Slip-On Flanges

Cost-Effective Flanged Connection for Low to Moderate Pressure Service

What Is a Slip-On Flange?

A slip-on flange is a flat flange with a bore slightly larger than the pipe outside diameter, allowing the pipe to slip through before welding. It is attached to the pipe by fillet welding both the inner and outer edges of the flange to the pipe.

Designed per ASME B16.5, slip-on flanges are simpler and less expensive than weld neck flanges. However, they provide approximately two-thirds the strength of a weld neck flange under internal pressure per the ASME code, and roughly one-third the fatigue life.

Key Design Features

  • Pipe slips inside the flange bore for easy positioning
  • Double fillet weld — one on the inside and one on the outside of the flange
  • No hub or taper, resulting in a compact, lightweight design
  • Easy alignment during installation, reducing field labor
  • Available in raised face (RF) and flat face (FF) facings

Common Applications

  • Low to moderate pressure systems
  • Utility and general service piping
  • Water treatment facilities
  • HVAC piping
  • Applications where frequent disassembly is needed

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Available Sizes and Pressure Classes

Slip-on flanges are available per ASME B16.5 in sizes NPS 1/2 through 24, in Classes 150, 300, 600, 900, 1500, and 2500. They are most commonly used in Class 150 and Class 300 applications where pressures and temperatures are moderate.

Common Materials

  • Carbon Steel (ASTM A105) — the most widely used material for general service
  • Stainless Steel (ASTM A182 F304/F316) — for corrosion-resistant applications in chemical and food processing
  • Low-Temperature (ASTM A350 LF2) — for cryogenic and low-temperature service

Slip-On vs Weld Neck Flanges

Key differences between slip-on and weld neck flanges:

  • Cost: Slip-on flanges are less expensive to purchase and easier to install, reducing overall project cost.
  • Strength: Weld neck flanges are significantly stronger and more resistant to fatigue, bending, and thermal cycling.
  • Alignment: Slip-on flanges are easier to align in the field because the pipe can slide through the bore before welding.
  • Welding: Slip-on flanges require two fillet welds (inside and outside), while weld neck flanges use a single butt weld with full penetration.

Related References