Mineral insulated thermocouple is a solid combination of thermocouple wire, insulating material and metal sheath, which has a fast response time, vibration and shock resistance and longer life compared to ordinary thermocouple, and is more suitable for harsh working conditions and are now widely used, there are some common problems as follows:
1. Inaccurate Instruction
(1) MI thermocouple is generally installed in important parts such as catalytic reactors and risers, and some are also involved in device interlock inspections, and extra care must be taken during operation. Methods are as follows:
First of all, the process operator needs to be notified to carry out the operation on site. If the faulty thermocouple participates in the interlocking of the device, it is necessary to apply for the interlocking removal ticket of the relevant device, and the on-site inspection can only be carried out after the confirmation of the process operator;
(2) Get the actual temperature of the thermocouple then compare this value with the value of the DCS display terminal and calculate the difference to see if the difference belongs to the normal error of the thermocouple. If it is within the error range, it can be judged that the indication deviation is caused by the process operation. If it exceeds and is too large, we should check whether the connection terminals of the junction box are secure and whether the positive and negative polarities are reversed. We should tighten and adjust if there is looseness or reversed connection, etc.
(3) Use a multimeter to check whether the insulation of the thermocouple compensation wire is normal. If there is a problem with the insulation, we need to confirm the faulty area of the compensation wire insulation in sections and replace it. It should be noted that the insulation problem of the compensation wire will cause the temperature indication of the thermocouple to fluctuate irregularly.
(4) Check the computer components in the measurement circuit such as the safety barrier and the input card.
(5) The polarity of the thermocouple compensation wire and the polarity of the thermocouple conductors are reversed. Generally the red wire is positive. The method of judging the polarity of thermocouple wire is as follows:
A. On-site maintenance can be judged according to the hardness of the thermocouple wire. Generally speaking, the harder pole is platinum rhodium wire, which is the positive pole of the thermocouple, and the softer pole is platinum wire, which is the negative.
B. The positive and negative polarity can also be judged by the millivolt block measurement of the digital multimeter.
2. Indicating volatility
(1) Loose connections. The temperature measurement point with loose wiring is usually located in the place where the vibration of the process equipment is large, or the thermocouple that has not been maintained for a long time. We should keep thermocouple be inspected and maintained regularly, and abnormalities can be found in time.
(2) Shorted or grounded. The insulation layer is exposed to high temperature for a long time, and it is easy to age and shed, so that the two cables are short-circuited, or they are grounded due to contact with the metal pipe that passes through the wire.
3. DCS no indication
(1) :The thermocouple is damaged. Disconnect the two wires of the thermocouple, and measure the resistance value between the two terminals with the electric barrier of the digital multimeter. If it shows infinity, it means that the thermocouple is broken.
(2) The DCS channel of the thermocouple is damaged. After checking that no problem with the wiring of the thermocouple and the field junction box, the card channel in the junction cabinet behind the field and the main control room should be jointly calibrated to check the DCS of the thermocouple. Whether the channel is intact, if it is damaged, it should be replaced, and check whether the thermocouple indication is normal after replacement.
4. Leakage or wear through of thermocouple well
Catalytic reaction thermocouple is mostly installed on pipelines with high-temperature and high-speed flowing media. Due to the long service life, high service environment temperature, and large erosion of the measurement medium, casing leakage often occurs at the end of a service cycle. Maintenance Personnel should strengthen the on-site monitoring of thermocouple. Once leakage is found, it should be dealt with in time under the premise of taking protective measures. In addition, when inspecting, maintaining, replacing and installing new thermocouple, the flow direction of the measurement medium should be confirmed, and the outer protective layer of the casing should be properly installed to prolong the service life of the thermocouple.
5. Conclusion
Thermocouple is a common and widely used instrument. As an instrument maintenance worker, the ability to analyze and eliminate common thermocouple faults is the guarantee of safe production. get best products from www.super-instrument.com.
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