How to distinguish between fixed flange-mounted probe thermocouples and fixed threaded-mounted probe thermocouples

Installation Structure and Connection Methods

The fixed flange mounting type connects to the equipment piping via a rigid flange plate, either by welding or bolting, forming a face-seal structure. This is suitable for large-diameter (DN50–DN300) high-pressure systems. The fixed threaded mounting type uses an external thread to screw directly into a pre-threaded hole in the equipment, forming a line-seal structure. This is commonly used in small-diameter (DN15–DN50) medium- and low-pressure applications. Flange-type connections do not require tapping on the equipment, while threaded connections require the equipment body to have standard threads (such as M27×2, 1″NPT).

 

Pressure Resistance and Sealing Reliability

Flange-type connections can withstand high pressures of 10–25 MPa, relying on metal-wound gaskets or flexible graphite for multi-point uniform sealing, suitable for extreme conditions such as catalytic cracking and main steam pipelines; the pressure limit of threaded connections is usually ≤10 MPa, and the sealing relies on refractory cement or asbestos rope, which is prone to thread seizing or seal failure due to thermal cycling, and is not suitable for environments with continuous high-pressure fluctuations.

 

Resistance to Mechanical Interference and Vibration Tolerance

Due to the overall rigid connection, flange-type connections can effectively absorb equipment vibration and shock, suitable for strong vibration environments such as steam turbines and electric arc furnaces, with a significantly lower failure rate than threaded connections; threaded connections are prone to loosening, stripping, or breakage under long-term vibration, requiring additional anti-loosening clamps, increasing maintenance costs.

 

Maintenance Convenience and Disassembly Efficiency

Flange-type connections support quick online replacement; the probe can be removed by simply loosening the bolts without affecting the pipeline integrity; although threaded connections are detachable, long-term high-temperature operation can easily lead to thread sintering, corrosion, and seizing, often requiring cutting or heat treatment for disassembly, increasing the risk of unplanned downtime.

 

Applicable Scenarios and Industry Preferences

Application Scenario	Flange Mounting Type	Threaded Mounting Type
Petrochemical	Catalytic reactors, hydrogenation pipelines	Small-diameter instrument pipelines
Power Systems	Boiler main steam, reheaters	Auxiliary temperature measurement points
Metallurgical Industry	Electric arc furnace walls, continuous casting crystallizers	Auxiliary temperature monitoring points
Nuclear Energy Systems	Primary circuit coolant monitoring	Low-risk auxiliary temperature measurement
Pharmaceutical/Food	Sanitary flange (mirror polished)	Rarely Used

 

Installation Costs and System Integration

Flange type has high initial installation costs, requiring matching flanges, bolt sets, gaskets, and alignment calibration; threaded type has lower costs, only requiring pre-fabricated threaded holes, making it suitable for standardized and modular equipment. Flange type is more conducive to system modular upgrades, while threaded type is limited by the interchangeability of thread specifications.

 

Lifespan and Corrosion Resistance

Flange type, due to its independently replaceable gaskets and the option of using corrosion-resistant materials such as duplex stainless steel and Hastelloy for the flange body, offers superior long-term corrosion resistance; the threaded type's threaded area is prone to accumulating corrosive media, and stress concentration at the thread root accelerates fatigue failure, resulting in a generally shorter lifespan than the flange type.

The fundamental difference between the two lies in their connection mechanics and sealing paths: the flange type is a "surface contact, high rigidity, maintainable" industrial-grade solution, while the threaded type is a "line contact, low-cost, convenient" basic configuration. In high-pressure, large-diameter, and high-reliability applications, the flange type is the preferred choice; in low-to-medium pressure, small-diameter, and budget-constrained applications, the threaded type still offers practical value.