Structural Design and Installation Mechanism
Fixed Thread Type: The probe is a rigid, integral structure. The protective tube and threaded interface are integrally formed. Mechanical fixing and sealing are achieved by forcibly screwing the probe into the pre-drilled threaded hole in the equipment. It is non-removable after installation, and the thread itself provides both sealing and pressure resistance.
Movable Thread Type: Uses a detachable threaded seat (thermowell) + floating probe structure. The threaded seat is fixed to the equipment wall, and the probe is inserted into the seat via an internal thread or ferrule-type interface. The probe body can move slightly along the axial direction to compensate for thermal expansion. Sealing is achieved by an O-ring or spring-loaded sealing ring, not relying on thread engagement.
Thermal Expansion Compensation Capability
|
Feature |
Fixed Thread Type |
Movable Thread Type |
|
Thermal Stress Release |
None, rigid connection easily leads to deformation or fracture of the protective tube |
Yes, the probe can float axially, absorbing thermal expansion and contraction stress |
|
Thermal Lag Effect |
Significant, heat needs to pass through the protective tube wall |
Smaller, the probe is in direct contact with the medium, resulting in faster response |
|
Applicable Temperature Difference Environment |
≤150℃ temperature difference (conventional working conditions) |
>300℃ temperature difference (e.g., steam pipelines, reactor start-up and shutdown) |
Sealing Method and Maintainability
Fixed Thread Type: Relies on high-temperature thread sealant (such as Loctite 577) or metal-to-metal sealing. It is non-removable after installation; maintenance requires shutting down the system, depressurizing, and disassembling the entire pipe section.
Movable Thread Type: Uses a fluororubber or PTFE O-ring + spring pre-tensioning structure to achieve zero leakage and online replacement. The probe can be removed while the system is under pressure, without interrupting the process flow, significantly improving maintenance efficiency.
Applicable Working Conditions and Industry Standards
Fixed Thread Type:
Suitable for medium and low pressure, stable working conditions (such as atmospheric pressure storage tanks, equipment with infrequent start-stop cycles);
Complies with GB/T 30423-2013 and IEC 60751, but without "removable" certification.
Movable Thread Type:
Designed specifically for high pressure, high temperature, and frequent thermal cycling scenarios (such as nuclear power plants, petrochemical reactors, and steam pipe networks);
Complies with IEC 60751 "removable connection" type specifications, and is listed as a standard product line by international manufacturers such as ABB, WIKA, and Endress+Hauser.
Typical Application Scenarios
Fixed Thread Type: Pharmaceutical sterilization tanks, chemical reactors (atmospheric pressure section), power plant boiler flues, food processing pipelines.
Movable Thread Type: Nuclear power plant primary cooling circuit, refinery high-pressure steam pipelines, large compressor cylinders, continuous production reactors.
The fundamental difference between the two lies in the engineering philosophy of "one-time installation" versus "maintainable design"-the former pursues compact structure and cost control, while the latter focuses on system availability and safety redundancy. The key to selection is not accuracy, but whether online maintenance is permitted, whether it can withstand thermal stress shock, and whether it needs to comply with industrial continuous operation standards.
