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An ASME B16.5 Class 150 Slip-On (SO) Flange is designed to slide directly over the outside diameter of the pipe. It is then securely attached using fillet welds at both the hub and the interior face of the flange. Engineers worldwide prefer slip-on configurations because they are easier to align than weld neck flanges and typically require lower initial fabrication costs.
Checklist for Ordering
Standard: ASME B16.5
Flange Type: Slip-On (SO)
Nominal Pipe Size: (e.g., 2", 4", 6")
Pressure Class: Class 150
Face Configuration: Raised Face (RF) or Flat Face (FF)
Material Specification: (e.g., ASTM A105 Carbon Steel, ASTM A182 F316L Stainless Steel)
1. The Bore Diameter (Y) Crux
The internal bore of a slip-on flange is slightly larger than the outside diameter of the matching pipe. This design choice provides just enough clearance for the pipe to slide smoothly inside the flange hub before welding. For example, a 4" schedule 40 pipe has a true outside diameter of 4.500 inches, which aligns perfectly with the 4.57-inch bore of the Class 150 SO flange.
2. Raised Face (RF) vs. Flat Face (FF)
By default, the vast majority of ASME B16.5 Class 150 flanges feature a 0.06-inch (1.6 mm) Raised Face. This raised portion concentrates gasket pressure, optimizing sealing performance.
However, if you are mating the flange to cast iron equipment-a common setup in municipal water treatment facilities across the UK and US-you must specify a Flat Face (FF) layout. Bolting a raised face flange to a brittle cast iron flat face flange risks cracking the cast iron flange when torque is applied.
3. Facing Finish (Roughness)
The contact surface of the flange face is explicitly textured to "bite" into the gasket material. ASME B16.5 specifies a serrated finish, typically ranging between 125 to 250 micro-inches AARH (Arithmetic Average Roughness Height). Avoid polishing or grinding this surface down during installation, as a smooth face will cause the gasket to slip under high pressure.
ASME B16.5 Class 150 Slip-On (SO) Flange
