Flange Connection Standards: ANSI vs DN Designations

What are ANSI and DN Flange Standards

ANSI flanges are governed by American National Standards Institute specifications. They define flange dimensions in inches and use a class-based system for pressure ratings. Common classes include 150, 300, 600, and 1500. The letter prefix often indicates the series, such as RT for ring-type joints or RF for raised face.

DN flanges follow a metric system. The DN stands for Diamètre Nominal, or nominal diameter. The number represents the internal diameter in millimeters of the pipe. A DN 50 flange fits a 50 mm nominal bore pipe. Pressure ratings in the DN world typically follow European or international standards, often labeled by pressure in bar or MPa at a specific temperature.

The core difference is the measurement base. ANSI uses inches. DN uses millimeters. This simple distinction changes the bolt hole spacing, the flange outer diameter, and the gasket face shape.

Why the Standard Matters in Procurement

Supply chains are global. A project in one region often sources parts from multiple manufacturers in different countries. If the buyer orders a “5” flange without specifying the standard, the vendor might supply an ANSI flange or a DN 125 flange. Both serve a similar nominal pipe size, but the bolt circle and face dimensions do not match.

This mismatch causes problems during assembly. Bolt holes will not align with the mating flange. The joint cannot be tightened evenly. The gasket, often a spiral-wound type or a flexible metal gasket, will not seat correctly. The result is a leak or a damaged flange face.

Buyers must specify the standard, the class or pressure rating, and the material grade on the purchase order. A simple line item like “Flange, 6 inch, Class 300, ASTM A16.5” is clear. “Flange, 6 inch” is not.

Bolt Patterns and Face Geometry

The bolt pattern is the most visible difference. For the same nominal pipe size, an ANSI flange and a DN flange have different bolt hole diameters and different bolt circle diameters.

Consider a nominal 6 inch size. An ANSI 6 inch flange has a specific bolt circle. A DN 150 flange, which is the metric equivalent for 6 inch pipe, has a different bolt circle. The bolt holes may even be offset in angle. If you place a DN 150 flange against an ANSI 6 inch flange, the bolts will not pass through the holes.

Face geometry also differs. ANSI raised face flanges have a specific height and diameter for the raised face. This face is designed to contain the gasket. DN flanges may have flat faces or raised faces with different dimensions. A gasket made for one standard will not seal properly on the other.

Pressure Ratings and Temperature Limits

Pressure ratings are not identical between the two systems. ANSI classes are defined at a reference temperature, often 100 degrees Fahrenheit or 38 degrees Celsius. As the temperature increases, the allowable working pressure drops. This is because the material strength decreases with heat.

DN pressure ratings are often listed in bar or MPa at a specific temperature, commonly 20 degrees Celsius. The rating changes as temperature rises. A DN 150 flange rated for a certain pressure at 20 degrees Celsius may have a lower allowable pressure at 300 degrees Celsius.

When selecting a flange for a high-temperature application, check the rating chart for the specific standard. Do not assume a Class 300 flange and a PN 16 flange have the same strength. They usually do not. The material grade also affects the rating. Cast iron, carbon steel, and stainless steel have different allowable stresses.

How to Match Standards in International Projects

International projects often involve a mix of standards. A plant in Europe might use DN flanges for new installations but retain legacy ANSI flanges from older equipment. A shipyard might require ISO flanges for certain sections.

The key is to identify the standard at every connection point. Walk the plant. Look at the existing flanges. Check the nameplates. Review the original drawings. If a new flange must mate with an old one, the standard must match exactly.

If you must connect two different standards, you need a reducer or a transition flange. This is a specialized part that has bolt patterns on both sides. It is not a standard off-the-shelf item. It must be custom fabricated. This adds cost and lead time. Avoid it if possible.

Feature	ANSI Flanges	DN Flanges
Measurement Base	Inches	Millimeters
Nominal Size Example	6 inch	DN 150
Pressure Rating System	Classes (e.g., 300, 600)	Pressure (e.g., PN 16, 30)
Common Standards	ASME B16.5, ASME B16.47	EN 1092-1, EN 1092-2
Bolt Pattern	Defined by class and size	Defined by pressure and size

Practical Sourcing Checklist

When sourcing flanges, use this checklist to avoid errors.

1. Confirm the nominal size and the standard. Is it a 6 inch ANSI flange or a DN 150 flange?
2. Check the pressure rating. Is the required class or PN rating listed?
3. Verify the material grade. Does the grade match the service conditions?
4. Look at the face type. Is it raised face, flat face, or ring-type joint?
5. Confirm the bolt specification. Does the bolt circle match the mating part?
6. Review the gasket requirement. Is the gasket compatible with the face geometry?
7. Check the documentation. Ensure the vendor provides the standard reference in the certificate of conformance.

A worked example makes this clear. Imagine a project requires a flange for a 6 inch pipe carrying steam at 250 degrees Celsius. The engineer selects an ANSI flange. The drawing specifies a 6 inch, Class 600, RT flange. The buyer sends the order to a supplier. The supplier confirms the part is made to ASME B16.5. The part arrives. The bolt circle is 11 inches. The face is ring-type. The gasket is a spiral-wound gasket with a 11 inch outer diameter. Everything matches.

If the buyer had ordered a DN 150 PN 40 flange, the bolt circle would be different. The face would be flat or raised with a different diameter. The gasket would not fit. The joint would leak. The cost of rework would far exceed the cost of the correct flange.

Common Mistakes to Avoid

The most common mistake is assuming size equivalence implies compatibility. A 6 inch pipe fits both an ANSI 6 inch flange and a DN 150 flange. The pipe size is the same. The flange is not. The bolt holes are in different positions. The face dimensions are different.

Another mistake is ignoring the temperature rating. A flange rated for 3000 psi at 100 degrees Fahrenheit may not be rated for that pressure at 600 degrees Fahrenheit. The rating charts show the allowable pressure at various temperatures. Check the chart for the operating temperature.

A third mistake is mixing materials without checking compatibility. A carbon steel flange may rust in contact with a gasket that is not suitable for that environment. A stainless steel flange may be required for corrosive services, but it is more expensive. Choose the material based on the service, not just the price.

Documentation and Traceability

Every flange should come with documentation. The manufacturer certificate of conformance should list the standard, the size, the class or pressure rating, the material grade, and the heat number. This document is your proof that the part meets the specification.

If the documentation is missing or unclear, ask the supplier for it before installation. Do not rely on visual inspection alone. A flange can look correct but be the wrong standard. The bolt holes may be slightly off. The face may be the wrong height. Only the documentation and a caliper can confirm compliance.

In international supply chains, language differences can cause confusion. “DN 50” is clear. “2 inch” can mean ANSI 2 inch or a metric equivalent. Always use the standard name and the specific designation. Write “ASME B16.5, 2 inch, Class 300” or “EN 1092-1, DN 50, PN 16”. This removes ambiguity.

The bottom line is simple. Know your standard. Match the standard on both sides of the joint. Verify the documentation. A flange is a small part, but it is the point where two large pressure vessels are joined. If the flange is wrong, the whole joint fails. Get the standard right from the start.