ISO Flanges: Complete Guide to Standards, Types, Dimensions & Selection

ISO flanges are piping connection components manufactured in accordance with standards published by the International Organization for Standardization (ISO). They provide a standardized, bolt-together method for joining pipes, valves, pumps, and other process equipment - ensuring dimensional interchangeability across manufacturers and national borders. Depending on the application context, the term "ISO flange" can refer to general industrial pipe flanges governed by ISO 7005, or to specialized vacuum-system flanges such as ISO-K, ISO-F, and KF flanges that are defined under ISO 2861 and related vacuum standards.

In both industrial and vacuum contexts, ISO flanges share a defining characteristic: they are designed to precise dimensional specifications that guarantee face-to-face compatibility between components from different suppliers. This interchangeability reduces procurement complexity and simplifies maintenance - particularly valuable in large-scale installations such as oil & gas processing plants, chemical reactors, water treatment facilities, and semiconductor fabrication cleanrooms.

Key properties that define ISO flanges:

• Dimensional standardization governed by ISO or equivalent national standards (DIN, EN)
• Specified sealing face types: Raised Face (RF), Flat Face (FF), or Ring-Type Joint (RTJ)
• Defined bolt-circle diameters, bolt hole counts, and flange outer diameters by nominal pipe size (NPS / DN)
• Material compliance with recognized forging standards (ASTM A182, EN 10222, etc.)
• Pressure-temperature ratings calibrated to specific pressure classes (PN 10 through PN 400 in ISO; Class 150 through Class 2500 in ASME)

For procurement engineers and plant designers, working with ISO-standard flanges means access to a globally recognized specification language - reducing the risk of dimensional mismatches and enabling competitive sourcing from qualified manufacturers worldwide, including specialized stainless steel producers in China such as Zhejiang Chengjiu Pipeline (CNCJ).

The abbreviation ISO stands for the International Organization for Standardization - the world's largest developer of voluntary international standards. Founded in 1947 and headquartered in Geneva, Switzerland, ISO currently maintains more than 24,000 published standards covering virtually every industry sector, from food safety and environmental management to engineering tolerances and flange dimensions.

A frequent point of confusion: "ISO" is not literally an acronym of the English name. The organization's founders chose ISO as a universal short form derived from the Greek word isos, meaning "equal" - a reflection of the organization's mission to promote equality through standardization. This single abbreviation works identically across the organization's three official languages (English, French, Russian), avoiding the need for multiple language-specific initialisms.

ISO vs Other Standard Bodies Relevant to Flanges

Understanding the landscape of ISO flange standards is essential before specifying components for any project. The following standards govern the most widely used flange families.

3.1 ISO 7005: The Core Piping Flange Standard

ISO 7005 is the primary international standard for metallic pipe flanges used in general industrial piping applications. It is published in three parts:

• ISO 7005-1 - Steel flanges (nominal pipe sizes DN 10 through DN 4000; pressure designations PN 2.5 to PN 400)
• ISO 7005-2 - Cast iron flanges (PN 2.5 to PN 63)
• ISO 7005-3 - Copper alloy and composite flanges (PN 10 to PN 40)

ISO 7005-1 for steel flanges is the most commercially significant part. It defines flange dimensions (outside diameter, bolt-circle diameter, bolt-hole size, flange thickness, and bore) in metric units, organized by DN (Diamètre Nominal / Nominal Diameter) and PN (Pression Nominale / Nominal Pressure) designations. The standard covers six types of flanges: flat face, raised face, tongue and groove, male and female, ring-type joint, and butt-weld end.

3.2 ISO 7005 vs EN 1092: What Is the Difference?

This is one of the most common questions engineers ask when working on cross-border projects. The short answer: EN 1092 largely supersedes ISO 7005 in European practice, and the two standards are closely harmonized but not identical.

Practical guidance: For European projects requiring CE marking or PED compliance, specify EN 1092. For projects in Asia, the Middle East, or global contracts where metric flanges are required but EU regulations do not apply, ISO 7005 remains a suitable reference. Many Chinese manufacturers - including CNCJ - can supply flanges certified to either standard upon request.

3.3 ISO 6164: Square and Rectangular Flanged Connections for Hydraulic Fluid Power

ISO 6164 is a narrower standard that defines four-bolt square and two-bolt rectangular flange heads and ports for hydraulic fluid power systems. It is not a general-purpose piping flange standard. ISO 6164 flanges are typically found in high-pressure hydraulic circuits operating at pressures up to 40 MPa (approximately 5,800 psi), where conventional circular flanges would be less practical due to spatial constraints. Common applications include industrial presses, mobile hydraulic machinery, and offshore drilling equipment.

Key characteristics of ISO 6164 flanges:

• Nominal sizes: DN 12.5 to DN 100 (½" to 4" port size)
• Two pressure ratings: Series 1 (25 MPa) and Series 2 (40 MPa)
• O-ring face sealing - no gaskets required
• Materials: carbon steel, stainless steel, ductile iron

3.4 ISO 2861 / DIN 28403: KF Vacuum Flanges

For vacuum system applications, ISO 2861 (aligned with DIN 28403 and the older Pneurop standard) defines the KF (Klein Flansch) or NW (Nominal Width) quick-release flange family. These are compact, elastomer-sealed flanges designed for rough to high vacuum service. The standard specifies dimensions for KF10, KF16, KF25, KF40, and KF50 sizes, using the nominal inside diameter (in millimeters) as the size designator.

The phrase "ISO flange" encompasses several distinct product families, each suited to different pressure ranges, vacuum levels, and assembly requirements. Understanding these categories is essential for correct specification.

4.1 ISO-K Flange

The ISO-K flange (also written ISO K) is a medium-to-large-bore vacuum flange designed for applications requiring frequent assembly and disassembly. It is standardized under ISO 1609 and covers nominal diameters from DN 63 to DN 630 (roughly 2.5 inches to 25 inches). ISO-K flanges use elastomeric O-ring seals (typically Viton, NBR, silicone, or EPDM) and are clamped together with double-claw clamps or centering rings - no bolt holes in the flange body itself.

Key technical characteristics of ISO-K flanges:

• Vacuum rating: Suitable for high vacuum applications down to approximately 10⁻⁸ mbar
• Temperature limit: Up to 150°C with fluoroelastomer (Viton) O-rings; higher with metal seal variants
• Materials: 304 stainless steel (most common), 316L stainless steel for corrosive environments, aluminum for weight-sensitive applications
• Assembly: Fast, tool-minimal connection using external clamp bands; ideal for systems that require frequent reconfiguration
• Common DN sizes: DN 63, DN 100, DN 160, DN 200, DN 250, DN 320, DN 400, DN 500, DN 630

ISO-K flanges are widely used in synchrotron beamlines, particle accelerators, industrial vacuum furnaces, coating systems, and large vacuum vessels in the semiconductor and flat-panel display manufacturing industries.

4.2 ISO-F Flange

The ISO-F flange (fixed-bolt variant) covers the same DN size range as ISO-K (DN 63–630) but uses a different connection method. Instead of external clamps, ISO-F flanges have a fixed bolt pattern machined directly into the flange body, providing a more rigid connection. They use the same centering ring and O-ring sealing as ISO-K but are secured with bolts through matching bolt holes - making them better suited for permanent or semi-permanent installations where vibration or mechanical loads might dislodge a clamped connection.

In practice, ISO-K and ISO-F flanges are dimensionally compatible on the sealing face and can be mixed within the same system (ISO-F on one side, ISO-K on the other), provided the centering ring and O-ring are the correct size. The ISO-F flange's bolt-down design makes it the preferred choice for vertical installations, large pumping ports, and high-vibration environments.

4.3 KF Flange (ISO-KF / NW Flange)

The KF flange (from German Kleinflansch, "small flange") is the most widely used quick-release vacuum flange for small-bore applications. Standardized under ISO 2861 and DIN 28403, KF flanges are also commonly called QF flanges, NW flanges, or ISO-KF flanges - all refer to the same product.

KF flanges are sexless (both mating faces are identical), joined by a tapered clamp ring over a centering ring that holds the O-ring seal in position. The flange itself has a 15° chamfered back that engages with the clamp. This design allows very fast assembly and disassembly without tools beyond a clamp handle.

Standard KF sizes and their nominal bore dimensions:

KF flanges achieve vacuum levels down to approximately 10⁻⁸ mbar with elastomeric seals. They are the dominant connector type in laboratory vacuum systems, mass spectrometers, electron microscopes, and industrial thin-film deposition equipment.

4.4 ConFlat (CF) Flange

The ConFlat (CF) flange is a metal-sealed vacuum flange designed for ultra-high vacuum (UHV) and extremely high vacuum (XHV) applications. The CF design uses a knife-edge that bites into an oxygen-free copper (OFHC) gasket, creating a virtually leak-free, all-metal seal capable of reaching vacuum levels below 10⁻¹² mbar. This makes CF flanges the standard choice for particle physics research, surface science instruments, space simulation chambers, and any UHV system where the lowest possible outgassing is essential.

Key differences between CF and KF/ISO flanges:

• Sealing mechanism: Metal-to-metal (copper gasket) vs. elastomeric O-ring in KF/ISO
• Vacuum capability: CF reaches 10⁻¹² mbar; KF/ISO limited to ~10⁻⁸ mbar
• Bake-out temperature: CF rated to 450°C bake-out; elastomeric seals typically limited to 150°C
• Assembly: CF requires torquing bolts to specification; cannot be assembled or disassembled as quickly as KF
• Cost: CF flanges and copper gaskets are more expensive per connection
• Reusability of seals: CF copper gaskets are single-use; KF/ISO O-rings are reusable if undamaged

4.5 Isolation Flanges (Dielectric Isolation Kits)

Isolation flanges (sometimes called dielectric flanges or specified with "isolation kit for flanges") are a specialized category designed to electrically isolate one section of piping from another. They are not defined by a single ISO standard but are manufactured to fit ISO, ANSI, or other standard flanges. An isolation flange kit typically consists of:

• An electrically isolating gasket (phenolic, PTFE, or mylar)
• Insulating sleeves for bolt holes
• Insulating washers under bolt heads and nuts

Isolation flanges are widely used in:

• Cathodic protection systems - preventing galvanic corrosion on buried pipelines
• Oil & gas distribution networks - electrically separating station piping from field pipelines
• Chemical processing plants - isolating dissimilar metal systems to prevent electrochemical corrosion
• Marine applications - protecting ship hull piping from stray current corrosion

4.6 Standard Industrial Piping Flanges Under ISO / EN Equivalents

In the context of general industrial piping, the phrase "ISO flange" most commonly refers to flanges manufactured to metric PN pressure designations - PN 6, PN 10, PN 16, PN 25, PN 40, PN 63, PN 100, PN 160, PN 250, and PN 400 - per ISO 7005 or EN 1092. These flanges are available in all standard configurations:

• Weld Neck Flange (WN) - long tapered hub welded to the pipe; best for high-pressure, high-cycle applications
• Slip-On Flange (SO) - slipped over the pipe and fillet-welded; lower cost, suitable for moderate pressures
• Socket Weld Flange (SW) - pipe inserted into a socket bore and fillet-welded; used for smaller bore, higher-pressure lines
• Blind Flange (BL) - solid disk for closing the end of a pipe or vessel nozzle
• Threaded Flange (TH) - screwed onto a threaded pipe end; no welding required
• Lap Joint Flange (LJ) - used with a stub end; allows bolt-hole rotation for alignment

 

 

 

 

 

 

Dimensional accuracy is the most critical attribute of any flange specification. The tables below provide representative dimensional data for the major ISO flange families. For full dimensional tables and ISO flange dimensions PDF equivalents, consult the complete standards documents or request a dimensional drawing from your supplier.

5.1 ISO 7005 / EN 1092 PN Pressure Class Dimensions (Steel Flanges, Selected DN Sizes)

All dimensions in millimeters. Bolt circle diameter (BCD), number of bolt holes (#BH), bolt hole diameter (BHD), and flange outside diameter (OD) for raised-face flanges.

Note: Dimensions above are representative reference values based on ISO 7005-1 / EN 1092-1. Always verify against the current published standard for safety-critical specifications.

5.2 ISO-K Vacuum Flange Dimensions (ISO 1609)

5.3 KF (NW) Vacuum Flange Standard Sizes

The comparison between ANSI (ASME) and ISO flanges is one of the most frequently asked questions from engineers working on international projects or specifying equipment for export. The two systems are not interchangeable - knowing the differences prevents costly procurement mistakes.

Unit System and Size Designation

ANSI/ASME flanges (governed by ASME B16.5 for sizes ½" to 24", and B16.47 for 26" to 60") use imperial units: nominal pipe size (NPS) in inches, flange dimensions in inches, and bolt sizes in fractional or unified thread (UNC/UNF). ISO flanges use metric units: DN (nominal diameter in millimeters), flange dimensions in millimeters, and bolt sizes in metric thread (M8, M10, M12, etc.).

Pressure Class Designations

ANSI/ASME uses pound-force per square inch (psi) class ratings: Class 150, 300, 600, 900, 1500, and 2500. ISO/EN uses PN (bar) ratings: PN 10, 16, 25, 40, 63, 100, 160, 250, 400. These are not direct conversions - a "Class 150" flange is not equivalent to a "PN 16" flange even though their pressure capacities are broadly similar, because the P-T rating curves differ.

Bolt Patterns and Dimensional Compatibility

For the same nominal bore size, ANSI and ISO flanges generally have different bolt-circle diameters, flange outside diameters, and bolt hole counts. This means that an ANSI Class 150 DN 100 flange will typically NOT bolt up directly to an ISO PN 16 DN 100 flange - even though both are described as "4-inch" flanges. Adapters or transition spools are required to connect the two systems. Always verify bolt-circle and OD dimensions from the relevant standard tables before assuming dimensional compatibility.

Facing Types

Both systems support raised face (RF) and ring-type joint (RTJ) facings, but the raised face height differs: ASME B16.5 specifies 1/16" (1.6 mm) raised face height for Class 150 and 300, and 1/4" (6.4 mm) for Class 600 and above. ISO/EN flanges specify raised face height per PN class, typically 2 mm for PN ≤ 40 and up to 3 mm for higher PN classes - meaning gasket seating dimensions differ.

ISO flange standards do not mandate specific material grades - instead, they reference broader material standards and leave grade selection to the designer based on process conditions. In practice, ISO flanges are manufactured from the same material families used for ASME flanges, with material certificates issued to either ASTM or EN standards depending on the project specification.

Stainless Steel Grades

Carbon and Alloy Steel

For standard industrial service: ASTM A105 (carbon steel forgings for ambient to 593°C service) is the most common piping flange material globally - equivalent to EN P245GH or P280GH under EN 10222-1. For low-temperature service, A350 LF2 is specified. For elevated-temperature alloy service, the A182 F5, F9, F11, F22, and F91 grades provide increasing Cr-Mo content for improved creep resistance.

CNCJ manufactures ISO-compatible stainless steel flanges in A182 F304/304L, F316/316L, F317L, F51, and F53 with mill test reports (MTR) and third-party inspection available for all orders.

The PN (Pressure Nominale) designation in ISO/EN standards is not a maximum pressure rating - it is a reference pressure at a specified reference temperature (typically 20°C for most materials). Actual allowable pressure decreases as temperature rises, following material-specific pressure-temperature (P-T) derating curves published in ISO 7005-1 or EN 1092-1.

ISO/EN PN Classes and Approximate Maximum Pressures at 20°C

Critical engineering note: Always use the P-T derating curve for the specific material group and temperature range - never apply the nominal PN value as a blanket maximum operating pressure without checking temperature-dependent allowable pressure.

When customers ask "what are the five most popular ISO standards," the answer depends heavily on industry context. In the industrial piping and process engineering sectors most relevant to flange specification, the following five ISO standards are encountered most frequently:

1. ISO 9001:2015 - Quality Management Systems. The world's most widely adopted ISO standard. Applicable to any organization supplying manufactured goods; ISO 9001 certification is a baseline quality assurance requirement for most flange manufacturers supplying to oil & gas, chemical, or power generation projects.
2. ISO 7005-1 - Metallic Flanges (Steel). The foundational dimensional standard for metric pipe flanges worldwide. Directly relevant to flange procurement and design.
3. ISO 14001:2015 - Environmental Management Systems. Increasingly specified by large end-users and EPCs as a supplier qualification requirement alongside ISO 9001.
4. ISO 45001:2018 - Occupational Health and Safety Management Systems. The ISO standard addressing workplace safety management; now supersedes the earlier OHSAS 18001 standard. Note: ISO 45000 does not exist as a standard - the correct designation is ISO 45001.
5. ISO 15156 (NACE MR0175) - Materials for Use in H₂S-Containing Environments. Critical for materials specification in sour service piping, where flange materials must be qualified for resistance to sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC).

Selecting the correct ISO flange for an application requires systematic evaluation of five key parameters. Working through these in sequence prevents the most common specification errors.

Step 1 - Determine the Applicable Standard

First, establish whether the project requires metric (ISO/EN) or imperial (ASME/ANSI) flanges. This is typically determined by: the country where the plant is located; the engineering standard specified in the project specification (EPC contract, data sheet, or P&ID general notes); and the existing pipe system standard if connecting to existing infrastructure.

Step 2 - Select the DN (Nominal Diameter)

The DN is determined by the connected pipe bore. In ISO/EN systems, DN corresponds directly to the nominal inside diameter of standard pipe. For example, DN 100 pipe has a nominal bore of approximately 100 mm (actual inside diameter varies slightly by wall thickness / schedule).

Step 3 - Select the PN (Pressure Class)

Calculate the required pressure class using the maximum allowable operating pressure (MAOP) at the maximum design temperature. Apply the appropriate P-T derating factor for the chosen material group. Select the lowest PN class whose allowable pressure at the design temperature exceeds the MAOP - selecting a higher PN class than necessary adds cost without operational benefit.

Step 4 - Select the Flange Type

Choose the end-connection type based on welding procedure, pipe schedule, disassembly requirements, and cost:

• Weld Neck: preferred for high-pressure, high-temperature, cyclic, or cryogenic service
• Slip-On: lower cost, suitable for moderate pressure and temperature; avoid in cyclic fatigue service
• Socket Weld: for small bore (DN ≤ 50) high-pressure lines; provides a clean internal bore
• Blind: for pressure testing, end-of-line closures, and future tie-in points
• Lap Joint: where bolt-hole alignment is difficult or frequent disassembly is needed

Step 5 - Select the Material

Match the material to the fluid service: temperature range, corrosion potential, hydrogen or sulfide content, and cleanliness requirements. For corrosive service involving chlorides, acidic fluids, or seawater, 316L or duplex stainless steel is the standard choice. For water treatment and general mild service, 304L provides excellent corrosion resistance at lower cost.

Step 6 - Specify the Facing Type and Gasket

For most ISO/EN applications, the standard facing is Raised Face (RF) with a spiral-wound gasket (SWG) for PN ≥ 25, or a flat elastomeric gasket for PN ≤ 16. Flat Face (FF) is required when mating to cast iron or ductile iron flanges to avoid cracking the brittle material. RTJ grooves are used for very high pressures or where zero-leakage is critical.

Q: What is the difference between KF and ISO flanges?

KF flanges (also known as NW or ISO-KF flanges) are compact quick-release vacuum connectors for small bore sizes (DN 10–50) used in rough to high vacuum service, standardized under ISO 2861. They use an elastomeric O-ring held in a centering ring and secured by a tapered clamp - no bolts required. ISO-K and ISO-F flanges (ISO 1609) are larger-bore vacuum flanges (DN 63–630) that share the same O-ring sealing principle but use external clamp bands (ISO-K) or through-bolts (ISO-F). In short: KF is for small-bore laboratory and instrument vacuum lines; ISO-K/F is for larger vacuum system ports and pump connections.

Q: Are ISO flanges and DIN flanges the same?

In most cases, yes - for practical purposes. DIN flanges (manufactured to DIN 2501 and related standards) predate ISO 7005 and served as one of the primary technical inputs to the ISO and EN standards. The bolt patterns, flange diameters, and PN pressure classes in DIN 2501, ISO 7005, and EN 1092-1 are closely harmonized for common PN/DN combinations, particularly PN 10 through PN 100 in the most common DN sizes. Minor differences exist in flange thickness and material group definitions. Always verify against the specific project data sheet.

Q: What are the three most common flange types in industrial piping?

Across global industrial applications, the three most commonly specified flange types are: (1) the Weld Neck flange, preferred for high-integrity, high-pressure service; (2) the Slip-On flange, widely used for moderate-service utility and process piping due to its lower cost; and (3) the Blind flange, used on virtually every piping system as an end closure, isolation device, or future connection point. Socket Weld flanges are also very common in small-bore high-pressure applications.

Q: Can ISO PN 16 flanges connect directly to ANSI Class 150 flanges?

Not reliably without verification. While PN 16 and Class 150 have broadly similar pressure-temperature capabilities, their bolt-circle diameters and flange outside diameters differ for many DN/NPS size combinations. Before mixing systems, check the BCD and OD from the respective standard tables for the specific size in question. For some sizes they may coincidentally align; for others they will not. The safest practice is to use a transition spool with one end drilled to ISO/EN bolt pattern and the other to ANSI bolt pattern.

Q: What does "ISO 160 flange" mean?

In a vacuum context, ISO 160 refers to an ISO-K or ISO-F vacuum flange with a nominal bore diameter of 160 mm (DN 160). In an industrial piping context, it could also refer to a PN 160 pressure class flange (160 bar nominal pressure) in DN sizes per ISO 7005-1. Always clarify which meaning applies for your application.

Q: What is the difference between ISO 21502 and ISO 21500?

Both are project management standards, not flange standards. ISO 21500:2012 provided high-level guidance on project management concepts and processes. ISO 21502:2020 is a more detailed, comprehensive project management guidance standard that supersedes ISO 21500 and aligns more closely with major project management frameworks (PMBOK, PRINCE2). Neither directly relates to flange specification - but both are relevant to EPC project execution methodology in large capital projects.

Q: What isolation flange kit components do I need?

A complete isolation kit for flanges (also called a dielectric isolation set) includes: one full-face isolating gasket sized to the flange bore and OD; a set of insulating bolt sleeves (one per bolt); insulating washers (two per bolt - one under the bolt head, one under the nut); and metallic washers (two per bolt - placed outside the insulating washers to distribute load). The assembled bolt sequence is: nut → metal washer → insulating washer → flange face → insulating sleeve → flange face → insulating washer → metal washer → bolt head. All components must be verified for the correct pressure class, bolt size, and flange standard (ISO or ANSI) before ordering.

Q: How many ISO standards are there?

As of 2024, the ISO catalogue contains more than 24,500 published international standards, covering virtually every industry from food safety (ISO 22000) to information security (ISO 27001), from quality management (ISO 9001) to environmental management (ISO 14001). New standards and revisions are published continuously - approximately 1,500 new or revised standards are published per year.

Reference Sources

1. ISO 7005-1:2011 - Metallic Flanges - Part 1: Steel Flanges. International Organization for Standardization, Geneva.
2. EN 1092-1:2018 - Flanges and Their Joints - Circular Flanges for Pipes, Valves, Fittings and Accessories, PN Designated - Part 1: Steel Flanges. CEN, Brussels.
3. ISO 1609:1986 - Vacuum Technology - Flange Dimensions (ISO-K / ISO-F). International Organization for Standardization, Geneva.
4. ISO 2861-1 / DIN 28403 - Vacuum Technology - Quick Release Couplings - Part 1: Dimensions (KF / NW flanges). ISO / Deutsches Institut für Normung.
5. ISO 6164:1994 - Hydraulic Fluid Power - Four-Screw and Two-Screw Flange Connections. International Organization for Standardization, Geneva.
6. ASME B16.5-2017 - Pipe Flanges and Flanged Fittings: NPS ½ Through NPS 24. American Society of Mechanical Engineers, New York.
7. ASTM A182/A182M - Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves for High-Temperature Service. ASTM International, West Conshohocken, PA.
8. GNB Vacuum Corporation - "Comparing KF, ISO, CF and ASA Flanges." vacuumchamber.com, 2020. Retrieved June 2026.
9. EVP Vacuum Technology - "Difference between CF, KF, and ISO Flanges." evpvacuum.com, 2018. Retrieved June 2026.