In the field of industrial temperature measurement, the installation method of thermocouples directly affects their measurement accuracy, environmental adaptability, and ease of maintenance. Fixed flange-mounted probe thermocouples, metal-protected insertion tube thermocouples with junction boxes, and fixed threaded-mounted thermocouples with junction boxes are three typical structures, whose design differences stem from varying requirements for sealing, ease of installation, and environmental adaptability. The following systematically analyzes the differences between the three from four dimensions: structural characteristics, installation mechanism, application scenarios, and maintenance characteristics.
I. Fixed Flange-Mounted Probe Thermocouples: The Cornerstone of Rigid Sealing in Industry
The core identifying feature of a fixed flange-mounted probe thermocouple is its integrated standardized metal flange at the end, typically using national standard or ANSI specifications from DN25 to DN50. The material is often 316L stainless steel or Inconel alloy to withstand long-term corrosion from high-temperature, high-pressure, and highly corrosive media in industries such as chemical and power generation. Its installation method is a rigid centered connection – the flange is pressed against the pre-existing flange surface of the equipment using bolts, with an asbestos gasket, metal spiral wound gasket, or flexible graphite gasket inserted in between to achieve zero-leakage sealing. This structure provides strong vibration resistance and long-term stability, making it suitable for core temperature measurement points in reactors, distillation columns, and large heat exchangers. Its advantage lies in reliable sealing, especially in flammable, explosive, or toxic media environments, where flange connection is a mandatory safety choice. However, the disassembly process is complex, requiring shutdown, depressurization, and removal of multiple bolts, and the flange surface is prone to micro-deformation due to thermal cycling, leading to subsequent sealing failure. In refinery pipelines at normal temperature and pressure, the lifespan of such thermocouples can reach more than 5 years, but in high-temperature furnaces above 1200°C, differential thermal expansion of the flange may still cause stress cracks, requiring regular inspection of bolt pre-tightening force.
II. Metal-Protected Insert-Type Thermocouple with Junction Box: A Flexible and Convenient Temporary Temperature Measurement Solution
The metal-protected insert-type thermocouple with a junction box, also known as the "non-fixed type," is characterized by its lack of external fixing structures. It consists only of a metal protection tube (usually 304 or 316 stainless steel), thermocouple wires, insulation sleeves, and a junction box. The insertion part is a smooth, straight rod without threads or flanges. During installation, the probe is simply inserted into the pre-reserved temperature measurement hole or protective sleeve of the equipment, without the need for additional fasteners. This structure provides high flexibility, making it particularly suitable for applications requiring frequent replacement, temporary testing, or adjustable insertion depth, such as laboratory testing, temperature verification during commissioning, or mobile temperature measurement equipment. Its advantages include easy installation, low cost, and fast response speed, and the measurement depth can be adjusted by using different lengths of insertion tubes. However, due to the lack of mechanical fixation, the probe is susceptible to loosening or even detachment due to fluid flow impact, especially in high-pressure or high-speed fluid environments, posing a safety hazard. Furthermore, the sealing performance is poor, usually relying on hole diameter matching or the addition of sealing rings for basic leak prevention, making it difficult to meet high sealing requirements. To improve stability, some models are used with movable flanges or ferrule structures to achieve adjustable insertion depth while maintaining a certain degree of fixation strength.
III. Fixed Threaded Installation Thermocouple with Junction Box: A Modular Screw-In Industrial Standard
The key feature of the fixed threaded installation thermocouple with a junction box is that its protective tube has standard external threads. Common specifications include M27×2, G1/2, M33×2, etc., and the material is mostly 304 or 316L stainless steel. The threaded part is integrally formed with the protective tube to ensure structural strength. During installation, the thermocouple is simply screwed into the pre-reserved internal threaded hole of the equipment, and an airtight connection is achieved with a high-temperature resistant sealing gasket (such as a graphite spiral wound gasket). The junction box is usually located at the end of the protective tube and is designed separately from the threaded part for easy wiring and maintenance. This compact structure, occupying minimal space, is suitable for conventional temperature measurement points such as pipe wall temperature, motor casings, and boiler flues, and is extremely common in petrochemical, metallurgical, and power industries. Its advantage lies in its strong detachability; maintenance or calibration only requires unscrewing for replacement, eliminating the need for welding or flange removal tools, greatly improving operation and maintenance efficiency. However, its key risks are thread stripping and cold welding – if an anti-seize compound (such as high-temperature lubricating paste containing graphite or boron nitride) is not used, stainless steel threads are highly susceptible to "cold welding" in high-temperature environments, leading to difficult disassembly or even breakage. Furthermore, threaded connections require high precision in the machining of the mounting hole; misalignment or burrs can lead to poor sealing or deformation of the protective tube.
IV. Selection Logic: Matching the Optimal Solution Based on Operating Conditions
The selection of the three types of thermocouples is essentially a result of engineering trade-offs. Fixed flange type is suitable for critical temperature measurement points requiring extremely high sealing and safety; metal-protected insertion tube type is suitable for temporary, debugging, or low-cost applications; while the fixed threaded connection box type prioritizes modular screw-on installation, suitable for space-constrained or frequently maintained applications. When selecting, comprehensive consideration should be given to medium pressure, temperature range, corrosiveness, vibration intensity, and maintenance frequency, avoiding decisions based solely on ease of installation, and ensuring the long-term stable operation of the temperature measurement system. For example, in a chemical reactor, the fixed flange type is the only option; in laboratory testing, the metal-protected insertion tube type may be more suitable; and for pipe measurement points requiring quick replacement, the fixed threaded connection box type is the best solution.
