ANSI Flange Guide: Fixed, Rotatable and ASME Flanges for Pipe Systems

An ANSI flange is often requested when a piping project needs standardized dimensions, bolt holes, pressure class and gasket sealing surfaces. In many modern specifications, buyers will also see ASME flanges, because common flange standards such as ASME B16.5 and ASME B16.47 are used for class-rated pipe flange systems.

This guide explains flange ANSI terminology, the difference between ANSI fixed flanges and ANSI rotatable flanges, and how to specify ANSI pipe flanges for industrial projects without confusion.

What Does ANSI Flange Mean?

The term ANSI flange is widely used in purchasing, plant maintenance and replacement orders. It usually refers to pipe flanges made to U.S. class-rated dimensional standards. Older drawings may say ANSI B16.5, while newer documents often say ASME B16.5. In real RFQs, both phrases can appear, so the safest approach is to confirm the exact standard, size, class and face type.

ANSI and ASME wording should not be treated as a complete specification by itself. A useful flange order also needs flange type, material, bore, pipe schedule, pressure class, gasket face and inspection requirements.

Common ANSI Pipe Flanges

ANSI pipe flanges are used to connect pipe sections, valves, pumps, tanks, pressure vessels and process equipment. The most common types include weld neck, slip on, blind, socket weld, threaded and lap joint flanges.

Flange Type	Typical Use	Key Buyer Check
Weld neck flange	High pressure, high temperature and cyclic service	Pipe schedule, bore and welding procedure
ปลอกสวมหน้าแปลน	Utility and moderate pressure piping	Pipe outside diameter and fillet weld requirement
หน้าแปลนปิด	Closing pipe ends, vessel nozzles and inspection openings	Pressure class, face type and bolt set
หน้าแปลนเชื่อมแบบซ็อกเก็ต	Small-bore high pressure connections	Socket bore, expansion gap and weld quality
Threaded flange	Non-welded low-pressure or small-bore service where allowed	Thread type and sealing method
Lap joint flange	Alignment-friendly removable connections with stub ends	Stub end compatibility and backing flange fit

ANSI Fixed Flanges vs ANSI Rotatable Flanges

The phrase ANSI fixed flanges usually describes flanges whose bolt-hole position becomes fixed after welding or installation. Weld neck, slip on, socket weld and threaded flanges are typical examples. Their orientation must be checked before final welding or assembly.

ANSI rotatable flanges usually refer to loose or backing flange designs that can rotate for bolt-hole alignment before tightening. Lap joint flanges with stub ends are the best-known example. Rotatable designs can make installation easier when bolt-hole orientation is difficult to predict.

Item	ANSI Fixed Flanges	ANSI Rotatable Flanges
Movement before bolting	Limited after welding or threading	Can rotate before bolt tightening
Common examples	Weld neck, slip on, socket weld, threaded	Lap joint flange with stub end, loose backing flange
Main advantage	Strong direct connection to pipe or equipment	Easier bolt-hole alignment and maintenance
Main risk	Wrong clocking can delay installation	Stub end and backing flange must match
Typical use	Process piping, valves, pressure systems	Corrosion-resistant lines, removable spools, maintenance sections

Why ASME Flanges Are Often Called ANSI Flanges

Many buyers still say ANSI flanges because old plant documents and industry habits use that term. Modern flange standards are commonly published as ASME standards. For example, ASME B16.5 covers many metallic pipe flanges and flanged fittings up to NPS 24, while ASME B16.47 covers large diameter steel flanges in Series A and Series B.

When a customer asks for “flange flange” or flange flanges in a quick message, the supplier should clarify whether the request means ANSI/ASME class-rated flanges, metric PN flanges, custom flange plates or another connection system.

What About ASME Tubes?

The phrase ASME tubes may appear in equipment and heat exchanger discussions, but flange orders normally focus on pipe size, pipe schedule, bore and flange standard. Tubes and pipes can have different dimensional systems, so a tube connection should not be assumed to fit an ANSI flange bore without checking actual outside diameter and wall thickness.

If a flange must connect to tubing, heat exchanger nozzles or OEM equipment, send drawings and measured dimensions. This prevents bore mismatch, gasket problems and bolt-hole alignment issues.

Pressure Class, Face Type and Gaskets

ANSI pipe flanges are often described by class numbers such as Class 150, 300, 600, 900, 1500 and 2500. These are pressure-temperature rating categories, not fixed working pressure values. Allowable pressure changes with material and operating temperature.

Face type is equally important. Raised face, flat face and ring type joint designs require suitable gaskets and mating flanges. A flange ANSI request should clearly state RF, FF or RTJ to avoid sealing problems.

Where ANSI Flanges Are Used

Flanges are used wherever piping systems need removable, inspectable or standardized connections. ANSI and ASME flanges are common in oil and gas, chemical plants, water treatment, power generation, marine systems, HVAC, food processing, fire protection, industrial pumps, valves and pressure equipment.

The same flange family can serve many industries, but the details change. A water line may need a coated carbon steel slip on flange, while a chemical line may require stainless steel or alloy weld neck flanges with full certificates and strict inspection.

How to Specify an ANSI Flange Correctly

A clear RFQ should include standard, flange type, nominal pipe size, pressure class, material grade, pipe schedule or bore, face type, quantity, inspection documents and destination. For replacement work, add photos, drawings and measured bolt circle dimensions.

Example: ASME B16.5 slip on flange, NPS 4, Class 150, raised face, ASTM A105N, bore for SCH 40 pipe, black paint, MTC required. This is much clearer than simply writing “ANSI flange 4 inch.”

Common Ordering Mistakes

• Using ANSI flange wording without naming ASME B16.5, ASME B16.47 or another exact standard.
• Forgetting whether the project needs ANSI fixed flanges or ANSI rotatable flanges.
• Ordering without pipe schedule, bore or face type.
• Mixing ASME flanges with EN, DIN or JIS flanges by pressure name only.
• Assuming tube dimensions and pipe dimensions are interchangeable.
• Skipping certificates, marking and inspection checks for export projects.

FAQ About ANSI and ASME Flanges

What is an ANSI flange?

An ANSI flange is a common purchasing term for a class-rated pipe flange made to U.S. dimensional standards, often now referenced through ASME standards such as ASME B16.5.

What are ANSI fixed flanges?

ANSI fixed flanges are flange types whose bolt-hole position is fixed after welding, threading or assembly. Weld neck, slip on, socket weld and threaded flanges are common examples.

What are ANSI rotatable flanges?

ANSI rotatable flanges are loose flange designs that can rotate for bolt-hole alignment before tightening, such as lap joint backing flanges used with stub ends.

Are ANSI flanges and ASME flanges the same?

In many RFQs, ANSI flange is legacy wording, while ASME flange is the modern standard reference. Always confirm the exact standard, class, size and dimensions before production.

Need ANSI Pipe Flanges?

Songhai Flange supplies ANSI pipe flanges, ASME flanges, ANSI fixed flanges, ANSI rotatable flanges, weld neck flanges, slip on flanges, blind flanges, lap joint flanges and custom flange products for industrial piping, chemical equipment, power generation, marine systems, water treatment and OEM projects. Our team can support drawing review, material selection, machining, inspection and export packing.