ANSI Flange Specification Guide: ANSI Flanges, Standards, Types, Dimensions, and Selection Tips
An ANSI flange specification is the technical language that tells a manufacturer, fabricator, and installer exactly what flange is required. It does more than name an ANSI flange. A complete specification defines the standard, size, pressure class, flange type, facing, material, bore, bolt pattern, inspection requirement, and surface treatment. Without those details, two flanges may look similar but fail to match in pressure rating, bolt hole layout, gasket seating, or pipe schedule.
This original guide explains what ANSI flanges are, how ANSI and ASME standards are used in flange procurement, which ANSI flange types are most common, and how to prepare a clear RFQ for fixed flanges, rotatable flanges, weld neck flanges, slip on flanges, blind flanges, and other industrial flange products.
What Does ANSI Mean for Flanges?
ANSI stands for the American National Standards Institute. In flange buying, the term “ANSI flange” is still widely used by engineers, distributors, and end users. In modern technical documents, many flange standards are issued under ASME, such as ASME B16.5 and ASME B16.47. Because the market has used ANSI terminology for decades, buyers often say ANSI flange when they mean an ASME-standard pipe flange.
The important point is not the label alone. The purchase order should clearly state the applicable standard and edition if required. For example, “ANSI flange Class 150” is less precise than “ASME B16.5 Class 150 raised face weld neck flange, ASTM A182 F316L, NPS 4, Schedule 40 bore.” The second description gives a manufacturer enough information to quote and produce the correct part.
What Should an ANSI Flange Specification Include?
| Specification Item | Example | Why It Matters |
|---|---|---|
| Standard | ASME B16.5, ASME B16.47, ANSI B16.1 | Defines dimensions, pressure-temperature ratings, tolerances, and bolt patterns |
| Size | NPS 2, NPS 12, NPS 36 | Matches the pipe and connected equipment |
| Pressure class | Class 150, 300, 600, 900, 1500, 2500 | Determines flange thickness, bolt size, and pressure-temperature capability |
| Flange type | Weld neck, slip on, blind, threaded, socket weld, lap joint | Controls installation method and service suitability |
| Facing | RF, FF, RTJ | Must match gasket and mating flange face |
| Material | ASTM A105, A350 LF2, A182 F304/F316/F51 | Controls strength, corrosion resistance, temperature range, and weldability |
| Bore or schedule | Schedule 40, Schedule 80, custom bore | Essential for weld neck, socket weld, and slip on flange fit-up |
| Inspection | MTC, PMI, UT, MT, PT, dimensional report | Supports traceability and project acceptance |
Common ANSI Standards for Flanges
When buyers search for ANSI standards for flanges, they usually need to identify which standard applies to the size, material, and service. The following standards are among the most common in industrial piping:
| Standard | Typical Scope | Common Applications |
|---|---|---|
| ASME B16.5 | Pipe flanges and flanged fittings from NPS 1/2 through NPS 24 | Oil and gas, chemical plants, power plants, general industrial piping |
| ASME B16.47 Series A | Large diameter steel flanges, generally NPS 26 and larger | Large pipelines, high-load applications, heavy industrial systems |
| ASME B16.47 Series B | Large diameter steel flanges with different dimensional pattern from Series A | Large diameter piping where lower weight or existing pattern compatibility is needed |
| ANSI/ASME B16.1 | Cast iron pipe flanges and flanged fittings | Water, steam, and low-pressure non-corrosive systems |
| ASME B16.36 | Orifice flanges | Flow measurement systems |
For export orders, the buyer should also confirm whether the project uses ANSI/ASME, EN, DIN, JIS, GOST, or a custom drawing. A flange manufacturer in China can often produce multiple standards, but the purchase description must be exact.
Common ANSI Flange Types
Weld Neck ANSI Flange
A weld neck ANSI flange has a long tapered hub and is butt welded to the pipe. It is commonly selected for high pressure, high temperature, vibration, thermal cycling, and critical process piping. The hub provides a smoother stress transition from pipe to flange, which is why weld neck flanges are common in refineries, chemical plants, power generation, and pressure equipment connections.
Slip On ANSI Flange
A slip on flange slides over the pipe and is normally fillet welded on both the inside and outside. It is economical, easy to align, and widely used in low to moderate pressure systems. Slip on ANSI flanges are popular in water lines, air systems, storage terminals, and general industrial piping where the piping class allows them.
Blind ANSI Flange
A blind flange closes the end of a pipe, valve, vessel nozzle, or inspection opening. It has no center bore and must resist pressure load across the full face. Blind flanges are often used for testing, isolation, maintenance access, and future expansion points. Correct thickness, class, material, and facing are especially important for blind flange safety.
Threaded ANSI Flange
A threaded flange connects to pipe without welding. It is usually used on small-diameter, low-pressure, non-critical systems where welding is undesirable. Threaded flanges can save installation time, but they are not ideal for severe vibration, high temperature cycling, or hazardous fluid service unless specifically approved by the piping specification.
Socket Weld ANSI Flange
A socket weld flange has a socket into which the pipe is inserted before fillet welding. It is often used for small-bore, higher-pressure piping where compact construction is useful. Socket weld flange specification should include pipe schedule, material, class, and required socket gap practice.
Lap Joint and Rotatable ANSI Flanges
Lap joint flanges are used with stub ends and can rotate around the pipe before bolting. This makes them useful when bolt-hole alignment is important or when frequent dismantling is expected. Buyers sometimes use the phrase ANSI rotatable flanges for lap joint flange systems or special swivel-type designs. If true rotation under service load is required, the drawing and design requirements must be stated clearly.
ANSI Fixed Flanges vs ANSI Rotatable Flanges
ANSI fixed flanges are welded or otherwise attached so the bolt-hole orientation is fixed after fabrication. Weld neck, slip on, socket weld, and threaded flanges are usually fixed once installed. ANSI rotatable flanges, in common purchasing language, often refer to lap joint flanges used with stub ends, where the backing flange can rotate for bolt alignment before tightening.
| Feature | ANSI Fixed Flanges | ANSI Rotatable Flanges |
|---|---|---|
| Typical examples | Weld neck, slip on, socket weld, threaded | Lap joint flange with stub end, swivel-style custom flange |
| Bolt-hole alignment | Must be controlled during fabrication | Easier to align before bolting |
| Maintenance | Stable and common for permanent joints | Useful for dismantling or difficult alignment locations |
| Sealing surface | Usually on the flange face itself | Often on the stub end face in lap joint systems |
| Ordering note | Specify bolt-hole orientation when required | Specify stub end type, flange standard, material, and facing |
ANSI Flange Dimensions: What Buyers Should Check
ANSI flange dimensions are standardized, but the buyer still needs to verify the correct size and class. Important dimensions include outside diameter, bolt circle diameter, number of bolt holes, bolt hole diameter, flange thickness, raised face diameter, hub diameter, hub length, bore, and gasket seating dimensions.
For weld neck flanges, the bore must match the pipe schedule or project requirement. For slip on and socket weld flanges, the bore and socket dimensions affect fit-up. For blind flanges, the thickness and pressure class are critical. For ASME B16.47 large diameter flanges, Series A and Series B are not automatically interchangeable because their dimensions and bolt patterns differ.
ANSI Flange Pressure Classes
ANSI/ASME flanges are commonly specified by pressure class, such as Class 150, 300, 600, 900, 1500, and 2500. The class is not a single pressure value for every temperature and material. Actual allowable pressure depends on material group and operating temperature. As temperature rises, allowable pressure normally decreases.
| Pressure Class | Typical Use | Buyer Reminder |
|---|---|---|
| Class 150 | Low-pressure water, air, general utility, some process services | Do not assume it is suitable without checking temperature and material |
| Class 300 | Moderate-pressure process lines, steam, oil and gas support systems | Common upgrade when Class 150 is not enough |
| Class 600 | Higher-pressure chemical, power, and hydrocarbon systems | Requires stronger bolting and correct gasket selection |
| Class 900 and above | Severe pressure/temperature services | Often needs stricter inspection and material control |
Material Choices for ANSI Flanges
Material selection should match pressure, temperature, corrosion, welding, and code requirements. Common ANSI flange materials include ASTM A105 carbon steel, ASTM A350 LF2 low-temperature carbon steel, ASTM A182 F304/F304L stainless steel, ASTM A182 F316/F316L stainless steel, alloy steels such as F11 and F22, and duplex stainless steels such as F51 and F53.
For corrosive media, stainless steel or duplex stainless steel may be more suitable than carbon steel. For low-temperature service, impact-tested materials may be required. For high-temperature steam or refinery service, alloy steel may be needed. A stainless steel flange supplier should provide material certificates and, when required, PMI testing to confirm grade identity.
How to Select the Right ANSI Flange
| Selection Question | Recommended Action |
|---|---|
| What standard applies? | Confirm ASME B16.5, ASME B16.47, ANSI B16.1, or project drawing |
| What is the pipe size? | Specify NPS/DN and pipe schedule or exact bore |
| What pressure and temperature? | Select class using the correct material pressure-temperature rating |
| What flange type is needed? | Choose weld neck, slip on, blind, threaded, socket weld, lap joint, or custom design |
| What face and gasket? | Match RF, FF, or RTJ face with the correct gasket type |
| What fluid and corrosion risk? | Select carbon steel, stainless steel, duplex, alloy, or special material accordingly |
| What inspection is required? | State MTC, PMI, NDT, dimensional report, hardness, or third-party inspection |
Common ANSI Flange Specification Mistakes
The first mistake is using the word “ANSI” without stating the exact standard. The second is omitting the pressure class. The third is failing to specify the face type. RF, FF, and RTJ flanges are not interchangeable in every joint. Another common issue is confusing ASME B16.47 Series A and Series B for large diameter flanges. They may have different bolt patterns and thicknesses, so confirming the series is essential.
Buyers should also avoid assuming that a “stainless steel ANSI flange” automatically means 316L. Stainless grades, heat treatment, corrosion requirements, and certification levels should be written clearly on the inquiry and purchase order.
RFQ Checklist for ANSI Flanges
To receive an accurate quotation from an ANSI flange manufacturer, include the following information:
- Standard: ASME B16.5, ASME B16.47 Series A/B, ANSI B16.1, or drawing
- Flange type: weld neck, slip on, blind, threaded, socket weld, lap joint, plate, or custom flange
- Size: NPS/DN and pipe schedule or bore
- Pressure class: 150, 300, 600, 900, 1500, 2500, or other requirement
- Facing: RF, FF, RTJ, tongue and groove, or special face
- Material: ASTM A105, A350 LF2, A182 F304/F316/F51, or project material
- Fixed or rotatable design: standard fixed flange, lap joint flange with stub end, or custom swivel design
- Surface treatment: anti-rust oil, black paint, galvanizing, passivation, or special coating
- Inspection and certificates: EN 10204 3.1 MTC, PMI, NDT, hardness, dimensional report, third-party inspection
- Quantity, delivery schedule, packaging requirement, and destination port
Conclusion
A clear ANSI flange specification prevents wrong dimensions, wrong bolt patterns, unsuitable pressure class, mismatched facing, and material errors. ANSI flanges are widely used because their dimensions and pressure classes support reliable interchangeability across industrial piping systems. But the best result comes from specifying the exact standard, size, class, material, flange type, facing, bore, and inspection requirement.
As a flange manufacturer in China and stainless steel flange supplier, Songhai Flanges can support ANSI flanges, ASME B16.5 flanges, ASME B16.47 large diameter flanges, weld neck flanges, slip on flanges, blind flanges, socket weld flanges, lap joint flanges, ANSI fixed flanges, and custom rotatable flange solutions for global industrial projects.