Slip On Flange

Slip on flanges are 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. It can be furnished without a hub if space is limited and the application allows for a “ring style” slip on. Although the hub style is more common when referring to a slip on, a ring style slip on without a hub still falls under the category of a slip on flange, and can be called out as you desire. Slip ons in higher pressure classes are often made with the height of a lap joint for a better connection. If the hub height is not a concern and a lap joint is not readily available, customers will sometimes opt for a slip on made to a lap joint style with a machined hub.

Slip on flanges are more commonly found in lower pressure assemblies and become less utilized when a higher stress service calls for a superior connection to the piping assembly. This is because a slip on style flange will accept a pipe with a fillet weld connection, whereas a weld neck flange will have a reinforced hub at the base and form a weld bevel connection directly to the butt weld end of a pipe. This tapered hub allows for stronger service in a mechanical stress environment.

The Weld Neck Flange vs Slip On Flange

The Weld Neck Flange is easily recognizable by the hub that gradually tapers from the bolted connection to the pipe. At the Point of Weld on the Weld Neck it is supplied with a specific bore to mate with the attaching pipe or fitting. This connection is made with a full penetration butt weld, this prevents turbulence at the joint, reduces erosion and restriction to product flow. The Weld Neck Flange offers increased strength under higher pressures, subzero or elevated temperatures that cause line expansion or retraction or other variable forces. The Slip On Flange is connected to the pipe by two welds, one on the back side of the Flange and one on the inside of the Flange. The inside weld of the Flange to the pipe is recessed a minimum of the wall thickness plus 3 millimeters or 3/16” of an inch. This is to avoid damage to the gasket seating surface or face. Although the initial cost of a Slip On Flange is slightly less than a Weld Neck Flange, you have two welds one on the back side and one on the inside of the Flange and the added inspection of the second weld doubling weld and inspection cost. The Slip On Flange is designed to be welded to pipe then a fitting as fittings are designed for butt weld connections. The calculated strength of a Slip On Flange under internal pressure is about two thirds that of a Weld Neck Flange, and their life under fatigue is about one third that of a Weld Neck Flange.