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

Ordering ASME B16.5 flanges involves more than choosing a pipe size and pressure class. The flange must match the mating connection, material specification, facing, bore and project requirements. A mismatch in any of these details can lead to quotation delays, installation problems or costly replacement. This guide helps engineers, distributors, stockists and EPC purchasing teams identify the information needed to specify ASME B16.5 flanges accurately and prepare a clear request for quotation (RFQ).

1. What Information Do You Need to Specify an ASME B16.5 Flange?

Before requesting a quotation, establish the basic technical requirements. A supplier needs enough information to identify the correct product rather than make assumptions about your application.

These details form the starting point for a technical quotation. For projects with special requirements, also provide approved drawings, the applicable piping specification and any additional inspection or documentation requirements.

If you are not certain which pressure class or material is appropriate, provide the design conditions and project specification for technical review rather than selecting a class based on its number alone.

2. How to Check ASME B16.5 Flange Dimensions

Dimensional compatibility is one of the first things to verify when purchasing replacement flanges or components for an existing piping system.

The nominal pipe size (NPS) alone does not establish that two flanges will fit together. The required dimensions depend on the specified size, pressure class, flange configuration and applicable standard.

Before approving a drawing or purchase order, check the following items.

• Outside diameter: Confirm that the flange dimensions match the applicable standard and mating connection.
• Thickness: Verify the specified flange thickness for the required size, class and design.
• Bolt circle and bolt holes: Check the bolt circle diameter, number of holes and hole diameter against the mating flange.
• Facing dimensions: Confirm that the facing configuration and dimensions are compatible with the mating flange and gasket.
• Bore and hub: For weld neck flanges, check the required bore against the pipe wall thickness or approved drawing.
• Overall geometry: Verify other relevant dimensions, including hub dimensions, where applicable.

For replacement orders, an approved drawing or verified dimensional record is often more useful than a product description alone. However, measurements from an existing component should be checked against the governing specification before being used as the sole basis for a new order.

For the standard’s scope, size ranges and technical background, see our ASME B16.5 standard guide.

3. How to Choose the Correct ASME B16.5 Pressure Class

ASME B16.5 uses pressure class designations including Class 150, 300, 400, 600, 900, 1500 and 2500, subject to the applicable size and component limitations.

The class number is not a direct pressure value in psi. The allowable pressure depends on the applicable material group and operating temperature, together with the rating provisions of the specified standard edition.

When reviewing a flange requirement, consider these points:

1. Confirm the design pressure and design temperature.
2. Identify the required material specification and grade.
3. Check the applicable pressure-temperature rating.
4. Verify the requirements of the governing piping code and project specification.
5. Confirm that the selected class is available for the specified size and flange configuration.

Class 300 should not automatically be treated as having twice the allowable pressure of Class 150. Nor should a supplier select a class based only on the normal operating pressure provided in an informal enquiry.

For distributors purchasing stock, it is also important to distinguish between frequently requested inventory specifications and the requirements of a particular project. A commonly stocked class is not necessarily suitable for every application.

4. Which Flange Type Should You Specify?

The connection design should be identified before the quotation is finalized. Common flange types include the following.

Weld Neck (WN) Flange

Used with a butt-weld connection. The tapered hub and welding arrangement make this a common option for demanding piping applications, subject to engineering design requirements.

Slip-On (SO) Flanges

Fits over the pipe and is attached using the specified welding arrangement. It is used in various general industrial piping applications.

Blind Flange (BL) Flanges

Closes a piping end, opening or compatible equipment connection. The specified class and design requirements must be suitable for the intended service.

Socket Weld (SW) Flanges

Designed for a socket-weld connection and commonly specified for small-bore piping.

Threaded Flange Flanges

Uses a threaded connection where permitted by the project design and service conditions.

Lap Joint (LJ) Flanges

Normally used with a compatible stub end and can allow easier bolt-hole alignment during assembly.

The type should be selected according to the piping design, connection method, service conditions and project requirements. Do not assume that different flange types are interchangeable simply because they share the same nominal size and class.

5. Material and Facing: Details That Should Not Be Left Unspecified

The material specification and facing are essential parts of a complete flange description.

Common industrial material specifications include ASTM A105 carbon steel forgings, ASTM A350 LF2 forgings for specified low-temperature applications, and ASTM A182 grades for covered stainless steel and alloy steel forgings. The correct choice depends on the design conditions, service environment and project requirements.

When reviewing the material requirement, confirm the exact grade rather than stating only “carbon steel” or “stainless steel.” Also identify any required supplementary testing, material traceability or documentation.

Facing options may include Raised Face (RF), Flat Face (FF) and Ring-Type Joint (RTJ), where applicable to the specified design.

The facing must be compatible with the mating flange and gasket arrangement. If the connection is being used in an existing system, confirm the facing details before placing the order.

6. When Should You Check ASME B16.47 Instead?

ASME B16.5 is not the correct dimensional reference for every flange size. Its flange coverage extends through NPS 24, subject to the applicable class requirements.

For larger-diameter flanges, check whether ASME B16.47 applies to the project. Do not assume that a larger flange can be specified simply by extending a B16.5 size designation.

The applicable standard, series where relevant, pressure class, dimensions and project requirements must all be confirmed before the order is finalized.

For a focused explanation of the differences and selection considerations, read our ASME B16.47 vs. B16.5 flange comparison.

7. How to Prepare a Clear Flange RFQ

A complete RFQ helps a manufacturer check the specification and quote the intended product. It also reduces the risk of repeated clarification between purchasing, engineering and the supplier.

For example, a purchase enquiry could specify:

Product: Weld Neck Flange
Standard: ASME B16.5
Size: NPS 6
Pressure Class: Class 300
Material: ASTM A105
Facing: RF
Bore: For the specified pipe schedule, subject to drawing confirmation
Quantity: 100 pieces
Documentation: Material test report as required by the purchase specification

This is an example of how to organize an enquiry, not a universal specification for every project. The actual bore, material, facing and documentation requirements must be confirmed against the approved technical documents.

For repeat orders, distributors and stockists should maintain a consistent specification record for each item. For project orders, provide the relevant line item, drawing revision and inspection requirements to reduce ambiguity.

8. What to Look for in an ASME B16.5 Flange Manufacturer

A low quotation is not enough to establish whether a flange is suitable for a project. Buyers should assess whether the supplier can meet the specified material, dimensions, manufacturing requirements and delivery conditions.

Before placing an order, consider asking the supplier to confirm:

• The applicable product standard and material specification
• Dimensional inspection requirements
• Material identification and traceability
• Required inspection and test documentation
• Marking, surface protection and export packing
• Production lead time and shipment arrangements

The required evidence will vary by project. Material certificates, inspection reports and third-party inspection should be agreed upon where they are specified; they should not be assumed to be included automatically in every order.

9. ASME B16.5 Flanges from JS FITTINGS

JS FITTINGS, manufactures steel pipe fittings and flanges for industrial customers and international markets.

When enquiring about ASME B16.5 flanges, buyers can provide the required size, class, material, facing, quantity and project documentation requirements for technical review and quotation.

Our ASME B16.5 flange product page provides a direct starting point for customers looking for the relevant product range. If your requirement involves a particular drawing, material grade or inspection plan, include those details in your enquiry so that the specification can be reviewed before an order is confirmed.

Frequently Asked Questions

What is the most important information when ordering ASME B16.5 flanges?

Start with the standard and required edition, NPS, pressure class, material, flange type and facing. Then confirm the bore, dimensions, quantity and any project-specific inspection or documentation requirements.

Can I select a flange based on NPS alone?

No. NPS is only one part of the specification. The class, flange type, facing, bore and mating dimensions must also be checked.

Does Class 300 mean 300 psi?

No. The class designation is not a direct psi rating. The allowable pressure must be determined using the applicable material and pressure-temperature rating provisions.

What should I provide when replacing an existing flange?

Provide the existing specification, approved drawing or verified dimensional information, including the size, class, material, facing, bolt-hole arrangement and bore where relevant. Confirm these details against the applicable standard before ordering.

Where can I request ASME B16.5 flange pricing?

Visit the JS FITTINGS ASME B16.5 flange product page and submit your required specifications, quantities and delivery information.

Final Checklist Before Placing Your Order

Before confirming an ASME B16.5 flange purchase, make sure the standard, size, class, material, type, facing and bore have been specified and checked. Confirm dimensional compatibility with the mating component, and agree on the required documentation, inspection and delivery conditions.

A clear specification gives both the buyer and manufacturer a common technical reference, helping reduce avoidable quotation questions and preventable ordering errors.

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