Pipe Fittings Types (ASME B16.9)

Quick Reference

ASME B16.9 buttweld fittings for pipes >2” diameter: elbows (45°/90°/180°, LR=1.5D, SR=1D), tees (equal/reducing/barred), reducers (concentric/eccentric), caps, stub ends. For pipes ≤2”, use ASME B16.11 forged fittings. Materials: ASTM A234 (carbon/alloy), A403 (stainless), A815 (duplex), B366 (nickel alloy).

What Are Pipe Fittings?

Pipe fittings connect, redirect, branch, or terminate sections of a piping system. An elbow changes direction; a tee branches flow; a reducer transitions between pipe sizes; a cap seals an open end. These are the building blocks that every piping engineer works with when laying out process piping, oil and gas pipelines, and industrial plant systems.

Types of Pipe Fittings

Buttweld Fittings Overview

Buttweld fittings change direction, branch off, or join sections of a piping system. They are welded directly to the pipe, producing a continuous, leak-free flow path with no sockets, threads, or mechanical joints in between.

These fittings dominate piping in oil and gas, power generation, chemical processing, and water treatment because the full-penetration butt weld delivers the strongest joint available. Selecting the correct fitting type, material, and size is a routine but critical part of piping design.

Bear in mind that for piping systems below 2 inches in diameter, socket weld and threaded (forged fittings) covered by the ASME B16.11 Specification, are used instead.

Buttweld fittings come in multiple shapes (elbows, tees, reducers, crosses, caps, stub ends), material grades (carbon, high-yield carbon, low-alloy, stainless, duplex, nickel alloy, and non-ferrous metals), and dimensions (2 to 24 inches seamless, welded for larger sizes).

Common Shapes of Buttweld Fittings

Asme b16.9 buttweld fittings classification

Fitting Type	Function	Variants
Elbows	Change flow direction	45°, 90°, 180° (return); LR (1.5D), SR (1D); reducing
Tees	Branch or combine flow	Equal, reducing, barred
Reducers	Connect different pipe sizes	Concentric (centered), eccentric (offset)
Caps	Seal pipe end	Standard, reducing
Stub Ends	Used with lap joint flanges	Type A, Type B, Type C

Elbows: Change flow direction. 45° and 90° most common; 180° (return elbows) for tight loops.

Tees: T-shaped fittings that split or combine flow. Equal tees have the same size on all three ends; reducing tees have a smaller branch diameter.

Reducers: Connect different pipe sizes:

• Concentric: Central axis of symmetry, uniform taper - standard where centerline alignment is acceptable
• Eccentric: Off-center taper - maintains same top (or bottom) level, prevents liquid accumulation in horizontal runs

Caps: Seal pipe ends, stopping flow. Critical when future expansion may be needed.

Stub Ends: Used with lap joint flanges for easy disconnection and inspection.

All these shapes are explored in greater detail below.

Materials for BW Fittings

Buttweld fittings are manufactured from a variety of materials, selected based on temperature, pressure, and corrosion requirements:

Material	Typical Specification	When to Use
Carbon Steel	ASTM A234 WPB	Standard service, high P/T, cost-effective
Low-Temp Carbon Steel	ASTM A420 WPL6	Service below -29°C
Alloy Steel	ASTM A234 WP11, WP22, WP91	High temperature (refineries, power plants)
Stainless Steel	ASTM A403 WP304, WP316	Corrosive service
Duplex/Super Duplex	ASTM A815	High strength + corrosion resistance
Nickel Alloys	ASTM B366	Severe corrosion, extreme temperatures
Non-Ferrous	Aluminum, Cupronickel, Copper	Seawater, desalination, specialty applications

Material specifications for buttweld fittings fall under ASME SA/A 234 (wrought carbon and alloy steel for moderate and high-temperature service) and corresponding ASTM standards.

These specifications govern manufacturing integrity and safety, covering everything from general low-pressure service to high-pressure, high-temperature critical service. Manufacturers, engineers, and end-users rely on these standards for selection, installation, and maintenance of buttweld fittings.

BW Fittings: Fields of Application

Buttweld fittings are the standard choice where the piping connection must withstand high pressure, extreme temperatures, or corrosive substances. Typical sectors include:

Industry	Reason for Buttweld
Petrochemical and refineries	High pressure, corrosive process fluids
Power plants	High-pressure steam and hot water systems
Water treatment plants	Durable, leak-free joints for large-volume water handling
Food and beverage	Stainless steel grades provide hygienic, corrosion-resistant surfaces

Advantages of Buttweld Fittings

The full-penetration weld creates the strongest joint type in piping. Once welded, the fitting and pipe behave as a single piece of metal, producing a smooth bore with no crevices for corrosion or turbulence. This translates to three practical benefits: the joint carries the full pressure rating of the pipe, the smooth internal surface minimizes friction losses, and there are no gaskets or threads to leak over time.

Specifications for BW Fittings (ASME/MSS)

ASME and MSS specifications govern dimensions, material grades, tolerances, manufacturing processes, testing, and marking of buttweld fittings. Always verify the correct specification applies to your project before ordering. Here is an overview of the key standards:

ASME Specifications

ASME B16.9 – Factory-Made Wrought Buttwelding Fittings:

This standard covers overall dimensions, tolerances, ratings, testing, and markings for wrought carbon and alloy steel buttwelding fittings of NPS 1/2 through NPS 48 (DN 15 through DN 1200). It includes elbows, tees, reducers, lap joint stub-ends, and caps.

ASME B16.28 – Wrought Steel Buttwelding Short Radius Elbows and Returns:

Covers design, dimensions, tolerances, and material requirements for short-radius elbows and returns. This standard applies to fittings that do not require the full range of material grades and design pressures in ASME B16.9.

ASME B16.25 – Buttwelding Ends:

Specifies weld-end preparation for piping components joined by welding. It defines weld types, dimensions, and finishes for the beveled ends.

MSS Specifications

MSS SP-43 – Wrought Stainless Steel Butt-Welding Fittings:

Covers dimensions, tolerances, and material requirements for stainless steel butt-welding fittings in NPS 1/2 through 24 inches. Focused on low-pressure, corrosion-resistant service.

MSS SP-75 – Specification for High-Test, Wrought, Butt-Welding Fittings:

Covers high-strength, seamless and welded butt-welding fittings for high-pressure, high-temperature service in oil and gas pipelines. Applies to carbon steel and alloy steel materials.

MSS SP-97 – Integrally Reinforced Forged Branch Outlet Fittings – Socket Welding, Threaded, and Buttwelding Ends:

Addresses branch outlet fittings intended to be butt welded, socket welded, or threaded to the run pipe. Covers design, dimensions, material, and testing requirements.

Pipe Fitting Types and Applications

Buttweld Elbow 45/90/180 Deg.

ASME B16.9 buttweld elbows are manufactured in a full range of outside diameter and wall thickness combinations to fit ASME B36.10 and ASME B36.19 carbon, alloy, and stainless steel pipes from 2 to 48 inches (and above).

Buttweld elbows below 24 inches are produced by cutting, heating, and bending seamless steel pipes. Elbows of larger sizes are manufactured from welded pipes or steel plates.

The three most common elbow types in piping are the 45, 90, and 180 degrees (return elbow):

Special Types of BW Elbows

Some special types of pipe elbows are:

Reducing elbow

A reducing elbow changes flow direction and reduces pipe size in a single fitting. It combines the function of an elbow (typically 90° or 45°) with that of a reducer. This fitting saves space and reduces the number of joints where a separate elbow-plus-reducer arrangement would be impractical or too costly.

Reducing Elbow

Feature	Description
Direction Change and Size Reduction	Allows for a directional change while transitioning to a smaller pipe size in a single fitting
Angle	Most commonly 90-degree and 45-degree; custom angles can be manufactured
End Connections	Designed to match differing pipe sizes; prepared for buttwelding, threaded, or other joining methods
Material Variety	Carbon steel, stainless steel, alloy steel, and others for different P/T and corrosion requirements

Reducing elbows appear in chemical processing, oil and gas pipelines, water treatment, and HVAC systems wherever the piping layout requires a direction change and size transition at the same point.

The single-fitting design saves space, lowers material cost and installation labor, and eliminates one weld joint, reducing both leak potential and pressure drop.

When selecting a reducing elbow, confirm the material is compatible with the process fluid and operating conditions, the size reduction and angle satisfy the design requirements, and the fitting conforms to the applicable ASME or MSS specification.

Mitered elbow

Mitered elbows are fabricated by cutting straight pipe sections at calculated angles and welding them together. Unlike standard factory-made elbows, mitered elbows are field- or shop-fabricated from multiple pipe segments (called “miters”). This approach allows for any angle, and it is often the most practical option for large-diameter piping where purchasing a factory-made elbow would be expensive or involve long lead times.

The number of miters depends on the required angle and bend radius. More segments produce a smoother flow path but require additional welding and fit-up work.

Characteristic	Description
Customization	Tailored to specific angle requirements, offering flexibility for unique or complex piping layouts
Cost-effectiveness	More economical than specially cast fittings for large-diameter pipes or uncommon angles
Versatility	Made from a variety of materials, matching the system’s P/T and corrosion resistance requirements

Mitered elbows are common in large-diameter water supply and distribution, HVAC ductwork with non-standard angles, and process piping in chemical, petrochemical, and power generation plants.

	Details
Advantages	Flexibility in design; cost-effective for large diameters; material matching with existing piping
Disadvantages	Increased leak potential (multiple welds); possible turbulent flow if poorly designed; requires precise fabrication

When specifying a mitered elbow, account for flow requirements (minimize turbulence and pressure drops), the number of miters (more miters produce smoother flow but add complexity), weld quality (critical for leak prevention), and inspection/testing scope (thorough NDE is necessary given the fabricated nature of these fittings).

Short Vs. Long-Radius BW Elbows

The center-to-face distance of a long-radius (LR) elbow is 1.5 times the nominal pipe size. For a short-radius (SR) elbow, that distance equals the nominal pipe size.

Example:

• For a 4 inches long radius (LR) butt weld elbow, the center-to-face distance is 4 x 25.4 x 1.5 = 152.4 mm;
• For a short radius butt weld elbow, the center-to-face distance is instead 4 x 25.4 x 1 = 101.6 mm. Short-radius pipe elbows are available only for the 90 and 180-degree configuration (and not for the 45 degrees); instead, long-radius elbows are available for all degrees.

90 Degrees Long Radius Elbow

Short-Radius (SR) Elbows

SR elbows have a center-to-face radius equal to the nominal pipe diameter (1D). A 4-inch SR elbow has a 4-inch radius. They are most commonly supplied in 90°. Use SR elbows where space is genuinely tight and a long-radius elbow will not fit. The trade-off is a sharper turn that creates higher pressure drop and more turbulence than an LR elbow of the same size.

Long-Radius (LR) Elbows

LR elbows have a radius of 1.5 times the nominal diameter (1.5D). A 4-inch LR elbow has approximately a 6-inch radius. Available in 90° and 45° as standard, LR elbows are the default specification for most piping systems. The gentler curve lowers pressure drop and reduces erosion at the outer wall of the bend. They appear in everything from water supply and HVAC to chemical and petrochemical processing.

Key Differences Summarized (SR Vs. LR Elbows)

Parameter	Short Radius (SR)	Long Radius (LR)
Center-to-face	1 x NPS	1.5 x NPS
Pressure drop	Higher (sharp turn)	Lower (gentle curve)
Space requirement	Compact	Requires more room
Available angles	90° and 180° only	45°, 90°, 180°
Default choice?	No (space-constrained use only)	Yes (standard specification)

Pro Tip

Always default to long-radius elbows unless space forces you to use short-radius. The pressure drop difference may seem small on paper, but over the life of a plant with dozens of elbows, it adds up in pump energy costs.

Select the elbow type based on the available space, the allowable pressure drop, and the process flow requirements. On most projects, the piping specification will already mandate LR elbows as the standard, with SR elbows permitted only where explicitly justified by the layout.

Pipe Elbows Manufacturing Process

The manufacturing of buttweld elbows transforms raw material into a finished product that meets ASME B16.9 dimensional and metallurgical requirements. The process follows these stages:

Step	Process	Description
1	Material Selection	Selection of pipe section or solid cylindrical steel billet based on temperature, pressure, and corrosion requirements
2	Cutting	Pipe cut to required length, or billet cut into discs of appropriate thickness
3	Heating	Heated in a controlled furnace to make material pliable without altering steel properties
4	Forming	Mandrel method (LR elbows), hot pushing method (LR and SR), or cold forming (smaller sizes)
5	Heat Treatment	Normalizing or annealing to relieve forming stresses and improve mechanical properties
6	Machining	Ends machined to create beveled edges for buttwelding
7	Cleaning/Finishing	Scale and contaminant removal; coatings or linings applied as needed
8	Inspection/Testing	Visual inspection, dimensional checks, NDT (radiography, ultrasonic), and pressure testing

The forming step (Step 4) uses one of three methods depending on size and material:

• Mandrel Method: Heated pipe placed over a shaped die and pushed through a forming die to bend into the elbow shape. Used primarily for long-radius elbows.
• Hot Pushing Method: Heated pipe pushed through a shaping die by hydraulic ram while being bent around a curved mandrel. Used for both LR and SR elbows.
• Cold Forming: Formed at room temperature; requires more force and may involve cold drawing. Used for smaller sizes or specific materials.

Mandrel Process

Cold Forming

Source: TK Bend

Pipe Bends

Pipe bends allow directional change in a piping system. Their function is the same as an elbow, but they are produced by bending a straight pipe rather than by factory-forming a separate fitting. This distinction matters in pipeline work, where long-radius bends (3D and 5D) reduce pressure drop and pig passage resistance more effectively than standard elbows.

Types of Pipe Bends

Pipe bends are classified by their bend radius relative to the nominal pipe diameter:

Type	Bend Radius	Typical Use
Short Radius (SRB)	1D	Tight spaces; higher pressure drop
Long Radius (LRB)	1.5D	General use; moderate pressure drop
3D Bend	3D	Pipelines; smoother flow, piggable
5D Bend	5D	High-pressure pipelines; lowest flow resistance, piggable

Manufacturing Processes

Three production methods cover most pipe bend requirements:

Method	Process	Best For
Hot bending	Pipe section heated in a furnace, bent over a mandrel or die	Large diameters, tight radii
Cold bending (induction)	Localized heating via induction coil; pipe pushed through bending machine	Precision bends; retains original pipe strength
Welded fabrication	Pipe segments cut at angles and welded together	Non-standard shapes where standard bends do not exist

Welded fabrication introduces multiple weld seams that represent potential failure points; thorough NDE is required.

Design Considerations

When incorporating pipe bends into a system, the following factors require attention:

Factor	What to Check
Flow dynamics	Bend angle, radius, and diameter affect velocity, pressure drop, and erosion/cavitation potential
Material compatibility	Bend material must suit the conveyed fluid in terms of corrosion, temperature, and pressure
Support and anchorage	Flow forces at bends can displace piping; proper supports and anchors prevent movement and fatigue
Thermal expansion	Temperature cycling stresses bends; accommodate this with expansion loops, bellows, or material selection

Applications

Pipe bends serve across multiple sectors. In oil and gas, they route pipelines across terrain and within processing facilities. In power generation, they form part of cooling systems, steam lines, and exhaust routes. In building construction, they appear in HVAC, water supply, and drainage systems where the piping must follow architectural constraints.

Selecting the correct bend radius and manufacturing method directly affects long-term system efficiency and maintenance costs.

Buttweld Tee

Equal Tee

An equal tee, otherwise called a straight tee, is a buttweld fitting used to branch a pipeline, or any other pipework, at 90 degrees.

A pipe tee is defined as “equal” when the bore size at the run and branch sides have the same diameter. An equal tee is, therefore, used to connect two pipes of the same nominal diameter.

Equal tees are available in sizes from half an inch to 48 inches (or larger) and in seamless and welded execution (seamless up to 24 inches, welded for tee sizes above 24 inches).

Buttweld tees are manufactured according to ASME B16.9 (carbon and alloy) and MSS-SP 43 (stainless steel and nickel alloy).

Reducing Tee

A reducing tee features a smaller bore size at the branched pipe side (generally 2/3 sizes smaller).

In case a larger bore size reduction is necessary, then a reinforced branch connection (such as a Weldolet) shall be preferred to prevent turbulence and have a smoother flow reduction.

The standard dimensions and tolerances of equal and reducing pipe tees are covered by the ASME B16.19 specification (carbon and alloy steel) and by the MSS SP 43 spec (for stainless steel and nickel alloy tees).

Barred Tee

A barred tee is a special type of (equal) tee used for pigging operations (reducing barred tees does not exist, as the pig would not pass through the reduced area anyway).

A barred tee features a welded restriction on the branch pipe side that prevents the pig from flowing from the run pipe into the branched pipe. Such barred restriction is welded on the internal side of the fitting and looks like a steel cage.

Pipe Tee Manufacturing Process

Manufacturing a buttweld tee follows a sequence of operations, each controlled to meet ASME B16.9 dimensional and metallurgical requirements:

1. Material Selection: Choose a grade that matches the service temperature, pressure, and corrosion environment. Common choices are carbon steel (A234 WPB), stainless steel (A403 WP304/316), and alloy steel (A234 WP11/22).

2. Cutting and Shaping:

   • The pipe is cut to the required run length using a saw or cutting machine.
   • The run is formed by extrusion (heating a pipe section and pushing it through a die) or, less commonly, by bending.

3. Branch Formation:

   • A hole is punched (hot-formed tees) or drilled (cold-formed tees) in the run.
   • For extruded tees, the branch grows directly from the run material. For fabricated tees, a separate pipe section is inserted and welded to the run.

4. Welding (if necessary): When the branch is not extruded integrally, it must be welded to the run. GTAW, SMAW, and GMAW are the common processes. Weld procedure qualification per ASME Section IX is mandatory.

5. Heat Treatment: Post-forming heat treatment (annealing, normalizing, or stress relieving) restores mechanical properties disturbed by hot or cold work.

6. Finishing and Inspection:

   • Weld excess is ground, surfaces are finished, and any protective coatings are applied.
   • Inspection includes dimensional checks, visual examination, NDE (radiography or ultrasonic testing of welds), and hydrostatic pressure testing.

7. Marking and Packaging: Finished tees are stamped with size, material grade, heat number, and manufacturer per ASME/ASTM requirements, then packaged for shipment.

BW Tee Manufacturing ProcessSource: TK Bend

Buttweld Cross

A buttweld cross creates a four-way branch in a piping system, splitting or collecting flow in four directions. The fitting has one inlet and three outlets (or the reverse), forming a cross shape. Its beveled ends accept buttweld connections to pipes of matching diameters.

The central intersection is typically reinforced to handle the stresses of diverging or converging flows. Buttweld crosses appear in chemical processing, petrochemical, wastewater treatment, and oil and gas distribution where multi-directional flow splitting is needed.

ASME B16.9 and MSS SP43 define pipe cross dimensions and tolerances. There are no specific limitations on available sizes and schedules.

BW Pipe Cross

Buttweld Reducers

Concentric and eccentric reducers both connect pipes of different diameters. They serve the same basic function but are specified for different piping orientations and process conditions.

Concentric Reducer

Concentric reducers are the most common type of buttweld reducer. They are cone-shaped, and both ends share the same centerline axis. This symmetrical design makes them the standard choice for vertical piping and any application where maintaining the pipe centerline is acceptable.

The uniform taper distributes flow evenly and minimizes localized erosion. The regular shape also simplifies insulation work.

The open ends of a concentric reducer are aligned and centered one to the other. Generally, this type of pipe reducer is used to modify the bore size of the pipeline by two (maximum three) measures, to avoid an excessive pressure drop in the pipeline.

If a larger reduction is needed, then a sequence of reducers shall be used to have a smooth and gradual adjustment of the pipeline bore size (vs. a drastic change).

For small bore size reductions, reducing flanges may be an alternative to buttweld reducers.

BW Concentric Reducer

Eccentric Reducer

Eccentric reducers are used, generally, for pipelines installed in a horizontal position (whereas concentric reducers are used for pipelines installed vertically or for the inlets of suction pumps, as top flat eccentric reducers).

Alternatively, to the concentric reducer, the open ends of an eccentric reducer are in an “offset” position from one to the other.

The offset design keeps one edge (top or bottom) flush with the connecting pipe. This prevents air pockets from forming at the top of liquid lines and prevents sediment from collecting at the bottom of gas lines. In pump suction piping, the flat-on-top orientation prevents vapor accumulation that would cause cavitation.

Common Mistake

Installing eccentric reducers upside down at pump suctions. For pump suction, use “flat-on-top” (FOT) to prevent vapor pockets. For drainage lines, use “flat-on-bottom” (FOB) to allow complete draining. Getting this wrong causes cavitation and pump damage.

BW Eccentric Reducer

Top and Bottom Flat Eccentric Reducer

Eccentric reducers can be top flat (“TF”) or bottom flat (“BF”).

The bottom flat eccentric reducer type is used for pipe racks (the bottom of the reducer is flat and at the same level as the bigger-sized run pipe), while the top flat pipe reducer type is used at the inlet of piping suction pumps as shown in the image below (this setup prevents the formation of vapor traps in the pipeline):

Top Flat and Bottom Flat Reducer

Concentric vs Eccentric Reducer

The fundamental difference is centerline alignment. A concentric reducer maintains a shared centerline between inlet and outlet; an eccentric reducer offsets the centerline so that one edge (top or bottom) stays flush.

Parameter	Concentric	Eccentric
Centerline	Shared between inlet and outlet	Offset; one edge flush
Orientation	Vertical piping or where centerline alignment is needed	Horizontal piping
Air/sediment	Can trap air in horizontal liquid lines	Flat-on-top prevents air pockets; flat-on-bottom prevents sediment buildup
Flow handling	Handles turbulent and laminar flow; symmetrical taper	Manages air entrapment and sediment issues in horizontal runs

BW Reducer Manufacturing Process

The manufacturing of buttweld reducers proceeds through these stages:

1. Material Selection: Choose a grade that matches the temperature, pressure, and corrosion requirements. Carbon steel, stainless steel, and alloy steels are the most common starting materials.

2. Cutting and Shaping:

   • The pipe or plate stock is cut to size by sawing, shearing, or plasma cutting.
   • The reducer shape is formed by one of two methods: extrusion (heating and pushing through a die, typical for seamless reducers) or pressing (plates pressed into shape in a die and seam-welded, used for large sizes).

3. Heat Treatment: The formed reducer undergoes annealing, normalizing, quenching, or tempering as needed to restore mechanical properties after hot or cold work.

4. Machining and Finishing: Ends are faced square and beveled per ASME B16.25 for butt welding. Surface finishing (sandblasting or pickling) improves surface quality and corrosion resistance.

5. Inspection and Testing: Dimensional checks, visual inspection, and NDE (radiography or ultrasonic) verify that the reducer meets all specification requirements. Pressure testing may follow for critical service.

6. Marking and Packaging: Approved reducers are stamped with size, material grade, heat number, and manufacturer identification per ASME/ASTM, then packaged for shipment.

Buttweld Reducer manufacturing process

Source: TK Bend

Pipe Cap

A buttweld pipe cap is used to blind or isolate the pipeline, permanently or temporarily (for example during the execution of maintenance and reparation works).

Buttweld pipe caps are manufactured using steel plates for most sizes and materials. The shape of a pipe cap is ellipsoidal and conforms to the requirements set by the «ASME Boiler and Pressure Vessel» code.

Buttweld pipe cap ASME b36.19

Pipe Cap Manufacturing Process

Buttweld pipe caps seal the end of a piping system and are manufactured as follows:

1. Material Selection: The cap material typically matches the pipe material. Carbon steel, stainless steel, and alloy steel are the standard choices, selected for the required pressure, temperature, and corrosion conditions.

2. Cutting and Shaping:

   • Plates or sheets are cut into circular blanks sized to the nominal pipe cap diameter.
   • The blank is formed into an ellipsoidal or hemispherical shape using one of three methods: deep drawing (punch and die, no seam, suits smaller caps), spinning (blank rotated at high speed against a roller or mandrel, suits larger seamless caps), or pressing (segments pressed into shape and seam-welded, used for the largest sizes).

3. Heat Treatment: Annealing or normalizing relieves forming stresses and achieves the specified mechanical properties.

4. Machining and Finishing: Cap edges are machined and beveled per ASME B16.25 for butt welding. Surface treatment (sandblasting or pickling) removes scale and improves corrosion resistance.

5. Inspection and Testing: Dimensional checks, visual inspection, and NDE (radiography or ultrasonic) confirm compliance. Pressure testing may be performed for critical applications.

6. Marking and Packaging: Caps are stamped with size, material grade, schedule, manufacturer ID, and applicable specification (ASTM, ASME), then packaged for shipment.

Buttweld cap manufacturing processSource: TK Bend

Seamless vs Welded BW Fittings

Butt weld fittings are available in seamless and/or welded execution (seamless for bore sizes below 24 inches, welded for sizes above 24 inches).

Seamless buttweld fittings have no seam welds and are therefore considered superior to welded BW fittings (a weld is always a weak point on the metal, prone to corrosion).

Seamless Buttweld Fittings

Seamless fittings are made from a single piece of metal, heated, and pushed or pulled over a form until the final shape is achieved. No welding is involved, and the resulting fitting has a uniform grain structure throughout.

This uniformity gives seamless fittings higher pressure ratings and better resistance to corrosion and high temperatures than their welded counterparts. They are typically available up to 24 inches in diameter. Specify seamless fittings for high-pressure, high-temperature, or corrosive service where the absence of a weld seam provides a measurable integrity advantage.

Welded Buttweld Fittings

Welded fittings are produced by forming and welding steel plate or by welding segments of pipe together. The weld seam introduces a zone of slightly different metallurgical properties compared to the parent material.

Welded fittings are available in a broader size range, including large diameters above 24 inches that cannot be produced economically as seamless. Their pressure ratings are somewhat lower than seamless equivalents due to the seam. They are the practical choice for moderate-pressure service, water supply, HVAC, and applications where the cost premium of seamless fittings is not justified.

Smls/Welded BW Fittings Differences Summarized

Parameter	Seamless	Welded
Manufacturing	Single piece, no welds	Formed from plate or pipe segments, seam welded
Material integrity	Uniform grain structure throughout	Variability possible at weld seam
Size range	Up to ~24” (standard)	Any size, including >24”
Pressure rating	Higher (no seam)	Lower (seam is weakest point)
Cost	More expensive	More economical, especially at large diameters
Preferred service	High P/T, corrosive, critical	Moderate P/T, utility, non-critical

Field Note

Always check fitting bevel angles before welding - ASME B16.25 specifies 37.5° for standard fittings, but some manufacturers ship with 30° bevels. Mismatched bevels cause incomplete root penetration and weld defects that show up in radiography.

The choice between seamless and welded fittings depends on the system’s pressure, temperature, fluid characteristics, and budget. For critical lines, seamless is the default. For utility and non-critical service, welded fittings offer a cost-effective alternative without sacrificing adequate performance.

Buttweld fitting pipe connection

Butt Weld Fittings Manufacturers

• Awaji
• APP/CB&I
• Bassi Luigi
• Canadoil
• Close Bend
• Colonial Machine
• Erne Fittings
• Ezeflow
• Hackney-Ladish
• Houston Pipe Benders
• IRC Spa
• Mills Iron Works
• OMR Raccordi (Officine Meccaniche Righi)
• Raccortubi
• Steel Forgings
• Sungkwang Bend
• Tectubi
• Tenaris
• TK Bend
• Tube Forgings of America

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Frequently Asked Questions

What are the main types of buttweld fittings?

The main types of ASME B16.9 buttweld fittings are: elbows (45°, 90°, 180° for direction changes), tees (equal, reducing, and barred for branching), reducers (concentric and eccentric for size transitions), caps (to close/seal pipe ends), and stub ends (used with lap joint flanges for easy alignment). These fittings are used for pipes NPS 1/2 and larger.

What is the difference between long radius (LR) and short radius (SR) elbows?

Long radius (LR) elbows have a center-to-face distance of 1.5 times the nominal pipe diameter, providing smoother flow with lower pressure drop. Short radius (SR) elbows have a center-to-face distance equal to the nominal diameter, used in space-constrained areas but with higher pressure drop and more turbulence. LR elbows are the default choice unless space is limited.

When should I use concentric vs eccentric reducers?

Use concentric reducers for vertical pipelines where centerline alignment is acceptable. Use eccentric reducers for horizontal pipelines: bottom-flat (BF) for pipe racks to maintain the bottom-of-pipe elevation, or top-flat (TF) at pump suctions to prevent vapor pockets that cause cavitation. Getting this wrong at pump suctions is a common and expensive mistake.

What is a barred tee used for?

A barred tee is a special equal tee used in pigging operations. It features internal welded bars (restriction cage) on the branch side that prevent the pipeline inspection gauge (pig) from entering the branch connection. This keeps the pig traveling through the main run during cleaning or inline inspection operations.

What is the difference between seamless and welded buttweld fittings?

Seamless fittings are made from a single piece of metal without welds, offering uniform grain structure, higher pressure ratings, and better corrosion resistance. They are available up to NPS 24. Welded fittings are fabricated from welded pipe or plate, available in larger sizes but with slightly lower pressure ratings due to the weld seam. Seamless is preferred for high-pressure, high-temperature, or corrosive service.

Related Articles

• ASME B16.9 Buttweld Fittings Dimensions & Weights
• Buttweld Fittings Materials (ASTM Grades)
• ASME B16.11 Forged Fittings (Socket Weld/Threaded)
• Stub Ends for Lap Joint Flanges
• Branch Fittings: Weldolet, Sockolet, Threadolet

Related quick answers: What Is a Pipe Reducer? | What Is a Pipe Cap? | What Is a Union?