Australian Standard Flanges: AS2129, AS4087 & AS4331 — Complete Guide

Australia uses not one but three primary flange standards - AS2129, AS4087, and AS4331 - plus ANSI B16.5 in the oil, gas, and mining sectors, and increasingly EN 1092-1 in European-influenced projects. For engineers and procurement teams working on Australian projects from outside the country, or for those trying to source compatible flanges internationally, this multi-standard landscape is genuinely confusing.

This guide explains each Australian standard clearly, shows how they relate to each other, identifies which industries use which standard, and answers the compatibility questions that come up most often - including whether AS2129 and AS4087 flanges can be bolted together, and how Australian standards compare to ANSI and PN-rated European flanges.

Overview: The Australian Flange Standard Landscape

Before diving into each standard individually, it helps to understand the big picture. Australian flange standards evolved from British standards (particularly BS10) and have been progressively updated and metricated over the decades. The result is a landscape where old and new standards coexist, and where the choice of standard depends heavily on the industry sector and the pressure rating required.

 In our experience supplying Australian export projects, the single most important question to ask is: what industry is this for? Water and wastewater projects almost always specify AS4087. General industrial projects tend to use AS2129. And anything in oil, gas, or mining will typically call for ANSI B16.5 - even in Australia.

What Is AS2129?

AS2129 is the Australian Standard for flanges used in general industrial piping systems. It originated from the British Standard BS10 and was progressively metricated and updated by Standards Australia. AS2129 uses a Table system - letters (D, E, F, H) that define progressively higher pressure ratings - rather than the PN or Class systems used by European and American standards.

AS2129 covers slip-on (bossed and flat), weld neck, blind, and threaded flanges in sizes from 15NB to 1200NB (approximately ½" to 48"). It is the standard most commonly encountered in general industrial, commercial, and water/wastewater applications in Australia.

AS2129 Table System: Current Status

AS2129 has been partially superseded over the years. Understanding which tables are still active - and which have been replaced - is essential for correct specification:

The practical implication: if you receive a project drawing that specifies AS2129 Table J, K, R, S, or T, the current equivalent is AS4331 - which aligns with ISO 7005-1 and is dimensionally compatible with ANSI B16.5 Class 150 to 2500. Always verify which edition of AS2129 the project specification references, as older documents may still circulate.

What Is AS4087?

AS4087 is the Australian Standard for metallic flanges specifically designed for waterworks service. It was developed as an update to AS2129 for the water industry, replacing the Table system with the PN (Pressure Nominale) rating system used in European standards. AS4087 covers sizes from 50NB to 1200NB in three pressure classes.

Despite being a newer standard, AS4087 maintains the same bolt hole drilling patterns as the corresponding AS2129 tables - which is why the two standards are largely compatible in the field. The main differences are in flange thickness (slightly increased in AS4087 to meet PN ratings) and raised face dimensions (also slightly larger, though raised face is rarely specified in Australian waterworks - flat face is the default).

AS4087 Pressure Classes

PN16 is by far the most commonly specified AS4087 class, covering the vast majority of municipal water mains, water treatment plants, and irrigation infrastructure. PN35 is used for elevated water towers and pump discharge lines where higher pressure is required.

AS2129 vs AS4087: Key Differences and Compatibility

This is the question we receive most often from engineers specifying Australian projects: are AS2129 and AS4087 flanges interchangeable? The answer is: mostly yes, with some important exceptions that are worth knowing before you place an order.

What Is the Same

• Bolt circle diameter (PCD) and bolt hole count are identical for corresponding tables and PN classes
• Table D = AS4087 PN16: fully compatible in all sizes
• Table F = AS4087 PN21: fully compatible in all sizes
• Table H = AS4087 PN35: fully compatible in all sizes

What Is Different

• Flange thickness: AS4087 flanges are slightly thicker than their AS2129 equivalents to meet PN pressure ratings
• Raised face dimensions: slightly larger in AS4087, but RF is rarely specified in Australian waterworks
• Table E has no direct AS4087 equivalent: Table E drilling matches AS4087 PN16 in most sizes, but not at 100NB, 250NB, and 450NB

💡 The most common mistake we see on Australian projects is assuming that Table E and AS4087 PN16 are fully interchangeable - they are not at 100NB, 250NB, and 450NB. At these three sizes, the bolt hole count or diameter differs. Always double-check these specific sizes if your project mixes AS2129 Table E with AS4087 PN16 flanges.

What Is AS4331?

AS4331 is the Australian Standard that covers higher-pressure flanges, from PN2.5 up to PN420. It is directly equivalent to ISO 7005-1, which is itself the metric equivalent of ASME B16.5 Class 150 through Class 2500. AS4331 flanges are 'deemed to comply' with ANSI B16.5 and are designed to be dimensionally interchangeable - though they are not absolutely identical in every detail.

AS4331 replaced AS2129 Tables J through T and is now the correct standard to reference for high-pressure piping applications in Australia. In practice, however, most Australian oil, gas, and mining projects specify ANSI B16.5 directly - partly because ANSI B16.5 has been dominant in these sectors for over 30 years, and partly because international equipment and valves are almost universally available to ANSI B16.5 dimensions.

How to Measure and Identify a Flange in Australia

If you are working on an existing Australian installation and need to identify an unknown flange - for maintenance, replacement, or expansion - follow these three steps:

Step 1: Measure the Outer Diameter (OD)

Measure the total outside diameter of the flange face in millimetres. This gives you the nominal size range to work with. Allow ±1–2mm for coating thickness, surface rust, or measuring error.

Step 2: Measure the Bolt Circle Diameter (PCD/BCD) and Count Bolt Holes

The bolt circle diameter (also called BCD or PCD) is the diameter of the circle passing through the centre of all bolt holes. Measure from centre to centre of two directly opposite bolt holes. Also count the total number of bolt holes.

Step 3: Match to the Standard

Use the OD, PCD, bolt count, and bolt size together to identify the standard. A single measurement is rarely enough - always cross-reference all four parameters.

Example: A flange with OD = 200mm, PCD = 160mm, 8 bolt holes, and M16 bolts matches AS4087 PN16 / AS2129 Table D at 80NB. The same PCD and bolt count also appear in EN 1092-1 PN16 at DN80 - so confirm the OD as well to distinguish the two standards.

What Does DN Stand For on a Flange?

DN stands for Diameter Nominal - the nominal (reference) diameter of a pipe or fitting in millimetres. It is used in metric piping systems including AS4087, AS4331, EN 1092-1, and ISO standards. DN is not the actual measured internal diameter; it is a reference number that corresponds to a family of pipe sizes.

Australian standards use NB (Nominal Bore) interchangeably with DN - 80NB and DN80 refer to the same size. ANSI standards use NPS (Nominal Pipe Size) in inches. The table below shows the most commonly used size conversions:

Australian Standard Flanges vs ANSI B16.5 and EN 1092-1

One of the most frequent questions from international engineers working on Australian projects - or from Australian engineers sourcing from overseas suppliers - is how Australian standards relate to ANSI and European flanges. The short answer: they are separate systems and are not directly interchangeable, though some have dimensional overlap.

Are ASME and ANSI flanges the same? Yes - ASME B16.5 and ANSI B16.5 refer to the same standard. The designation changed when the American National Standards Institute (ANSI) transferred maintenance of the standard to ASME (American Society of Mechanical Engineers). The technical content is identical; the name difference is administrative only.

Is PN40 the same as PN16? No - PN40 and PN16 are different pressure classes within the same PN rating system. PN40 (40 bar / 580 psi cold working pressure) is rated for significantly higher pressure than PN16 (16 bar / 232 psi). They share the same DN size notation but have different flange dimensions - PN40 flanges are thicker, heavier, and have more bolt holes at larger sizes.

Which Australian Standard Applies to Your Project?

The standard you need depends primarily on the industry sector and the service conditions. Use the guide below as a starting point - always confirm against the specific project specification or engineering code:

💡When in doubt, the safest approach is to ask the project engineer for the piping class specification document. This document will explicitly state the flange standard and pressure class required for each service line. Specifying the wrong standard - even at the correct pressure rating - can result in flanges that cannot be bolted to existing equipment.

References & Sources

• AS2129:2000 (incorporating Amendment 1): Flanges for Pipes, Valves and Fittings. Standards Australia.
• AS4087:2021: Metallic Flanges for Waterworks Purposes. Standards Australia.
• AS4331.1:2005: Metallic Flanges - Steel Flanges (ISO 7005-1:1992, MOD). Standards Australia. 
• Global Supply Line: Flange Dimensions Table Types - AS4087, AS2129, ANSI B16.5. 
• ASME B16.5-2017: Pipe Flanges and Flanged Fittings. American Society of Mechanical Engineers.