Structural Differences: The movable threaded connection type thermocouple consists of three parts: an independently removable metal protection tube, a rotating threaded interface, and a sealed junction box. The thermoelements are insulated from the protection tube by insulating material, and the signal is led out through the junction box terminals, achieving both mechanical and electrical separation. The fixed flange-mounted probe type thermocouple, on the other hand, is an integrated, single-piece structure. The thermoelements are encapsulated within a metal sheath, with the wires directly led out from the tail end, without any separate connection unit. Its base is fixed to the equipment flange with bolts via a rigid flange plate, forming a non-removable rigid connection. It is a single-use temperature sensing unit where the probe itself is the mounting body.
Installation Method and Operation Logic: The movable threaded type requires manual screwing into a pre-set threaded hole in the equipment. After installation, the probe can still be freely rotated 360° to precisely align with the temperature measurement point, adapting to curved pipe surfaces, spatial interference, or installation deviations. Sealing is ensured by torque control. The fixed flange type requires the equipment to have a pre-welded or pre-reserved flange interface. During installation, the probe flange can only be aligned and locked with the equipment flange using bolts. The angle and insertion depth are completely non-adjustable. Operation depends on the manufacturing precision of the equipment and is suitable for fixed temperature measurement points in standardized, high-pressure, large-diameter pipelines, such as steam main pipes and reactor flange interfaces.
Maintenance and Replacement Capabilities: For the movable threaded type, maintenance only requires unscrewing the protection tube and the temperature sensing probe. The junction box, cable, and seals can be reused for a long time. A single replacement takes 5-10 minutes, supporting periodic calibration and fault isolation. For the fixed flange type, if the sheath corrodes, the wires age, or there is poor contact, the entire probe must be removed and replaced. The removal process requires removing flange bolts and gaskets, and it is impossible to repair locally. This is a high-cost, high-downtime design, with long replacement cycles and low frequency, but high cost per replacement.
Environmental Adaptability and Reliability: The movable threaded type, with its metal protective tube and IP65 or higher sealed junction box, can withstand high temperatures (up to 1200℃), high pressure, strong vibration, condensation, and cleaning environments, making it suitable for continuously operating industrial systems. Although the fixed flange type often uses 316L stainless steel or Inconel for its sheath material, with a rated temperature of over 1300℃, and the flange structure can withstand extremely high system pressures (such as PN40 and above), it lacks electrical shielding and adjustable damping structures. Long-term thermal cycling can easily lead to deformation of the flange sealing surface, accumulation of thermal expansion stress, and fatigue cracking at the probe base.Therefore, it is only suitable for high-pressure, high-temperature main process temperature measurement in stable operating conditions without severe vibration or frequent starts and stops.
Typical Application Scenarios: The movable threaded type is widely used in demanding environments requiring long-term stability, maintainability, and industrial compliance, such as pharmaceutical GMP reactors, food CIP cleaning lines, nuclear power auxiliary pipelines, and chemical high-pressure reactors. The fixed flange type dominates in critical temperature measurement points that require rigid fixation, high internal pressure resistance, and no micro-movement or leakage, such as large steam pipelines, high-pressure boiler outlets, refinery reactor flange interfaces, and chemical main process pipelines. It is the "hard connection" standard solution for temperature monitoring in industrial main processes.
Standards and Specifications:The movable threaded type strictly adheres to GB/T 30429-2013 "Replaceable Sheath" and IEC 60584-3 "Removable Installation" standards, featuring standardized interfaces and traceability; although the fixed flange type does not have a separate standard number, its installation method is clearly classified as a "fixed connection method" in GB/T 30429-2013. Its flange dimensions, sealing surface type, and material requirements refer to GB/T 9112 (Steel Pipe Flanges) and JB/T 8622 (Technical Conditions for Industrial Thermocouples), making it a formally recognized installation type under industrial standards, not a non-standard custom design.
Selection Decision Core: If long-term operation, repeatable maintenance, vibration resistance, and industrial compliance are required, the movable threaded installation with a junction box must be selected; for high-pressure, high-temperature, large-diameter main process pipelines requiring absolute sealing and rigid fixation, and without a maintenance plan for permanent temperature measurement points, the fixed flange-mounted probe type is the only compliant and reliable solution.
Operation Precautions: Welding or forcibly fixing the junction box of the movable threaded type is strictly prohibited, as this will completely negate its core advantages; for the fixed flange type, forcibly tightening the flange without proper alignment, clean sealing surfaces, and the use of specified gaskets is prohibited, as this will lead to leakage, stress concentration, or probe fracture. During installation, a torque wrench must be used to tighten according to the standard torque; the use of impact tools is strictly prohibited.
