A reducing flange line item carries two nominal sizes, and ambiguity about which is which is the single largest cause of wrong parts. Everything else follows ordinary flange practice.
Say both sizes, and label them
"8 x 6 reducing flange" is ambiguous in the field even though most people will read it correctly. Better to be explicit:
8 inch Class 300 reducing weld neck flange, bore for 6 inch Schedule 40 pipe, RF, A105.
Flange size, class, type, bore size with schedule, facing, material. Nothing to interpret.
The convention is flange size first, then bore size. The flange size is the one that governs the class, the thickness, the drilling and the facing.
The eight items
- Flange nominal size. The larger size. This sets outside diameter, thickness, bolt circle, hole count and hole size.
- Pressure class. Belongs to the larger size. 150 through 2500, or API 5K through 20K.
- Flange type. Reducing weld neck, slip-on, threaded, socket weld or lap joint. There is no reducing blind.
- Bore nominal size. The smaller size, the pipe that welds or screws in.
- Schedule or wall thickness of the smaller pipe. The item most often omitted. On a weld neck it is essential, because the hub bore has to match the pipe inside diameter. Nominal size alone does not determine it.
- Facing and finish. Raised face, flat face or ring joint, plus the surface finish if the specification calls one out. Ring joint needs the ring number.
- Material grade and specification. A105, A350 LF2, F316, F51 and so on, with the ASTM specification and revision if it is called out.
- Design pressure, temperature and fluid. Not strictly needed to machine the part, but it lets us flag an application where the abrupt transition is going to cause trouble.
Then the commercial items
- Quantity and required delivery date
- Certification: MTRs, a specific specification revision, third party inspection or witness
- Any domestic origin requirement, Buy American Act, BABA or AIS. See domestic and import flanges
- Ship-to location, and whether the part needs preservation or face protection
- Marking requirements beyond the standard
Identifying an existing flange
Most reducing flange enquiries are retrofits, and they start with an unmarked flange already installed. Five measurements off the existing flange, plus two off the pipe, and we can identify the pattern:
- Flange outside diameter
- Bolt circle diameter
- Number of bolt holes
- Bolt hole diameter
- Flange thickness, and whether there is a hub or a raised face
- Outside diameter of the pipe that has to connect
- Wall thickness of that pipe, or its schedule
Send those with a photograph and we will tell you what the existing flange is and confirm whether the reduction you need is sound.
Off-pattern work
Past the published tables, or where the drilling does not match any standard pattern, the part becomes a drawing. Send a sketch, a marked-up print, a scan of an old drawing, or a sample we can measure. We will issue a dimensioned drawing for approval before anything is cut. See CAD and 3D drawings.
What we will tell you back
If the reduction is more than two nominal sizes, or the service looks like one where the abrupt step causes problems, pump suction, metering, slurry, high velocity or cyclic duty, we will say so at the quote and suggest a reducer with a standard flange instead.
We supply both, so there is no commercial reason for us to sell you the one that does not work. See when to use a pipe reducer instead.