ANSI/ASME B16.5 Flanges: Dimensions, Pressure Classes, Types and Selection Guide

When specifying flanges for industrial piping systems, ANSI/ASME B16.5 is one of the most widely recognized standards in the world. It provides the dimensional, pressure-temperature rating, material, tolerance, marking, testing, and designation requirements needed to ensure that flanges can be properly matched and safely installed in piping systems.

ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 through NPS 24, with pressure classes including 150, 300, 400, 600, 900, 1500, and 2500. Class 2500 flanges are covered from NPS 1/2 through NPS 12, while the other major classes extend through NPS 24.

For industrial projects, choosing the correct ANSI/ASME B16.5 flange is not simply a matter of matching pipe diameter. The flange must be selected according to the pipe size, pressure class, material, operating temperature, flange type, facing, bore, gasket, bolting, and applicable project specifications.

What Is ANSI/ASME B16.5?

ASME B16.5 is the American Society of Mechanical Engineers standard titled Pipe Flanges and Flanged Fittings: NPS 1/2 Through NPS 24 Metric/Inch Standard.

The standard establishes requirements for:

• Flange dimensions
• Pressure-temperature ratings
• Materials
• Tolerances
• Bolt hole dimensions
• Flange facing dimensions
• Marking
• Testing
• Designation of flange openings
• Flanged fittings

The term “ANSI flange” is still widely used in the international piping industry, although ASME is the organization responsible for developing and maintaining B16.5. Therefore, phrases such as ANSI flange, ASME flange, and ANSI/ASME B16.5 flange are commonly used by engineers, buyers, and suppliers when referring to flanges manufactured to this dimensional and rating standard.

One important point is that B16.5 does not mean every flange has the same pressure capacity at every temperature. Pressure-temperature ratings depend on the applicable material group and operating conditions. The Class designation should therefore be considered together with material and temperature rather than treated as a simple PSI value.

What Sizes and Pressure Classes Does ANSI/ASME B16.5 Cover?

ASME B16.5 covers flange sizes from NPS 1/2 to NPS 24.

The principal pressure classes are:

ASME’s current B16.5 standard specifically identifies Class 150, 300, 400, 600, 900, and 1500 flanges in NPS 1/2 through NPS 24, while Class 2500 is covered from NPS 1/2 through NPS 12.

This size range makes B16.5 suitable for a broad range of industrial piping applications, including oil and gas, petrochemical plants, chemical processing, power generation, water treatment, refineries, and general process piping.

For sizes above NPS 24, engineers generally need to refer to ASME B16.47, rather than B16.5. B16.47 is intended for larger-diameter flanges and has its own dimensional and rating requirements.

NPS Is Not the Same as Actual Pipe Outside Diameter

One common purchasing mistake is assuming that NPS is the actual outside diameter of the pipe.

For example, a NPS 6 pipe does not have a 6-inch outside diameter. The nominal pipe size is a standardized designation, while the actual outside diameter depends on the NPS system and remains standardized across different pipe schedules.

This is why flange selection should always be based on the correct NPS, pipe OD, wall thickness or schedule, flange bore, and applicable standard.

A weld neck flange, for example, needs a bore compatible with the pipe to achieve the required welding and flow characteristics.

What Is the Difference Between Class 150 and Class 300 Flanges?

Class 150 and Class 300 are two of the most commonly specified ASME B16.5 flange pressure classes.

The most important thing to understand is that Class 300 does not simply mean “300 PSI,” and Class 150 does not simply mean “150 PSI.”

The Class designation is part of the ASME pressure-temperature rating system. The actual allowable working pressure depends on factors such as the flange material and operating temperature.

In general, a Class 300 flange has a higher pressure-temperature rating than a comparable Class 150 flange when the same material and temperature conditions are considered.

Key Differences Between Class 150 and Class 300

A Class 300 flange generally has a more robust geometry than a Class 150 flange. Depending on the NPS, the difference can include flange thickness, bolt size, bolt-hole arrangement, hub dimensions, and overall weight.

For example, selecting a Class 300 flange simply because it appears “stronger” is not necessarily the correct engineering decision. The flange class should be determined from the design pressure, design temperature, material group, piping specification, gasket, bolting, and project requirements.

Does Class 300 Always Mean Twice the Pressure of Class 150?

No.

This is one of the most common misunderstandings about ASME flange classes.

Class 150 and Class 300 are designation systems, not direct PSI measurements. The pressure-temperature rating tables in ASME B16.5 determine the allowable pressure for a particular material at a particular temperature.

Therefore, it is incorrect to select a flange simply by saying:

“The system operates at 250 PSI, so I need a Class 250 flange.”

There is no Class 250 flange designation in ASME B16.5.

Instead, the engineer should determine the applicable ASME class based on the piping material, design temperature, design pressure, and applicable rating table.

What Flange Types Are Covered by ANSI/ASME B16.5?

ASME B16.5 covers several common flange configurations used in industrial piping systems. The major types include:

1. Weld Neck Flanges
2. Slip-On Flanges
3. Socket Weld Flanges
4. Threaded Flanges
5. Lap Joint Flanges
6. Blind Flanges

Different flange designs provide different connection methods and performance characteristics.

1. Weld Neck Flange

A Weld Neck (WN) flange has a long tapered hub and is designed to be butt welded directly to the pipe.

The tapered hub provides a gradual transition between the flange and pipe, making the weld neck configuration particularly suitable for high-pressure, high-temperature, and cyclic service.

Typical applications include:

• Oil and gas pipelines
• Refineries
• Petrochemical plants
• Power plants
• High-pressure process piping
• Steam systems
• Critical industrial services

Weld neck flanges are often selected when fatigue resistance, structural integrity, and reliable stress distribution are important.

2. Slip-On Flange

A Slip-On (SO) flange slides over the outside of the pipe and is normally welded around the pipe.

Compared with a weld neck flange, the slip-on design is simpler to install and can be more economical for general industrial applications.

Typical applications include:

• Water treatment
• Utility piping
• General process piping
• Low-to-moderate pressure systems
• Industrial maintenance projects

For demanding cyclic or high-pressure applications, engineers may prefer a weld neck configuration depending on the design requirements.

3. Socket Weld Flange

A Socket Weld (SW) flange has a recessed socket into which the pipe is inserted before welding.

Socket weld flanges are commonly used for small-bore piping systems, especially where relatively high pressure is required.

Typical applications include:

• Hydraulic systems
• Chemical injection lines
• Instrument piping
• Steam lines
• Small-diameter process piping

The socket geometry must be properly considered during fabrication and installation, particularly in corrosive service where crevice effects may be a concern.

4. Threaded Flange

A threaded flange connects to a threaded pipe without requiring a conventional welding operation.

This can be useful in applications where welding is undesirable or impractical.

Threaded flanges are commonly associated with:

• Small-diameter piping
• Utility systems
• Maintenance applications
• Areas where welding is restricted

However, threaded connections require careful consideration of pressure, temperature, vibration, sealing, and service conditions.

5. Lap Joint Flange

A Lap Joint (LJ) flange is normally used together with a stub end.

The flange can rotate around the pipe/stub-end assembly, which makes bolt-hole alignment easier during installation.

This configuration is particularly useful where:

• Frequent disassembly is required
• Alignment is difficult
• Corrosion-resistant pipe materials are being used
• The flange material can be different from the pipe material

The ability to rotate the flange provides an important practical advantage during installation and maintenance.

6. Blind Flange

A Blind Flange (BL) has no center bore and is used to close the end of a piping system, valve, vessel nozzle, or pipeline section.

Blind flanges are widely used for:

• Pipeline isolation
• Pressure testing
• Equipment maintenance
• Future pipeline extensions
• Vessel and nozzle closures

Because a blind flange must withstand the pressure acting against the closed end, its thickness and pressure rating are critical design considerations.

What Are the Most Common ANSI/ASME B16.5 Flange Facings?

Flange facing is another important specification that should not be overlooked.

Common flange facing configurations include:

• Raised Face (RF)
• Flat Face (FF)
• Ring Type Joint (RTJ)

Raised Face Flange

Raised Face is one of the most commonly specified flange facings in industrial piping.

The raised surface provides a defined gasket seating area and is widely used in process piping, oil and gas, chemical, and power applications.

Flat Face Flange

Flat Face flanges have a flat gasket seating surface and are commonly used where the mating equipment or piping system requires a flat-face connection.

When connecting different flange configurations, compatibility between the flange faces and gasket must always be confirmed.

Raised Face Flange vs. Flat Faced Flange, click here

How Do I Find the Right Flange Size and Class for My Pipe?

Choosing the correct ASME B16.5 flange size and pressure class requires more than matching the pipe diameter.

A practical selection process is to check the following parameters.

Step 1: Identify the Nominal Pipe Size

Determine the pipe’s NPS, such as:

• NPS 2
• NPS 4
• NPS 6
• NPS 8
• NPS 12
• NPS 24

Do not select a flange based only on the actual outside diameter. Confirm the nominal pipe size and applicable pipe standard.

Step 2: Check the Pipe Schedule or Wall Thickness

For many flange types, especially weld neck flanges, the flange bore must be compatible with the pipe wall thickness and inside diameter.

Common pipe schedules include:

• SCH 10
• SCH 20
• SCH 40
• SCH 80
• SCH 120
• SCH 160

The exact bore requirement should be confirmed against the project specification and flange manufacturer’s dimensional data.

Step 3: Determine the Design Pressure

Identify the maximum design pressure of the piping system.

Do not confuse design pressure with normal operating pressure. The flange selection should be based on the engineering design conditions and applicable code requirements.

Step 4: Determine the Design Temperature

Temperature has a direct effect on allowable pressure.

A flange that is suitable at one temperature may have a different allowable pressure at a higher operating temperature.

This is why pressure class alone is not sufficient to determine whether a flange is suitable.

Step 5: Select the Flange Material

Common industrial flange materials include:

• ASTM A105 carbon steel
• ASTM A350 LF2
• ASTM A182 F304/F304L
• ASTM A182 F316/F316L
• Alloy steel grades such as F11 and F22

The material should be selected based on pressure, temperature, corrosion resistance, toughness, process fluid, and project requirements.

For carbon steel piping systems, ASTM A105 forged flanges are among the most commonly specified options.

Step 6: Select the Flange Type

The connection method should match the application.

For example:

High-pressure or cyclic service → Weld Neck

General industrial service → Slip-On

Small-bore high-pressure service → Socket Weld

No-welding applications → Threaded

Frequent dismantling or alignment requirements → Lap Joint

Pipeline termination or isolation → Blind

This selection should always be verified against the applicable piping specification.

Step 7: Select the Facing

The flange facing must match the mating flange, gasket, pressure class, and service conditions.

Common choices include RF, FF, and RTJ.

Step 8: Confirm Bolt Hole and Dimensional Compatibility

Before purchasing, verify:

• Outside diameter
• Flange thickness
• Bolt circle diameter
• Number of bolt holes
• Bolt hole diameter
• Bore diameter
• Facing dimensions
• Hub dimensions
• Overall length where applicable

These dimensions are critical for ensuring that two mating flanges can be assembled correctly.

ANSI/ASME B16.5 Flange Dimensions: What Should Buyers Check?

When purchasing ASME B16.5 flanges, a dimensional drawing or technical datasheet should normally include the information necessary to verify compatibility.

For a typical flange, buyers may need to confirm:

For international projects, buyers should also confirm whether dimensions are required in inch-based or metric presentation, while recognizing that ASME B16.5 is a metric/inch standard with bolt and flange-hole dimensions expressed according to the requirements of the standard.

ASME B16.5 vs ASME B16.47: What Is the Difference?

One of the most important distinctions for flange buyers is the difference between ASME B16.5 and ASME B16.47.

ASME B16.5

• NPS 1/2 through NPS 24
• Classes 150, 300, 400, 600, 900, 1500
• Class 2500 from NPS 1/2 through NPS 12
• Widely used for standard industrial piping flanges

ASME B16.47

• Primarily used for larger-diameter flanges
• Covers NPS 26 and larger
• Includes Series A and Series B configurations
• Used for large-diameter pipeline and process applications

Therefore, if a project requires a 30-inch, 36-inch, or 48-inch flange, B16.5 is generally not the dimensional standard to start with; ASME B16.47 should be investigated instead.

Common Materials for ASME B16.5 Forged Flanges

Material selection is just as important as flange dimensions.

For industrial piping, commonly specified materials include:

Carbon Steel

ASTM A105 is widely used for forged carbon steel flanges in industrial piping.

It is commonly selected for:

• Oil and gas
• Petrochemical
• Power generation
• Water treatment
• General process piping

Low-Temperature Carbon Steel

Materials such as ASTM A350 LF2 may be selected for applications where low-temperature toughness is required.

Stainless Steel

Common stainless steel flange grades include:

• ASTM A182 F304
• ASTM A182 F304L
• ASTM A182 F316
• ASTM A182 F316L

These materials are often selected where corrosion resistance is more important than the properties of conventional carbon steel.

Alloy Steel

Grades such as ASTM A182 F11 and F22 may be specified for elevated-temperature applications, depending on the engineering requirements.

The correct material should always be determined from the applicable material specification, design conditions, process medium, and project requirements.

Why Are Forged Flanges Preferred for Critical Industrial Piping?

The manufacturing method has a direct impact on flange performance.

A forged flange starts from a steel forging billet or forged ring, followed by machining to achieve the required dimensions and facing.

For critical industrial applications, forged construction is widely used because it provides a robust manufacturing route for components exposed to pressure, mechanical loads, and elevated temperatures.

At JS FITTINGS, forged flanges are manufactured for industrial applications where dimensional consistency, material traceability, and reliable performance are important.

Our production capabilities include carbon steel flanges and other steel piping products manufactured to major international standards. JS FITTINGS operates a manufacturing facility exceeding 35,000 m², with more than 40 years of experience, annual production capacity of up to 30,000 tons, and monthly output of approximately 700 tons of flanges.

How to Order ANSI/ASME B16.5 Flanges From a Manufacturer

When requesting a quotation, providing complete technical information can significantly reduce communication time and prevent incorrect quotations.

A typical RFQ should include:

Product: Weld Neck Flange / Slip-On Flange / Blind Flange / Socket Weld Flange

Standard: ASME B16.5

Size: NPS 6

Pressure Class: Class 300

Material: ASTM A105

Facing: RF / FF / RTJ

Bore: According to pipe schedule

Quantity: Required number of pieces

Additional requirements: MTC, NACE, PMI, UT, third-party inspection, coating, marking, etc.

For example:

100 pcs ASME B16.5 Class 300 WN Flanges, NPS 6, ASTM A105, RF, bore for SCH 40 pipe, with EN 10204 3.1 MTC.

This type of specification gives the manufacturer enough information to prepare an accurate quotation and technical confirmation.

Why Choose JS FITTINGS for ASME B16.5 Flanges?

Selecting the right standard is only the first step. The manufacturing quality, dimensional control, material traceability, inspection process, and delivery capability of the supplier are equally important.

JS FITTINGS is a China-based manufacturer and supplier of steel pipe flanges, butt weld fittings, and steel pipes. The company serves industrial customers in international markets and manufactures products according to major international standards, including ASME, ANSI, GOST, EN, BS, DIN, JIS, and SANS.

Our flange production capability includes commonly requested configurations such as:

• Weld Neck Flanges
• Slip-On Flanges
• Blind Flanges
• Socket Weld Flanges
• Threaded Flanges
• Lap Joint Flanges
• Custom industrial flange solutions

JS FITTINGS reports monthly production of approximately 700 tons of flanges and annual production capacity of up to 30,000 tons, supported by manufacturing, inspection, finishing, and export capabilities. The company also lists certifications and approvals including CE, ISO, GOST-R, PETROBRAS, NIOC, and ADNOC.

For distributors, EPC contractors, stockists, and industrial end users, the objective is not simply to purchase an ASME B16.5 flange. It is to obtain the correct flange, manufactured to the correct standard, material, dimensions, pressure class, and project requirements.

FAQ About ANSI/ASME B16.5 Flanges

Is ANSI B16.5 the same as ASME B16.5?

In the piping industry, the terms are often used interchangeably. However, ASME B16.5 is the actual ASME standard for pipe flanges and flanged fittings. “ANSI flange” remains a widely used commercial term, particularly when referring to ANSI/ASME dimensional and pressure-class requirements.

What is the largest flange size covered by ASME B16.5?

ASME B16.5 covers flanges through NPS 24. For larger sizes, ASME B16.47 is generally the applicable standard. Class 2500 under B16.5 is limited to NPS 12.

What pressure classes are available under ASME B16.5?

The standard includes Class 150, 300, 400, 600, 900, 1500, and 2500, subject to the applicable size range and rating requirements.

Is Class 300 a 300 PSI flange?

No. The number in the Class designation should not be interpreted as the flange’s direct pressure rating in PSI. The allowable pressure depends on the applicable material and temperature.

Which is better, Class 150 or Class 300?

Neither is universally “better.” Class 300 is generally suitable for higher pressure-temperature requirements than Class 150, but the correct selection depends on the design pressure, temperature, material, piping specification, and applicable rating tables.

Which ASME B16.5 flange is best for high-pressure service?

A weld neck flange is commonly selected for high-pressure, high-temperature, cyclic, and critical service because of its butt-welded configuration and tapered hub. However, the appropriate flange type and class must ultimately be determined by the project engineering requirements.

Can ASME B16.5 flanges be used internationally?

Yes. ASME B16.5 is widely used in international oil and gas, petrochemical, power generation, chemical, water treatment, and industrial piping projects. However, individual projects may specify additional requirements for materials, testing, certification, inspection, coating, or regional regulations.

Final Thoughts

ANSI/ASME B16.5 flanges are a fundamental component of modern industrial piping systems. The standard provides a consistent framework for flange dimensions, pressure-temperature ratings, materials, tolerances, marking, testing, and connection requirements.

However, choosing the correct flange requires more than selecting a nominal size and pressure class.

Engineers and buyers should consider the complete combination of:

Pipe Size + Pressure Class + Material + Temperature + Flange Type + Facing + Bore + Gasket + Bolting + Project Specification

Getting these details right helps ensure proper flange fit-up, reliable sealing, safe operation, and long-term piping system performance.

If you are sourcing ASME B16.5 forged flanges for an oil and gas project, EPC contract, industrial plant, stock inventory, or distribution business, JS FITTINGS can provide technical support and quotation based on your required size, class, material, facing, bore, quantity, and inspection requirements.

Need an ASME B16.5 flange quotation? Contact JS FITTINGS with your flange specifications and project requirements.

JS FITTINGS — Industrial Steel Pipe Fittings & Flanges Manufacturer

Website: https://jsfittings.com/

Email: admin@jsfittings.com

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