notizie sull'azienda ASME B16.47 Series A vs Series B: Selecting Large-Diameter Flanges from NPS 26 to NPS 60

Above NPS 24, a flange stops being a catalogue line item and becomes a dimensional commitment. The bolt circle, gasket seating dimensions and facing fixed at the design stage determine which spare blind, gasket and replacement forging will fit that nozzle for the next twenty years. ASME B16.47 governs this range — NPS 26 through NPS 60 — and it carries a trap that catches experienced buyers: it publishes two complete, independent dimensional series, Series A and Series B, and the two do not bolt together.

This article explains where the two series came from, how they differ in practice, and how to specify a large-diameter flange that matches what is already installed.

Why large-diameter flanges need a separate standard

ASME B16.5 ends at NPS 24, and the boundary is not arbitrary. Below it, proportions can be tabulated from bore size and pressure class, and one set of tables serves slip-on, weld neck, blind, lap joint, socket weld and threaded designs. Above it, the joint is sized less by pipe wall than by the bolt load needed to seat a gasket across a very large circumference, and by the stiffness required to keep seating stress uniform during tightening.

A large flange is also a handling problem. It must be lifted with equipment, machined on a large-bore lathe, welded to heavy-wall pipe under controlled heat input, and supported so that its own mass does not load the nozzle it hangs from. That combination of requirements justifies a separate document.

Series A and Series B: two histories in one standard

B16.47 does not define one flange. It defines two, because two traditions already existed in industry and neither could be retired without breaking installed plants.

Series A reproduces MSS SP-44, the traditional heavy large-diameter flange found in older refinery, terminal and pipeline specifications. It is dimensionally generous: larger outside diameter, thicker section, larger bolt circle.

Series B reproduces API 605, developed as a lighter, more compact alternative for new large-bore piping. It uses more bolts of smaller diameter on a smaller bolt circle, with a reduced outside diameter and a thinner section, so it weighs less and costs less to build and support.

Keeping both as Series A and Series B is why an order that says only “ASME B16.47” is incomplete. The standard is a container for two incompatible geometries.

Dimensional differences that change the design

Outside diameter and bolt circle

A Series A flange occupies more radial space, which changes pipe-rack spacing, nozzle projection, insulation clearance and bolt access. On a revamp, that extra diameter can decide the series on its own.

Bolt count and bolt diameter

Series A reaches its bolt load with fewer, larger bolts; Series B with more, smaller ones. This changes field work as much as design. More bolts mean more tightening steps and a longer assembly sequence, while larger bolts need heavier tooling and, at big sizes, hydraulic tensioning rather than a torque wrench. Neither pattern is better in the abstract; the trade is assembly time against tooling envelope.

Mass, handling and nozzle load

Series A carries more metal in the ring, so it is heavier at the same size and class. That mass is carried by the pipe, by the support, or on a vessel by the nozzle, where the resulting bending moment can influence reinforcement design.

Stiffness and gasket seating

The thicker Series A section resists rotation under bolt load better than the thinner Series B section. Rotation is what opens the outer edge of a large gasket, because a long circumference turns a small angular deflection into a measurable gap. Where the service is hot, cyclic or emission-limited, that margin is usually worth the extra weight.

Series A versus Series B at a glance

What ASME B16.47 does not cover

The standard covers weld neck and blind flanges only. Slip-on, socket weld, lap joint and threaded designs are outside its scope at these sizes. A requisition asking for a 30-inch Series B slip-on flange is asking for something B16.47 does not define; the item will be refused or quietly substituted with something that does not match the drawing. If a large-bore connection genuinely needs a non-weld-neck configuration, it is a custom or code-designed component.

The class range is bounded as well. B16.47 tabulates classes up to Class 900; beyond that, a large-diameter joint is an engineered component rather than a standard one. The document also does not supply gaskets, bolting or an installation procedure — those come from the gasket manufacturer, the bolting standard and the project's joint integrity programme.

Material and documentation choices

Dimensions are only half the specification. Carbon steel large-diameter flanges are normally ordered to ASTM A105/A105M, which covers forged carbon steel piping components for ambient and higher-temperature service. Stainless and alloy grades come from ASTM A182/A182M, which covers forged or rolled alloy and stainless flanges for high-temperature service and includes grades such as F304L, F316L, F51 and F55 duplex, and the high-alloy austenitic grades used in aggressive environments. Low-temperature service normally moves to an impact-tested ferritic grade such as A350 LF2.

Three details belong in the order rather than in an assumption. The weld neck bore must match the pipe schedule actually used, referenced to ASME B36.10M or B36.19M; on heavy-wall or non-standard pipe the nominal schedule is not enough. The facing type and finish must be stated, because a raised face and a ring joint groove are not interchangeable and a groove cannot be corrected afterwards. And the inspection package should be defined explicitly — typically a mill test certificate to EN 10204 3.1 with heat number traceability, positive material identification where the alloy grade matters, and any ultrasonic or magnetic particle examination required by the purchase specification.

Gaskets and bolting: where large joints actually fail

Most large-diameter joint failures are assembly failures, not flange failures. The gasket belongs to the series: because bolt circle and seating dimensions differ, a gasket cut for Series A will not seat correctly in Series B, even at the same size and class. Order by series, class, size and facing, and check the centering ring against the actual flange.

Bolt load has to be calculated from the gasket manufacturer's seating stress and maintenance factor for the specific gasket construction — not copied from a smaller joint or a different gasket type. The load then sets the tightening procedure: cross-pattern passes at increasing pressure, with hydraulic tensioning or torque multipliers at the largest sizes, so that preload scatter stays inside what the gasket can tolerate. A single pass around a 48-inch bolt circle leaves the last bolts carrying a different load from the first.

Five specification mistakes to avoid

• Writing “ASME B16.47” without a series. The standard contains two incompatible geometries; the series is part of the identity of the part.
• Assuming the two bolt patterns interchange. Bolt circle, bolt count and gasket dimensions all differ.
• Specifying a flange type the standard does not make. No slip-on, socket weld, lap joint or threaded flanges at NPS 26 and above under B16.47.
• Carrying B16.5 gasket and torque data into a large joint. Scaling a small-joint procedure up does not preserve gasket seating stress.
• Leaving bore, facing and documentation to the supplier. Those three items decide whether the flange fits, seals and passes inspection.

Frequently asked questions

Is ASME B16.47 the same as MSS SP-44?

No, but Series A reproduces MSS SP-44 dimensions for the sizes it covers. If a specification calls for SP-44 dimensions, Series A is the equivalent B16.47 designation. Series B follows API 605 and is a different geometry.

Can a Series A flange be bolted to a Series B flange?

Not as a pressure-tight joint. The bolt circles, bolt counts and gasket seating dimensions are different, so the holes will not line up and the gasket cannot seat correctly in both flanges at once.

Does ASME B16.47 include slip-on flanges?

No. It covers weld neck and blind flanges only. A large-bore slip-on, lap joint, socket weld or threaded flange is outside its scope and must be handled as a custom or code-designed component.

Which series should a new project use?

The decision usually follows from constraints rather than preference. Series B reduces mass, cost and radial space, which suits new-build lines with weight and layout targets. Series A provides a stiffer, heavier flange with MSS SP-44 continuity, which suits brownfield tie-ins and services where resistance to rotation matters. Choose once, at the piping class level, and apply it consistently across the line.

What facing is standard on B16.47 flanges?

Raised face is the common default, with ring joint grooves available where the service requires a metal-to-metal seal. The facing must be stated on the order together with its finish, because the two configurations are not interchangeable after machining.

Conclusion

ASME B16.47 is a standard with two answers. Series A carries the MSS SP-44 tradition: larger, heavier, stiffer, and dimensionally continuous with a great deal of installed plant. Series B carries the API 605 tradition: compact, lighter and cheaper to build and support, at the cost of more bolts and a thinner section. Both are correct; neither is universal. The real failure mode is not choosing the wrong series on technical grounds, it is failing to choose at all and letting a vague purchase description decide by default.

For buyers, the practical rule is to specify size, class, series, flange type, facing, bore schedule, material specification and documentation package, and to verify the mating dimensions against what is already on the line before the forging is cut.

Request a large-diameter flange quotation

If you are sourcing flanges for a large-bore line, send the complete specification — NPS, pressure class, series, flange type, facing, material grade and documentation requirements — and our team will review the dimensions against your drawing and respond with a technical and commercial offer. Our alloy steel flange range covers weld neck, blind and other forged configurations in carbon, stainless, duplex and nickel alloy grades; for a comparable weld neck reference, see this nickel alloy weld neck flange. Send the inquiry through our contact page and include the design temperature and medium, so that the material and gasket recommendations match the service.

This article provides general information only. Final selection must be based on the applicable code, the project specification, and confirmation by qualified engineering personnel.