Necked butt welding flanges are a type of flange fitting that can be numbered and produced according to certain methods and standards, playing an important role and value in various fields and industries, ensuring significant contributions during use. Gaskets are used for sealing between flanges. Flange fittings refer to fittings with flanges. Necked butt welding flanges and necked flat welding flanges refer to the welding methods when connecting flanges and pipes. Necked flat welding flanges require only single-sided welding without needing to weld the inner mouth of the pipe and flange connection, while necked butt welding flanges require double-sided welding for installation. Therefore, necked flat welding flanges are generally used for low and medium-pressure pipelines, while necked butt welding flanges are used for medium and high-pressure pipeline connections, with necked butt flanges typically rated at least PN2.5MPa. The use of butt welding is to reduce stress concentration. In pipelines that need to be connected, various installation methods for flanges are used. Low-pressure pipelines can use threaded flanges, while flanged welds are used for pressures above 4 kg. In pipeline engineering, flanges are mainly used for connecting pipes. Sealing points are added between two flanges, which are then fastened with bolts. Flanges of different pressures have different thicknesses and use different bolts. Necked butt welding flanges are suitable for pipelines with significant pressure or temperature fluctuations, or for high-temperature, high-pressure, and low-temperature pipelines, and are also used for transporting expensive, flammable, and explosive media. Necked butt welding flanges are not easily deformed, provide good sealing, and are widely used.
Comparison of Necked Butt Welding Flanges and Necked Flat Welding Flanges: Specific Differences
1. Different welding methods: Flat welds cannot be radiographically tested, while butt welds can. The welding method for necked flat welding flanges and pipes is a fillet weld, while the welding method for necked butt welding flanges and pipes is a butt weld; flat welding involves two fillet welds, while butt welding involves a single butt weld. The difference between necked flat welding flanges and flat welding flanges without necks is that necked flat welding flanges have an additional part at the pipe welding point compared to flat flanges without necks, which are simply flat plate flanges without a flange projection, also resulting in a flat fillet weld. The welds connecting necked butt welding flanges are classified as Type B seams, while those for necked flat welding flanges are classified as Type C seams, with different non-destructive testing requirements after welding.
2. Different nominal pressures: The nominal pressure for necked flat welding flanges is 0.6—4.0MPa, while the nominal pressure for necked butt welding flanges is 1–25MPa. Clearly, the pressure rating for necked flat welding flanges is lower.
3. Different connection methods: The main structural difference between necked flat-weld flanges and necked butt-weld flanges lies in the method of connection with the flange. Necked flat-weld flanges are generally connected at the flange angle, while necked butt-weld flanges are connected directly with the flange.
Necked slip-on flanges can generally only connect to pipes and cannot directly connect to butt-weld fittings; butt-weld flanges can generally connect directly to all butt-weld fittings (including elbows, tees, reducers, etc.), of course, including pipes.
The stiffness of necked butt-weld flanges is greater than that of necked slip-on flanges, and butt-weld flanges are stronger than slip-on flanges, making them less prone to leakage.
Necked slip-on flanges and necked butt-weld flanges cannot be replaced casually. From the manufacturing perspective, necked slip-on flanges (SO is the abbreviation for SLIP ON) have a larger inner diameter, which means lower weight and cost. Additionally, necked butt-weld flanges (WN is the abbreviation for WELDING NECK) with a nominal diameter greater than 250mm require inspection, while SO flanges do not require inspection, so the cost is relatively lower.