When you compare types of flanges, the names can become confusing quickly. Weld neck and slip-on describe how the flange connects to pipe. Raised face and ring-type joint describe the sealing surface. Class 150 and Class 600 indicate rating classes, while ASME B16.5 and API 6A define different product scopes.
Treating those labels as interchangeable can lead to a joint that does not fit, seal, or satisfy your project specification.
This guide helps you separate those decisions. You will see the six common pipe flange types, five specialty designs, the main flange faces, and the standards used in industrial and oilfield systems. You will also get a practical selection process you can use before sending an RFQ.
The short answer is that most process-piping work starts with weld neck, slip-on, socket weld, threaded, lap joint, or blind flanges. Your final choice still depends on pressure and temperature, pipe size and schedule, fluid, loads, corrosion, inspection access, installation method, and the governing standard.
Key Takeaways
- The six common types of flanges are weld neck, slip-on, socket weld, threaded, lap joint, and blind.
- A flange type is not the same as a flange face. Raised face (RF), flat face (FF), and ring-type joint (RTJ) describe how the gasket or metal ring seals.
- Pressure class is not a fixed pressure in psi. The allowable pressure depends on material group, temperature, standard, size, and other design conditions.
- Weld neck flanges are a common starting point for demanding pressure, temperature, cyclic, or high-load service, but the piping specification controls the final choice.
- Slip-on flanges simplify fit-up, while socket weld and threaded flanges are generally associated with smaller-bore applications within the limits of the governing code.
- Lap joint flanges help with bolt-hole alignment and can reduce the amount of expensive alloy in a joint because the backing flange may not contact the process fluid.
- API 6A Type 6B and Type 6BX flanges belong to wellhead and tree equipment service. Do not select them by treating an API working-pressure rating as an ASME pressure class.
- You should quote the complete joint: flange standard, size, bore or pipe schedule, rating, facing, material, gasket, bolting, coating, testing, documentation, and mating connection.
What Is a Flange?
A flange is a projecting rim or forged connection used to join pipe, valves, pumps, vessels, manifolds, and other equipment. In a typical flanged joint, two compatible faces clamp a gasket or ring with bolts or studs. This gives you a connection that can be assembled, inspected, and dismantled without cutting the piping.
The flange is only one part of the joint. Sealing also depends on the mating face, gasket or ring, bolting, surface finish, alignment, assembly procedure, and operating conditions.
If you want a closer look at the hub, face, bore, bolt circle, holes, and other features, see Alienfrac’s guide to flange parts. For the operating principle, read how flanges work.
First, Separate Type, Face, Class, Material, and Standard
Before you compare flange designs, keep these five labels separate:
| Selection item | What it tells you | Examples |
|---|---|---|
| Flange type | How the component connects or what function it performs | Weld neck, slip-on, socket weld, threaded, lap joint, blind |
| Face | How the joint seals | Raised face, flat face, ring-type joint |
| Rating | The pressure–temperature rating system used by the standard | ASME Class 150–2500; API-rated working pressure |
| Material | The metallurgy and related properties | Carbon steel, low-temperature carbon steel, stainless steel, duplex, nickel alloy |
| Standard | The dimensional and technical rules | ASME B16.5, ASME B16.47, API 6A, EN 1092-1 |
You need all five to define a flange. “Four-inch RTJ flange” is incomplete because RTJ is a facing, not an attachment method, and the phrase does not identify the standard, rating, bore, material, or exact ring groove.
Quick Comparison of Common Flange Types
| Flange type | Pipe connection or function | Common reason to choose it | Main point to check |
|---|---|---|---|
| Weld neck | Butt-welded tapered hub | Demanding pressure, temperature, cyclic, or load conditions | Match the bore to the pipe schedule and control weld quality |
| Slip-on | Pipe slips through the flange and is secured with fillet welds | Easier fit-up for suitable general-service applications | Confirm service limits, weld access, and internal setback |
| Socket weld | Pipe enters a machined socket and is secured by an external fillet weld | Compact connection for small-bore welded piping | Confirm the assembly gap and suitability for crevice-corrosion conditions |
| Threaded | Screws onto threaded pipe | Installation without welding | Check the thread standard, pressure and temperature limits, and leakage risk |
| Lap joint | Loose backing flange used with a stub end | Easier alignment or reduced use of costly alloy material | Specify the matching stub end and compatible flange face |
| Blind | Solid closure with no through bore | Line isolation, pressure testing, or future access | Check thickness, facing, tapping requirements, and stored-energy controls |
The Six Main Types of Flanges
1. Weld Neck Flange
A weld neck flange has a long, tapered hub and joins the pipe with a full-penetration butt weld. The bore is normally specified to suit the pipe inside diameter or schedule, giving you a smoother transition through the joint than designs with an internal ledge.
You will often consider a weld neck flange for high pressure, elevated or low temperature, cyclic loading, vibration, bending, or other demanding service. The tapered hub helps transfer stress between the pipe and flange, and the butt weld can be examined with the non-destructive examination method required by your piping specification.
The trade-off is more exact fit-up, welding, and inspection. You must state the pipe schedule or wall thickness because the flange bore and weld preparation need to match the pipe. If the bore is wrong, you may create internal mismatch, poor weld geometry, or an unwanted flow restriction.
2. Slip-On Flange
A slip-on flange has a bore larger than the pipe outside diameter, allowing the flange to slide over the pipe for alignment. It is normally attached with fillet welds at the outside and inside, subject to the applicable standard and fabrication procedure.
You may choose this type when fit-up speed and lower initial flange cost matter and the service is suitable for a slip-on connection. It is common in general industrial piping where design pressure, temperature, fatigue, and external loads are less demanding than services that call for a weld neck flange.
Do not compare flange purchase price alone. A slip-on design uses two fillet weld locations, and the internal weld must not damage the gasket seating surface. Welding labor and inspection can narrow the apparent cost gap.
Before you specify a slip-on flange, check whether your piping code or project specification permits it for the fluid, pressure, temperature, corrosion allowance, cyclic duty, and examination category. Also confirm the pipe insertion and setback dimensions with the approved fabrication detail.
3. Socket Weld Flange
A socket weld flange contains a counterbored recess. You insert the pipe into the socket, withdraw it to the assembly gap required by the approved procedure, and make an external fillet weld.
This compact design is generally used in smaller-bore welded piping. It can avoid the butt-weld preparation required by a weld neck flange when the governing specification allows it.
The internal socket creates a crevice. That detail matters if your fluid can promote crevice corrosion, solids can collect, or cleanliness is critical. The pipe end and socket also need correct dimensions; pushing the pipe hard against the shoulder and welding it in that position can leave no allowance for thermal expansion during welding.
State the nominal pipe size, schedule, material, rating, facing, and required socket dimensions. Do not treat socket weld and slip-on flanges as interchangeable just because both use fillet welds.
4. Threaded Flange
A threaded flange has an internal pipe thread that mates with externally threaded pipe. It lets you assemble a flanged connection without welding, which can be useful where hot work is restricted or where the material is difficult to weld.
Threaded flanges are usually considered for smaller sizes and less severe duties within code and specification limits. They can appear in utilities, air, water, instrumentation, and other services where the selected thread form and sealing method are acceptable.
Threads reduce the effective pipe wall and may loosen or leak under temperature cycling, vibration, bending, or repeated assembly. A sealant cannot correct incompatible thread forms or a connection used outside its permitted service.
Specify the exact thread standard and form, not simply “threaded.” NPT, BSPT, and other threads are not automatically compatible. Confirm whether seal welding is required or prohibited and whether the fluid service permits a threaded joint.
5. Lap Joint Flange
A lap joint assembly uses two pieces: a stub end that is butt-welded to the pipe and a loose backing flange that sits behind it. The backing flange can rotate, making bolt-hole alignment easier during installation or repeated dismantling.
This arrangement is attractive when you use corrosion-resistant pipe. The wetted stub end can match the process material while the backing flange may be made from a less costly material, provided the design, environment, and specification allow it. You can also reuse a sound backing flange when replacing a worn or corroded stub end.
The stub end forms the sealing surface, so order the two parts as a compatible assembly. Check stub-end type and length, lap thickness, corner radius, face finish, gasket, backing-flange bore, and rating.
Use this type where alignment, frequent access, lined piping, or alloy-cost control makes the two-piece design worthwhile.
6. Blind Flange
A blind flange is a solid closure used to terminate a pipe, valve, vessel nozzle, or equipment opening. Because it is bolted, you can remove it for inspection, cleaning, testing, or future expansion.
Blind flanges see bending from internal pressure and bolt loading. Large diameters and high ratings can become heavy, so you should plan lifting, handling, bolting, and safe removal. Never loosen a blind until the isolated volume is depressurized, drained, and verified under the site’s energy-isolation procedure.
You may also encounter tapped blind flanges for drains, vents, instrumentation, or test connections. The design, drawing, machining control, and acceptance requirements must cover the tapped hole.
Your RFQ should identify the standard, size, rating, face, material, thickness or drawing, and whether tapping, lifting points, coating, or special testing is required.
Five Specialty Flange Designs
7. Long Weld Neck Flange
A long weld neck flange extends the hub of a standard weld neck design. It is often used as an equipment or vessel nozzle where the extended neck can replace a separate pipe neck and flange combination.
You should provide the full neck length, bore profile, wall thickness, weld end, facing, material, and design basis. Because many long weld neck flanges are made to project drawings, a familiar nominal size and class may not define the entire component.
8. Reducing Flange
A reducing flange connects a larger flange pattern to a smaller bore. It can save space compared with installing a separate reducer and flange, but the sudden change in flow area may increase turbulence, pressure loss, erosion, or solids accumulation.
Use it only after checking the process and piping implications. State both the flange size and reduced bore, along with pipe schedule, facing, rating, material, and flow direction if the design is not symmetrical.
9. Expander Flange
An expander flange combines a weld neck-style connection with an enlarged flange end, allowing you to connect a smaller pipe to a larger mating flange without a separate concentric reducer in some layouts.
The compact arrangement can reduce weld count and overall length. Check transition geometry, stress, flow effects, availability, and the manufacturer or project standard.
10. Orifice Flange
An orifice flange set supports an orifice plate used for differential-pressure flow measurement. The pair normally includes pressure-tapping provisions and requires controlled bore, plate, gasket, tapping orientation, and upstream/downstream installation.
You are specifying a measurement assembly, not two ordinary flanges with holes. Include the metering standard, line size and schedule, rating, facing, material, tap details, orifice plate, jack screws if required, and inspection documents.
11. Spectacle Blind, Spade, and Spacer
Spectacle blinds, spades, and spacers provide visible line isolation between two flanges. A spectacle blind joins a solid blade and an open ring; rotating it places either the blind or open side in the line. A separate spade and spacer perform the same isolation/open functions as two components.
These are line blanks rather than pipe-attachment flange types, but buyers often group them under types of flanges. Specify the matching flange standard and rating, line size, thickness, material, facing or surface finish, handle arrangement, and required identification. Allow enough space to spread the joint safely during changeover.
Flange Face Types: RF, FF, and RTJ
The flange face determines where and how the seal is formed. You cannot choose it independently of the mating flange, gasket or ring, rating, material, and service.
Raised Face
A raised face flange has a gasket seating area elevated above the bolting circle. It concentrates bolt load over a smaller gasket area and is widely used in process piping. The required surface finish and gasket type depend on the standard and service.
Flat Face
A flat face flange has a sealing surface in the same plane as the surrounding face. It is commonly associated with full-face gaskets and equipment or piping materials that may be damaged by the bending created when a raised face is tightened against them.
Do not mate RF and FF flanges unless the governing design and equipment manufacturer explicitly approve the arrangement. The joint geometry and gasket loading may not behave as intended.
Ring-Type Joint
An RTJ flange has a machined groove for a metal ring gasket. It is used in pressure and temperature services where the selected standard calls for a ring-joint connection. The ring designation and groove geometry must match; similar-looking R, RX, and BX rings are not general substitutes.
RTJ describes the face and sealing system. It does not tell you whether the flange is weld neck, blind, or another design. For a direct comparison, see RF vs RTJ flange and Alienfrac’s broader flange face guide.
Other facings include tongue-and-groove and male-and-female designs. Use them as matched pairs with the correct gasket and dimensions.
Standards and Rating Systems You Will Meet
ASME B16.5
ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 through NPS 24. Its scope includes pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and methods of designating openings. The standard uses rating classes rather than a single universal working pressure.
ASME B16.47
ASME B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60. It includes Series A and Series B dimensional families. Two flanges with the same nominal size and class but different series do not share the same dimensions, so the series must appear on your drawing and RFQ.
API Specification 6A
API 6A applies to wellhead and tree equipment. API 6A Type 6B and Type 6BX are ring-joint flange families used for defined nominal bore sizes, working-pressure ratings, temperature classes, material classes, product specification levels, and other specification requirements.
Do not convert an API rated working pressure into an assumed ASME class. Match the complete API connection, including flange type, nominal bore, rated working pressure, ring gasket, material class, temperature class, PSL, and mating equipment requirements. Alienfrac explains the application boundary in API vs ASME flanges and compares API 6B and 6BX flanges.
Standards change. Before you order, confirm the edition and any addenda, errata, project supplements, or purchaser requirements named in the contract. Alienfrac’s flange standards guide provides a useful starting map.
How to Choose the Right Type of Flange?
1. Start with the Governing Design Documents
Check the piping code, line class, equipment specification, datasheet, drawing, and purchaser requirements. These documents may already limit flange type, facing, material, and rating.
2. Define Pressure and Temperature Together
Give the design pressure, design temperature, operating range, start-up and upset conditions, vacuum if applicable, and any pressure cycling. Then select the rating from the correct standard’s pressure-temperature tables for the material group. Read more about why class is not a direct psi value in the flange pressure class guide.
3. Define Pipe and Mating Dimensions
State nominal pipe size, outside diameter, schedule or wall thickness, bore, corrosion allowance, and mating connection. For weld neck, socket weld, reducing, and expander designs, the pipe dimensions directly affect the flange bore and weld detail.
4. Describe the Fluid and Environment
List the process fluid, concentration, water content, solids, H2S or CO2 exposure, chlorides, cleaning chemicals, and external environment. Material selection may need corrosion allowance, hardness control, impact properties, sour-service controls, overlay, cladding, coating, or a corrosion-resistant alloy. Use Alienfrac’s flange materials guide to organize the discussion.
5. Check Loads and Duty
Include piping weight, thermal expansion, vibration, pulsation, bending, nozzle loads, seismic or wind loads, and cyclic service. A flange pressure rating does not by itself approve every external load combination.
6. Choose a Compatible Face, Gasket, and Bolting Set
Treat the two flanges, gasket or ring, studs, nuts, lubricant, and tightening procedure as one joint. Confirm face compatibility, surface finish, ring number or gasket dimensions, bolt material, coating, and assembly method.
7. Plan Fabrication, Inspection, and Maintenance
Decide whether welding is allowed, which procedures and welder qualifications apply, what NDE is required, and how the joint will be inspected or accessed later. Consider alignment, tool clearance, lifting, insulation, coating repair, and the space required to remove bolts or insert a line blind.
8. Send a Complete RFQ
For each flange, state:
- Standard and edition
- Flange type
- Nominal size or API nominal bore
- Pipe schedule, wall thickness, or required bore
- ASME class or API rated working pressure
- Facing and surface-finish requirement
- Material grade, heat treatment, impact or hardness requirements
- Sour-service, corrosion, coating, or overlay requirements
- Gasket or ring designation
- Bolting scope
- Quantity and mating connection
- Dimensional drawing for nonstandard items
- NDE, pressure testing, dimensional inspection, marking, and document requirements
- Third-party inspection, packing, preservation, and delivery terms
If you need API oilfield connections, Alienfrac manufactures API flanges and API 6A flanges for project-specific requirements. Send the complete connection data rather than asking for an “API flange” without its bore, rating, material, and seal.
Common Selection Mistakes
Calling RTJ a Flange Type
RTJ is a facing and metal-ring sealing system. You still need the attachment or functional type, such as weld neck or blind.
Treating Class as PSI
Class 300 does not mean 300 psi. Allowable pressure varies with material and temperature under the applicable standard.
Ordering by Nominal Size Alone
Nominal size does not define bore, pipe schedule, class, face, bolt pattern, or standard. A flange can share the same size label and still be physically incompatible.
Mixing Standards or Series
ASME, API, EN, JIS, and other systems use different dimensions and rating rules. Even within ASME B16.47, Series A and Series B differ.
Ignoring the Complete Joint
A suitable flange cannot compensate for the wrong gasket, ring, bolts, surface finish, alignment, or assembly procedure.
Adding Requirements After Production
Material traceability, special marking, NDE, third-party witness, and data-book requirements should be agreed before manufacturing. Late requests may require rework or may be impossible to verify retrospectively. Confirm identification needs with the flange marking guide.
FAQ
What are the main types of flanges?
The six main pipe flange types are weld neck, slip-on, socket weld, threaded, lap joint, and blind. Specialty designs include long weld neck, reducing, expander, and orifice flanges. Spectacle blinds, spades, and spacers are line-isolation devices often discussed with flange types.
Which flange type is best for high pressure?
A weld neck flange is often selected for demanding high-pressure or cyclic process piping because of its butt weld and tapered hub. In wellhead and tree service, an API 6A connection such as Type 6B or 6BX may apply instead. The correct choice comes from the governing standard, material, temperature, loads, size, and project specification—not pressure alone.
What is the difference between a weld neck and a slip-on flange?
A weld neck flange butt-welds to the pipe through a tapered hub, while a slip-on flange slides over the pipe and is normally attached with fillet welds. Weld neck designs suit more demanding loads and cyclic service; slip-on designs can simplify alignment in suitable general service. Compare total fabrication and inspection cost, not only flange price.
Is an RTJ flange a separate flange type?
RTJ is a flange-facing and sealing arrangement that uses a metal ring in machined grooves. A flange can be both weld neck and RTJ, or blind and RTJ. You must specify both the flange type and the facing.
What is the difference between raised face and flat face flanges?
A raised face concentrates gasket load on an elevated sealing area. A flat face keeps the sealing surface in the same plane as the surrounding face and is often used with a full-face gasket. The mating equipment, material, gasket, and governing standard determine which face you should use.
Can you connect API and ASME flanges?
Do not assume they mate because the nominal size or pressure looks similar. Bolt patterns, bores, faces, ring grooves, dimensions, and rating systems may differ. Use an engineered adapter or crossover designed for the two exact connections when the system requires a transition.
What information should you send to a flange manufacturer?
Send the standard and edition, type, size or bore, pipe schedule, rating, face, material, service conditions, gasket or ring, mating connection, quantity, drawing, tests, inspection, marking, documents, and delivery terms. That information lets the manufacturer check compatibility and quote the correct scope.
Choose the Connection as a Complete System
Knowing the names of the types of flanges is useful, but your purchasing decision goes beyond the name. A weld neck, slip-on, or blind flange only works as intended when its standard, dimensions, rating, face, material, gasket, bolting, and mating component belong together.
Start with the design documents and real service conditions. Then define the complete joint in the RFQ and ask the supplier to list every assumption or deviation. You will receive quotations that are easier to compare and reduce the chance of discovering a mismatch during installation.
For an application-specific review, send Alienfrac your flange standard, size or bore, rating, face, material class, temperature, fluid, mating connection, quantity, drawing, inspection plan, and document list. The team can review the connection and prepare a line-by-line quotation for the flange, sealing components, testing, documentation, and export packing you require.