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Flanges - API vs. ASME/ANSI

API and ASME/ANSI flanges - comparing dimensions and pressure ratings.

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Although the dimensions of ASME and API flanges may sometimes be compatible for bolting - using the same bolt circle and the same number of bolts - they do not share the same pressure rating system.

  • ASME rates the pressure of a class based on the material of construction and the design temperature
  • API specifies allowable materials and gives it a specific pressure rating

The difference between ASME/ANSI and API is the fabrication material and a higher rated API operating pressure.

ASME/ANSI flanges are commonly used in industrial process systems handling water, steam, air and gas. API flanges are manufactured for high strength operating refinery systems with products such as oil and explosive gases.

The flange standards API 6A and ASME/ANSI B16.5 have the same dimensions - but the API 6A flanges are rated for higher pressures as indicated below:

API vs. ASME/ANSI Flanges with the same Dimensions
FlangePressure Class Rating
(psi)
Nominal Size Range
(inches)
ASME/ANSI B16.5API 6AASME/ANSI B16.5API 6A1)
Weld neck 600 2000 1/2 - 24 113/16 - 11
900 3000
1500 5000
Blind and Threaded 600 2000 113/16 - 211/4
900 3000 113/16 - 203/4
1500 5000 113/16 - 11

1) In the old API standard, flanges ranged from 1 1/2 to 10(20) inches.

  • 1 inch = 25.4 mm
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