Flanges up to NPS 24 and flanges above NPS 24 are covered by different ASME standards, and the large-diameter standard has two series that do not bolt to each other. Getting the standard and series wrong on a large-flange order is one of the most expensive mistakes in a piping package. This page explains which standard applies, what changes above NPS 24, and what a large-flange RFQ must state.
Which Standard Covers Which Size
- ASME B16.5 — pipe flanges and flanged fittings from NPS ½ through NPS 24, in pressure classes 150, 300, 400, 600, 900, 1500 and 2500.
- ASME B16.47 — large-diameter steel flanges from NPS 26 through NPS 60, in Series A and Series B.
Both standards set dimensions, tolerances, marking and pressure–temperature ratings by material group. The material itself — for example ASTM A105, A182 F316L or A350 LF2 — is specified separately.
Series A and Series B Are Not Interchangeable
ASME B16.47 contains two sets of dimensions for the same nominal sizes:
- Series A — derived from the older MSS SP-44 flanges. Generally heavier, with larger bolt circles and fewer, larger bolts.
- Series B — derived from the older API 605 flanges. Generally more compact, with smaller bolt circles and more, smaller bolts.
For the same NPS and class, Series A and Series B flanges have different outside diameters, bolt circles and bolt hole patterns, so they will not mate. The series must match the flange it connects to — on the existing line, the valve or the equipment nozzle — and must be written on the order. An RFQ that says only "B16.47" is incomplete.
Other Differences to Check Above NPS 24
- Available types: B16.47 covers weld neck and blind flanges. Other types in large sizes, such as slip-on or lap joint, are usually made to the purchaser's drawing or another standard, and must be specified clearly.
- Pressure classes: the classes offered in B16.47 differ by series and size. Confirm that the class you need exists for the size and series before ordering.
- Bore: weld neck flanges must match the wall thickness of the connecting pipe; state the bore or the pipe schedule or wall.
- Facing: state raised face, ring type joint (with ring number) or flat face. See RF, RTJ and FF flange facings.
- Edition: if your project specification names an edition year of the standard, include it.
What a Large-Flange RFQ Should Include
- Flange type (weld neck or blind, or another type with drawing)
- NPS, pressure class and facing
- Standard and series — for example ASME B16.47 Series A
- Bore or pipe wall for weld neck flanges
- Material specification and grade, including class where applicable
- Quantity per size and delivery date
- Certificate type (EN 10204 3.1 or 3.2), impact testing or hardness limits if required, and NDT
- Coating, marking and packing requirements
For the full line-by-line format, see our flange RFQ worked example. Browse our flanges by material, or send your large-flange list for a quotation.
Send Your Flange RFQ
