Seeking help with heating furnace thermocouple issues
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On the roof of our furnace, there are 3 S-type thermocouples arranged in an east-west direction; they are referred to as the West Furnace Thermocouple, Middle Furnace Thermocouple, and East Furnace Thermocouple (spaced apart by about 4 meters). (The thermocouples are directly connected to the PLC module via compensation wires.) 1. Five days ago, the temperatures measured by these three thermocouples were normal (according to the operator’s experience); they were around 1230°C. 2. Starting at some point 5 days ago, over a period of nearly 10 minutes, the temperatures of the 3 thermocouples located on the top of the furnace decreased simultaneously, dropping to around 1150°C. Over these 5 days, no matter how the air-fuel ratio was set, the temperature wouldn’t rise (the temperature controller was set to 1230°C, but the thermocouple detected around 1150°C). 3. There was really nothing we could do in terms of thermodynamics today; they asked us to go check the heat exchange thermocouples. The questions I would like to ask are also listed below: ① Before the heat exchange thermocouple, the thermocouple on the west side of the furnace showed 1180°C, the thermocouple inside the furnace showed 1120°C, and the thermocouple on the east side of the furnace showed 1160°C. ②We replaced the thermocouple in the furnace; we didn’t touch the other 2 thermocouples at all. ③About 10 minutes after the replacement, the temperature of the newly installed thermocouple in the furnace rose to 1230°C, reaching a maximum of 1260°C. At this time, everyone thought that the thermocouple was broken. ④What’s incomprehensible is that the thermocouple temperatures on the west and east sides of the furnace have also increased. The temperature on the west side of the furnace rose to around 1250°C, while that on the east side rose to 1200°C. ⑤There is another issue: the temperature shown by the thermocouple on the furnace side keeps dropping, according to the thermal engineers; it doesn’t rise at all, and the temperature remains unchanged for a long time. I observed it for a while, and indeed that’s what happened. Later, I went to the main control room (at this time, the temperature indicated by the thermocouple on the west side of the furnace on the screen was 1198°C). I disconnected the positive wire of the thermocouple from the PLC terminals. At that moment, the temperature displayed on the control screen was 3277°C. After about 20 seconds, I reconnected the wire that had been removed, and at that point the temperature in the furnace rose to 1215°C (an increase of almost 20°C). And the temperature began to change; it was no longer the same as before. For example, it can rise to 1235°C (during the previous 5 days of thermal monitoring, no such significant change was observed). In summary, I have no idea what the cause of the problem is. I would like to ask the experts here.
This post was last edited by hxch150804 on 2021-9-16 08:21. To resolve such issues, it’s first necessary to clarify the approach and determine where the problem lies. The first step is to check whether, in the absence of interlock functionality and with only display available, the equipment can be maintained online by removing the compensation cable from the thermocouple’s field wiring box and directly measuring the output in MV units. Using a reference table, the temperature can be determined; thereafter, the field ambient temperature must be subtracted from this value. You can use a glass tube thermometer to measure the ambient temperature, but multiple readings will be required as the response might be slow. This allows one to determine the actual measured temperature, and then the temperatures of the three thermocouples can be checked individually for comparison. If the temperature here is indeed low, then it’s a process issue. In other words, our measurement system must first ensure the authenticity and accuracy of the measurements of the individual components; subsequent processes such as signal transmission, conversion, and display are merely representations.
The inspection direction is the common circuit section of the three thermocouples; based on the observed phenomena, this direction is preliminarily determined to be the correct one
Are the three thermocouples connected to the same card? Are there any alternative connection points on other cards that I could try using?