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 movable flange 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 Thermocouple: The Cornerstone of Rigid Sealing in Industry
The core characteristic of a fixed flange mounted probe thermocouple is its integrated standardized metal flange at the end, usually using national standard or ANSI specifications from DN25 to DN50. The material is often 316L stainless steel or Inconel alloy to withstand long-term erosion 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-reserved 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 extremely 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 its 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. Furthermore, 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℃, the 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 Versatile 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-velocity fluid environments, posing a safety hazard. Furthermore, the sealing performance is relatively poor, usually relying on hole diameter matching or added 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.
III. Movable Flange Mounted Thermocouple with Junction Box: A Balanced Solution for Adjustable Depth
The core feature of the movable flange mounted thermocouple with a junction box is its separate design of the flange and the protection tube. The flange can move axially along the protection tube and is fixed to the equipment with bolts, allowing for flexible adjustment of the insertion depth. The junction box is usually independent of the flange and located at the end of the protection tube, facilitating wiring and maintenance. This design allows users to adjust the insertion depth according to actual needs without disassembling the entire thermocouple, making it particularly suitable for applications requiring precise control of the temperature measurement position, such as in certain reactors or piping systems. Its advantages include ease of installation and adjustability, adapting to different temperature measurement depths while maintaining good sealing performance. However, its sealing performance, while superior to the insert-type, is slightly inferior to the fixed flange type, and the movement of the flange may introduce additional leakage risks. Furthermore, the movable flange structure may experience reduced stability in high-pressure or high-temperature environments due to frequent adjustments, requiring regular inspection of the flange tightening status.
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. The fixed flange type is suitable for core temperature measurement points with extremely high requirements for sealing and safety; the metal protection insertion tube type is suitable for temporary, debugging, or low-cost application scenarios; while the movable flange with junction box type prioritizes adjustable depth and convenient maintenance, making it suitable for situations requiring flexible adjustment of the temperature measurement position. When selecting a type, comprehensive consideration should be given to media pressure, temperature range, corrosiveness, vibration intensity, and maintenance frequency, avoiding decisions based solely on ease of installation, to ensure 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 protection insertion tube type may be more suitable; and in systems requiring frequent adjustment of the temperature measurement depth, the movable flange with junction box type is the best solution.
