A weld neck flange is butt-welded to the pipe, so the bore of the flange hub has to match the inside diameter of the pipe it joins. ASME B16.5 defines the flange’s outside dimensions for each size and class, but the bore depends on the pipe wall – which is why a weld neck flange order is incomplete without the pipe schedule or wall thickness.
Why the Bore Must Match
- Weld quality: a step between flange bore and pipe bore makes root fit-up and full penetration harder and can leave a stress concentration at the weld.
- Flow and erosion: an internal step disturbs flow and can trap deposits or create erosion in high-velocity service.
- Inspection: a large mismatch can make radiographic or ultrasonic interpretation more difficult.
- Pigging: in pipelines, a bore step can obstruct pigs and inspection tools.
What to Check Before Approving the Order
| Item | Check against |
|---|---|
| Nominal pipe size and pressure class | Piping specification / line class |
| Pipe schedule or wall thickness | Piping specification (for example Sch 40, Sch 80, Sch 10S or a specified wall) |
| Flange bore | Pipe inside diameter for that schedule, as stated on the order |
| Weld end (hub) outside diameter and bevel | Pipe outside diameter; weld bevel to ASME B16.25 or the order |
| Facing and finish | RF, FF or RTJ, and the gasket surface finish required |
| Dimensional standard | ASME B16.5 up to 24"; ASME B16.47 Series A or Series B above 24" (the two series are not interchangeable) |
When Flange and Pipe Walls Differ
Sometimes the flange is ordered with a heavier wall than the pipe – for example where a higher-class flange is used, or where a common flange bore is chosen for several pipe schedules. In that case the thicker part is normally tapered at the weld end so the transition is gradual; ASME B16.25 shows acceptable weld-end transitions. Agree the transition on the order or drawing rather than leaving it to the fabricator.
How to Write It on the Order
A complete line item reads like this: Weld neck flange, 6" Class 300 RF, bore to suit Sch 40 (6.065" ID), weld bevel to ASME B16.25, ASTM A105, ASME B16.5, EN 10204 3.1. Adjust the bore wording to your own piping specification.
Related guides: RF vs RTJ vs FF flange facing, flange RFQ template and ordering to ASME B16.5 vs B16.47.
Frequently Asked Questions
Why does a weld neck flange need the pipe schedule?
Because the flange bore must match the inside diameter of the pipe it is welded to, and that depends on the pipe wall thickness. The flange outside dimensions come from ASME B16.5, but the bore must be stated on the order.
What happens if the flange bore does not match the pipe?
A step between the bores makes weld fit-up and full penetration harder, disturbs flow and can complicate inspection. Larger differences are handled with a tapered weld-end transition agreed on the order.
Are ASME B16.47 Series A and Series B flanges interchangeable?
No. Above 24" the two series have different dimensions, so the series must be stated on the order and match the mating flange.
