Slip-on flange is a popular type of pipe flange designed to accept pipe into the bore/center of the flange for welding around both the Outer Diameter (OD) of the pipe and on the interior side of the connection. You may recognize them:
The slip-on flange is a simple and excellent alternative to a weld neck as it does not have a weld bevel, and therefore allows the pipe to be adjusted in length relative to its position of the flange.
The bore of the slip on will give ample amount of space for the matching pipe. This allows for enough working space for the welder and fabricator to make the connection.
This flange type is extremely common in lower pressure applications. Most slip on style flanges will have a hub which will often appear similar dimensionally to a raised face.