ASME B16.5 or B16.47 Series A? The Class Number Alone Is Not Enough

When a purchase requisition says “Class 300 flange,” you do not yet have a complete description. In the ASME world the same Class number appears in two separate flange standards: B16.5 and B16.47. They are not alternatives to each other but documents that divide the diameter range between them — and where they meet, they give different dimensions for the same Class.

The split is by size, not by pressure

B16.5 covers small and medium sizes; that is the range industrial piping meets every day. Above it the standard stops and dimensions are read from B16.47. The reason for the split is scale, not pressure. On a large flange, the distribution of bolt load along the gasket, the tendency of the flange ring to rotate out of its own plane, and the absolute value of manufacturing tolerances cannot be handled with the same approach that works at small sizes. Large diameters need their own calculation basis and their own dimension tables.

The result is that a line's flange list quietly crosses from one standard to the other as size increases. The engineer who wrote the design knows this. The person raising the order often does not, because products from both standards are called “Class 150” or “Class 300.”

Series A and Series B are not the same thing

B16.47 itself divides into two series. For the same size and the same Class, Series A and Series B define different flange geometry: outside diameter, bolt circle, bolt count and thickness all change. They do not bolt to each other. So “B16.47 Class 300” is also incomplete — without the series, two different products have been described.

The distinction is most awkward when extending an existing plant, because series information is the first data to drop out of plant records. You have to confirm which series is installed by measuring the bolt circle and counting the bolts. Guessing by eye does not work here; flanges from the two series look alike.

What to confirm before ordering

Asked at tender stage, these four lines take a few minutes. Asked after the material has arrived on site, they mean returns, remanufacture and delay.

Keeping design and site on the same wording

Most flange mismatches in a plant come not from calculation errors but from losses in transmission. The design says B16.47 Series A, the enquiry list carries only Class and size, the manufacturer supplies its own default, and the site ends up with two geometries. The chain breaks in the gap between documents.

Preventing it is a matter of writing discipline rather than engineering: the standard number, and the series where one exists, never travel apart from the size and the Class. Where two standards will coexist on one line — normal enough when a large header feeds smaller branches — the changeover point is marked on the isometric. That mark may be the only information available to whoever opens the line years later.