Selection and installation methods for thermocouples and thermal resistors

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I. Selection of thermocouples and thermal resistors 1. Thermal resistors should be used when the normal operating temperature range of the object being measured is below 300°C. 2. Thermocouples should be used for the objects being measured, whose normal operating temperature is above 300°C. II. Practical Training on the Installation and Maintenance of Thermocouples and Thermal Resistors – Master the methods for installing thermocouples and thermal resistors. Learn to use thermocouples and thermal resistors for temperature measurement. Master the wiring of thermocouples, thermal resistors, and secondary instruments. III. Installation requirements for thermocouples and thermal resistors. When installing thermocouples and thermal resistors, it is important to ensure accurate temperature measurement, safety, reliability, and ease of maintenance, while also avoiding any interference with the operation of the equipment and production processes. To meet these requirements, the following two points should be taken into consideration when selecting the installation location and insertion depth for thermocouples and thermal resistors: 1. Thermocouples and thermal resistors equipped with protective sleeves experience heat transfer and dissipation losses; to reduce measurement errors, they should have an adequate insertion depth. (1) When the insertion depth of the sensing element exceeds 1 meter, it should be installed as vertically as possible, or support frames and protective sleeves should be used. (2) If it is necessary to measure the temperature of the smoke in the flue, even though the flue diameter is 1000 mm, a thermocouple or thermal resistor with an insertion depth of 500 mm will suffice. (3) For thermocouples used to measure the temperature of the fluid at the center of a pipe, their sensing ends should generally be inserted into the center of the pipe, either vertically or at an angle. If the diameter of the pipe containing the fluid to be measured is 100 millimeters, the insertion depth of the thermocouple or thermal resistor should be between 50 and 100 millimeters (taking into account the dimensions of the mounting base or flange). (4) For temperature measurement of high-temperature, high-pressure, and high-speed fluids (such as main steam temperature), in order to reduce the resistance exerted by the protective sleeve on the fluid and prevent the sleeve from breaking under the influence of the fluid, a shallow insertion method for the protective sleeve or a heat-shielded thermocouple can be used. For thermocouples with a shallow-insertion type of protective sleeve, the depth of insertion into the main steam pipeline should be no less than 75 mm; the standard insertion depth for heat-shielded thermocouples is 100 mm. 2. To ensure adequate heat exchange between the measuring ends of thermocouples and thermal resistors and the medium being measured, it is necessary to select appropriate locations for these sensors; efforts should be made to avoid installing them near valves, elbows, as well as in dead corners of pipes and equipment. IV. Installation methods for thermocouples and thermal resistors: 1. First, it is necessary to measure the dimensions of the flanges or thread threads of the thermocouples and thermal resistors, and then fabricate the corresponding flanges or threaded bases. 2. Make holes in the pipeline to be measured according to the dimensions of the flange or threaded base. 3. Welding of the flange or threaded base. Insert the flange base or threaded base into the pre-drilled hole, and weld it to the pipeline being measured. 4. Secure the thermocouple or thermal resistor with bolts or screw it into the welded threaded base. 5. Connect the wires of the thermocouple or thermal resistor in the terminal box according to the wiring diagram, and link them to the corresponding display instruments on the gauge panel. Be careful that the terminal box does not come into contact with the wall of the pipeline containing the medium being measured, to ensure that the temperature inside the terminal box remains below 100°C. The wire exit holes in the terminal box should be protected so as to prevent short circuits in the wiring terminals due to poor sealing or the accumulation of moisture and dust. 6. The location where thermocouples or thermal resistors are installed should take into account ease of inspection and maintenance.