ANSI ASME Flanges Guide: ASME Class, ANSI and ASME Differences Explained

Buyers often use ANSI ASME as one phrase when ordering pipe flanges, but ANSI and ASME do not play exactly the same role. ANSI is associated with U.S. standards coordination and accreditation, while ASME develops many of the mechanical engineering standards used for pressure equipment, piping and flanges.

For flange purchasing, the practical question is not only ASME vs ANSI. It is whether the flange standard, size, pressure class, material, face type and documentation are clearly specified. This guide explains what an ASME class means and how to write a clearer RFQ for industrial flange projects.

ANSI and ASME: What Do They Mean?

ANSI stands for American National Standards Institute. It is widely associated with standards coordination and accreditation in the United States. ASME stands for American Society of Mechanical Engineers. ASME publishes many engineering standards used for boilers, pressure vessels, piping, fittings and flanges.

In flange work, buyers may see old drawings marked ANSI B16.5, while newer documents usually say ASME B16.5. This is why the terms ANSI ASME, ANSI flange and ASME flange often appear together in RFQs and plant records.

ASME vs ANSI for Flanges

The phrase ASME vs ANSI can be confusing because many older flange specifications used ANSI wording, while modern flange standards are commonly published under ASME. In day-to-day purchasing, a request for ANSI B16.5 flanges often points to ASME B16.5 dimensions and pressure classes, but the project specification should always be checked.

If a replacement flange is for an old plant, do not rely only on the word ANSI. Confirm the actual dimensions, bolt circle, hole size, face type and pressure class. Legacy drawings, modified equipment and local fabrication practices can create differences that are not obvious from the title block.

What Is an ASME Class?

ASME class is a pressure-temperature rating category used for class-rated flanges. Common classes include Class 150, 300, 400, 600, 900, 1500 and 2500. The class number does not mean the flange can always operate at that exact pressure in every service.

The allowable working pressure depends on material group and operating temperature. For example, a Class 150 carbon steel flange and a Class 150 stainless steel flange may have different allowable pressure at elevated temperature. Hot service normally reduces allowable pressure compared with ambient conditions.

Why ASME Class Matters in Purchasing

ASME class affects flange thickness, bolt size, bolt circle, gasket selection, pressure-temperature rating and total weight. A Class 600 flange is not just a heavier version of a Class 150 flange. It has different dimensions and must match the mating flange, gasket and bolting set.

When ordering ANSI ASME flanges, always send the class together with nominal pipe size, flange type, material and face type. A short request such as “4 inch ASME flange” is not enough for accurate manufacturing or quotation.

Common ANSI ASME Flange Standards

Several standards may appear in flange and piping orders. ASME B16.5 is commonly used for NPS 1/2 through NPS 24 metallic pipe flanges and flanged fittings. ASME B16.47 is commonly used for larger diameter steel flanges. ASME B16.9 covers factory-made butt-welding fittings, not ordinary bolted flanges.

Standard	Typical Scope	Buyer Check
ASME B16.5	Common metallic pipe flanges and flanged fittings	NPS, class, face, material and bore
ASME B16.47	Large diameter steel flanges	Series A or Series B, class and dimensions
ASME B16.9	Butt-welding fittings	Fitting type, schedule and material
EN/DIN/PN standards	Metric flange systems	DN, PN, drilling pattern and face type

ANSI ASME Flange Types

Common ANSI ASME flange types include weld neck, slip on, blind, socket weld, threaded and lap joint flanges. Weld neck flanges are often selected for high pressure, high temperature or critical service. Slip on flanges are common in utility and moderate service. Blind flanges close pipe ends or pressure openings.

Socket weld and threaded flanges are common in smaller sizes when the design allows them. Lap joint flanges are useful when bolt alignment or frequent disassembly matters. The correct flange type should match the service, piping code and installation method.

ASME Class and Face Type

Face type is another key detail. Raised face, flat face and ring type joint faces are not interchangeable without engineering review. The ASME class and face type together influence gasket selection and sealing behavior.

For example, a raised face flange may use a ring gasket or spiral wound gasket. A flat face flange often uses a full-face gasket, especially with cast iron or softer mating components. RTJ flanges use a metal ring gasket and are common in higher pressure services.

ASME vs ANSI in Export Documentation

Export orders should use consistent wording across the quotation, invoice, packing list, drawing, marking and material certificates. If the buyer writes ANSI and the project specification writes ASME, clarify the required standard before production.

Good documentation helps customs clearance, site receiving inspection and final project handover. It also prevents confusion when the same shipment includes ASME class flanges, EN PN flanges and custom flange products.

How to Write a Clear RFQ

A complete RFQ for ANSI ASME flanges should include standard, flange type, NPS size, ASME class, material grade, pipe schedule or bore, face type, quantity, inspection requirement and destination. For custom products, include drawings, bolt circle, hole quantity, hole diameter and any machining tolerance.

Example wording: ASME B16.5 weld neck flange, NPS 6, Class 300, raised face, ASTM A105N, bore for SCH 40 pipe, MTC required, export packing. This is much clearer than “ANSI ASME flange class 300” alone.

Common Mistakes to Avoid

• Using ANSI and ASME wording without confirming the exact standard.
• Treating ASME class as a fixed working pressure value.
• Forgetting that allowable pressure changes with temperature and material.
• Mixing ASME class flanges with PN flanges by pressure name only.
• Ordering without face type, pipe schedule or material grade.
• Ignoring marking and material certificate requirements for export projects.

FAQ About ANSI and ASME Flanges

What is the difference between ANSI and ASME?

ANSI is associated with standards coordination and accreditation, while ASME develops many mechanical engineering standards used for pressure equipment, piping and flanges.

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

In many flange RFQs, ANSI B16.5 is legacy wording for the same general flange standard family now commonly referenced as ASME B16.5. Always confirm the required edition and dimensions before ordering.

What does ASME class mean?

ASME class is a pressure-temperature rating category such as Class 150, 300 or 600. The allowable pressure depends on material and operating temperature, so it is not a single fixed pressure value.

How should I specify ANSI ASME flanges?

Specify standard, size, class, material, flange type, face type, pipe schedule or bore, quantity, inspection documents and destination. Drawings are recommended for replacement and custom flange orders.

Need ANSI ASME Flanges?

Songhai Flange supplies ASME class flanges, ANSI ASME replacement flanges, weld neck flanges, slip on flanges, blind flanges, socket weld flanges, threaded flanges and custom flange products for oil and gas, chemical, power, marine, water treatment and OEM projects. Our team can support drawings, material selection, machining, inspection and export packing.