In temperature measurement, when operating conditions exceed the suitable limits of threaded connections (e.g., high pressure, high temperature, large diameter, strong corrosion, or frequent disassembly), flanged connections become the preferred solution. Renowned for their high structural strength, exceptional sealing reliability, and standardized interchangeability, flanged connections hold a central position in demanding industrial processes. A complete flanged joint system consists of mating flanges, a sealing gasket, and connecting bolts. Its classification is primarily based on three dimensions: attachment method, facing type, and pressure-temperature rating.
Part I: Classification by Method of Attachment to Pipe/Equipment
This is the most fundamental classification, determining the load-bearing capacity and application scenarios of the flange.
1. Weld Neck Flange
This type offers the highest structural integrity and performance among all flanges. It features a tapered hub that transitions to a butt-welding end, which is welded to the pipe. The tapered hub provides excellent structural reinforcement, smoothly distributing stresses and avoiding concentration at the flange neck. Consequently, it offers exceptional resistance to fatigue loads caused by mechanical vibration, thermal shock, or pressure cycling.
Primary Applications: High-pressure, high-temperature (or cryogenic) piping systems for toxic, flammable, or explosive media. The preferred choice for critical units in petrochemical, power generation, and oil & gas transmission pipelines. It is the ideal connection method for introducing temperature sensors via thermowells into systems under extreme conditions (e.g., high-pressure steam).
2. Slip-On Flange
The bore of a slip-on flange is slightly larger than the pipe's outer diameter. During installation, the pipe slides through the flange, and fillet welds are applied on both the inside and outside of the flange to the pipe end. Its structure is simpler than a weld neck, cheaper, and relatively easier to install, but it has lower strength.
Primary Applications: Widely used in medium- to low-pressure liquid and gas systems (e.g., cooling water, low-pressure steam, air). The most commonly used and numerous flange type in plants. Due to its economy, it is extensively used for temperature measurement points in non-extreme conditions.
3. Threaded Flange
This flange has a central bore with internal pipe threads (e.g., NPT or BSPT) and can be screwed directly onto a pipe with matching external threads, eliminating the need for welding. This connection is removable.
Primary Applications: Primarily used where welding is not desirable. Examples include adding measurement points to existing pipelines, alloy steel pipes (complex welding procedures), areas where hot work is prohibited (e.g., parts of refineries, chemical plants), or locations requiring regular disassembly for inspection. Its pressure rating is generally lower than that of weld neck flanges.
4. Socket Weld Flange
Similar to a slip-on flange, but its bore has a recessed shoulder (socket). The pipe is inserted into the socket and then fillet welded on the outside only. This construction offers better fatigue strength than slip-on flanges and a more robust weld.
Primary Applications: Mainly used for small-bore (typically DN ≤ 50) high-pressure piping systems, such as instrument impulse lines or hydraulic lines. It provides a more reliable connection for small pipes compared to slip-on flanges.
5. Lap Joint Flange
This flange is not directly welded to the pipe. Instead, it "laps" over the end of the pipe, which is fitted with a separate "lap joint stub end" or "backing flange." The flange itself can rotate freely, facilitating bolt hole alignment during assembly. The pressure is borne by the stub end, while the flange serves only to connect and compress the gasket.
Primary Applications: Suitable for expensive or non-ferrous piping (e.g., copper, nickel alloys, plastic-lined pipes), as only an inexpensive carbon steel flange is needed, while the stub end can be of the same material as the pipe, saving cost. Also used where frequent disassembly or precise bolt hole orientation is required.
Part II: Classification by Sealing Face Type
The facing is the heart of the flange system's leak-tightness. Its form dictates the gasket type and sealing integrity.
1. Raised Face
The most common type, featuring a raised, smooth ring on the sealing surface. It is typically used with non-metallic soft gaskets (e.g., compressed asbestos, PTFE, graphite-reinforced). The raised height provides space for gasket compression.
Applications: The most widely used, suitable for general services like water, steam, air, and oil at nominal pressures from PN1.6 to PN16 (or Class 150 to Class 300).
2. Male-Female Face
Consists of a mating pair: one flange with a recessed face (female) and one with a raised face (male). The gasket sits inside the female face, and the male face fits into it. This design provides good gasket positioning, prevents gasket blow-out, and limits over-compression.
Applications: Offers better sealing than RF, suitable for higher pressures (e.g., PN4.0-PN10.0, Class 300-Class 600) or where media have a tendency to seep.
3. Tongue and Groove Face
This offers the best sealing integrity. One flange has a precision-machined tongue, and the other has a matching groove. The gasket (often a metal ring-joint or serrated metal gasket) is placed in the groove, confined on four sides by the tongue and groove walls, making blow-out virtually impossible and providing a long sealing path.
Applications: Used for extreme and hazardous services: high pressure (PN10.0 and above, Class 600 and above), high temperature, toxic, flammable, or high vacuum. Common in core reactor units in the petrochemical industry.
4. Flat Face
The entire face of the flange is flat and smooth, with no raised area. It typically requires a full-face gasket that covers the entire surface.
Applications: Primarily used with flanges made from low-strength materials like cast iron or plastic, or for very low-pressure applications (PN0.6, PN1.0), such as circulating water systems. It avoids high localized stress on the flange.
Part III: Classification by Pressure-Temperature Rating
A flange's pressure-containing capability is not a fixed value but varies with material and operating temperature. This is indicated by its "pressure class."
PN Series: European and Chinese system, metric units. Examples: PN10, PN16, PN25, PN40, PN63. The number represents the maximum allowable working pressure in bar at 20°C. For example, PN16 indicates a design pressure of 16 bar.
Class Series: American system, imperial units. Examples: Class 150, 300, 600, 900, 1500, 2500. The Class number is a dimensionless rating designation, not a direct pressure value. Class 150 corresponds to a pressure rating of approximately 20 bar at ambient temperature, but the allowable pressure for that same flange decreases at higher temperatures. The specific pressure-temperature relationship must be checked in rating tables from standards like ASME B16.5.
Selection Principle: The flange class must be selected based on the maximum working pressure at the design temperature, ensuring that the mating flanges, gasket, and bolts are all rated for the same class service.
Conclusion
For temperature sensor applications, flange selection is a comprehensive decision-making process: First, determine the required facing type (RF, MFM, TG) based on media pressure and characteristics. Then, choose the attachment method (WN, SO, Threaded, etc.) based on pipe material, installation requirements, and cost. Finally, select the correct pressure class (PN or Class) from the relevant standards (e.g., GB/T, HG/T, ASME B16.5) based on design temperature and pressure. A correctly selected flanged joint is the cornerstone for ensuring the long-term safe and leak-free operation of a temperature measurement point.
