Thermocouple application and overhaul inspection notes!

Thermocouple thermometers are the most widely used temperature detection instruments in the industry, but they all have the same measurement principle. As industrial workers must be familiar with the thermocouple temperature measurement principle, in order to face the fault when dealing with ease.
The importance of temperature detection
Chemical production is a very complicated material balance change process. The degree of production is usually not intuitively observed. It can only be recognized indirectly through some parameters that can reflect the process changes, and then the corresponding operations can be performed to ensure the smooth operation of production. . Although there are many parameters that characterize the extent of production, it is difficult for a specific characterization parameter to fully reflect the extent of production for a complex production process. Changes in material balance also occur at the same time as the generation and transfer of energy. Whether it is a physical reaction or a complex chemical change, the process of production can be judged by the detection of energy. Temperature is a physical parameter that characterizes the size of energy. Therefore, for chemical production, we only need to detect the temperature of some key points to judge the degree of production, so that we can control production, increase production capacity, and reduce energy consumption to ensure safety. Therefore, temperature detection is a part of chemical production. A very important parameter.
Temperature detection means
Temperature detection in chemical production is divided into two categories: local indication and remote display control. The local indication is relatively simple. The use of bimetal thermometers with poor accuracy is usually not the basis for production control adjustment. Far-distance temperature meter transmits the temperature of the spot to the control room display and control is an important parameter of production, as an important means of production control requires accurate and reliable.
Compared with various types of instrumentation used in chemical production, temperature detection is the simplest instrument with the least number of types in the traditional four major detection parameters. There are only three general categories of remote temperature detection instruments used in the field:
High-temperature radiation thermometers are used in production sites where the production temperature is above 1000 degrees for a long period of time. Such meters are used less frequently in most productions.
Thermal resistance temperature meters are commonly used in low temperature applications below 200 degrees Celsius. The most widely used is the temperature detection of various pumping equipments themselves, which is used to detect the operating temperature of the equipments to prevent overheating damage to the equipments.
Thermocouple temperature detection instrument, because the wide temperature range can be used from -200 - 1200 degrees range is the most widely used temperature detection instrument in chemical production. Thermocouples are divided into many models depending on the temperature range used. The most widely used type is the K-type thermocouple. Although the materials of different types of thermocouples are different, their temperature measurement principle is the same.
Thermocouple temperature measurement principle
A thermocouple is a very simple test instrument. As long as two ends of two different materials are welded together, a thermocouple thermometer can be made. The temperature measurement principle of such a simple structure instrument is the thermoelectric effect of a metal conductor, and this thermoelectric effect consists of two parts: the contact potential and the temperature difference potential.
Thermocouple hot end as shown in the figure:
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Contact potential
Contact potential is the main reason why thermocouples can measure temperature. Like the diode PN junction in the electronic device, the hot end of the thermocouple is welded together by two conductors of different materials. Due to the difference in conductor density, an electron density difference is generated. Under this density difference, the two conductor contact surfaces are Because of the different diffusion rates of electrons, a changing electric field will be generated. When the temperature of the contact surface is constant, electron diffusion will reach a dynamic equilibrium, resulting in a constant electrostatic field—contact potential.
Contact potential under different materials at the same temperature
When the temperature is constant, the contact potential is only related to the material of the two conductors of the thermocouple. Just as the voltage across the diodes of the silicon tube and the helium tube is different, the contact potential of thermocouples made of different materials at the same temperature is not the same. Therefore, different types of thermocouples have different potentials at the same temperature, so the thermocouples are divided into many types and different ranges of use.
Contact potentials at different temperatures of the same material
Thermocouples of the same type have different contact potentials at different temperatures. This is because when the thermocouple contact (the two conductors are welded together) the electron diffusion during the temperature change is balanced, the new electron diffusion reaches a new equilibrium point. At this time, the electrostatic field generated by the two conductors - the contact potential is not the same as the previous potential. Just as the static parameters of the diodes differ at different temperatures, the thermocouple will produce a changing potential.
For a finished finished thermocouple, the magnitude of the contact potential corresponds to the temperature at the contact end of the thermocouple. The temperature is high and the potential is large. Therefore, this contact potential is called the thermoelectric potential.
Thermoelectric thermoelectric potential
The thermoelectric potential is the main reason why thermocouples can characterize temperature. Temperature is an artificially stipulated physical quantity used to represent the size of energy. Although it has an absolute zero point, it is too different from our life. Like atmospheric pressure, the temperature zero we normally call is zero degrees Celsius. It is an artificial setting. The zero point. The signal of the temperature measuring instrument needs a reference temperature to reproduce the temperature, which is the zero degree Celsius used in our daily life.
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Thermocouple temperature measurement composition
Although the thermocouple itself is simple in construction, a good temperature measurement system needs to be constructed to accurately measure the temperature. The thermocouple is mainly composed of the following three parts: a hot electrode, a sheath tube, and a compensation wire.
Hot electrode
Thermo Electrodes, often called thermocouples, are the core components that thermocouples can measure. For manufacturing cost considerations, the length of the hot electrode is usually limited to about 1 meter, and the spot temperature measurement point is mostly a few hundred meters away from the control room. Therefore, the thermocouple needs a connection terminal to pass the signal to the control through other forms. room.
Thermal electrodes can be divided into different models according to different materials. Industrial field applications usually use K-type and E-type thermocouples. Type K thermocouples are nickel-chromium-nickel-silicon thermocouples, which have a wide range of temperature measurement, can be used within -200-1000 degrees for a long period of time, and can be used in a large number of applications to meet most conditions of industrial production. E-type thermocouples are nickel-chromium-nickel-copper thermocouples, and the range of use is less than -50-600 degrees than that of K-type. However, since the thermoelectric potential generated at the same temperature is larger than that of K-type, the error at low temperature is relatively small. There are also many applications in the 400 degree production.
Thermal electrode structures are generally divided into two categories: assembled and armored.
The assembled thermocouple is an ordinary thermocouple. Both ends of the hot electrode use a high-temperature-resistant insulating ceramic column as an insulating sleeve to prevent short-circuit at both ends of the thermocouple. The end of the hot electrode is connected to the insulating terminal column. This kind of structure is convenient for on-site installation. When the length of the hot electrode and sheath tube are different, they can be cut freely according to the length of the sheath tube, which is convenient and quick for field installation. The use of ceramic insulating sleeve is easy to damage, it will cause poor insulation, and even cause the thermocouple two conductors short circuit, so be careful when removing and installing, as far as possible to ensure the integrity of the ceramic casing.
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The hot end contact potential generated during the use of the thermocouple needs to be remotely transmitted to the control room for temperature reproduction. The temperature reproduction needs to be based on the reference Celsius zero degree, but it is difficult and impossible to achieve a constant zero degree Celsius temperature reference in the control room. In order to ensure that the thermoelectric power is not distorted by the temperature gradient during transmission, a measure that can extend the on-site thermal potential into the control room is required—thermoelectric potential extension.
Temperature difference potential
If a conductor has different temperatures at both ends, then the electron energy at the two ends of the conductor will be different. A high temperature electron energy will diffuse into the low temperature direction, thereby generating a positive potential at high temperature and a negative potential at low temperature when electrons. After the diffusion reaches the dynamic equilibrium, an electrostatic field is formed - the temperature difference potential. The thermocouple achieves the purpose of extending the thermoelectric potential generated by the hot-end temperature to the control room through this principle, thereby avoiding the measurement distortion caused by the temperature gradient during the transfer process.
Thermocouple contact potential and temperature difference potential superposition is the whole thermocouple temperature measurement principle, the difficult physical temperature detected into a convenient measurement of the millivolt voltage signal processing, in order to achieve the temperature of the measured point remote transmission.
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Sheath tube
Temperature is the physical quantity of material energy. Due to the energy transfer diffusibility, the energy of different measuring points leads to different temperatures. To accurately and reliably measure the temperature of the material, the accuracy of the detection is improved. In engineering, the temperature measurement point is generally selected as far as possible in the middle of the material. Thermocouples need to go deep inside the pipelines of the tested equipment during use. To ensure the safety of production, thermocouples need to be installed with protective tubes.
The installation of protective sleeves usually has two connection methods: thread mounting and flange mounting.
The threaded installation is usually applied in the pipeline temperature measurement. This has the advantage of convenient pipeline construction. It can be installed in small pipelines in the inclined flow direction to improve the accuracy of temperature measurement. The disadvantage is that the safety is poor, and the sealing is achieved through threads and gaskets. Can not be used in high pressure environments.
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Armored thermocouple, the hot electrode is inserted into a slender metal sheath tube, which is filled with high-temperature insulating aluminum oxide, magnesium oxide, etc., and the terminal lead is connected to the terminal board. Because armored thermocouples have a metal shell to protect them from high mechanical strength, they can be bent and installed, and the insulation is not easily damaged, so they are used in large quantities. However, the lead wire of the armored thermocouple is relatively fragile, and it is difficult to handle the repair once fractured. Therefore, be careful when taking the armored thermocouple to prevent the breakage of the lead wire.
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Flange installation is achieved through the connection between the two flanges, usually used in the installation of tower equipment and high-pressure pipelines, but due to the presence of flanges, its fastening bolts are easy to rust and are not conducive to disassembly and installation, and can only be perpendicular to Tower equipment installation direction of the pipeline, can not achieve the installation in the direction of flow is not conducive to heat exchange, temperature measurement is not as accurate as threaded connection.
Compensation wire
Compensation wire is a special transmission cable for thermocouple temperature measuring instruments. Different from ordinary transmission copper cables, as long as the transmitted signal is not distorted, the use of the compensation wire is a compensating cable used to continue the thermoelectric potential of the thermocouple. Because of the high material and manufacturing cost of the hot electrode, it is unrealistic to use a few hundred meters of the thermocouple of the finished product to extend from the site to the control room. Therefore, a transmission line that can both have the same thermo-electric potential as the on-site hot electrode performance is required and can be reduced. Cost of cable - compensating wire. The use of the compensation wire is not only the transmission of the hot electrode contact potential but also the continuation of the thermoelectric temperature difference potential, so its purpose is different from the conventional copper cable. In order to ensure that the temperature difference potential of the compensation wire is consistent with the temperature difference potential of the on-site hot electrode, the compensation wire of the same type as the hot electrode should be used in the transmission and cannot be mixed or misused.
Compensation wire use matters needing attention
First, the reason that the compensating wire is different from the copper wire is not only the material, but also the purpose of the compensating wire is to compensate for the thermoelectric potential in the transmission in addition to the thermoelectric potential of the thermocouple. The compensation wire itself is divided into positive and negative poles. It must be correctly connected to the thermocouple wiring, control room and secondary meter or control system at the scene, and cannot be connected as easily as the copper cable.
Second, compensation wire transmission as much as possible without joints, inevitably when possible to do a good job in the connection at the joints. Because the compensation wire is essentially a cheap thermocouple, if there is a contact in the middle of the compensation wire, a contact potential will be generated. If the contact resistance at the contact point is large, the contact potential will increase, causing the compensation wire to introduce additional contact potential. If the contact point is in poor contact, this contact potential will undergo a large jump, resulting in distortion of the temperature measurement. Similarly, the wiring at the compensating wire and thermocouple terminal must also be well connected to prevent the introduction of contact potential.
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Common Faults and Handling in the Use of Thermocouples
The thermocouple is a temperature measurement system. The signal transmission process is composed of a hot electrode, a terminal, a compensation wire, and a secondary treatment. Any part of the problem may cause inaccurate temperature measurement. The actual use of the phenomenon of failure can be roughly divided into the following categories: display value is too large, the value is too small, the value of the jump, full-scale value.
The thermal electrode itself causes a malfunction
The hot electrode is a metal conductor. During use, long-term exposure to high temperature smoke and the influence of ambient gas (corrosion, water vapor) will deteriorate and deteriorate. This will cause the thermo-electromotive force of the hot electrode to change in accordance with the temperature, resulting in temperature - millivolt. Voltage deviation from the table, resulting in temperature error.
Thermal electrode insertion depth is not enough. This fault usually occurs during the overhaul of the new device or device. The length of the sheath tube in the thermocouple installation is not enough. As a result, the distance to the pipeline equipment does not reach the center of the medium, causing the thermocouple's temperature measurement hot end to not reach the highest temperature point. As a result, the measured temperature is low. If the insertion depth of the thermocouple is too close to the outer wall of the pipeline equipment, the error of its temperature measurement is even greater, and even the sense of temperature measurement is lost completely.
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If the protective sleeve is long enough and the hot electrode is not long enough to handle, a sufficiently long hot electrode can be inserted. If the sheath tube itself is not long enough, it can not be processed, can only be replaced after waiting for the device to stop, so the length of sheath tube installation is very important, determines the accuracy of thermocouple temperature measurement, but often the problem of thermocouple is exactly This site sheathing pipe is installed.
Terminals
From the thermocouple temperature measurement principle, we can see that the composition of the thermoelectric potential includes the contact potential and the temperature difference potential. If the thermocouple field terminal and the compensation wire and compensating wire are not in good contact with the wire in the secondary processing table of the control room, then an additional contact potential will be generated in the thermocouple signal transmission process, causing a temperature measurement error. If the wire connection is poor, There will also be phenomena of large temperature jumps.
With the extension of time, the contact potential of the thermocouple terminal and the compensation wire at the site may become large due to aging, and the phenomenon that the measured temperature appears distorted or even jumps. At this time, when the compensation wire is reconnected to the site, the fault will occur. Exclusion, this is also the reason why many field instrumentation engineers are puzzled.
If there is a joint in the middle of the compensating wire, the influence of this contact potential on the temperature measurement will also appear. Therefore, it is better not to have a joint in the use of the compensating wire, which cannot be avoided. In addition to the connection processing of the joint, the connector must be connected. The location is marked clearly. Once the temperature measurement fault occurs, the position of the compensation wire connector can be quickly found and checked. Otherwise, the fault caused by the compensation wire is difficult to find and remove.
Compensation wire
The compensation wire is an open, inexpensive thermocouple of the same type as the field thermocouple and its performance is basically the same as that of a standard thermocouple. The type of compensating wire used in the field must be consistent with the installed thermocouple. This specific type can be found on the cable sheath of the compensating wire, and can also be distinguished by compensating the color of the insulated sheath of the wire inside the wire. If the type of compensating wire is incorrect, temperature measurement error will be introduced. For example, if a type K thermocouple is connected to an E-type compensation wire, the temperature displayed in the control room will be high. Similarly, the E type thermocouple on the spot will be used to access the K type compensation. The wire shows a low temperature.
The compensating wire is a cheap thermocouple and therefore has its own positive and negative points. It must be correct when wiring the field and the thermocouple and connecting with the secondary table of the control room. If one end of the compensation wire is connected in the opposite polarity to the thermocouple or the second meter, the thermopotential transmitted to the secondary meter is negative. After processing, the display temperature is negative or minimum (minimum in the configuration). If the polarity of the compensating wire site and the control ends are reversed at the same time, it is equivalent to a negative temperature difference potential in the line of the field thermocouple millivolt signal, then the millivolt potential of the secondary wire entering the secondary table becomes smaller, indicating that The temperature will be too small.
Secondary treatment
The secondary processing instrument compares the introduced millivolt voltage signal with the temperature and performs cold junction temperature compensation of the thermocouple. If the selected thermocouple type in the secondary table configuration is inconsistent with the field, then the converted measured temperature will deviate. If the ambient temperature measured by the temperature measurement thermistor of the secondary watch itself is inaccurate, then the converted field temperature will also have a deviation. In addition, if the secondary meter is an output current temperature transmitter, the temperature range of the output current signal should be consistent with the control room display configuration.
Overhaul of thermocouple overhaul
The overhaul of thermocouples is usually divided into the calibration of the accuracy of the thermode itself and the wear inspection of the sheath. The calibration of the hot electrode is very important, but it is not necessary to perform the work when the overhaul is needed. If the device malfunctions during the normal operation, it can be removed from the site for troubleshooting and calibration.
The inspection of the thermocouple sheath tube is the key work during the overhaul, but many companies actually ignore it. Thermocouple sheathing pipe is inserted into the equipment and pipeline in daily use, and is subject to the erosion and corrosion of the medium for a long time. The outer wall of the thermocouple sheath is gradually worn to thin the wall thickness, and even the extreme conditions such as perforation and fracture occur. Once the accident of the thermocouple sheath tube is running, the accident will cause the equipment to stop. If it is serious, it will cause a fire accident. Therefore, the inspection of the degree of corrosion of the thermocouple sheath tube is the key task of the overhaul.
There are usually two methods for inspection of sheathed tubes, artificial eye inspection and pressure test. The pressure test method is usually used in high pressure devices such as high pressure hydrogenation devices. The sealing performance of the thermocouple sheath tube cannot be accurately judged by the naked eye, and it needs to be disassembled and used for professional pressure test equipment.
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If the production equipment is not operating in a high pressure environment, the wear detection of the thermocouple jacket tube is usually confirmed by visual inspection. This method is quick and easy, but it is prone to waste. After dismantling the thermocouple sheath on the site and removing the dirt on the sheath surface with a rag, observe whether the outer wall of the sheath tube shows signs of wear, corrosion, hemp holes, etc. If it is not obvious, replace the gasket. After recovery, if it is noticed that the sheath tube is seriously corroded, replace the new sheath tube. In order to ensure safety, a new sheath tube is replaced whenever the thermocouple sheath tube is inspected on site, as long as there are obvious signs of corrosion, to ensure the safety of the device operation, but at the same time, a certain amount of thermocouple sheath tube is wasted.
Temperature measurement range and allowable error
Thermocouple Type Code Index No. Measuring Range °C Allowable Deviation Δt °C
Platinum crucible 30-platinum crucible 6WRRB0-800±1.5°C or ±0.25%│t│
Platinum rhodium 10-platinum WRPS0~1600±1.5°C or ±0.25%│t│
Nickel-chromium-nickel-silicon WRNK0~1300±2.5°C or ±0.75%│t│
Nickel-Chromium-Copper-Nickel WREE 0~800±2.5°C or ±0.75%│t│
to sum up
Thermocouple thermometers are the most widely used temperature detection instruments in the industry, but they all have the same measurement principle. As industrial workers must be familiar with the thermocouple temperature measurement principle, in order to face the fault when dealing with ease.

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