production can proceed safely, steadily, and in an orderly manner. 2. 2 Redundancy technology: In the operation of chemical process instrument automation control systems, the use of redundancy technology significantly enhances the safety and operability of such systems. Redundancy technology includes the following elements. 2. 2.1 Hardware redundancy: Multiple identical modules or components are connected in parallel. In hot standby mode, one of them is in operation (the active card), responsible for tasks such as system data acquisition, computation, control output, and network communication; while the other is in standby mode (the standby card), which continuously monitors the internal control status of the active card (i.e., status synchronization). The positive/negative logic between the active/standby card units is mutually exclusive – that is, one is the active card, and the other must be the standby card. Moreover, there are redundant control circuits (also known as active/standby control circuits) and communication circuits between them, to ensure that the two card units operate simultaneously and in an orderly manner, thus maintaining consistency in their input and output characteristics. The development of modern communication technologies has driven the advancement of redundancy technologies. These technologies feature online fault detection capabilities, enabling fault detection, fault location, fault isolation, and fault alarms through human-machine interfaces. Fault detection includes the power supply, microprocessor, data communication links, data buses, and I/O status, etc. Among them, fault diagnosis includes self-diagnosis of faults and mutual inspection of faults (inspection between active and spare cards), enabling seamless automatic switching in the event of a system failure. On the one hand, redundancy technology helps operation and maintenance personnel diagnose system failures, enabling timely maintenance of the system to eliminate fault points. On the other hand, it enhances the reliability of the system, making a significant contribution to ensuring safe production. 2. 2.2 Logical voting for software multiple redundancy: Voting refers to the process and method by which a redundant system determines a conclusion using the majority principle. For measurements of the same medium, 2 or more instruments are installed to carry out separate measurements; the data from these instruments are sent to a redundant system for voting purposes, in order to determine the system’s operating status. The use of redundant voting improves the reliability of the system and reduces the impact that unnecessary device interlocks can have on production. Common voting methods include the one-of-two voting logic (1oo2), the two-of-two voting logic (2oo2), the one-of-three voting logic (1oo3), and the two-of-three voting logic (2oo3). Taking the three-to-two voting logic as an example, under normal conditions, the status of all 3 control instruments shows as 1; whenever the combined signals of any 2 of these control instruments are both 0, the voter will command the actuator to carry out the corresponding interlock action. A two-out-of-three voting logic is a reasonable choice; it overcomes the flaw of dual-system designs, which are unable to distinguish between correct and incorrect signals. No matter what fault occurs in any channel, the system can continue to function properly after voting, thereby maintaining its safety and reliability at a reasonable level. 3 Conclusion The widespread use of automatic control systems for chemical instrumentation is of great significance for achieving intelligent production, freeing up labor forces, and preventing production accidents in a safe and effective manner. With the advancement of science and technology and the continuous upgrading of chemical process control instruments, there are increasingly more fault points in these control systems. Only by starting with basic faults, accumulating experience over time, and actively learning new technologies to fully understand any new problems that arise, can one perform maintenance tasks more effectively, address faults at an early stage, and ensure the proper operation of chemical process control systems. This article is reprinted from the original URL: http://www.jssanchang.com/hangye/1114.html