Distinguishing Between Flat Welding Flanges and Welding Neck Flanges

How do flat welding flanges differ from welding neck flanges?

The following points outline the key distinctions:

1. Variations in the application scope between flat welding flanges and welding neck flanges are included.
2. The distinction between a flat welding flange and a welding neck flange is significant and pertains to several vital factors.

The difference in scope of the application

• Flat welding flange

It is suitable for connecting steel pipes with a nominal pressure not exceeding 2.5MPa. The flange's sealing surface can be smooth, concave-convex, tenon-groove, among other types. The smooth flange finds the widest application, particularly in relatively mild-medium conditions such as low-pressure non-uniform compressed air and low-pressure circulating water systems. Its notable advantage lies in its comparatively lower cost.

• Welding neck flange

It is employed to weld the neck of the flange and pipe. Characterized by a well-designed structure, high strength, and rigidity, it can endure repeated bending and temperature fluctuations under high temperatures and pressures. Moreover, it boasts reliable sealing performance. The welding neck flange, with a nominal pressure of 1.0 to 16.0 MPa, features a concave-convex sealing surface. 

Differences in welding methods

The welding process encompasses flat welding flanges, neck flanges, and pipe connections. Only single-side welding is necessary when welding a flat welding flange, eliminating the need to weld the inner mouth of the pipe and the flange connection. Conversely, the welding installation of welding neck flanges necessitates double-sided welding of the flange. Consequently, flat welding flanges are typically employed for connecting low and medium-pressure pipes, while welding neck flanges are utilized for linking medium and high-pressure pipes. Welding neck flanges generally possess a minimum rating of PN2.5MPa. The welding neck serves to reduce stress concentration. Most conventional welding neck flanges feature a neck design known as nipple flanges. As a result, the installation, labor, and auxiliary material costs associated with welding neck flanges are usually higher due to the additional process involved.

Not all welding neck flanges require welding both inside and outside. Typically, only one weld is made on the outside without specific demands. I haven't come across 1/2" welding neck flanges. Flat welding is preferable because it ensures vertical alignment of the pipe and flange, preventing any pipe inclination.

The welding neck flange is crafted from steel, with the interface end featuring the same pipe diameter and wall thickness as the steel pipe to be welded. This ensures alignment between the two steel pipes. On the other hand, the flat welding flange comprises a concave surface slightly larger than the outer diameter of the pipe at the interface, allowing the pipe to be inserted for welding. Welding neck flanges exhibit superior welding performance and are less susceptible to corrosion.

Flat welding involves a boss-like welding neck, where a groove is created on the boss for pipe insertion and subsequent welding. In socket welding, no boss is present, and a groove is directly opened on the flange body, akin to a hole on a blind flange, followed by groove creation. While flat welding is slightly superior to socket welding, welding neck flanges are generally preferred due to their versatility across applications.

Welding neck and flat welding flanges are the two most utilized types. Welding neck flanges are distinguished by their extended neck, while flat welding flanges lack this feature. Regarding safety, welding neck flanges are typically preferred due to their reinforced design. However, flat welding flanges may be considered viable alternatives for applications where the medium is standard, pressure levels are moderate, and potential leakage poses no significant risk.

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