What is Pipe fitting

What is Pipe fitting?

Pipe fitting is the operation of repairing or installing tubing or piping systems which convey gas, liquid, and solid materials occasionally. It also involves selecting and preparing tubing or pipe, joining it by several means and location and repairs of leaks. Fitters work with various tubing and pipe materials such as copper, aluminum, steel, plastic and iron. Pipe fitting is not referred to as plumbing although the two are related, but have separate trades. Stainless steel pipe fittings are made of stainless steel, so it is made of stainless steel pipe fitting, which contains: stainless steel elbow, stainless steel tee, stainless steel cross, stainless steel reducer, stainless steel pipe cap, etc.

Socket weld pipe fittings   In what industries are pipe fittings used
Buttweld pipe fittings   What does “schedule” mean for pipe fittings
Threaded pipe fittings   How are pipe fittings manufactured
How are pipe fittings measured

What is Pipe fitting

According to the connection method Divided into socket weld pipe fittings, buttweld pipe fittings, threaded pipe fittings. Stainless steel elbow for pipe turning place; flange for the pipe and pipe interconnected parts, connected to the tube end, stainless steel tee for three pipe collection place; stainless steel pipe for four pipe collection Place; stainless steel reducer for the different diameter of the two pipes connected to the place. Main stainless steel Material: 304, 304L, 316, 316L, 904L, 2205, 2507, 310S……

1. Socket weld pipe fittings

Socket weld pipe fittings

A Socket Weld is a pipe attachment detail in which a pipe is inserted into a recessed area of a Valve, fitting or flange. In contrast to buttweld pipe fittings, Socket Weld fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.

To join pipe to Valves and fittings or to other sections of pipe, fillet-type seal welds be used. Socket Welded Joints construction is a good choice wherever the benefits of high leakage integrity and great structural strength are important design considerations.

Fatigue resistance is lower than that in butt-welded construction due to the use of fillet welds and abrupt fitting geometry, but it is still better than that of most mechanical joining methods.

Some details of socket weld pipe fittings

SW Fittings are family of high pressure fittings are used in various industrial processes.

· They are used for lines conveying flammable, toxic or expensive material where no leakage can be permitted, and for steam 300 to 600 PSI.

· They are used only in conjunction with ASME Pipe and are available in the same size range.

· They are used in areas where pipe-work is permanent and are designed to provide good flow characteristics.

· They are are produced to several ASTM standards and are manufactured in accordance with ASME B16.11. The B16.11 standard covers pressure-temperature ratings, dimensions, tolerances, marking, and material requirements for forged carbon and alloy steel. Acceptable material forms are forgings, bars, seamless pipe, and seamless tubes which conform to the fittings chemical requirements, melting practices, and mechanical property requirements of ASTM A105, A182, or A350.

· They are available in three pressure ratings: Class 3000, 6000 and 9000.

Advantages and disadvantages of socket weld fittings


· The pipe need not be beveled for weld preparation.

· Temporary tack welding is no needed for alignment, because in principle the fitting ensures proper alignment.

· The weld metal can not penetrate into the bore of the pipe.

· They can be used in place of threaded fittings, so the risk of leakage is much smaller.

· Radiography is not practical on the fillet weld; therefore correct fitting and welding is crucial. The fillet weld may be inspected by surface examination, magnetic particle (MP), or liquid penetrant (PT) examination methods.

· Construction costs are lower than with butt-welded joints due to the lack of exacting fit-up requirements and elimination of special machining for butt weld end preparation.


· The welder should ensure for a expansion gap of 1/16 inch (1.6 mm) between de pipe and the shoulder of the socket.
ASME B31.1 para. 127.3 Preparation for Welding (E) Socket Weld Assembly says:
In assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16″ (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.

· The expansion gap and internal crevices left in socket welded systems promotes corrosion and make them less suitable for corrosive or radioactive applications where solids buildup at the joints may cause operating or maintenance problems. Generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.

· Socket welding are unacceptable for UltraHigh Hydrostatic Pressure (UHP) in Food Industry application since they do not permit full penetration and leave overlaps and crevices that are very difficult to clean, creating virtual leaks.

The purpose for the bottoming clearance in a Socket Weld is usually to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal, and to allow for differential expansion of the mating elements.

2. Buttweld pipe fittings

Buttweld pipe fittings

A pipe fitting is defined as a part used in a piping system, for changing direction, branching or for change of pipe diameter, and which is mechanically joined to the system. There are many different types of fittings and they are the same in all sizes and schedules as the pipe.

Fittings are divided into three groups:

• Buttweld (BW) pipe fittings whose dimensions, dimensional tolerances et cetera are defined in the ASME B16.9 standards. Light-weight corrosion resistant fittings are made to MSS SP43.

• Socket Weld (SW) pipe fittings Class 3000, 6000, 9000 are defined in the ASME B16.11 standards.

• Threaded (THD), screwed fittings Class 2000, 3000, 6000 are defined in the ASME B16.11 standards.

Applications of buttweld pipe fittings

A piping system using buttweld pipe fittings has many inherent advantages over other forms.

• Welding a fitting to the pipe means it is permanently leakproof

• The continuous metal structure formed between pipe and fitting adds strength to the system

• Smooth inner surface and gradual directional changes reduce pressure losses and turbulence and minimize the action of corrosion and erosion

• A welded system utilizes a minimum of space

Bevelled ends

The ends of all buttweld pipe fittings are bevelled, exceeding wall thickness 4 mm for austenitic stainless steel, or 5 mm for ferritic stainless steel. The shape of the bevel depending upon the actual wall thickness. This bevelled ends are needed to be able to make a “Butt weld”.

ASME B16.25 covers the preparation of buttwelding ends of piping components to be joined into a piping system by welding. It includes requirements for welding bevels, for external and internal shaping of heavy-wall components, and for preparation of internal ends (including dimensions and dimensional tolerances). These weld edge preparation requirements are also incorporated into the ASME standards (e.g., B16.9, B16.5, B16.34).

Material and performance

The most common materials used in fittings produced is carbon steel, stainless steel, cast iron, aluminium, copper, glass, rubber, the various types of plastics, etc..

In addition, fittings, like pipes, for specific purposes sometimes internally equipped with layers of materials of a completely different quality as the fitting themselves, which are “lined fittings”.

The material of a fitting is basically set during the choice of the pipe, in most cases, a fitting is of the same material as the pipe.

What is a seamless buttweld pipe fitting ?

· A seamless butt weld pipe fitting is made directly from a section of seamless pipe material, by applying heat and pressure to transform pipe directly into an elbow or other shape.

· A seamless fitting has no welded parts by itself, a characteristic demanded in some critical applications.

· By comparison, a regular butt weld fitting has one, two or more welds, depending on dimension, shape and manufacturing method.

· No weld means less risk of weld yield and porosities.

3. Threaded pipe fittings

Threaded pipe fittings

Threaded joints probably represent the oldest method of joining piping systems. Like Socket Weld fittings, threaded fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.
The dimensional standards for taper pipe threads are given in ASME B1.20.1. That document gives all required dimensions including number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters.

· Threaded piping is commonly used in low-cost, noncritical applications such as domestic water, fire protection, and industrial cooling water systems.

· Threaded fittings are normally made of cast gray or malleable iron, cast brass or bronze, or forged alloy and carbon steel.

· They are available in three pressure ratings: Class 2000, 3000 and 6000.

Npt thread

All fittings on this page are described, are provided with NPT thread, ASME B1.20.1. The American National Pipe Thread Tapered, is the best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal.

Advantages and disadvantages of threaded pipe fittings


· Installation productivity is moderately high, and specialized installation skill requirements are not extensive.

· Leakage integrity is good for low-pressure, low-temperature installations where vibration is not encountered.


· Rapid temperature changes may lead to leaks due to differential thermal expansion between the pipe and fittings.

· Vibration can result in fatigue failures of screwed pipe joints due to the high stress intensification effects caused by the sharp notches at the base of the threads.

· In hazardous piping systems threaded connections should be avoided, if possible. Their vulnerability to fatigue damage is significant, especially where exposed threads are subject to corrosion.

In what industries are pipe fittings used ?

Fittings are used wherever liquids, gases, chemicals and other fluids are created, processed, transported, or used. Range of industries is very wide, as shown below:

· Energy related. Oil and gas industries, including upstream (exploration and production), midstream (transportation by ships, pipeline, rail, trucks; storage and wholesale) and downstream (refining into fuels or chemicals; marketing and distribution). Power plants using fossil or nuclear fuels. Geothermal energy for harnessing heat from the earth.

· Food and beverage production. From basic ingredients to elaborate desserts. From milk, to juices and other beverages. Cold chain to keep produce fresh, drinks and frozen foods at a set temperature. Desalination plants to transform seawater into drinking water for arid areas and various tropical islands.

· Pharmaceuticals and biotech production. Medicines and vaccines that treat illnesses and help restore bodies and minds to their top conditions.

· Semiconductor manufacturing: Microchips that go into all electronics, including smartphones, tablets, computers, television sets, telecom networks, …

· Pulp and paper production. Newsprint, glossy paper, cardboard boxes, books, writing paper, …

· Production of construction materials and plastics. From roofing materials, to glassmaking for windows, to copper wiring for transporting electricity, and various plastics throughout houses and office buildings.

How are pipe fittings measured ?

· Pipe fittings are measured by their diameter, wall thickness (known as “schedule”), and shape or configuration. (Fittings are also defined by their material grade and whether they are welded or seamless.)

· Diameter refers to outside diameter of a pipe or fitting.

· The North American standard is known as Nominal Pipe Size (NPS). The International Standard is known as Diameter Nominal (DN). Pipes and fittings are actually made in similar sizes around the world: they are just labeled differently.

· From ½ in to 12 inch “Nominal Pipe Size”, outside diameters are slightly larger than indicated size; inside diameters get smaller as schedules grow.

· From 14 in and larger “Nominal Pipe Size”, outside diameters are exactly as indicated size; inside diameters get smaller as schedules grow.

· As with other North American standards (inch, foot, yard, mile, …), many pipe standards (diameters up to 12 inch and wall thickness) are based on historical precedents (a toolmaker’s dies during US Civil War) rather than a “scientific” method.

What does “schedule” mean for pipe fittings ?

What is schedule 40, sch80 ?

· Schedule, often shortened as sch, is a North American standard that refers to wall thickness of a pipe or pipe fitting. Higher schedules mean thicker walls that can resist higher pressures.

· Pipe standards define these wall thicknesses: SCH 5, 5S, 10, 10S, 20, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160, STD, XS and XXS. (S following a number is for stainless steel. Sizes without an S are for carbon steel.)

· Higher schedules are heavier, require more material and are therefore more costly to make and install.

Why are fittings sometimes thicker and heavier than pipes to which they are connected ?

· Fittings are sometimes thicker than their connecting pipes to meet performance requirements or due to manufacturing reasons.

· Due to fitting geometry, stress is very different when compared to a pipe. Using extra material is often necessary to compensate for such additional stress, especially for tees and tight curve elbows.

· Fitting manufacturers may not always stock plates or pipes for all metal grades or sizes. When responding to an order, manufacturers always use the right metal or alloy, but sometimes made with next-higher available plate or pipe size while still respecting specified inside diameters.

What does 45 SR or 90 LR Elbow mean for pipe fittings ?

What does 45 SR or 90 LR Elbow mean for pipe fittings  

· 45 and 90 refer to angles for changing direction of pipe flows: 45 degrees elbow or 90 degrees elbow. Most elbows are 45˚ or 90˚.

· SR means Short Radius; LR means Long Radius. Centerline radius of long radius (LR) elbows is smoother than of short radius (SR).

· In SR fittings, radius equals nominal diameter size. A 6 inch pipe will have a 6 inch radius center-to-end, giving it a sharp curve.

· In LR fittings, radius equals 1.5x nominal diameter size. A 6 inch pipe will have a 9 inch centerline radius, for a smoother curve than SR.

What is the difference between a 3R and a 3D Bend?

What is the difference between a 3R and a 3D Bend? 

· There is no difference between 3R and 3D. Both have a centerline radius of three times the nominal diameter size. Both have a smoother curve than an SR or LR.

· (In fittings, 3D does not refer to modeling software or screens.)

What does 3D or 6D mean for pipe fittings ?

· Larger numbers have smoother curves.

· Centerline radius is calculated based on diameter and number that precedes it. For example, a 10″ 6D has a 60″ centerline radius.

· Smoother curves are often used for applications with higher pressure, abrasion or corrosion.

What is the difference between concentric and eccentric reducers ?

What is the difference between concentric and eccentric reducers 

· A concentric reducer is symmetrical: both ends are aligned along the center.

· An eccentric reducer is not symmetrical: ends are off center of one another.

What is a lateral tee pipe fitting ?

What is a lateral tee pipe fitting 

· A lateral tee separates one line into two; the second line branches at a 45˚ angle.

How are pipe fittings manufactured ?

Key steps in manufacturing process are as follows:

1· Planning: based on fitting specifics and client requirements; includes creating part identification in manufacturing system.

2· Selection of raw material: metal; plate or pipe.

3· Cutting.

4· Heating.

5· Forming.

6· Welding: for fittings that are not simple elbows, returns, bends, reducers or caps.

7· Inspection and tests: according to standards and client requirements.

8· Coating (if required).

9· Preparation of documentation.

10· Shipping.

The technical requirements for pipe fittings are as follows:

Geometric dimensions include outer diameter, inner diameter and wall thickness. It is also required to control the radius of curvature. If the radius is 1.5d, the radius of curvature must be within the required tolerances. As most of these pipe fittings are connected to pipes by welding, in order to improve the quality of welding, the end of the pipe fitting should be cut into a groove, leaving a certain angle with a certain edge; this requirement is relatively stringent and there are regulations for edge thickness, angle and range of variation. The surface quality and mechanical properties of pipe fittings are basically the same as those of the tubes. The steel grade of pipe fitting is the same as the connected pipe in order to facilitate welding. There should be surface treatment for all pipe fittings, removing the inner and outer scales by shot blast and then coating with anti-corrosion paint. This process is not only for exporting to other countries, but also for preventing oxidation rust during domestic transportation. There are also some requirements for packing. For small size pipe fittings, we need to make a wooden box which is about 1 cubic meter and the total weight of pipe fittings inside should not exceed one ton; this standard allows a mixed package, that is, putting the small size pipe fittings into the big ones, but the total weight can not be over 1 ton. For big size pipe fittings, we need to have a single package, for example, pipe fittings with size of 24 inch should be packed alone.  In addition, the marks on packages, is to indicate the size, steel number, batch number and manufacturer trademark. Stamp the pipe fitting with steel seal along with documents of packing list and written warranty.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Ltd (www.steeljrv.com)


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