The pipe flange is the most common bolted pipe-joint connection in AU industry — used to join two pipe lengths, attach a valve to a line, connect a pump to a manifold, blank off the end of a run, or provide a tie-in point for future expansion. The principle is simple: two mating flanges sandwich a gasket, bolts pull the flanges together to compress the gasket, and the joint is leak-tight.
The complexity is in the standards. Australia uses three overlapping flange standard families — AS 2129 (general purpose, Tables A through T), AS 4087 (waterworks, PN16/21/35), and ANSI B16.5 (oil and gas, Class 150/300/600). They have different bolt circle diameters, different numbers of holes, different bolt sizes, and different pressure ratings. A DN100 Table E flange will NOT bolt up to a DN100 ANSI Class 150 flange. A DN100 AS 4087 PN16 will NOT bolt up to a DN100 AS 2129 Table D. Get the standard wrong and you've ordered the wrong flange — or worse, you've installed a joint that looks fine but isn't compliant with the system design.
This guide covers flange types (slip-on, weld neck, blind, threaded, lap joint, reducing), face types (flat face, raised face, ring joint), the three standard families with comprehensive bolt-pattern reference tables, cross-standard compatibility, blind flange applications, gasket selection, bolting sequence, common installation mistakes and the AIMS supply story.
AIMS stocks 20+ pipe flange product families across AAP house brand and Dixon, covering AS 2129 Table D + Table H, AS 4087 PN16 + PN21 + PN35, and ANSI B16.5 Class 150 + Class 300 + Class 600. Slip-on, weld neck, blind, threaded (BSP and NPT), roll-grooved adaptor, and TTMA buttweld variants — see the AIMS supply section below.
Important disclaimer: The dimension and bolt pattern values in this guide are typical and indicative. For safety-critical specification, engineers must verify all flange dimensions against the current edition of the relevant standard (AS 2129:2000, AS 4087:2011, ASME B16.5, etc.) and the manufacturer's product datasheet. Flange selection for pressure-piping systems is regulated under AS 4041 (Pressure Piping) and AS 1210 (Pressure Vessels) — confirm class rating against system design conditions.
How a pipe flange joint works
A flanged pipe joint has four components: two mating flanges (one welded or threaded to each pipe end), a gasket between them, and a set of bolts pulling the flanges together.
- The pipe ends are flanged — either by welding a flange to the pipe end (slip-on, weld neck), threading the pipe into a screwed flange, or rolling a groove and clamping a flange adaptor onto the pipe (Victaulic-style).
- A gasket sits between the two flange faces — sized to match the flange's gasket seating surface. The gasket material is selected for the fluid + temperature + pressure (see the Spiral Wound Gasket Guide for material selection and the O-Ring Guide for elastomer chemical compatibility).
- Bolts pass through aligned holes in both flanges. Bolt size, number of bolts, and hole diameter are all standard-specific — every flange standard has its own bolt pattern.
- The bolts are tightened in a controlled sequence (cross-pattern, multi-stage torque) to compress the gasket uniformly. Even compression = bubble-tight joint.
The key engineering insight: the joint's pressure rating is governed by the weakest of (a) the flange itself, (b) the gasket's seating compression, (c) the bolt's tensile capacity. All three must be matched to the system's design pressure + temperature.
The AU standards reality — AS 2129, AS 4087, ANSI B16.5
Australia uses three flange standard families, each derived from different historical origins and each serving different industries. The wrong-standard-for-the-application is the single most common flange ordering mistake in AU industry.
| Standard | Origin | Industries | Pressure ratings | Sizing convention |
|---|---|---|---|---|
| AS 2129:2000 | British BS 10 (1962 base), Australian adoption | General industrial pipework, light water systems, mining, building services, agriculture | Tables A, B, C, D, E, F, H, J, K, R, S, T (low to high pressure) | Imperial nominal pipe size with metric DN equivalents |
| AS 4087:2011 | Australian water industry-specific (replaces older Australian water flange specs) | Waterworks, water utility mains, sewerage, treatment plants, hydrants | PN16, PN21, PN35 (pressure nominal in bar) | Metric DN |
| ASME B16.5 | American, ANSI/ASME global standard | Oil and gas, petrochemical, process plants, refineries, LNG, mining process | Class 150, 300, 600, 900, 1500, 2500 | NPS (Nominal Pipe Size, imperial) |
| EN 1092-1 / DIN 2501 | European | Imported European equipment, some HVAC | PN6, PN10, PN16, PN25, PN40 etc. | Metric DN |
| JIS B 2220 | Japanese | Imported Japanese equipment, marine, some industrial | 5K, 10K, 16K, 20K, 30K (kg/cm²) | Metric A-series |
Which standard for which application — AU industry practice
- General workshop, building services, light industrial: AS 2129 Table D (low pressure) or Table E (general)
- Water utility, council water mains, sewerage, fire ring main: AS 4087 PN16 or PN21
- Higher pressure water industry, pump stations: AS 4087 PN35 or AS 2129 Table H
- Oil and gas pipelines, refinery, process plants: ASME B16.5 Class 150 / 300 / 600 (per pressure class design)
- Imported European machinery (pumps, valves, vessels): EN 1092-1 (PN16, PN25, PN40 common)
- Mining process water, slurry, dewatering: AS 2129 Table E or Table H (per pressure design)
- Compressed air ring main, instrument air: AS 2129 Table D or ANSI Class 150
When connecting two existing flanges of unknown origin, always identify both flanges' standards before ordering replacement gasket or hardware. Bolt-up of mismatched standards (visually similar but bolt-pattern different) is the most common cause of "the new flange doesn't fit" calls.
Flange types — slip-on, weld neck, blind, threaded, lap joint, reducing
Seven major flange types are commonly specified in AU industry. Each is suited to specific service conditions, installation requirements and pressure class.
| Flange type | How it attaches to pipe | Strength (relative) | Best for | Avoid |
|---|---|---|---|---|
| Slip-on (plate) | Pipe slides through flange; flange welded with two fillet welds (front + back) | ~70% of weld neck | General-purpose pipework, low to medium pressure, easy alignment during install | High-pressure service, fatigue-cycling service, severe thermal cycling |
| Weld neck (WN) | Tapered neck butt-welds to pipe end; smooth bore transition | 100% (the reference) | High-pressure service, fatigue service, thermal cycling, all critical pipework | Cost-sensitive low-pressure work where slip-on is acceptable |
| Blind | Solid plate — no pipe bore. Used to terminate a line or blank off a connection | N/A (solid plate) | Line termination, future tie-in points, equipment isolation, pressure test endpoints, maintenance access blanks | Through-flow service (blind = no flow) |
| Threaded (screwed) | BSP or NPT thread cut into flange bore; pipe threads in | Limited by thread strength | Galvanised pipe, low-pressure non-hazardous service, small bore (≤2"), where welding is impractical | High pressure, vibration, thermal cycling (threads loosen) |
| Socket weld (SW) | Pipe slides into recessed bore; single fillet weld | ~80% of weld neck | Small bore (≤2"), instrument lines, sample taps, branch connections in process service | Large bore, corrosive service (crevice corrosion in socket) |
| Lap joint | Flange slides freely over stub end; stub end butt-welds to pipe | ~80% of weld neck (stub end controls) | Stainless steel or expensive alloy lines (lap flange in cheap carbon steel), frequent disassembly, hole alignment headaches | High-pressure service requiring full strength |
| Reducing | Two different bore sizes in one flange (e.g. 4" flange with 2" bore) | Per smaller bore rating | Pipe size transitions in tight spaces, eliminating an extra reducer fitting | Where flow stream uniformity matters |
For ordering, the most common AU industrial flange families are slip-on (general purpose) and blind (line termination + isolation). Weld neck is specified for higher-pressure service and any installation where the joint sees fatigue cycling or thermal stress. Threaded flanges remain common in galvanised pipe water service.
Blind flanges — what they're for, when to use them
A blind flange is a solid disc with the same outside diameter, bolt circle, hole pattern, thickness and rating as the equivalent slip-on or weld neck flange — but with no pipe bore. The blind flange bolts onto a mating flange to terminate the line. Inside the blind flange, flow stops. Outside, the bolted joint behaves identically to any other flanged connection.
Where blind flanges are specified
- End-of-line termination — the simplest case. A header, manifold or distribution line that ends at a point. The blind flange caps the end. Easy to remove later if the line is extended.
- Future tie-in points — pipework designed with future expansion in mind. A spool piece + blind flange is welded in at design stage. When the future connection is added, the blind flange is removed and the new branch piped in. No hot work near live equipment.
- Equipment isolation blanks — replacing a valve or piece of equipment temporarily. The pipework is opened, the equipment removed, and blind flanges installed on both upstream and downstream sides to seal the system while the equipment is out of service.
- Pressure test endpoints — a pipework system requires hydrostatic test before commissioning. Blind flanges seal the line ends during the pressure test, with one blind drilled for a fill/test connection.
- Maintenance access blanks — a flanged opening in a tank, vessel or large pipe that's normally bolted closed with a blind flange. Removing the blind opens an inspection or cleanout access.
- Spectacle blinds — a specialty form. A figure-eight shaped plate sits between two mating flanges; one half is a blind, one half is a hole (open). Rotating the spectacle blind 180° switches between "line open" and "line blanked" without removing the assembly. Used at storage tank inlets, transfer station blocking, and any point where positive isolation is required for confined-space entry under WHS LOTO procedures.
Critical sizing rule
A blind flange MUST match the standard, rating and bolt pattern of the mating flange exactly. A DN100 Table E blind flange bolts to a DN100 Table E mating flange. It will NOT bolt to a DN100 Table D mating flange (different bolt circle), and it will NOT bolt to a DN100 ANSI Class 150 mating flange (different bolt count + circle + hole size). Always identify the mating flange's exact standard and rating before ordering a blind.
AIMS blind flange range
AIMS stocks four blind flange product families across AU and US standards:
- AAP Blind Steel Plate Flange — AS 2129 Table D, 2" to 5" sizes, 113 units in stock
- AAP Blind Steel Plate Flange Table-H — AS 2129 Table H high pressure, 2" to 6", 84 units
- AAP Blind Flat Face Flange PN16 — AS 4087 PN16 water industry, 3" to 10", 160 units (largest blind range in stock)
- AAP Blind Flanges 316/316L SS C150 — ANSI Class 150 stainless steel, 1" to 4", 96 units (chemical + marine + food service)
For blind flange sizes or ratings not in stock — AS 2129 Table E blinds, AS 4087 PN21 or PN35 blinds, ANSI Class 300 or 600 blinds, EN 1092 PN25 blinds, spectacle blinds — contact our team for sourcing through our supplier network.
Slip-on vs weld neck — strength + cost trade-off
Slip-on and weld neck are the two dominant non-blind flange types. Both end up with a flange face on the end of the pipe — the difference is how they get there.
Slip-on plate flange
The pipe slides through the bore of the flange to a stop point, then two fillet welds are made — one outside the flange (between the flange OD and the pipe OD), one inside (between the pipe end and the flange face). The flange is mechanically held by both welds.
Advantages: Cheaper than weld neck (lower flange cost, simpler manufacture). Easier alignment during installation — the pipe can be positioned and rotated before welding. Lower skill requirement for the welder (two fillet welds vs one critical butt weld).
Disadvantages: Strength is approximately 70% of equivalent weld neck. The fillet weld at the flange face creates a stress concentration. Not approved for sustained high-pressure service in most pressure-piping codes. Bore transition is abrupt (90° step), which can cause flow disruption + erosion in some services.
AIMS stocks the AAP Slip-On Weld Flange ANSI B16.5 Class 150 (290 units, 19 size variants — the deepest single flange product in stock), the AAP Slip-On Plate Flange (Table D), the AAP Slip-On Forged Flange Table-H for higher pressure, and the AAP 4" Slip-on Forged Steel Plate Flange BS10 Table-D.
Weld neck flange
The pipe end butt-welds to a tapered neck that's integral to the flange. The neck wall thickness matches the pipe wall thickness (specify XS for extra-strong, STD for standard schedule, etc.). The bore transitions smoothly from pipe ID to flange bore — no step.
Advantages: Full 100% strength relative to the pipe (the joint is as strong as a continuous pipe). Smooth bore transition (low flow disruption). Excellent fatigue + thermal cycling performance. The default choice for high-pressure pipework, refinery + petrochemical service, and any pressure-piping system designed to AS 4041 + ASME B31.3.
Disadvantages: Higher flange cost (more material, more complex forging). Requires a skilled welder for the critical butt weld + correct pre-heat + post-weld heat treatment per WPS. The pipe must be cut accurately to length before welding (no length adjustment after).
AIMS stocks the AAP Weld Neck Flange ANSI B16.5 Class 150 (XS schedule) (88 units, 2" to 6") and the AAP Weld Neck Flange ANSI B16.5 Class 600 (96 units, 1" to 4") for high-pressure service.
When to specify each
- Slip-on: Low to medium pressure (Class 150 / Table D / PN16). Workshop service. Building services. Irrigation. Compressed air. Cost-sensitive installations where weld neck is not strictly required.
- Weld neck: High pressure (Class 300+ / Table H / PN35+). Process plant service. Refinery, petrochemical. Steam (any pressure). Sustained vibration or thermal cycling. Any pressure-piping system where AS 4041 or ASME B31.3 design rules apply.
Threaded (screwed) flanges — BSP vs NPT
A threaded flange has a tapered or parallel pipe thread cut into its bore. The pipe is threaded with a matching male thread and screwed in. Used where welding is impractical (galvanised pipe — welding burns the zinc coating), where the pipe is field-cut to length without weld equipment, and where the pressure rating is low to moderate.
BSP vs NPT — the AU + US thread reality
| Thread standard | Origin | Industries | Sealing method |
|---|---|---|---|
| BSP (G or R) — British Standard Pipe | British / Australian default | AU plumbing, water, gas (limited), general industrial, hydraulics (some) | BSPT (R) tapered — seals on thread interference. BSPP (G) parallel — seals on washer/O-ring at face. |
| NPT — National Pipe Tapered | American | US oil and gas, imported US equipment, process plants spec'd to ASME | Tapered thread — seals on thread interference with thread sealant or PTFE tape |
BSP and NPT are NOT interchangeable. Pitch is different (BSP at 11, 14, 19 TPI; NPT at 11.5, 14, 18, 27 TPI) and thread angle is different (BSP 55°; NPT 60°). A BSP male in an NPT female may thread several turns but will not seal and will likely leak under pressure or fail under thermal cycling.
AIMS threaded flange range
AIMS stocks BOTH BSP and NPT threaded flange options to cover AU + imported US equipment service:
- AAP Screwed Flange ANSI B16.5 Class 150 — BSP — 1" to 4" (5 variants), 72 units. The AU industrial workhorse.
- AAP Screwed Flange ANSI B16.5 Class 150 — NPT — 1/2" to 2" (9 variants), 120 units. For imported US equipment + process service.
- AAP Screwed Flange ANSI B16.5 Class 300 — NPT — 1/2" to 2" (8 variants), 121 units. Higher-pressure US-thread service.
- AAP Screwed Forged Flange Table-H — 3/4" to 2" (6 variants), 68 units. AS 2129 Table H high-pressure BSP service.
- Dixon Screwed Flange Round Drilled Table D BSP — Galvanised Malleable Iron — 1/2" to 2" (10 variants), 96 units. The galvanised plumbing + water service standard.
For pressure-piping context on thread standards (BSPP vs BSPT, NPT, sealing methods, AS 1722), see the Hydraulic Fittings Guide which covers all pipe thread standards used across AU industry.
Face types — flat face, raised face, RTJ, tongue & groove
The flange face is the surface that contacts the gasket. Five face types are commonly specified:
| Face type | Description | Best for | Avoid |
|---|---|---|---|
| Flat Face (FF) | The entire flange face is flat. Full-face gasket extends to the bolt holes. | Cast iron flanges (mandatory — see warning below), low-pressure service, AU water industry default (AS 4087) | High-pressure service where flange face stress would crush the gasket |
| Raised Face (RF) | A small raised ring around the bore (typically 2mm high). Gasket sits on the raised ring only, not extending to the bolt holes. | The default for ANSI B16.5 Class 150 and above, most process service, where higher gasket compression is required | Cast iron flange mate (cracks under bolt torque — see warning) |
| Ring-Type Joint (RTJ) | A machined groove around the bore accepts a soft metal ring gasket (octagonal or oval cross-section). | High-pressure oil + gas (Class 600+), refinery service, high-temperature steam | Low-pressure service (RTJ is expensive overkill) |
| Tongue & Groove (T&G) | One flange has a raised tongue, the mate has a corresponding groove. Self-aligning. | Severe service where gasket blow-out must be prevented, high-pressure or high-temperature applications | General-purpose service (cost not justified) |
| Male & Female (M&F) | One flange has a male shoulder, the mate has a recessed female. Aligns gasket precisely. | Where gasket positioning under high pressure must be guaranteed | Where flange disassembly + reassembly orientation matters (asymmetric) |
CRITICAL WARNING — Never bolt a raised face (RF) flange to a flat face (FF) cast iron flange.
The raised face concentrates bolt-up force on a small area of the cast iron flange. Cast iron is brittle in tension and cracks at the raised-face contact zone, sometimes during initial bolt-up and often weeks or months later under thermal cycling. Cast iron flanges (gate valves, pumps, fittings with cast iron bodies) must always mate to a flat face flange — never raised face. If the mating equipment has a cast iron flat face flange, specify a flat face (FF) mate, not raised face.
For gasket selection covering all face types (flat face fibre, spiral wound for raised face, soft metal rings for RTJ), see the Spiral Wound Gasket Guide covering material selection by service and AS 4087 + ASME B16.20 standards.
AS 2129 — Tables A through T explained
AS 2129:2000 (Flanges for pipes, valves and fittings) is the Australian general-purpose flange standard derived from the British BS 10 standard. AS 2129 organises flanges into pressure-rating "Tables" lettered A through T, each rated to a specific working pressure at standard temperature.
| Table | Working pressure (typical) | Industry use |
|---|---|---|
| Table A | ~310 kPa | Very low pressure — almost obsolete in modern installations |
| Table B | ~620 kPa | Low pressure water + air |
| Table C | ~930 kPa | Light-duty water |
| Table D | ~1,400 kPa (14 bar) | Standard low-medium pressure — most common AU general-purpose flange. Building services, irrigation, general process. |
| Table E | ~1,800 kPa (18 bar) | The AU water industry default — most pump connections, valve flanges, hydrant inlets. Slightly higher pressure than Table D, slightly larger PCD on some sizes. |
| Table F | ~2,800 kPa (28 bar) | Higher pressure water and general industrial |
| Table H | ~5,500 kPa (55 bar) | High pressure — AIMS stocks Table H slip-on, blind, screwed and forged options. Mining process service, hydraulics, high-pressure pumps. |
| Table J | ~7,000 kPa (70 bar) | High pressure |
| Table K | ~10,300 kPa (103 bar) | Very high pressure |
| Table R, S, T | 14,000 kPa+ (140 bar+) | Very high pressure — specialist service |
The most commonly specified AS 2129 Tables in AU general industry are Tables D, E and H. Tables D and E differ slightly in bolt circle diameter on some sizes — the PCD reference table below shows the exact values.
AS 2129 PCD + bolt pattern reference — Tables D, E and H
The bolt circle diameter (PCD — Pitch Circle Diameter), number of bolt holes, hole diameter and bolt size for the three most commonly stocked AS 2129 Tables. These are typical published values — engineers must verify against AS 2129:2000 current edition for safety-critical specification.
AS 2129 Table D — general purpose (low-medium pressure)
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 1/2" | DN15 | 95 | 67 | 4 | 14 | M12 |
| 3/4" | DN20 | 100 | 73 | 4 | 14 | M12 |
| 1" | DN25 | 115 | 83 | 4 | 14 | M12 |
| 1-1/4" | DN32 | 120 | 87 | 4 | 14 | M12 |
| 1-1/2" | DN40 | 135 | 98 | 4 | 18 | M16 |
| 2" | DN50 | 150 | 114 | 4 | 18 | M16 |
| 2-1/2" | DN65 | 165 | 127 | 4 | 18 | M16 |
| 3" | DN80 | 185 | 146 | 4 | 18 | M16 |
| 4" | DN100 | 215 | 178 | 4 | 18 | M16 |
| 5" | DN125 | 255 | 210 | 8 | 18 | M16 |
| 6" | DN150 | 280 | 235 | 8 | 22 | M20 |
| 8" | DN200 | 335 | 292 | 8 | 22 | M20 |
| 10" | DN250 | 405 | 356 | 12 | 22 | M20 |
| 12" | DN300 | 455 | 406 | 12 | 26 | M24 |
AS 2129 Table E — AU water industry default
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 1/2" | DN15 | 95 | 67 | 4 | 14 | M12 |
| 3/4" | DN20 | 100 | 73 | 4 | 14 | M12 |
| 1" | DN25 | 115 | 83 | 4 | 14 | M12 |
| 1-1/4" | DN32 | 125 | 95 | 4 | 14 | M12 |
| 1-1/2" | DN40 | 135 | 105 | 4 | 18 | M16 |
| 2" | DN50 | 165 | 127 | 4 | 18 | M16 |
| 2-1/2" | DN65 | 185 | 140 | 4 | 18 | M16 |
| 3" | DN80 | 205 | 165 | 4 | 18 | M16 |
| 4" | DN100 | 230 | 191 | 8 | 18 | M16 |
| 5" | DN125 | 270 | 235 | 8 | 18 | M16 |
| 6" | DN150 | 305 | 260 | 8 | 22 | M20 |
| 8" | DN200 | 370 | 324 | 8 | 22 | M20 |
| 10" | DN250 | 430 | 387 | 12 | 26 | M24 |
| 12" | DN300 | 490 | 438 | 12 | 26 | M24 |
AS 2129 Table H — high pressure
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 1/2" | DN15 | 105 | 75 | 4 | 18 | M16 |
| 3/4" | DN20 | 115 | 83 | 4 | 18 | M16 |
| 1" | DN25 | 120 | 89 | 4 | 18 | M16 |
| 1-1/4" | DN32 | 135 | 98 | 4 | 18 | M16 |
| 1-1/2" | DN40 | 150 | 114 | 4 | 18 | M16 |
| 2" | DN50 | 165 | 127 | 4 | 22 | M20 |
| 2-1/2" | DN65 | 185 | 146 | 4 | 22 | M20 |
| 3" | DN80 | 205 | 165 | 8 | 22 | M20 |
| 4" | DN100 | 230 | 191 | 8 | 22 | M20 |
| 6" | DN150 | 305 | 260 | 8 | 26 | M24 |
| 8" | DN200 | 370 | 324 | 8 | 26 | M24 |
Key observation: Tables D and E often share OD and PCD on small sizes (DN15 to DN25) but diverge on larger sizes. Table E flanges at DN50 onwards are physically larger with bigger PCD than Table D — they are NOT interchangeable from DN50 up.
AS 4087 — PN16, PN21, PN35 (AU waterworks)
AS 4087:2011 (Metallic flanges for waterworks purposes) is the Australian waterworks-specific flange standard, separate from AS 2129. AS 4087 uses PN (Pressure Nominal in bar) ratings, with three classes common in AU water utility service:
- PN16 — 16 bar working pressure. Standard water distribution mains, pump suction, low-pressure water service.
- PN21 — 21 bar working pressure. Higher-pressure water distribution, pump discharge, hydrants. Roughly equivalent in pressure to ANSI Class 150 but with a different bolt pattern.
- PN35 — 35 bar working pressure. High-pressure water service, mining process water, fire ring mains under elevated pressure. Roughly equivalent in pressure to ANSI Class 300 but with a different bolt pattern.
AS 4087 PN16 bolt pattern reference
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 2" | DN50 | 165 | 125 | 4 | 18 | M16 |
| 3" | DN80 | 185 | 145 | 4 | 18 | M16 |
| 4" | DN100 | 220 | 180 | 8 | 18 | M16 |
| 6" | DN150 | 285 | 240 | 8 | 22 | M20 |
| 8" | DN200 | 340 | 295 | 8 | 22 | M20 |
| 10" | DN250 | 395 | 350 | 12 | 22 | M20 |
| 12" | DN300 | 445 | 400 | 12 | 22 | M20 |
AS 4087 PN21 bolt pattern reference
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 2" | DN50 | 165 | 125 | 4 | 18 | M16 |
| 3" | DN80 | 185 | 145 | 4 | 18 | M16 |
| 4" | DN100 | 220 | 180 | 8 | 18 | M16 |
| 6" | DN150 | 285 | 240 | 8 | 22 | M20 |
| 8" | DN200 | 340 | 295 | 8 | 22 | M20 |
| 10" | DN250 | 405 | 355 | 12 | 26 | M24 |
| 12" | DN300 | 455 | 410 | 12 | 26 | M24 |
AS 4087 PN35 bolt pattern reference
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 2" | DN50 | 175 | 135 | 4 | 22 | M20 |
| 3" | DN80 | 205 | 160 | 8 | 22 | M20 |
| 4" | DN100 | 235 | 190 | 8 | 22 | M20 |
| 6" | DN150 | 305 | 250 | 8 | 26 | M24 |
| 8" | DN200 | 375 | 320 | 12 | 26 | M24 |
| 10" | DN250 | 440 | 385 | 12 | 30 | M27 |
| 12" | DN300 | 490 | 438 | 12 | 30 | M27 |
AIMS stocks AS 4087 PN16 in the AAP Blind Flat Face Flange PN16 range (3" to 10", 160 units), the AAP Plate Flat Faced Flange PN21 (4" to 12", 192 units) and the AAP Plate Flat Faced Flange PN35 (4" to 12", 112 units).
ANSI B16.5 — Class 150, 300, 600 (US standard)
ASME B16.5 is the American flange standard used globally in oil + gas, refinery, petrochemical, and process plant service. Ratings are given as "Class" numbers approximating the maximum allowable working pressure in psi at moderate temperature, but the actual pressure-temperature rating is published in ASME B16.5 pressure-temperature tables per material grade.
| Class | Approximate pressure rating | Industry use |
|---|---|---|
| Class 150 | ~20 bar / 285 psi at room temp | General process, low-pressure oil and gas, building services, light industrial. The most commonly specified ANSI class in AU general industry. |
| Class 300 | ~52 bar / 740 psi at room temp | Higher-pressure process service, pump discharge, vessel inlets, refinery service |
| Class 600 | ~104 bar / 1,480 psi at room temp | High-pressure pipeline service, oil and gas transmission, fire-safe API 6D pipeline minimum |
| Class 900 | ~155 bar / 2,220 psi at room temp | Very high pressure pipeline service |
| Class 1500 | ~260 bar / 3,700 psi at room temp | Severe service pipeline, wellhead |
| Class 2500 | ~430 bar / 6,170 psi at room temp | Extreme high pressure |
ANSI B16.5 Class 150 bolt pattern reference
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 1/2" | DN15 | 90 | 60.5 | 4 | 16 | 1/2" |
| 3/4" | DN20 | 100 | 70 | 4 | 16 | 1/2" |
| 1" | DN25 | 110 | 79 | 4 | 16 | 1/2" |
| 1-1/4" | DN32 | 115 | 89 | 4 | 16 | 1/2" |
| 1-1/2" | DN40 | 125 | 98 | 4 | 16 | 1/2" |
| 2" | DN50 | 150 | 121 | 4 | 19 | 5/8" |
| 2-1/2" | DN65 | 180 | 140 | 4 | 19 | 5/8" |
| 3" | DN80 | 190 | 152 | 4 | 19 | 5/8" |
| 4" | DN100 | 230 | 191 | 8 | 19 | 5/8" |
| 6" | DN150 | 280 | 241 | 8 | 22 | 3/4" |
| 8" | DN200 | 345 | 298 | 8 | 22 | 3/4" |
| 10" | DN250 | 405 | 362 | 12 | 25 | 7/8" |
| 12" | DN300 | 485 | 432 | 12 | 25 | 7/8" |
ANSI B16.5 Class 300 bolt pattern reference
| Size | DN | OD (mm) | PCD (mm) | No. of holes | Hole dia (mm) | Bolt size |
|---|---|---|---|---|---|---|
| 1/2" | DN15 | 95 | 67 | 4 | 16 | 1/2" |
| 3/4" | DN20 | 115 | 83 | 4 | 19 | 5/8" |
| 1" | DN25 | 125 | 89 | 4 | 19 | 5/8" |
| 1-1/4" | DN32 | 135 | 98 | 4 | 19 | 5/8" |
| 1-1/2" | DN40 | 155 | 114 | 4 | 22 | 3/4" |
| 2" | DN50 | 165 | 127 | 8 | 19 | 5/8" |
| 2-1/2" | DN65 | 190 | 149 | 8 | 22 | 3/4" |
| 3" | DN80 | 210 | 168 | 8 | 22 | 3/4" |
| 4" | DN100 | 255 | 200 | 8 | 22 | 3/4" |
| 6" | DN150 | 320 | 270 | 12 | 22 | 3/4" |
| 8" | DN200 | 380 | 330 | 12 | 25 | 7/8" |
| 10" | DN250 | 445 | 387 | 16 | 29 | 1" |
| 12" | DN300 | 520 | 451 | 16 | 32 | 1-1/8" |
Cross-standard compatibility — which flanges can mate
The single most common flange-related sourcing question in AU industry is: "I have flange standard X — will flange standard Y mate with it?" The answer is almost always NO unless the standards are identical, but there are a few specific overlaps worth knowing.
| If your existing flange is... | Compatible with | NOT compatible with |
|---|---|---|
| AS 2129 Table D | AS 2129 Table D (same size) | Table E (PCD differs from DN50 up), Table H, AS 4087 PN16/21/35, ANSI Class 150/300/600 |
| AS 2129 Table E | AS 2129 Table E (same size) | Table D (PCD differs from DN50 up), Table H, ANSI Class 150 (PCD + bolt count differ), AS 4087 |
| AS 2129 Table H | AS 2129 Table H (same size) | All other AS Tables, all ANSI Classes, all AS 4087 PN ratings |
| AS 4087 PN16 | AS 4087 PN16 (same size). Same bolt pattern as PN21 in many sizes — but PRESSURE RATING DIFFERS. | AS 2129 (all Tables), ANSI Class 150/300, PN35 |
| AS 4087 PN21 | AS 4087 PN21 (same size). Same bolt pattern as PN16 in many sizes — but rated for higher pressure. | AS 2129, ANSI Class 150 (looks similar — PCD often differs by a few mm), PN35 |
| AS 4087 PN35 | AS 4087 PN35 (same size) | All other standards |
| ANSI B16.5 Class 150 | ANSI Class 150 (same size, same face type RF/FF) | AS 2129 Tables, AS 4087 (PCD + holes differ), Class 300/600 |
| ANSI B16.5 Class 300 | ANSI Class 300 (same size, same face type) | Class 150/600, AS 2129, AS 4087 |
| ANSI B16.5 Class 600 | ANSI Class 600 (same size, same face type) | Class 150/300/900, AS 2129, AS 4087 |
The most common AU mistake
Specifying AS 2129 Table E for a connection to existing ANSI Class 150 equipment because they have similar pressure ratings. The pressure ratings are similar (~18 bar vs ~20 bar at room temp) but the bolt patterns do not align. Table E and Class 150 have different PCD on most sizes, different numbers of bolts on some sizes, and different bolt diameters on most sizes. A Table E flange will NOT bolt onto a Class 150 flange.
The exception — small-bore ANSI Class 600 vs Class 900
For sizes 2" and below, ANSI Class 600 and Class 900 share bolt dimensions in some sizes (the standard reuses the 1500 lb hardware on smaller higher-class flanges). This is the only common interchangeability across pressure classes. Verify against ASME B16.5 table 2-1.2 before assuming compatibility.
Lap joint flanges — the workaround
If you have a flange in one standard and need to connect to a different standard's pipe end, a lap joint flange + stub end combination is sometimes the cleanest solution. The lap flange rotates freely on the stub end, eliminating bolt-hole alignment headaches. The stub end is welded to the pipe in whatever orientation works. The trade-off is reduced strength (lap joint is ~80% of weld neck) and higher cost (two-piece assembly).
Materials — carbon steel, 316 SS, galvanised, aluminium
Flange material is selected to match the pipe material, the fluid being transported, and the temperature + corrosion environment. Common AU options:
| Material | Best for | Avoid |
|---|---|---|
| Carbon steel (A105 forged / A216 WCB cast) | Oil and gas pipework, refinery, general industrial, structural pipework, building services. The default for most ANSI flanges. | Marine, severe chloride, food/dairy/beverage service, low-temperature service (impact rating drops) |
| Stainless steel 316/316L | Chemical service, marine, food/dairy/beverage, pharmaceutical, hot water, chloride exposure, low-temperature service. The default for any non-mild environment. | Where strength of carbon steel is needed at low cost |
| Stainless steel 304/304L | Mild chemical, food/dairy where chloride exposure is low, cosmetic-grade pipework | Marine, chloride exposure, swimming pool plant (pits) |
| Galvanised malleable iron | Cold potable water, low-pressure plumbing, irrigation, light industrial water — the AU plumbing default | Hot water above 60°C (zinc layer degrades), chemical service, high pressure |
| Aluminium (TTMA buttweld) | Tank truck flange connections (TTMA standard), road tanker mounting, mobile equipment where weight matters | High pressure, structural pipework, fixed installations |
| Ductile iron | Water industry, waterworks (AS 4087), buried pipework, fire ring mains | High-pressure oil and gas, chemical service |
| Forged alloy steel | High-temperature service (steam, refinery), severe service piping | Where carbon steel is acceptable (cost penalty) |
AIMS stocks carbon steel flanges across all AS and ANSI ranges, 316/316L stainless blind flanges (AAP Blind Flanges 316/316L SS Class 150, 1" to 4", 96 units), Dixon malleable iron galvanised threaded flanges (Table D BSP, 96 units), and Dixon aluminium TTMA flanges for tank truck applications.
For stainless steel fastener selection covering grades 304/A2-70, 316/A4-70, A4-80, and the galling prevention rules for stainless flange bolting, see the Stainless Steel Fasteners Guide.
Gasket selection — flat fibre, spiral wound, ring joint
The gasket is the sealing element between two flanges. The gasket material and style is selected to match the flange face type (FF, RF, RTJ), the pressure class, the temperature, and the fluid being sealed. The flange and gasket are a matched pair — neither works without the right partner.
| Gasket type | Face type | Pressure range | Temperature range | Best for |
|---|---|---|---|---|
| Compressed fibre (insertion) | Flat face | Low to medium (PN16 / Class 150 typical) | -40 to 200°C | Water, oil, general industrial. Cost-effective workshop default. |
| Rubber sheet (NBR, EPDM) | Flat face | Low (PN16 / Class 150) | NBR -40 to 100°C; EPDM -40 to 150°C | Water (EPDM), oil/fuel (NBR), low-pressure ductile iron flange joints |
| PTFE / Teflon | Flat face or raised face | Low to medium | -200 to 260°C | Chemical service, food/dairy, anywhere chemical inertness is required |
| Spiral wound (CG/CGI) | Raised face | Medium to high (Class 150 through Class 2500) | -200 to 800°C+ | Industrial process, refinery, petrochemical, steam, high-pressure oil and gas. The workshop-standard for most ANSI flange service. |
| Kammprofile / corrugated metal | Raised face | Medium to high | To 600°C+ | High temperature where spiral wound alternatives are sought |
| Ring-type joint (RTJ) | RTJ groove only | High (Class 600+) | To 800°C+ | API 6A wellhead, high-pressure oil and gas, refinery severe service |
For comprehensive gasket selection including ASME B16.20 colour codes, AS 4087 standards, material selection by service and the practical installation procedure, see our dedicated Spiral Wound Gasket Guide.
AIMS stocks AAP insertion gaskets (compressed fibre + natural rubber, Table E) in the Gaskets collection, plus AAP spiral wound gaskets for raised-face service.
Bolting + tightening sequence — the cross-pattern rule
Even bolt torque around the flange is what makes a leak-tight joint. Sequential tightening (going around the bolt circle in order) distorts the flange and crushes the gasket unevenly — guaranteed leak. Cross-pattern tightening in multiple stages is the only correct method.
The 30/60/100% cross-pattern rule
- Snug all bolts finger-tight to position the flange and gasket.
- Stage 1 — 30% of final torque applied in cross-pattern (opposite-bolt sequence: 1, 5, 3, 7, 2, 6, 4, 8 for an 8-bolt flange).
- Stage 2 — 60% of final torque in the same cross-pattern.
- Stage 3 — 100% of final torque in the same cross-pattern.
- Stage 4 — Verify by passing around the flange once more in sequence to verify all bolts hold final torque.
Bolt grade selection
The bolt grade must match the flange's pressure-temperature rating. Common pairings:
- AS 2129 Tables D/E + AS 4087 PN16/21: Grade 4.6 mild steel or Grade 5.8 zinc-plated (low-pressure water + general)
- AS 2129 Table H, AS 4087 PN35: Grade 8.8 high-tensile HDG
- ANSI Class 150: A307 / Grade 5.8 / Grade 8.8 (specification-dependent)
- ANSI Class 300 and above: A193 B7 (carbon-moly), A193 B16 (high-temp), or A320 L7 (low-temp impact rated)
- Marine, food, chemical: 316 stainless A4-70 or A4-80
For bolt grade markings, Australian + ASTM specification cross-reference, see the Bolt Grade Chart. For metric bolt torque values across all common grades and sizes, see the Metric Bolt Torque Chart.
Bolt holes straddle the centerline
The conventional flange orientation is "two-flat" — the horizontal and vertical centerlines pass midway between bolts, not through bolts. This applies to standard flange installations and is referenced in ASME B16.5 and AS 2129. Drilling holes on the centerline instead of straddling is non-standard and creates installation headaches with the pipe fitter.
Common installation mistakes
| Mistake | Consequence |
|---|---|
| Mixing AS 2129 Table D with AS 4087 PN16 (same DN, different PCD) | Bolt holes don't align — re-drill or replace flange. Production delay + scrapped flange. |
| Mixing Table E with ANSI Class 150 in water service | Pressure rated OK but bolt pattern wrong. Flange won't bolt up. |
| Bolt holes drilled on centerline instead of straddling | Non-standard install — pipe fitter rework. Equipment alignment headaches. |
| RF flange on flat-face cast iron mate | Cast iron flange cracks under bolt torque. Complete equipment replacement. |
| Wrong gasket OD for raised face | Gasket overhang into bolt circle, leak under pressure. |
| Slip-on flange used where weld neck is specified | Reduced fatigue life, premature joint failure under cycling load. |
| Through-bolt loose fit (oversized hole) | Joint vibration loosens bolts over time. Leak develops in service. |
| Cross-tightening skipped, sequential tightening used | Flange distortion, uneven gasket compression, leak. |
| Galvanised bolt + chloride exposure (marine, swimming pool plant) | Galvanic corrosion of bolt, joint fails over months. Specify 316 SS. |
| BSP flange + NPT pipe (or vice versa) | Thread interference but no seal. Leaks immediately or under cycling. |
AIMS pipe flange supply — AAP + Dixon
AIMS stocks over 20 distinct pipe flange product families across AS 2129, AS 4087 and ANSI B16.5 — one of the deepest flange ranges in AU industrial supply. AAP dominates by SKU count and volume; Dixon supplies specialty configurations (Table D screwed galvanised, roll-grooved adaptor, TTMA buttweld).
Slip-on flanges
- AAP Slip-On Weld Flange ANSI B16.5 Class 150 — 1/2" to 2"+ (19 size variants), 290 units. The single deepest flange product in stock.
- AAP Slip-On Plate Flange — AS 2129 Table D, 1/2" to 2"+ (13 variants), 192 units
- AAP Slip-On Forged Flange Table-H — high-pressure, 1/2" to 2"+ (9 variants), 155 units
- AAP 4" Slip-on Forged Steel Plate Flange BS10 Table-D — 16 units
- AAP Plate Flat Faced Flange PN21 — AS 4087 water, 4" to 12" (12 variants), 192 units
- AAP Plate Flat Faced Flange PN35 — AS 4087 water, 4" to 12" (7 variants), 112 units
Weld neck flanges
- AAP Weld Neck Flange XS ANSI B16.5 Class 150 — 2" to 6" (6 variants), 88 units
- AAP Weld Neck Flange ANSI B16.5 Class 600 — high-pressure, 1" to 4" (6 variants), 96 units
Blind flanges
- AAP Blind Steel Plate Flange — AS 2129 Table D, 2" to 5" (7 variants), 113 units
- AAP Blind Steel Plate Flange Table-H — AS 2129 Table H, 2" to 6" (5 variants), 84 units
- AAP Blind Flat Face Flange PN16 — AS 4087, 3" to 10" (11 variants), 160 units
- AAP Blind Flanges 316/316L SS Class 150 — stainless, 1" to 4" (6 variants), 96 units
Threaded (screwed) flanges
- AAP Screwed Flange ANSI B16.5 Class 150 BSP — 1" to 4" (5 variants), 72 units
- AAP Screwed Flange ANSI B16.5 Class 150 NPT — 1/2" to 2" (9 variants), 120 units
- AAP Screwed Flange ANSI B16.5 Class 300 NPT — 1/2" to 2" (8 variants), 121 units
- AAP Screwed Forged Flange Table-H — 3/4" to 2" (6 variants), 68 units
- Dixon Screwed Flange Round Drilled Table D BSP Galvanised Malleable Iron — 1/2" to 2" (10 variants), 96 units
Specialty + adaptor flanges
- Dixon 4" Roll Grooved Flange Adaptor Table D Galvanised — Victaulic-to-flanged transition
- Dixon Roll Grooved Flange Adaptor — multi-size, 40 units, 304/316 SS + galvanised
- Dixon TTMA Buttweld Flange 4" Aluminium — tank truck industry standard
- Dixon TTMA Buttweld Flange 3 x 3/8" Aluminium — tank truck
Honest scope — sourced through supplier network on request
The following are NOT in standard AIMS stock and are sourced through our supplier network:
- AS 4087 PN16 + PN35 weld-neck flanges specifically (PN21 stocked as flat face plate)
- AS 2129 Tables A, B, C, F, J, K, R, S, T (lower-traffic and high-pressure ratings)
- ASME B16.5 Class 900 / 1500 / 2500 (extreme high-pressure)
- ASME B16.47 large diameter flanges (>24" / DN600)
- ASME B16.48 line blanks (spectacle blinds)
- EN 1092-1 / DIN 2501 European flanges (PN6, PN10, PN25, PN40)
- JIS B 2220 Japanese flanges (5K, 10K, 16K)
- Lap joint flanges + stub ends
- Orifice flanges (with tap connections for flow measurement)
- Spectacle blinds + figure-8 line blanks
- Ring-type joint (RTJ) flanges for high-pressure oil and gas (Class 600+)
- Reducing flanges (e.g. 4" body with 2" bore)
For any of these, contact our team or call (02) 9773 0122 with the specification + size + service conditions, and we'll quote the right product through our supplier network.
Selection checklist — 9 questions before ordering
- What standard does the mating flange use? AS 2129 (which Table?), AS 4087 (which PN?), ANSI B16.5 (which Class?), EN 1092, JIS B 2220? Identify exactly before ordering — never assume.
- What size? Nominal pipe size (NPS or DN). Confirm both ID and OD on the existing flange match the spec.
- What flange type? Slip-on, weld neck, blind, threaded, lap joint, reducing? Match to service + installation method.
- What face type? Flat face (FF) for cast iron mate, raised face (RF) for ANSI default, RTJ for high-pressure oil and gas?
- What material? Carbon steel general, 316 SS for chemical/marine/food, galvanised malleable iron for water plumbing, aluminium for TTMA?
- What pressure + temperature? Confirm operating + design pressure against the flange's rating at the actual operating temperature (rating de-rates with temperature).
- What gasket? Specified together with the flange — face type + pressure class + media determines gasket selection.
- What bolts? Grade matched to flange class. 4.6/5.8 for low pressure, 8.8 for higher, A193 B7 for ANSI process service, 316 SS for marine/chemical.
- Any regulatory standard? AS 4041 (Pressure Piping), AS 1210 (Pressure Vessels), AS 4087 (waterworks). Confirm certification + traceability.
For complex or safety-critical flange specifications — high-pressure process, marine, food/dairy/pharmaceutical, hazardous chemicals — work directly with the design engineer and confirm all flange specifications against the current standard edition + manufacturer datasheet. The values in this guide are typical and indicative.
For sizing assistance, cross-standard compatibility, or unusual service conditions, contact our team or call (02) 9773 0122.
Frequently asked questions
What is a pipe flange?
A pipe flange is a bolted joint connection used to join two pipe lengths, attach a valve or piece of equipment to a pipe, or terminate a pipe run. Two mating flanges sandwich a gasket; bolts pull the flanges together to compress the gasket and seal the joint. Pipe flanges are specified to one of three major Australian standard families — AS 2129 (general purpose, Tables A through T), AS 4087 (waterworks, PN16/21/35), or ANSI B16.5 (oil and gas, Class 150/300/600). Each standard has its own bolt circle diameter, hole count, hole diameter and bolt size — flanges of different standards do NOT mate together.
What are the main types of pipe flange?
Seven major flange types are commonly specified in AU industry: slip-on (pipe slides through, two fillet welds, general purpose), weld neck (tapered neck butt-welds to pipe, full strength, high-pressure default), blind (solid plate, terminates a line), threaded or screwed (BSP or NPT thread, low-pressure water plumbing), socket weld (small-bore process service), lap joint (rotating flange + stub end for easy alignment), and reducing (two different bore sizes for pipe transitions). Slip-on and blind are the most common AU general-purpose types. Weld neck is specified for high-pressure or critical service.
What's the difference between AS 2129 Table D and Table E?
AS 2129 Table D and Table E are both general-purpose Australian flange ratings. Table D is rated for ~1,400 kPa (14 bar) working pressure — the standard low-medium pressure flange for building services, irrigation, and general industrial. Table E is rated for ~1,800 kPa (18 bar) — the AU water industry default for pump connections, valve flanges and hydrant inlets. The two tables share OD and PCD on small sizes (DN15 to DN25) but diverge from DN50 upward — Table E flanges become physically larger with bigger PCD. A Table D flange will NOT bolt onto a Table E flange at DN50 and larger sizes.
AS 2129 vs AS 4087 — which one do I need?
AS 2129 is the general-purpose Australian flange standard (Tables A through T, pressure ratings from very low to very high), used for general industrial pipework, building services, irrigation, mining, and most non-water-utility applications. AS 4087 is the Australian waterworks-specific standard (PN16, PN21, PN35) used by water utilities, councils, water treatment plants, and sewerage. If you're working on a council water main, water utility distribution, fire ring main, or sewerage, use AS 4087. If you're working on general industrial pipework, building services, or irrigation, use AS 2129 (typically Table D, E or H). The two standards have different bolt patterns — they are NOT interchangeable.
What's the difference between ANSI Class 150 and AS 2129 Table E?
Both ANSI Class 150 and AS 2129 Table E are general-purpose, medium-pressure flange ratings (~20 bar and ~18 bar at room temperature respectively). The pressure ratings are similar but the bolt patterns are NOT — they have different bolt circle diameters (PCD), different numbers of bolts on some sizes, and different bolt sizes. An ANSI Class 150 flange will NOT bolt onto an AS 2129 Table E flange even though their pressure ratings are close. This mismatch is one of the most common AU industry flange ordering mistakes. Always identify the exact standard of the mating flange before ordering.
How do I read a flange PCD chart?
PCD (Pitch Circle Diameter) is the diameter of the imaginary circle that passes through the centres of all the bolt holes around a flange. Reading a flange chart: find the nominal pipe size in the first column (e.g. DN100 / 4"), then read across to get the flange OD (overall outside diameter), PCD (bolt circle diameter), number of bolt holes, hole diameter, and bolt size. To measure an existing flange in the field: measure the OD with a tape across the outside; measure the PCD as the distance from the centre of one bolt hole to the centre of the bolt hole directly opposite (or use the chord method for odd hole counts). Match all values against the standard chart to identify which standard the flange belongs to.
What is a blind flange used for?
A blind flange is a solid disc with the same OD, PCD, hole pattern and rating as the equivalent open flange — but with no pipe bore. It bolts to a mating flange to terminate the line, blank off a connection, or seal an equipment opening. Common uses: end-of-line termination on a header or manifold, future tie-in points designed into pipework for later expansion, equipment isolation blanks (when removing a valve or piece of equipment temporarily), pressure test endpoints (sealing line ends during hydrostatic testing), maintenance access blanks on tanks and large pipes, and spectacle blinds for positive isolation under LOTO procedures. A blind flange must match the mating flange's standard, rating and bolt pattern exactly.
Slip-on vs weld neck — which is stronger?
Weld neck flanges are stronger — approximately 100% of pipe strength, vs slip-on at approximately 70% of equivalent weld neck. The difference is in how the flange attaches to the pipe. Slip-on uses two fillet welds (one outside the flange, one inside), creating stress concentrations at the flange face. Weld neck uses a single critical butt weld at the tapered neck, with smooth bore transition and no stress concentration. Slip-on is cheaper and easier to install (forgiving alignment), but is not approved for sustained high-pressure or fatigue-critical service in most pressure-piping codes. Specify weld neck for ANSI Class 300+, AS 2129 Table H+, steam service, and any pressure-piping system designed to AS 4041 or ASME B31.3.
Can I bolt AS 2129 Table D to AS 4087 PN16?
No. Even though they're both Australian general-purpose flange ratings at similar pressure (Table D ~14 bar, PN16 = 16 bar), they have different bolt circle diameters (PCD) on most sizes. The bolt holes will NOT align between a Table D flange and a PN16 flange of the same nominal size. If you need to connect existing AS 2129 Table D pipework to AS 4087 PN16 equipment (or vice versa), you either need a transition spool with one Table D flange and one PN16 flange welded together, or you need to replace one of the flanges to match the other standard. Always identify both flange standards before assuming they'll bolt up.
What gasket goes with a raised face flange?
Raised face (RF) flanges are the default ANSI B16.5 configuration above Class 150 and pair with several gasket types depending on service: compressed fibre insertion gaskets for low-pressure non-critical service, PTFE for chemical service, spiral wound (CG/CGI) for medium-to-high pressure industrial process (the workshop default for most ANSI flange service), kammprofile/corrugated metal for high temperature, or ring-type joint (RTJ) for very high pressure oil and gas (Class 600+). The gasket OD must match the raised-face OD and the gasket must NOT extend into the bolt circle. For comprehensive gasket selection by service, see the AIMS Spiral Wound Gasket Guide. Never use a flat-face gasket on a raised face flange — the gasket will be over-compressed at the centre and under-compressed at the edges, causing leaks.
How do I measure a flange to identify the standard?
Five measurements identify a flange: (1) Flange overall diameter (OD) — measure across the outside with a tape; (2) Number of bolt holes — count them; (3) Pitch circle diameter (PCD) — measure centre-to-centre across opposite holes (or use the chord method for odd hole counts); (4) Hole diameter — use a calliper to measure one hole; (5) Bolt size — measure a bolt or compare against the hole diameter. Then match these against the standard charts (AS 2129 Tables D/E/H, AS 4087 PN16/21/35, ANSI B16.5 Class 150/300) until you find a match. If the OD looks like ANSI Class 150 at a given DN but the hole count is wrong, you may have an AS standard flange. Cross-checking against multiple standards is normal — flanges are not always marked.
How many bolts does a 4-inch flange need?
It depends on the standard. A DN100 / 4" flange has: AS 2129 Table D — 4 bolts; AS 2129 Table E — 8 bolts; AS 2129 Table H — 8 bolts; AS 4087 PN16 — 8 bolts; AS 4087 PN21 — 8 bolts; AS 4087 PN35 — 8 bolts; ANSI B16.5 Class 150 — 8 bolts; ANSI B16.5 Class 300 — 8 bolts. The 4-bolt Table D is the only standard 4-bolt DN100 flange in common use — all others use 8 bolts. This is one reason Table D and Table E flanges of the same nominal size are NOT interchangeable. Always verify the exact bolt count + PCD against the standard chart for the specification you're working to.
What's the difference between threaded and slip-on flange?
Threaded (screwed) flanges attach to the pipe via a BSP or NPT thread cut into the flange bore — the pipe is threaded with a matching male thread and screwed into the flange. Slip-on flanges attach via two fillet welds — the pipe slides through the flange bore and is welded both inside and outside the flange. Threaded is used where welding is impractical (galvanised pipe, where zinc would burn off; field installations without weld equipment; low-pressure non-hazardous service). Slip-on is used where welding is available and somewhat higher pressure is required. Slip-on is generally stronger than threaded for higher-pressure service. Threaded flanges are limited to small bore (typically 2" and below) and lower pressure ratings due to thread strength limitations.
What is a backing flange?
A backing flange (also called a lap joint backing flange or stub end backing flange) is a free-rotating flange used with a stub end. The stub end is butt-welded to the pipe; the backing flange slides freely over the stub end and rotates to align with the mating flange. Used in two main scenarios: (1) Stainless steel or expensive alloy lines where the flange is made of cheap carbon steel and the stub end is the expensive alloy — only the wetted stub end contacts the fluid; (2) Hole alignment headaches where the mating flange can't be rotated to align bolt holes — the backing flange rotates freely to suit. Backing flanges are typically rated at ~80% of equivalent weld neck due to the lap joint geometry.
What does TTMA flange mean?
TTMA stands for Tank Truck Manufacturers Association — a US standards body that publishes specifications for tank truck (road tanker) fittings and flange connections. TTMA flanges are flat-face, aluminium or stainless steel buttweld flanges with a specific bolt pattern used for tank truck manholes, fittings, valve mounts, and hose connections on road tankers. Common sizes are 3" and 4". AIMS stocks Dixon TTMA buttweld flanges in aluminium for tank truck applications. TTMA is not interchangeable with AS 2129, AS 4087 or ANSI B16.5 — it's a niche standard specific to tank truck and mobile equipment service.
