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Flange Dimension Quick View — Printable Chart & Key Measurements

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Flange Dimension Quick View: Bolt Circle, Face, and Bore Guide

Flanges connect pipes, valves, pumps, and other equipment in piping systems. Correct flange dimensions are essential for fit, sealing, and mechanical integrity. This quick guide highlights the three most commonly referenced flange features—bolt circle, flange face, and bore—so you can check compatibility quickly and confidently.

1. Bolt Circle (BC / PCD)

  • What it is: The bolt circle is the imaginary circle that passes through the centers of the flange bolt holes. Also called Pitch Circle Diameter (PCD).
  • Why it matters: Proper BC ensures bolt holes line up between mating flanges or equipment; mismatched BC prevents assembly.
  • Quick check steps:
    1. Measure center-to-center distance between two opposite bolt holes across the flange.
    2. If bolts are unevenly spaced, measure from flange center to hole center and multiply by 2 for diameter.
    3. Compare to standard tables for the flange class and nominal pipe size (NPS).
  • Tolerance & common standards: ANSI/ASME B16.5 and ISO standards specify BC by flange type and NPS.

2. Flange Face (Raised Face, Flat Face, RTJ)

  • What it is: The flange face is the contact surface where the gasket seals. Common face types:
    • Raised Face (RF): Most common; a raised sealing area concentrates gasket load.
    • Flat Face (FF): Entire face is flat; used when mating a flange to a flange or equipment with a flat surface.
    • Ring-Type Joint (RTJ): Uses a metal ring gasket in a machined groove for high pressure.
  • Why it matters: Face type determines gasket choice and required sealing surface finish.
  • Quick check steps:
    1. Visually identify RF, FF, or RTJ (RF has a raised ring; RTJ has a groove).
    2. Measure raised face height if RF (typical heights defined in standards).
    3. Note surface finish: smoother finishes are required for certain gasket materials.
  • Common compatibility notes: Never mate RF to FF without appropriate gasket and engineering review.

3. Bore (Inside Diameter / ID)

  • What it is: The flange bore is the internal opening that the pipe passes through; it can be full bore (matching pipe ID) or reduced.
  • Why it matters: Mismatched bore can create flow restrictions or turbulence; it must align with pipe ID or connected equipment bore.
  • Quick check steps:
    1. Measure internal diameter across the flange opening.
    2. Compare to pipe ID for full- or reduced-bore applications.
    3. Verify hub/neck dimensions on weld neck flanges for proper butt-weld preparation.
  • Notes on raised vs. reduced bore: Reduced bore flanges are used to control flow or for specific design reasons—ensure allowed by system design.

Quick Reference Checklist (use before assembly)

  • Bolt circle: Matches mating flange/bolt pattern.
  • Bolt hole size & number: Correct for the specified flange rating.
  • Flange face type: RF, FF, or RTJ — compatible with gasket and mating face.
  • Bore ID: Matches pipe or equipment bore (note reduced bore if present).
  • Face finish & raised face height

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