Regarding the temperature measurement of chemical processing units, when the temperature sensing element without a transmitter at the site becomes disconnected, the control systems (SIS and DCS) take action
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The last edit to this post was made by 328104062 on 2015-11-26 at 23:01. As the title suggests: 1. Why are pressure and flow measurements both carried out through transmitters (pressure transmitters, differential pressure transmitters) that output 4-20mA signals? These signals then go to the devices in the field cabinet (safety barriers, converters), where the range is adjusted. In contrast, for temperature measurement (using thermal resistors and thermocouples), there are only two or three wires, along with protective wiring boxes and compensation wires that are sent to the field cabinet. Is it simply to save money that no transmitters are used for temperature measurement? 2. Temperature measurement: What hardware devices must be passed through in sequence after entering the cabinet room, and how is it different from flow measurement? Discuss thermocouples and thermal resistors separately (remember something about balance bridges and contact electromotive force – how is this implemented between cabinets?). 3. If a break in the wire occurs when the thermal resistor measures 100°C, the SIS and DCS systems display IOP (open circuit input), and the displayed value remains at 100°C. Then why does the SIS system trigger a high-temperature interlock? Which part might be problematic? What is temperature variation between cabinet rooms? Where should the maximum and minimum values be set when the connection is interrupted, and how to do it? Why can’t modern smart meters detect an open circuit of thermal resistors (thermocouples) on-site, in order to avoid device interlock problems? 4. Regarding the hardware and software connections for the above flow rates, pressures, and temperatures (thermal resistors, thermocouples) within the on-site cabinet rooms as well as in the DSC and SIS systems, please explain the common connection methods used – such as the hardware and cards involved, and how data is converted – and it would be great if there could be diagrams illustrating these connections. Thank you very much; I really appreciate it (since I don’t have the opportunity to work with the connections inside the cabinet rooms, I can’t analyze the issues accurately without understanding the process). Automation instruments
1. Both pressure and flow measurement are based on pressure differences; these pressure differences are converted into 4-20mA signals using transmitters (pressure transmitters, differential pressure transmitters), and then sent to the cards in the DCS cabinet. Temperature measurement (using thermal resistors and thermocouples) yields electrical signals directly; thermal resistors measure the change in resistance as temperature changes, while thermocouples measure the change in potential as temperature changes. It can be directly fed into the DCS panel without the need for a converter. ——Temperature measurement does not require transmitters; of course, it’s not to save money. It is because an electrical signal needs to be transmitted between the field primary meter and the DCS cabinet. 3. The resistance should be at its maximum when the thermal resistor is broken, and the potential should be at its minimum when the thermocouple is broken. 4. I don’t know anything else; I’m not majoring in instrumentation.
Those from the electrical and instrumentation field who don’t understand how to learn*
For temperature measurement (using thermal resistors and thermocouples), there are only two or three wires, along with a protective wiring box; compensation wires, etc., are taken into the on-site cabinet. No transmitter is required for temperature measurement
Let me help answer part of this for your reference only: 1. Why are pressure and flow measurements both carried out through transmitters (pressure transmitters, differential pressure transmitters) that output 4-20mA signals? These signals then go to the devices in the field cabinet room, such as safety barriers and converters, where the range is adjusted. In contrast, temperature measurements (using thermal resistors and thermocouples) involve only two or three wires, along with protective wiring boxes and compensation wires that are sent to the field cabinet room. Answer: The main reason is cost savings. Firstly, there are also transmitters for temperature, but their cost is much higher compared to those ordinary DCS systems that come equipped with thermocouple and thermal resistance cards; for example, those from Rosemount – the cost of such transmitters is roughly equivalent to half the price of a thermocouple card. In terms of transmission, since interference from long-distance transmission of thermoresistor and thermocouple signals can be ignored. Save wherever possible. Is temperature measurement done without transmitters just to save money? 2. Temperature measurement: What hardware devices must be passed through in sequence after entering the cabinet room, and how is it different from flow measurement? Discuss thermocouples and thermal resistors separately (remember something about balance bridges and contact thermoelectromotive forces – how are these implemented between cabinets?). Answer: They go directly into the thermal resistor and thermocouple I/O cards; no safety barrier is required. As for the last two questions, you need to resolve them using the cards themselves. 3. If a wire break occurs when the thermal resistor at the site measures 100°C, and the SIS and DCS display IOP (open circuit input) with a value of 100°C still, then why does the SIS system trigger a high-temperature interlock? Which part might be causing the problem? What is temperature variation between cabinet rooms? Where should the maximum and minimum values be set when the connection is interrupted, and how to do it? Why can’t modern smart meters detect an open circuit of thermal resistors (thermocouples) on-site, in order to avoid device interlock problems? Answer: I will only answer what I know. I haven’t worked with SIS before, so I’m not familiar with it. 1. Compared to a broken resistor, the resistance value defaults to its maximum value; therefore, the signal received by the DCS is definitely at full scale, which will inevitably trigger a high-level interlock. 2. In DCS, it can only be set on the card corresponding to the thermal resistor; there is usually a fluctuation switch, similar to the option in PLCs to choose whether to display the maximum or minimum value in case of a fault. Within the DCS, I found that it’s not possible to make such settings. 3. Smart meters can detect field disconnections right away, and there are corresponding jumper switches available outside the transmitter for you to use. 4. Regarding the hardware and software connections for the above flow rates, pressures, and temperatures (thermal resistors, thermocouples) within the on-site cabinet rooms as well as in the DSC and SIS systems, please explain the common connection methods used (such as the hardware and cards involved, and how data is converted), and it would be great if there could be diagrams illustrating these connections. Thank you very much; I really appreciate it (since I don’t have the opportunity to work with the connections inside the cabinet rooms, I can’t analyze the issues accurately without understanding the process). Answer: It’s too complicated; let someone more experienced downstairs handle it. My knowledge is limited; I hope experts can add more!
The last edit to this post was made by ssln123 on 2015-11-27 at 15:07; adding temperature control in the field will undoubtedly increase costs and also create an additional point of failure. But why is temperature compensation still needed sometimes? Mainly, the thermocouple signals weaken during long-distance transmission, and the compensation wires are also very long. The cost of three thermal resistance cables is also not low. However, in some areas, temperature sensors are necessary, as current signals do not attenuate easily; especially in areas with many temperature points, a temperature sensor box is installed at each location, which communicates with the corresponding DCS communication devices via networking. 2 The temperature can be fed directly into the thermocouple or thermal resistance card, or it can be sent to the card via a safety barrier; this approach requires a higher level of security. 3 Regarding the issue of wire disconnection, one approach is to set in the temperature safety gate whether to use the maximum value, the minimum value, or the original value in case of a disconnection. Those that go directly into the card holder without a safety barrier usually operate at the highest speed. 4 This is a rather complex issue; find some materials related to instrumentation engineering on your own and take a look at the layout of cabinets in the cabinet room.
1. It is not solely to save money; it is mainly the working principle that determines whether a transmitter is needed or not. 2. Temperature measurement: After entering the cabinet room, one can access the temperature display instruments; to access the DCS, it is necessary to go through a temperature transmitter first. Unlike the principle of flow measurement, different instruments are used. 3. If a wire break occurs when the on-site thermoresistor reads 100°C, the SIS and DCS will display IOP (open circuit at the input), and the displayed value will still be 100°C (which indicates that there is a problem with the display instrument). The SIS system and the DCS system are independent of each other and do not affect one another. When the thermal resistor is disconnected, it should display the maximum value; in the case of a short circuit, it should show the minimum value ； Only thermocouple temperature measurement comes with short-circuit protection, which is set in the instrument’s internal secondary parameters; refer to the instrument’s user manual for details. Why can’t modern smart meters detect an open circuit of thermal resistors (thermocouples) on-site, in order to avoid device interlock problems? (Then it is not a high-grade intelligent instrument; it is an intelligent instrument with incomplete functions.) 4. Regarding the hardware and software connections for flow, pressure, and temperature (thermoresistors, thermocouples) in the on-site cabinets as well as within DSC and SIS, please explain the common connection methods used – such as the hardware and cards involved, and how conversions are carried out. The main components include UPS, power supply modules, isolation modules, insulated gates, etc....