How to Read an Industrial Flange Drawing

An industrial flange drawing contains the information needed to identify, manufacture and inspect a flange. Before sending an RFQ or approving a quotation, review the following details.

1. Check the Standard

First identify the applicable standard, such as ASME B16.5, ASME B16.47 or EN 1092-1.

The standard affects the flange dimensions, pressure rating, bolt pattern, facing and tolerances. Flanges with the same nominal size may not be interchangeable if they follow different standards.

2. Confirm Size and Pressure Rating

Check the nominal size, usually shown as NPS or DN, together with the pressure rating, such as Class 150, Class 300 or PN16.

Nominal size is not the same as the actual flange outside diameter or bore. The pressure rating must also suit the piping system’s operating pressure, temperature and design requirements.

3. Read the Main Dimensions

Important dimensions normally include:

• Outside diameter
• Flange thickness
• Bore diameter
• Hub diameter and length
• Raised face diameter and height
• Overall length for weld neck flanges

For a weld neck flange, the bore and hub dimensions must match the pipe outside diameter and wall thickness.

4. Check the Bolt Pattern

The drawing should show:

• Number of bolt holes
• Bolt hole diameter
• Pitch circle diameter, or PCD
• Bolt arrangement
• Bolt size and length

A flange can have the correct nominal size but still fail to connect if its hole count, hole diameter or PCD does not match the mating flange.

5. Identify the Facing

Common facing types include:

• Raised Face, RF
• Flat Face, FF
• Ring Type Joint, RTJ
• Male and Female face
• Tongue and Groove

The facing must match the gasket and the mating flange. Surface finish, serration or groove details may also be specified on the drawing.

6. Verify Material and Documents

Check the material grade, such as ASTM A105, ASTM A182 F304/L or ASTM A182 F316/L.

The drawing may also require heat treatment, testing, marking or material traceability documents. For export and engineering projects, the RFQ may need to specify a Material Test Certificate, inspection report or other quality documents.

7. Review Notes and Tolerances

Always read the general notes. They may define:

• Dimensional tolerances
• Surface finish
• Bevel or chamfer requirements
• Marking and stamping
• Inspection requirements
• Drawing revision and project specifications

If any information is unclear, confirm it with the supplier before production.

Final Checklist

Before approving a flange drawing, confirm the standard, size, pressure rating, material, main dimensions, bolt pattern, facing, tolerances and documentation requirements.

A careful drawing review helps prevent flange mismatch, installation delays and incorrect quotations. These details should also be included in the technical RFQ.