ANSI Flange Guide: Dimensions, Sizes, Ratings and Class 150 vs Class 600

ANSI flanges are widely specified in oil and gas, petrochemical, power, water treatment and general industrial piping. In current engineering practice, the phrase usually refers to flanges manufactured to ASME dimensional and pressure-temperature standards, especially ASME B16.5 and ASME B16.47. Buyers still use terms such as ANSI flange, ANSI 150 flange and ANSI 600 flange because the older terminology remains common in drawings, catalogs and international RFQs.

This guide explains ANSI flange dimensions, ANSI flange sizes, flange ratings, materials, facings and selection. It also corrects a costly misunderstanding: an ANSI pressure class is not a single pressure value in psi. The allowable pressure changes with material and temperature.

What Is an ANSI Flange?

An ANSI flange is a bolted piping component described in commercial language as conforming to an ANSI dimensional system. ANSI is the American National Standards Institute, which coordinates and accredits standards development in the United States. The detailed flange requirements commonly used today are published by ASME.

For a precise purchase order, write the actual standard designation rather than only “flange ANSI.” Examples include:

• ASME B16.5: Pipe flanges and flanged fittings in nominal sizes NPS 1/2 through NPS 24 within its scope
• ASME B16.47: Large-diameter steel flanges in nominal sizes NPS 26 through NPS 60
• ASME B16.36: Orifice flanges used with differential-pressure flow measurement
• ASME B16.48: Line blanks, including spectacle blinds, spades and spacers

The standard designation must be combined with size, class, type, facing, material and bore information. “ANSI flange” alone is not enough to manufacture or purchase the correct component.

ANSI Standards for Flanges: B16.5 vs B16.47

The most frequently requested ANSI standard for flanges is ASME B16.5. It defines dimensions, tolerances, materials, markings, testing and pressure-temperature ratings for common pipe flanges and flanged fittings within its size range. It includes pressure classes 150, 300, 400, 600, 900, 1500 and 2500, subject to the size and product limitations in the standard.

For NPS 26 through NPS 60, engineers commonly use ASME B16.47. This standard includes Series A and Series B flange dimensions. Series A and Series B are not interchangeable. They can have different outside diameters, bolt circles, thicknesses and bolting, even when nominal size and class are the same.

Selection item	ASME B16.5	ASME B16.47
Typical size range	NPS 1/2 through NPS 24	NPS 26 through NPS 60
Common pressure classes	150 through 2500, subject to standard scope	75, 150, 300, 400, 600 and 900, subject to series and size
Series designation	Not divided into Series A and B	Series A or Series B must be stated
Typical use	General process and utility piping	Large-diameter pipelines and equipment connections

Always check the project-specified edition. Dimensions and rating tables should be taken from the controlled standard, not from an unverified online chart.

How to Read ANSI Flange Dimensions

ANSI flange dimensions are a coordinated set of values. A flange cannot be identified reliably by outside diameter alone. The main dimensions include:

• Nominal pipe size: NPS identifies the matching piping system but is not a direct measurement of flange outside diameter.
• Outside diameter: The maximum diameter of the flange body.
• Flange thickness: The body thickness, excluding or including facing features as defined by the standard.
• Bolt circle diameter: The diameter of the circle through the centers of the bolt holes.
• Number and diameter of bolt holes: These must match the mating flange.
• Hub dimensions: Hub diameter and length depend on flange type.
• Bore: The internal opening, often affected by pipe schedule for weld neck, socket weld and slip-on configurations.
• Facing dimensions: Raised-face diameter and height, ring-joint groove or other sealing details.

As pressure class increases at the same nominal size, the flange usually becomes thicker and may use larger or more numerous bolts. Outside diameter and bolt circle can also change. A Class 150 flange therefore does not normally bolt directly to a Class 600 flange.

ANSI Flange Sizes

ANSI flange sizes are stated in NPS, or nominal pipe size. NPS is a nominal designation rather than the measured outside diameter of the flange or pipe. For international projects, an approximate DN equivalent may also appear, but NPS and DN should not be converted casually when exact drilling and bore dimensions matter.

A complete size description might read:

ASME B16.5, NPS 6, Class 300, weld neck, raised face, ASTM A105, bore for Schedule 40 pipe.

Each part of that description controls a different manufacturing detail. Removing the pipe schedule can leave the weld neck bore undefined. Removing the facing can create a gasket mismatch. Removing the material prevents a correct pressure-temperature rating review.

What Do ANSI Flange Ratings Mean?

ANSI flange ratings are pressure classes, commonly written Class 150, Class 300, Class 600 and so on. The class number is a designation used with standard pressure-temperature tables. It is not the universal maximum working pressure in psi.

The actual allowable working pressure depends on:

• Material specification and material group
• Metal temperature
• Applicable standard edition
• Flange class
• Service limitations imposed by the piping code or project

As temperature rises, allowable pressure commonly decreases because material strength changes. Two Class 150 flanges made from different materials may therefore have different allowable pressures at the same temperature.

ANSI 150 Pressure Rating Explained

The search phrases ANSI 150 pressure rating and 150 ANSI pressure rating often lead to the assumption that Class 150 means 150 psi. That assumption is incorrect.

Class 150 is a pressure class designation. For some common carbon steel material groups at lower temperatures, the table value can be higher than 150 psi. At elevated temperatures, the allowable value decreases. Stainless steel and alloy steel groups follow their own rating tables.

To determine a Class 150 flange rating correctly:

1. Identify the exact material specification and grade.
2. Find its material group in the applicable standard.
3. Locate the design metal temperature.
4. Read the Class 150 pressure-temperature table value.
5. Confirm that the piping code and project specification do not impose a lower limit.

Do not use a generic statement such as “150 lb flange equals 150 psi” for design. The old term “150 pound flange” is still heard in trade language, but Class 150 is the correct modern designation.

ANSI 600 Pressure Rating Explained

The same rule applies to the ANSI 600 pressure rating. Class 600 does not mean that every flange is limited to exactly 600 psi. It identifies a stronger dimensional and rating class whose allowable pressure must be selected from the table for the material and temperature.

Compared with Class 150 at the same nominal size, a Class 600 flange normally has a thicker body, heavier hub and more substantial bolting arrangement. The exact differences vary by size and flange type. Class 600 components are used in higher-pressure process piping, oil and gas facilities, steam systems, chemical plants and other demanding services when the design conditions require them.

A higher class does not solve every engineering problem. Material compatibility, toughness, corrosion allowance, fatigue, gasket selection, bolting and installation quality remain essential.

Quick Comparison of Common Flange Ratings

Flange class	General characteristic	Typical decision point
Class 150	Lightest common ASME B16.5 class	General utility and process service within table limits
Class 300	Heavier dimensions and higher rating than Class 150	Moderate pressure or temperature requirements
Class 600	Substantially heavier construction	Higher-pressure and more demanding process service
Class 900	High-pressure flange system	Critical process and energy applications
Class 1500/2500	Very high-pressure construction	Specialized high-pressure systems with strict engineering controls

This table describes relative classes only. It is not a pressure chart. Final design values must come from the applicable pressure-temperature tables.

Common ANSI Flange Types

Weld neck flange

A weld neck flange has a tapered hub and a butt-weld connection. It provides a gradual stress transition and is widely selected for high pressure, high temperature, cyclic loading and critical service.

Slip-on flange

A slip-on flange slides over the pipe and normally uses internal and external fillet welds. Fit-up is convenient, but engineers should evaluate fatigue, temperature cycles and service criticality before selecting it.

Blind flange

A blind flange has no center bore. It closes a pipeline, nozzle or valve end and must withstand pressure loading over its full internal area.

Socket weld flange

A socket weld flange receives the pipe into a recessed socket and uses a fillet weld. It is generally associated with smaller pipe sizes where permitted by the piping specification.

Threaded flange

A threaded flange connects without a flange-to-pipe weld. Its use depends on service, material, temperature, code rules and leakage risk.

Lap joint flange

A lap joint flange works with a stub end. The backing flange can rotate for bolt-hole alignment, which is useful in systems requiring frequent dismantling or costly corrosion-resistant wetted materials.

Flange Facings and Gasket Compatibility

Dimensions and class are only part of flange compatibility. The facing controls the gasket interface.

• Raised face: Common in ASME process piping and used with several gasket styles.
• Flat face: Often used where mating equipment or material limitations require full-face gasket support.
• Ring-type joint: Uses a precision-machined groove and metallic ring for suitable high-pressure or high-temperature applications.

Do not bolt mismatched facings together unless an approved engineering design specifically allows it. Gasket dimensions, material and pressure-temperature capability must match the joint.

Materials for ANSI Flanges

Common forged flange materials include:

• ASTM A105: Carbon steel forgings for ambient and higher-temperature piping applications within material limits
• ASTM A350 LF2: Carbon steel forgings requiring notch toughness for suitable low-temperature service
• ASTM A182 F304/F304L: Austenitic stainless steel for suitable corrosion and temperature conditions
• ASTM A182 F316/F316L: Molybdenum-bearing stainless steel for improved resistance in many corrosive environments
• ASTM A182 alloy grades: Chromium-molybdenum and other alloys for specified elevated-temperature service

Never select flange class independently from material. The material group is necessary to determine the applicable rating.

ANSI vs PN Flanges

ASME Class and EN PN designations come from different standards. A Class 150 flange is not automatically identical to PN16, and Class 300 is not simply PN40. Even where approximate pressure capability appears similar, outside diameter, bolt circle, hole count, hole diameter, thickness and facing dimensions can differ.

Adapters or engineered transition pieces may be required when connecting systems built to different standards. Do not redrill a pressure flange or force mismatched bolting without an approved engineering procedure.

How to Select the Correct ANSI Flange

1. Confirm the governing code and flange standard. Identify ASME B16.5, ASME B16.47 Series A or B, or another project standard.
2. Define design pressure and temperature. Include startup, shutdown, upset and thermal-cycle conditions.
3. Select the material. Consider corrosion, toughness, weldability and code allowable stresses.
4. Determine the pressure class. Use the correct material-group pressure-temperature table.
5. Choose the flange type. Consider loads, fatigue, installation method and maintenance requirements.
6. Specify facing and gasket. Confirm compatibility with the mating component.
7. State pipe schedule or bore. This is particularly important for weld neck and socket weld flanges.
8. Define testing and documentation. Include material certificates, NDE, PMI, impact tests and third-party inspection where required.

RFQ Checklist for ANSI Flanges

For an accurate quotation, provide:

• Standard and edition
• NPS or DN and quantity
• Pressure class
• Flange type and facing
• Material specification and grade
• Pipe schedule or required bore
• Surface finish and dimensional tolerances
• Heat treatment and impact-test temperature
• NDE, PMI, hardness and supplementary testing
• Coating, marking, packing and certification requirements

Frequently Asked Questions

What is the ANSI standard for flanges?

The phrase commonly refers to ASME B16.5 for NPS 1/2 through NPS 24 and ASME B16.47 for NPS 26 through NPS 60. Other standards cover specialized products such as orifice flanges and line blanks.

Is ANSI 150 rated for only 150 psi?

No. Class 150 is a class designation, not a fixed psi limit. Its allowable pressure depends on material group and temperature.

What is the ANSI 600 pressure rating?

Class 600 is not one universal pressure value. Use the applicable ASME pressure-temperature table for the exact material and design temperature.

Can Class 150 and Class 600 flanges connect?

Normally no. Their thickness, drilling and other dimensions generally differ. Both mating flanges should conform to the same size, class, facing and dimensional standard unless an engineered adapter is used.

Are ANSI and ASME flanges the same?

In everyday trade language, ANSI flange often means an ASME B16 flange. For technical accuracy, specify the ASME standard number, edition, size and class.

Where can I find an ANSI flange dimension chart?

Use the controlled edition of the applicable ASME standard or a manufacturer chart verified against it. Confirm class, flange type and facing before using any dimension.

Conclusion

Correctly specifying flange ANSI products requires more than naming a size and class. Buyers must identify the ASME standard, nominal size, pressure class, type, facing, material, bore and testing requirements. Most importantly, flange ratings must be evaluated against material and temperature rather than treated as fixed psi values.

Songhai Flange manufactures standard and custom forged ANSI/ASME flanges in carbon steel, stainless steel and alloy steel. Send your standard, size, class, material, facing, quantity and inspection requirements for a technical review and quotation.

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