When to prefer Weld Neck Raised Face (WNRF) flange over Slip On Raised Face (SORF) flange?

Purpose of Raised Face: The raised face refers to a flat, elevated surface on the flange. This raised face serves several important purposes:

• Improved Sealing: The raised face creates a designated area for the gasket to sit on. When the flanges are bolted together, the raised face compresses the gasket, forming a tighter seal and preventing leaks between the connected pipes.

SORF Design:

• Has a cylindrical body with a flat, raised face on one end for the gasket.
• Why is it called “Slip on”?
• The pipe slips over the body and is welded to the flange on the outside diameter. Flange and pipe are welded at both inside and outside as shown in the above figure.
• Easier and faster to install. The pipe simply slides over the flange body, and fillet welds are applied from the outside.
• Weaker Connection: The weld on a SORF flange is placed on the flange body itself. This area is typically thinner compared to the pipe wall. When under pressure, the stress concentrates at the weld zone, making the connection more susceptible to failure at higher pressures.
• Stress Concentration: The fillet weld on a SORF flange creates a sharp geometric change where the pipe meets the flange body. This point acts as a stress concentrator, further amplifying the stress on the weld.
• Drawback:
  • SORF flanges are not directly compatible with elbows or tees. Because the mating surface of a SORF flange is flat, whereas elbows and tees have curved or angled shapes. These shapes are incompatible with the flat mating surface of a SORF flange.
  • A 100 per cent Radiography Test cannot be performed on the weld between the flange and pipe since it is a lap joint.
• Strength: The strength estimated from a Slip-On flange when subjected to internal pressure is approximately two-thirds that of a Weld Neck flange.
• Advantage: Lower price than Weld Neck flange, Easier and faster to install.

WNRF Design:

• Features a cylindrical body with a tapered neck that extends outward and connects smoothly to the pipe. The weld neck design, with its tapered neck that transitions smoothly into the pipe, distributes stress more evenly. This makes them ideal for applications with high pressure, high temperatures, or cyclic loading.

• Requires more complex and time-consuming installation. The pipe needs to be bevelled to match the neck angle, and full penetration welds are applied around the entire neck circumference.
• The entire neck is welded to the pipe for a stronger connection.