How to choose the form and installation method of thermocouple bushing and thermal resistance bushing

Thermometer bushing is a temperature sensor installation piece that is sheathed outside the temperature measurement element of a thermocouple, thermal resistance or thermometer or is pre-installed on the equipment to resist the pressure and corrosion of the measured medium.

Thermometer bushing is a temperature sensor installation piece that is sheathed outside the temperature measurement element of a thermocouple, thermal resistance or thermometer or is pre-installed on the equipment to resist the pressure and corrosion of the measured medium.

1. The selection of the production process of thermocouple bushing and thermal resistance bushing

There are two processes for making thermocouple bushings and thermal resistance bushings: one is the steel pipe welding type, and the other is the integral drilling type.

①The welded steel pipe is formed by sealing one end of the steel pipe. The casing is generally thinner with a larger inner hole, so the compressive capacity is not high, only 6.4MPa. If the casing is installed on the pipeline, even if the operating pressure is small, the casing will be broken and damaged under the vibration of the impact force due to the high fluid flow rate, such as the casing at the outlet of the circulating water pump. Damages or even accidents occur. At the same time, because the inner holes of the steel pipe are larger, there are more heat conduction intervals inside the casing, which results in a slow measurement response speed. The assembled thermocouple and assembled thermal resistance use steel pipe welded casing.

How to choose the form and installation method of thermocouple bushing and thermal resistance bushing

②The overall drilling type is solid bar drilling and forming, the inner hole of the casing is small, and it is closely matched with the measuring element. Therefore, the reaction speed is fast, the anti-vibration effect is good, and the pressure resistance is high, which can reach more than 30MPa. Based on the advantages and disadvantages of the above two processes, for the thermowell process, it is recommended to choose a full-hole drilling thermocouple or an integral rotary hole thermal resistance. At present, the domestic advanced drilling technology adopts the single-ended deep blind hole integral drilling process. The inner hole diameter of the integrally drilled thermocouple casing is only φ6.5mm, which is perfectly matched with the international standard φ6mm measuring element.

How to choose the form and installation method of thermocouple bushing and thermal resistance bushing

2. Selection of thermowell and thermal resistance bushing form

Under the same length, the tapered casing has a higher natural frequency than the straight casing, so the ability to resist fluid impact and vibration is stronger. And because the diameter of the front end of the tapered sleeve is smaller than that of the straight sleeve, a faster response speed is ensured. According to the above two points, it is recommended to use tapered casing for the casing form, and the diameter difference between the large and small ends is not less than 6mm.

How to choose the form and installation method of thermocouple bushing and thermal resistance bushing

3. Selection of thermowell and thermal resistance sleeve diameter

According to different pressure levels, the casing diameter should also be selected reasonably. For example, the diameter of the casing used in medium and low pressure applications can be 25~19mm in the large and small end diameters, and the diameter of the casing used in the high pressure field can be 32 in the large and small end diameters. ~21mm etc.

How to choose the form and installation method of thermocouple bushing and thermal resistance bushing

4. Selection of installation methods for thermocouple bushings and thermal resistance bushings

There are three installation methods for thermocouple bushings and thermal resistance bushings: flange, thread and welding. The welding type is generally determined according to the needs of the site. For the two methods of flange and embossing, flange installation is recommended for the following reasons:

① For occasions with higher pressure levels, threaded installation is not suitable and can only be installed with flanges.

②For corrosive occasions, the threaded installation is easy to be disassembled due to corrosion. At this time, flange installation must be selected.

③Used on the equipment, all use flange installation.

④For occasions where the temperature is higher than 230℃, the thread is prone to high-temperature gap deformation, so flange installation should be used.

⑤Combining the above points, it is recommended to install thermocouple bushings and thermal resistance bushings with flanges between thread and flange installation.

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