For thermal resistors and thermocouples in ordinary thermal power plant projects, is it necessary to install transmitters?
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If asked whether it is necessary to use a transmitter when only thermal resistors and thermocouples are installed on-site and PLC control is used, with the distance to the control cabinet being within 200 meters, the answer is that a transmitter may not be required. The main reason for not using a transmitter is to save costs. But could this lead to very inaccurate or unstable temperature measurements, or other problems? Could there be additional cost increases as well? Thank you
According to the 4th edition of the Petrochemical Process Control Design Manual, with a cable resistance of 5Ω, 3*1.5mm2 triple-shielded cables are used (to enhance interference resistance), and the cable length can reach around 400 meters
Are you saying that if the distance between the two is within 400 meters, there are no technical issues?
High requirements are placed on thermocouple compensation wires, as described in API551; the maximum length of cable that can be used is not specified
No problem means that the resistance of the transmission cable is low enough, meeting the 5 ohms specified in the manual; if there are concerns, a 3*2.0mm2 cable can be chosen. Sure, the larger the cable cross-sectional area, the lower the resistance in the transmission line. It is also necessary to choose a triple-twisted shielded cable to eliminate electromagnetic interference
There is no problem in connecting thermocouples/thermoresistors directly to the PLC/DCS over a distance of 200m, provided that the cables are designed and installed according to standards. On-site temperature monitoring has become popular in the past decade or so; before that, people would go directly to the control room. The existing installation specifications date back to a long time ago, with the signal transmission distance being set at 2 km.
Hello, what you said should be consistent with what I encountered; our previous small power plant didn’t seem to have any specific requirements regarding this. But it seems that those in the petrochemical industry prefer to use transmitters, and rarely use thermocouples or thermal resistors
The technical staff at power plants focus on power transmission and distribution, large-scale mechanical equipment, and thermal efficiency, adopting a conservative approach to conventional control methods. In the chemical industry, when transmitters are used, many power plants also employ pressure pipes to transmit pressure to the control room where it is displayed on pressure gauges ； When chemical plants began to widely adopt DCS, power plants were still using unit combination instruments. Modern power plants are probably much better now.
Do you mean that although this approach used in power plants is also feasible and economically viable, from the perspective of technical advancement and innovation, the use of temperature sensors equipped with transmitters in chemical plants represents the direction in which development should proceed?
Imported temperature control devices are also not cheap, and using such devices means there is an additional point of failure at the site. Heating can improve interference resistance (thermocouples) and save on cable costs. So, taking everything into account, I still prefer straight-pull cables for heat resistance; I’m not too confident in domestically produced cables due to their temperature sensitivity, as there can be significant variations.