Series A vs Series B Flanges: Key Differences & Applications

The ASME B16.5 Specification

The ASME B16.5 standard covers steel pipe flanges and flanged fittings for nominal pipe sizes (NPS) ranging from ½ to 24 inches, and for pressure classes 150 to 2500. It details pressure‑temperature limits, dimensional tolerances, component layout, and the designation of flange openings. This specification is widely referenced in the industry, and the term "B16.5" (or "B16 5") is used interchangeably with ANSI B16.5. For larger diameters—NPS 26 through 60—ASME B16.47 is the applicable standard, covering pressure classes 75, 150, 300, 400, 600, and 900.

ASME B16.47 Series A vs Series B Flanges

ASME B16.47 splits its designations into Series A and Series B flanges. Both series include welded‑neck and blind flanges, but they differ in size, weight, bolt circle, and bolt count, which directly influence cost and performance.

Series A Flanges

• Designed for higher external loads and larger pipe sizes.
• Feature a larger bolt circle diameter and more bolt holes.
• Heavier and thicker construction, often used for critical or high‑pressure applications.
• Include Ring‑Type Joint (RTJ) flanges for classes 300 to 900.
• Typically more expensive due to increased material and machining requirements.

Series B Flanges

• Optimized for lighter service and moderate pressure ranges.
• Have a smaller bolt circle diameter and fewer bolt holes.
• Smaller, lighter construction, which reduces material cost.
• Widely adopted for retrofit, repair, and non‑critical piping.
• Cost‑effective choice for many piping suppliers and end users.

Both Series A and B flanges comply with API 605, ensuring compatibility across projects that require standard API flange designs.

Choosing the right flange depends on the application's pressure rating, external load, and budget. For new pipelines or high‑stress systems, Series A is often preferred, while Series B offers a more economical solution for retrofit or lower‑pressure installations.