Help: What should I do if S-type thermocouples break frequently?
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There is an S-type dual thermocouple in the plant that frequently gets damaged and causes an open circuit. This thermocouple is used to measure the temperature in the flue of the heating furnace; the normal operating temperature ranges from 900 to 950 degrees. There are three temperature monitoring points in the flue, all equipped with dual thermocouples – one connected to the DCS system and another to the SIS system. A logic of \"three out of two\" is used for interlocking. One of these thermocouples gets damaged from time to time, sometimes every month and sometimes every three months; four thermocouples have already been replaced, but the cause of the problem cannot be identified. The other two thermocouples installed at the same level have not suffered any damage. Each time a replacement thermocouple is connected to the DCS line, it burns out after about 10 minutes. Does anyone know what should be done? Currently, only the SIS connection is in use; if both the DCS and SIS connections are used simultaneously, it also causes the SIS connection to burn out.
The DCS maintenance team insists that their cards will not cause the thermocouples to be damaged, and they asked our field instrumentation team to find out the reason
The wire at the card connector was disconnected and then reconnected on-site; a stable millivolt value could be measured on-site, but the value fluctuated at the disconnected part of the card connector. About 10 minutes after the wire was reconnected, the thermocouple on-site got damaged
The key is to modify the interface and select the thermocouple properly. We use the S scale for about 3 years. 1150 degrees.
It’s still necessary to check the installation location on site, and to carefully examine whether the point where the thermocouple gets damaged is always the same. At our factory, we installed a thermocouple on the roof of the hot air furnace, and it would get damaged after two to three days; later, upon closer inspection, I found that it was the corundum on the outside of the protective tube
I’m sorry; the post was interrupted. The breakage points in the external corundum tubes were similar, which suggested that the protective tubes had deformed. After cutting open those protective tubes, it was found that the insulating bricks inside had deformed as well, causing the protective tubes to deform. This led to the thermocouples not being aligned properly, resulting in vibrations that struck the tube walls and damaged the thermocouples. I hope this helps you
Try changing the SIS safety barrier to a two-output type; if it continues to get damaged, it might be related to the installation location on site
A temperature of 950 degrees is not considered high when using an S-type model; you could try a product from another manufacturer and pay attention to factors such as vibration at the installation location. It seems likely that the burnout was caused not only by heat but also by external forces that broke the thermocouple; further observation and analysis are needed.
What about installation issues? The temperature inside the furnace is so high; I remember that when removing the thermocouple used in high-temperature conditions, it needs to be removed slowly, as the insulation material is sensitive to thermal expansion and contraction. Only a small section of the thermocouple can be pulled out at a time. Once it cools down, a cold thermocouple can be inserted, but uneven expansion can still cause damage
Check the interface again, measure the current and voltage, as well as the nuts, etc. Careful attention must be paid during installation: the interface must be correct, the temperature and scale values must match, and the appropriate signal wires must be selected.
Bro, I think it might be interference. How’s the shielding connection? Also, are there any high-voltage cables along the wires of the thermocouples? If so, replace them. In our factory, each thermocouple designed for use in hot and abrasive environments costs over 2000, and they can last half a year without any problems