ANSI Flanges

American National Standards Institute Pipe Flange Specifications

What Are ANSI Flanges?

ANSI (American National Standards Institute) is the organization that historically published pipe flange standards in the United States. While these standards are now maintained by ASME (American Society of Mechanical Engineers), the term “ANSI flanges” is still widely used in the industry. When someone refers to an “ANSI flange,” they typically mean a flange manufactured to one of the ASME B16 standards.

Key ANSI/ASME Flange Standards

  • ASME B16.5 — Pipe Flanges and Flanged Fittings (NPS 1/2 through 24)
  • ASME B16.47 — Large Diameter Steel Flanges (NPS 26 through 60) with Series A (MSS SP-44) and Series B (API 605)
  • ASME B16.36 — Orifice Flanges
  • ASME B16.1 — Gray Iron Pipe Flanges and Flanged Fittings
  • ASME B16.42 — Ductile Iron Pipe Flanges and Flanged Fittings

Pressure Classes

ANSI/ASME flanges are organized by pressure class: 150, 300, 400, 600, 900, 1500, and 2500. The class number does not directly equal the working pressure in psi — actual pressure-temperature ratings depend on the material and temperature.

Class 150 is the most common for general service. Class 300 and above are used for higher pressure applications.

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ANSI vs ASME: What's the Difference?

ANSI is a standards organization that accredits other organizations (like ASME) to develop standards. ASME is the organization that actually writes and maintains the flange standards. The standards were originally published as “ANSI B16.5” but are now officially “ASME B16.5.”

In practice, “ANSI flange” and “ASME flange” refer to the same products. Both terms are correct.

Common ANSI Flange Types

  • Weld Neck — High-integrity connection with a tapered hub
  • Slip-On — Slides over the pipe and is welded in place
  • Blind — Used to close the end of a piping system
  • Socket Weld — For small-bore, high-pressure applications
  • Threaded — Screwed connection without welding
  • Lap Joint — Used with a stub end for easy alignment

Common Materials

  • Carbon Steel (A105) — The most common flange material for general service
  • Stainless Steel (A182 F304/F316) — For corrosion resistance
  • Alloy Steel (A182 F11/F22) — For high-temperature service
  • Low-Temperature (A350 LF2) — For cryogenic and low-temperature applications

Related References