The socket-welded flange with neck and the butt-welded flange with neck have significant differences in terms of weld form, material, nominal pressure, connection method, structural stiffness, manufacturing cost and inspection requirements. The detailed analysis is as follows: 

Welding seam form 

Butt-weld flange with neck: The welding seam between the pipe and the flange is of the fillet weld type. This type of weld seam is relatively simple, but its sealing performance and strength are relatively low. 

Butt-weld flange with neck: The welding seam of the flange and the pipe forms a circumferential weld. This weld form is more complex, but it provides higher sealing performance and strength, and is suitable for harsh working conditions such as high pressure and high temperature. 

Material

Butt-welded flange with neck: The material is usually machined from ordinary steel plates that meet the required thickness. This material has a relatively low cost, but its strength and corrosion resistance may be relatively weak. 

Socket-welded flange with neck: The material is mostly machined from forged steel parts. The forged steel parts have higher strength and corrosion resistance, and can better adapt to harsh working conditions such as high pressure and high temperature. 

Nominal pressure 

Butt-weld flange with neck: The nominal pressure range is usually 0.6 - 4.0 MPa. This pressure rating is suitable for medium and low-pressure pipeline systems. 

Socket-weld flange with neck: The nominal pressure range can reach 1 - 25 MPa. This pressure rating can meet the requirements of high-pressure pipeline systems and is suitable for working conditions with high temperature, high pressure, low temperature, and significant temperature-pressure fluctuations. 

Connection method

Butt-weld flange with neck: Generally, it can only be connected to the pipeline, but not directly to the butt-welded pipe fittings. Its connection method is relatively simple, but the flexibility is lower. 

Socket-welded flange with neck: It can be directly connected to all socket-welded pipe fittings (including elbows, tees, reducers, etc.), including pipelines. This connection method is more flexible and can adapt to more complex piping systems. 

Structural stiffness and strength 

Flanged neckless butt-weld connection: Although the neck design enhances the strength of the flange, compared to the necked butt-weld flange, its structural stiffness remains relatively low. 

Socket-welded flange with neck: It is welded to the pipeline through a conical neck structure, featuring higher structural stiffness and strength. The height of the neck increases with the increase of nominal pressure, reaching up to 89mm at the maximum. The structural stiffness is improved by approximately 50% compared to the flat-welded flange. 

Manufacturing cost and inspection requirements 

Butt-weld flange with neck: The manufacturing cost is relatively low because its material and manufacturing process are relatively simple. Moreover, for butt-weld flanges with a nominal diameter greater than 250mm, non-destructive testing is usually not required, which further reduces the cost. 

Butt-weld flange with neck: The manufacturing cost is relatively high because its material and manufacturing process are more complex. Moreover, for butt-weld flanges with a nominal diameter greater than 250mm, 100% radiographic testing and 20% ultrasonic testing are required to ensure the weld quality. These testing requirements increase the manufacturing cost and time.