ASME B16.47 Series A vs Series B Flanges: Selection and Removal Guide

Large-diameter flange selection is not complete when the nominal pipe size and pressure class are known. Under ASME B16.47, the purchaser must also identify Series A or Series B. Both series cover large-diameter steel flanges, but they use different outside diameters, bolt circles, bolt quantities, bolt sizes, thicknesses, and gasket dimensions. A flange from one series generally cannot be bolted directly to the other.

This guide explains Series A vs Series B flanges, how to identify the installed series, how to specify a replacement, and how to remove an ASME B16.47 flanged joint safely. It is written for piping engineers, maintenance teams, EPC contractors, and buyers sourcing large-diameter weld neck or blind flanges.

What Is ASME B16.47?

ASME B16.47 is the dimensional standard commonly used for large-diameter steel pipe flanges from NPS 26 through NPS 60. It addresses dimensional requirements, pressure-temperature ratings, materials, tolerances, marking, and related requirements for covered flange types and rating classes. The available size and class combinations must be checked in the applicable edition rather than assumed.

The standard contains two independent dimensional families:

Series A: historically associated with MSS SP-44 dimensions.
Series B: historically associated with API 605 dimensions.

These historical references explain why two large-diameter flange patterns remain in service. For new procurement, the governing requirement should be stated as ASME B16.47 with the required series and edition.

Quick Comparison: ASME B16.47 Series A vs Series B

Comparison point	Series A	Series B
General construction	Larger and heavier pattern in many comparable sizes	More compact and lighter pattern in many comparable sizes
Outside diameter	Generally larger	Generally smaller
Flange thickness	Generally thicker	Generally thinner
Bolting pattern	Commonly fewer, larger-diameter fasteners	Commonly more, smaller-diameter fasteners
Bolt circle and holes	Series A dimensions	Different Series B dimensions
Weight and handling	Usually heavier, with greater lifting demand	Usually lighter and more compact
Typical project preference	Frequently specified for robust pipeline and process duties	Often selected where lower weight and compact geometry are priorities
Interchangeability	Not normally interchangeable; verify every mating dimension

?Generally? matters because exact relationships depend on nominal size, pressure class, flange type, and standard edition. Approved dimensional tables and project specifications always control.

Key Differences in Detail

1. Outside diameter and installation envelope

Series A flanges usually have a larger outside diameter than Series B flanges of the same nominal size and class. That difference affects clearance around structures, adjacent piping, valve operators, insulation, and bolt-tensioning equipment. A replacement review should include the full installation envelope, not only the pipe bore.

2. Flange thickness and weight

Series A is normally the heavier construction. The difference becomes important at large diameters, where a single blind flange can require engineered lifting points, a crane plan, and temporary support. Series B may reduce component and handling weight, but lower weight does not make it a universal substitute. The complete joint must match the specified standard and design basis.

3. Bolt circle, hole count, and fastener size

The two series use different bolting arrangements. Series A commonly uses fewer fasteners of larger diameter, while Series B commonly uses more fasteners of smaller diameter. The bolt-circle diameter and hole dimensions also differ. Therefore, matching NPS and class do not create a matching bolt pattern.

Fastener quantity affects tightening time and tooling access. Fastener diameter affects torque, tensioning equipment, washer selection, and replacement inventory. Joint assembly procedures should be prepared for the actual series rather than copied from another flange of the same nominal size.

4. Gasket dimensions and sealing geometry

A gasket must match the flange series, nominal size, class, facing, and applicable gasket standard. A gasket selected only by NPS and class may have the wrong inside diameter, outside diameter, or centering relationship. This can reduce seating control, expose gasket material to flow, or interfere with bolts.

Facing type and finish remain separate requirements. Raised-face, ring-type-joint, and other specified configurations require compatible mating faces and gasket systems.

5. Pressure-temperature rating

Series A is often described as ?stronger? because it is heavier, but this shortcut is not a valid selection method. Pressure-temperature capability depends on the applicable standard table, material group, class, temperature, flange type, and all limitations in the governing code and project specification. Use the published rating for the actual material and edition.

6. Cost and availability

Series B can offer lower raw-material and shipping weight in many sizes. Series A may be preferred because of an established pipeline specification, installed equipment interface, owner standard, or desired joint configuration. Actual lead time can reverse the apparent cost advantage if one pattern is uncommon in the local supply chain.

Are Series A and Series B Flanges Interchangeable?

No, not as a general rule. Their bolt circles, hole quantities, hole diameters, outside diameters, thicknesses, and gasket dimensions are different. Even if the pipe bore and pressure class appear compatible, the flanges usually will not bolt together.

Do not solve a mismatch by slotting bolt holes, omitting fasteners, using undersized bolts, forcing alignment, or installing an improvised gasket. Such modifications change the load path and can compromise joint integrity. A transition should use a properly engineered component, spool, adapter, or flange arrangement approved for the design conditions.

How to Identify Series A or Series B in the Field

Read the flange marking. Look for the standard, series, size, class, material grade, manufacturer, and heat identification. Do not rely on paint color.
Count the bolt holes. The hole count is a useful first clue but is not enough by itself.
Measure the bolt circle. Measure between opposite hole centers or calculate the bolt-circle diameter using a controlled field method.
Measure outside diameter and thickness. Compare several dimensions because corrosion, coating, and facing height can affect readings.
Confirm the facing and bore. Check raised-face or RTJ geometry, hub bore, pipe wall, and any special machining.
Compare with the correct standard edition. Record the edition used by the project; dimensions should not be taken from an unidentified online chart.
Verify the mating component. A valve, vessel nozzle, or equipment flange may follow a manufacturer-specific pattern.

When markings are missing, create a dimensional inspection report and have engineering approve the identification before ordering a replacement.

How to Select Series A vs Series B Flanges

Choose Series A when:

The piping class, owner standard, or equipment interface explicitly requires it.
The existing system uses a Series A bolt pattern and gasket.
A project has standardized tooling, spares, and procedures around Series A.
The heavier configuration is part of the approved design basis.

Choose Series B when:

The project specification explicitly permits or requires Series B.
Reduced weight and installation envelope provide a verified project benefit.
The mating valve, equipment, or pipeline component has a Series B pattern.
Compatible fasteners, gaskets, tooling, and replacement stock are available.

The correct series is the one that matches every mating component and the approved piping design. It should never be chosen by price alone after equipment has already been purchased.

ASME B16.47 Flange Purchasing Checklist

A complete inquiry for a large-diameter flange should state:

ASME B16.47 edition and Series A or Series B
Nominal pipe size and pressure class
Flange type, such as weld neck or blind flange
Facing type and required surface finish
Material specification, grade, and supplementary requirements
Weld-neck bore, pipe outside diameter, and wall thickness
Corrosion allowance and any special machining
Design pressure and temperature where manufacturer review is required
Impact testing, heat treatment, hardness, PMI, and NDE requirements
Material certificates, traceability, marking, and coating
Gasket and bolting specifications when supplied as a package
Quantity, drawing approval, inspection, packing, and delivery requirements

Writing only ?36-inch Class 300 flange? is incomplete. The missing series can produce a component that cannot be installed even though its nominal size and class appear correct.

How to Remove an ASME B16.47 Flanged Joint Safely

The phrase how to remove ASME B16.47 usually means how to dismantle a large-diameter flanged connection. ASME B16.47 defines the flange; the plant isolation, line-breaking, and bolted-joint procedures control removal.

1. Confirm positive isolation

Identify all pressure and material sources, shut down the system, isolate it under the approved procedure, depressurize, drain, vent, purge, cool, and test the atmosphere where required. Treat the joint as live until the authorized checks prove otherwise.

2. Plan lifting and support

Large-diameter flanges, especially blind flanges, can be extremely heavy. Obtain the actual component weight, choose rated lifting equipment, support both sides of the joint, and control the center of gravity. Never allow pipe strain to release unpredictably when bolts are removed.

3. Record the installed arrangement

Photograph markings, count bolts, record orientation, identify the series, and mark mating positions when required. Confirm that replacement gaskets and fasteners match the actual flange.

4. Inspect and prepare the fasteners

Remove loose corrosion, apply an approved penetrant if suitable, and confirm the correct tools. Severely corroded bolts may require controlled cutting under a hot-work procedure. Provide containment for falling nuts and washers.

5. Loosen in a controlled sequence

Use the site-approved cross-pattern or staged sequence so gasket load is released gradually. Initially loosen fasteners only enough to confirm the joint is free of pressure. Do not stand in the potential release path or pry directly toward personnel.

6. Separate the joint without damaging faces

Use approved flange spreaders at designated gaps. Do not hammer sealing surfaces or drive unsuitable wedges into the facing. Keep hands clear and install safety blocks before working between separated flanges.

7. Remove and inspect the gasket

Dispose of compressed gaskets according to site rules. Inspect faces for corrosion, radial scratches, erosion, distortion, and coating damage. Protect exposed flange faces immediately.

8. Store or transport the flange safely

Use rated lifting points or an engineered sling arrangement. Do not lift a heavy blind flange by temporary bolts unless the method has been approved. Preserve identification and protect the machined face during storage.

Reassembly After Removal

Verify that both mating flanges are the same series, size, class, and facing.
Confirm flange alignment without using bolts to force the pipe into position.
Install the specified new gasket centered correctly.
Use the correct bolt diameter, length, material, nuts, washers, and lubricant.
Apply the approved multi-pass tightening pattern and target load.
Record tool calibration and final values for critical joints.
Complete required leak or pressure testing before startup.

Torque values should not be copied from a generic chart without confirming fastener material, coating, lubricant, gasket, flange condition, and the project’s joint-assembly procedure.

Common Specification and Maintenance Errors

Omitting Series A or Series B from the purchase order
Assuming the heavier series always has a higher allowable pressure
Ordering a gasket by size and class without identifying the series
Using bolt count alone to identify an unmarked flange
Mixing dimensions from different standard editions
Trying to connect Series A directly to Series B
Ignoring the weight of a large blind flange during removal
Using bolts to correct piping misalignment
Reusing a compressed gasket after opening the joint
Removing fasteners before positive isolation is verified

Frequently Asked Questions

What is the main difference between ASME B16.47 Series A and Series B?

Series A is generally larger, thicker, and heavier, commonly using fewer larger bolts. Series B is generally more compact and lighter, commonly using more smaller bolts. Exact dimensions depend on size and class.

Can Series A bolt to Series B?

Normally no. Their bolt circles, hole patterns, and related dimensions differ. Use a properly engineered transition rather than modifying bolt holes.

Which series is better for high pressure?

Neither series is universally ?better.? Use the pressure-temperature rating for the actual class, material group, size, and standard edition, together with the governing piping code and project specification.

Does ASME B16.47 cover NPS 24 flanges?

ASME B16.47 is generally used from NPS 26 through NPS 60. NPS 24 and smaller steel flanges are commonly covered by ASME B16.5, subject to the project requirements.

How can I identify an old unmarked flange?

Measure the outside diameter, thickness, bolt count, bolt-hole diameter, bolt-circle diameter, facing, and bore, then compare the complete set with the applicable standard tables. Engineering should approve the identification.

Can the old gasket be reused after flange removal?

No. A compressed gasket may not reproduce the required seating stress and should normally be replaced under the approved joint procedure.

Conclusion

The essential point in Series A vs Series B flanges is compatibility. Two ASME B16.47 flanges with the same NPS and pressure class can still have different bolt patterns, outside diameters, thicknesses, fasteners, and gaskets. Always state the series in drawings, inquiries, purchase orders, inspection records, and maintenance plans.

For safe removal, combine positive process isolation with engineered lifting, controlled bolt release, proper flange spreading, face protection, and complete joint records. Accurate identification before dismantling prevents the wrong replacement flange from arriving when the system is already offline.