Series A vs Series B Flanges

Quick Answer

Series A flanges (originally MSS SP-44) are larger, heavier, and have more bolt holes than Series B flanges (originally API 605) at the same NPS and class. Series A is the default in most refinery and petrochemical projects. Series B is lighter and cheaper, commonly used in pipelines and storage terminal piping. The two series are NOT interchangeable—bolt patterns differ.

Dimensional Comparison: NPS 30 Class 150

Parameter	Series A	Series B
Flange OD	787.4 mm (31.00 in)	749.3 mm (29.50 in)
Thickness	47.6 mm (1.88 in)	38.1 mm (1.50 in)
Bolt circle	723.9 mm (28.50 in)	692.2 mm (27.25 in)
Number of bolts	28	20
Bolt size	1-1/4 in	1-1/4 in
Approx. weight (WN, A105)	120 kg (265 lb)	85 kg (187 lb)
Relative cost	Higher (+25-40%)	Lower (baseline)

Dimensions from ASME B16.47.

Origin and History

Series A and Series B trace back to two separate industry standards that ASME B16.47 consolidated:

Series	Original Standard	Origin	Industry Focus
Series A	MSS SP-44	Manufacturers Standardization Society	Refinery, petrochemical, power
Series B	API 605	American Petroleum Institute	Pipeline, tank farm, terminals

When ASME merged both into B16.47, it preserved both dimensional series because the installed base was too large to standardize on one. The pressure-temperature ratings are identical—both use the same ASME B16.5 P/T tables.

When to Specify Each Series

Series A is preferred for:

• Refinery process piping and petrochemical plants
• High-integrity service (high temperature, cyclic loading)
• Projects where the piping class or client specification mandates Series A
• Systems connected to equipment with Series A nozzle flanges

Series B is suitable for:

• Cross-country pipelines
• Tank farm and terminal piping
• Low-to-moderate pressure utility lines
• Applications where weight and cost savings matter
• Pipeline systems designed to API standards

Common Mistake

Ordering Series B flanges for a project that specifies Series A (or vice versa). The flanges will arrive with the wrong OD, different bolt count, and mismatched bolt circle. Always verify the series designation in the piping class before issuing the purchase order. Look for “B16.47 Ser. A” or “B16.47 Ser. B” in the line class specification.

Bolt Pattern Differences

The higher bolt count in Series A provides more uniform gasket compression and better leak resistance:

NPS	Class	Series A Bolts	Series B Bolts
26	150	24	20
30	150	28	20
36	150	32	24
30	300	28	28
36	300	32	28

Pro Tip

If the project spec simply says “ASME B16.47” without specifying a series, assume Series A. It is the more conservative choice and the default in most EPC project piping specifications. Always request clarification from the piping engineer to avoid costly procurement errors.

Read the full guide to flanges