Summary of issues often overlooked in instrument control systems

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Any malfunction in a measuring instrument in a chemical plant can cause problems for the entire production process, and may even lead to interruptions in production or safety issues. So, how can a large-scale production system be maintained effectively to prevent failures in the instrumentation control system? Let’s take a look at these pitfalls in instrument control systems! 1. Never use a single multi-core cable for both signal cables and power cables! 2. When maintaining instruments and equipment on oxygen pipelines, ensure no oil adheres to them. Oil-free transmitters and pressure gauges must not be installed together with ordinary gauges. 3. When disconnecting wires from the instrument for maintenance, be sure to wrap the wire ends properly to prevent short circuits! 4. Cables should not have intermediate joints. 5. Point shielding grounding is generally carried out on the control room side. 6. The protective hose must be placed below the instrument inlet to prevent water from entering the instruments. 7. Instruments exposed to the elements should be fitted with protective enclosures or wrapped in nylon plastic bags. 8. When cables are laid in cable trays, intrinsically safe cables, power cables, and signal cables should be separated using partitions. 9. When wiring, compensation wires must not be used with wire nuts (terminals); this prevents two different conductors from coming into contact, which could cause measurement errors. 10. During production, if there are issues that need to be addressed with the instruments, whether indoors or outdoors, it is necessary to follow the established procedures or protocols. The operators must be informed, and in some cases, a written signature is also required. 11. In areas with lightning protection, where field instruments are connected to a safety barrier via surge protectors and then to control systems such as DCS and SIS, in order to avoid unnecessary wiring in additional cabinets, the surge protectors located in the field cabinet room should be installed on the same side of the cabinet as the safety barriers for the corresponding circuits. 12. The control room must have measures in place to prevent small animals, as rats urinating on ESD cards can cause the entire system to stop operating, resulting in significant losses. 13. Single-unit testing must be completed before installing the instruments, and circuit testing must be finished after installation before joint testing can be carried out. 14. When the device is in operation, process technicians must be present for the maintenance of the instruments. Keep this in mind: if a problem arises, it’s not a minor matter. 15. When performing on-site maintenance of instruments, it is essential to contact the process engineers to understand the current operating conditions. When removing instruments that are connected to power supplies, be sure to turn off the power first, and then use a multimeter to confirm that the power has indeed been turned off – after all, your life is your own. 16. When designing flow meters, it is essential to select the appropriate type of meter based on the medium being measured, as well as temperature and pressure factors, and to implement proper flow compensation. Attention should be paid to the various special requirements of flow meters during installation. 17. When installing the instrument panels in the control room, to prevent rainwater from entering the control room, it is necessary to consider upward and downward bends as well as ensure proper sealing. 18. When instrument air is introduced from the main pipe, the valve must be positioned above the exact center of the pipeline; ideally at a 90-degree angle above the pipeline, to prevent contaminants in the air stream from entering the instrument valve. 19. The shielding layer shall not be grounded at both ends ； The outlets of outdoor cable protection conduits should have rainproof measures ； In explosion-proof environments, pay attention to the sealing of pipe openings. 20. Alarm instruments and audio equipment must be properly maintained and in working order; otherwise, in the event of a process accident, the personnel in charge of instrumentation will face serious consequences, as the operators fail to detect the issue due to faulty alarms. 21. In situations involving ammonia, copper and copper alloys are prohibited ； For power supply of the DCS system, a dual-power supply setup should be designed! 22. For temperature measurement using thermal resistors, the two-wire system cannot be used for long-distance transmission. 23. The insulation resistance of the cable should be greater than 5 megohms ； The minimum bending radius of cables should generally be greater than 10 times the cable diameter; for optical cables, it should be 15 times the diameter ； When instrument cables are laid parallel to electrical cables, a certain distance should be maintained between them (greater than 0.8 meters), and the distance from such cables to equipment and pipes should be more than 150 millimeters. 24. For the hydraulic testing of instrument pipelines, when testing austenitic stainless steel pipelines, the chloride content in water must not exceed 25 PPM. The working ground resistance of the instruments should be less than 1 ohm, while the resistance of other grounds should be less than 4 ohms. 25. The protection of instruments should be done using fire-resistant cloth, namely “asbestos cloth”. Plastic bags should not be used. 26. The instruments used in the hydrogen unit must meet both the explosion-proof rating and the protection rating requirements; neither can be lacking. Intrinsically safe signals (cables) and flameproof signals (cables) must not be routed into the same field intermediate junction box. 27. The insulation resistance of the cable should be greater than 5 megohms ； The minimum bending radius of cables should generally be greater than 10 times the cable diameter; for optical cables, it should be 15 times the diameter ； When instrument cables are laid parallel to electrical cables, a certain distance should be maintained between them (greater than 0.8 meters), and the distance from such cables to equipment and pipes should be more than 150 millimeters. 28. For the hydraulic testing of instrument pipes, when conducting tests on austenitic stainless steel pipes, the chloride ion content in water must not exceed 25 PPM (parts per million). The grounding resistance for instrument circuits should be less than 1 ohm; for other grounds, it should be less than 4 ohms. 29. The instruments used in the hydrogen unit must meet both the explosion-proof rating requirements and the protection rating requirements; neither can be neglected. Intrinsically safe signals (cables) and flameproof signals (cables) must not be routed into the same field intermediate junction box. 30. When designing the FF bus, terminators (a resistor and a capacitor connected in series) must be installed at the ends of the power regulator and the Fieldbus junction box. 31. The solenoid valves used for interlocking should be fail-safe type, remaining powered under normal conditions and de-energized during interlocking. 32. When designing and installing instruments, if the pipeline for the temperature instrument has a diameter of less than DN80, a thermometer expansion tube should be used to increase the pipeline diameter to over 80. 33. The flow measurement element is not included in the plant’s hydrostatic testing. 34. When measuring steam flow and condensers are used for the positive and negative pressure transfer lines, the installation heights of the two condensers must be kept the same. 35. For bending the pressure guiding tubes of on-site instruments, cold bending should be used; hot bending methods such as gas welding are not permitted. 36. The occurrence of flashing must be fully considered in the design and selection of control valves, the design of pipelines, and the determination of pressure distribution. Regarding control valves, the following points should be noted. (1) Increase the hardness of the material. (2) Reduce the flow velocity of the fluid. (3) Select the appropriate type of control valve and flow direction. For example, for fluids that vaporize easily, balls with high pressure recovery should not be used. 37. Valves and butterfly valves; single-seat valves with low pressure recovery, etc., can be selected. Measures to eliminate and reduce what actually occurs: (1) Control the pressure drop to prevent cavitation. For example, a multi-stage pressure reduction method is employed to divide the pressure drop across the control valve into several stages. (2) Reduce the effects of cavitation. A similar method to that used to prevent flashing is employed. For example, by increasing the hardness of the material or reducing the flow rate, the impact caused by cavitation can be minimized. (3) Distribute pipeline pressure reasonably to increase the downstream pressure. 38. Issues to be noted when overheadly laying cables and wires into the control room: (1) Before entering the control room, a fixed support point must be provided for the trough board to prevent stress from developing on indoor equipment due to climate changes ； (2) Before the trough panel enters the control room, it must have a slope of at least 1/100, directed outdoors, to prevent rainwater from flowing into the control room along the trough panel ； (3) All holes in the walls for entry and exit to the control room must be sealed to prevent rats, mosquitoes, and similar pests from getting in. 39. When selecting local pressure gauges, it is essential to determine the nature of the pressure source: whether it is a shock load or normal pressure. If there aren’t enough gauges available, replacing them can be a problem. The installation dimensions of the in-situ thermometer must be discussed with the process team; when measuring liquids that are not fully filled in the pipe, it is essential to choose the appropriate dimensions ； When designing, the selection of materials for high temperature and high pressure must differ from those used under normal temperature and pressure conditions. Do not disassemble equipment while it is still powered on; carry a voltage tester with you to avoid the risk of electric shock. 40. Do not disassemble equipment while it is still powered on; carry a voltage tester with you to avoid the risk of electric shock. 41. DCS must implement proper anti-static measures to prevent accidents caused by static electricity. 42. To service the interlock equipment, a forced command from the DCS is required before any action can be taken. 43. The solenoid coil must not be removed while it is powered, otherwise the coil will be damaged. 44. When rotors and wheel flowmeters are installed vertically, it is essential to ensure that the fluid flows from bottom to top. 45. After installing a new control valve, the air supply pipeline should be drained for a while before being connected to the valve positioner, to prevent oil from entering and causing damage to the positioner. 46. Copper is also prohibited in instruments used for acetylene gas; therefore, in applications involving acetylene gas, in addition to meeting the requirements regarding explosion protection ratings, it is necessary to pay attention to the fact that some device labels may indicate “Not suitable for acetylene gas”. 47. Current signals between DCS and electrical systems. Since the electrical signals that are sent over are usually active, it is best to use an isolator for isolation. On one hand, this prevents the instrument’s I/O cards from being connected to the electrical circuit; on the other hand, without an isolator, it may be impossible to achieve proper synchronization between the two systems. 48. Provide steam tracing for the instrument pipelines; it is advisable to use tracing pipes with an outer diameter of 12 inches or more, as otherwise, if the pipeline length is large, it is easy for the steam to not be hot enough, resulting in poor tracing efficiency. 49. To correct a common error in design: some people install safety barriers regardless of the type of anti-riot system in use. In fact, safety barriers are used in intrinsically safe explosion-proof systems. Intrinsic safety explosion-proof system: (1) The field instruments must be intrinsic safety type instruments ； (2) There must be a safety barrier on the control room side. (3) The intermediate cable must be an intrinsically safe signal cable. 50. For intrinsically safe instruments and flameproof instruments, signal isolation can be achieved using isolators if necessary. 51. For Zener barriers, attention must be paid to grounding issues. 52. In fact, instruments and the manufacturing process are inseparable; when performing online maintenance on instruments, it is essential to minimize any disruption to the manufacturing process. 53. Selection of flow meters: When the electrical conductivity of the process medium to be measured is low, electromagnetic flow meters should not be used ； When high-level factory-level measurement requirements exist, a mass flow meter should be selected. 54. When measuring the pressure of a medium, a condenser or desiccant must be used when the temperature of the medium being measured is above 60 degrees. 55. After installing the pneumatic control valve, be sure to pay close attention not to get the operating settings for air-open and air-close types wrong in the DCS. 56. If the reading of the temperature instrument system suddenly reaches the maximum or minimum value, it is generally due to a fault in the instrument system. Because the temperature instrument system has a large measurement lag, no sudden changes occur. At this time, the causes of failure are usually broken thermocouples, thermal resistors, compensation wires, or a malfunctioning transmitter amplifier. 57. Be sure to label the wire numbers properly when wiring instruments! Previously, I made such a silly mistake while working; it was quite troublesome to correct later. If temperature sensors are installed at pipe bends or at an angle, they should be positioned against the direction of flow. 58. If there are both pressure primary points and temperature primary points on the same pipeline, the pressure primary point should be located on the upstream side of the temperature primary point. 59. The rotameter must be installed vertically on the pipeline, and the flow direction of the medium must be from bottom to top. 60. Straight pipes require 5DN on the upstream side and 3DN on the downstream side (DN is the pipe diameter). Disclaimer: The copyright of this article belongs to the original author. If copyright issues are involved, please contact us for removal