Dimensions of Restriction Orifice Plates for ASME B16.5 Raised Face Flanges


D1 & D2 = ± 1%
D3 = ± 1 mm
D4 = 0/-0.4 mm
L = ± 0.1 mm
N = see note 6
P = ± 0.1 mm
S = ± 0.1 mm
T = see note 2


  1. Marking
    The Upstream side of the orifice tab shall be stamped at the indicated locations with:
    - the word UPSTREAM
    - the Tag Number
    - the ASME flange class, followed by RF
    - the nominal pipe size (NPS)
    - the measured orifice D1 (in mm)
    - the material of the orifice plate
    - the type of orifice plate (Square, Restriction etc.)
    Letter height approx. 4 mm
  2. Welding of tab
    Tab may be integral with orifice plate up to and including 7 mm plate thickness.
    In other cases full penetration joint to be applied.
  3. Finish of plate
    The upstream and downstream faces of the orifice plate to a diameter equal to D3 shall have a surface roughness Ra ≤ 0.8 µm according to ASME B46.1.
    The remaining annular part (between D4 and D3) of upstream and downstream faces shall have a surface roughness Ra between 3.2 and 3.6 µm according to ASME B46.1. This finish shall be obtained by machining with a round nosed cutting tool having a 0.8 mm radius at a feed rate between 0.35 and 0.45 mm per revolution.
  4. Concentricity
    The centre of the orifice shall not differ from the centre of the plate by more than 0.1 mm for ≤ NPS 8 and 0.2 mm for > NPS 8.
  5. Finish of throat
    The throat and outlet cone shall have a surface roughness Ra ≤ 0.4 µm according to ASME B46.1.
    All edges shall be sharp and free from burrs, so that when viewed with the naked eye no light is reflected by the corners.
  6. Plate thickness
    The difference between any two measurements of plate thickness, for annular part D1-D3, shall not differ by more than 0.001 DN in mm.
    The plate shall not depart from flatness along any diameter by more than 0.01 D3.
    Tolerance for N shall be:
    N ≤ 6 mm +0.1/-0.25 mm
    N > 6 mm +0/-1.5 mm
    Thickness for annular part D3-D4, may be slightly less, max 0.05 mm, due to surface finishing difference between D1-D3 and D3-D4. (see note 3)

General Notes:

  • Dimensions are in millimeters unless otherwise indicated.
  • Allowable pressure difference 20 bar for temperatures up to 400 °C and D1/D3 ≥ 0.1. The thickness of the plate needs to be verified for strength in case the pressure difference is higher than 20 bar and/or the temperature is beyond 400 °C. Based on the outcome of the calculation the thickness of the plate needs to be adjusted.
  • DIA D1 & DIA D2 - to be specified by purchaser.
  • Material to be specified by purchaser.