Butt weld pipe fittings: Frequently Asked Questions
These FAQs were prepared as an introduction to answer why / where / what / how / who / when questions from:
- Managers or non-technical people who need to have a high-level understanding of the pipe fitting industry.
- Technical specialists who need to explain what they do to senior managers or to their non-technical co-workers.
- Newcomers to the pipe fitting industry who want to get up to speed quickly.
FAQs are grouped below in six categories:
What is a butt weld pipe fitting ?
- A butt weld pipe fitting is designed to be welded on site at its end(s) to connect pipe(s) together and allow change in direction or pipe diameter, or branching or ending.
- This fitting then becomes part of a system for transporting fluids (oil, gas, steam, chemicals, …) in a safe and efficient manner, over short or long distances.
- Other types of fittings can be threaded or socket welded.
What are advantages of butt welding over flanges ?
- Flanges connect pipes by bolting them together, whereas butt weld fittings connect pipes by welding them together.
- Depending on size, butt weld fittings require 10% to 75% less material than a flanged connection, which means lower material cost and lower weight.
- Welded construction permits greater freedom in designing a layout; this system uses less space, is more compact, and has greater strength.
- Butt weld fittings provide optimum flow characteristics, without pockets or sharp corners, creating less frictional resistance, no pressure loss, less turbulence, …
What is a seamless butt weld 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.
What is PVF ?
- PVF means Pipe, Valve and Fittings, and is widely used as short-hand to describe this industry.
- PVFs can be made in a variety of metals or plastics.
- Manufacturers typically specialize in special types of materials; distributors can carry narrow or very broad product lines.
Fittings applications and markets
In what industries are pipe fittings
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.
Fitting sizes and dimensions
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 mean for pipe fittings ?
- 45 and 90 refer to angles for changing direction of pipe flows: 45 degrees or 90 degrees. 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 elbow?
- 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 ?
- 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 pipe fitting ?
- A lateral separates one line into two; the second line branches at a 45˚ angle.
Fitting standards and certifications
Who decides on standards and specifications for butt weld fittings ?
- Manufacturers of butt weld pipe fittings have to meet specifications and requirements of many organizations, to assure quality, compatibility and performance of their products.
- Key national and international standards organizations focus on materials and end-user industries, including American Society for Testing and Materials (ASTM), American Society of Mechanical Engineers (ASME), International Organization for Standardization (ISO), Norway’s NORSOK, …
- Some international organizations also focus on quality standards across all industries, such as International Organization for Standardization (ISO).
- Large industrial end-users also have their own programs to certify individual manufacturing plants as meeting their criteria to be listed on their Approved Manufacturers List (AML).
- All these certifications and approvals are valid for a specific period, and must be renewed regularly.
What is an MTR ?
- Material Test Reports (MTRs) are provided by manufacturers to certify physical properties and metal grade or alloy for each fitting, flange, pipe, or valve. This MTR is essential for demanding applications (pressure, temperature, corrosion, abrasion,…)
Why do manufacturers offer such a large variety of metals and materials ?
- Pipes and fittings are used in many applications and many industries that have widely different operating conditions.
- To reach best performance (resistance to various types of corrosive fluids, operating temperature, …) at acceptable costs, many metals have found unique niche applications in fittings.
Why choose a specific material, or grade, or schedule for a fitting ?
- Choice of material or grade or schedule is not made by distributors or by fitting manufacturers, but by engineers who design pipelines or systems based on performance requirements and acceptable costs.
- Engineers take into account key operating conditions and environment: pressure, temperature, abrasion, corrosion, vibrations, …
- Fitting materials also have to be chemically compatible with pipe materials to which they are connected to avoid inter-metal corrosion, and for weldability.
- Finally, cost of various materials varies greatly.
What does “high yield” (“HY”) mean for pipe fittings ?
- Yield strength refers to ability of materials to maintain a load or pressure without permanent deformation. (In this case, yield does not refer to manufacturing productivity of materials or equipment.)
- Fitting design is based on yield strength.
- Steel products do not all have similar potential. Yield strength can be changed by steel thickness, its chemical composition and heat treatments.
What is the difference between a manufacturer and a supplier of fittings ?
- Manufacturers own machines, presses and equipment to actually make fittings from pipe or plate.
- Suppliers are intermediaries between manufacturers and end-users: suppliers can be wholesalers, distributors, EPCs, …
- In some cases, end-users need to speak directly to manufacturers to get the fittings they need when they need them.
Can fitting manufacturers in North America and Europe compete with low-labor-cost countries ?
- Butt weld fittings are used in the most demanding applications: oil & gas, nuclear, chemicals, …
- A butt weld pipe fitting looks like a simple product, but many things can go wrong during design and manufacturing, causing problems later in the field: scratched inner surface, thinner wall at outside radius, oval fittings, out-of-specs welding-end finish, invisible cracks, wrong chemical composition of steel grade, …
- Manufacturing close to end-users in North America or Europe with experienced and highly skilled workers assures highest quality.
- Also Western manufacturers can compete by emphasizing technology, product development and highly efficient manufacturing processes.
- (Of course, dumping by foreign competitors of imported fittings at prices below production costs is not an acceptable competing practice.)
How are pipe fittings manufactured ?
Key steps in manufacturing process are as follows:
- Planning: based on fitting specifics and client requirements; includes creating part identification in manufacturing system.
- Selection of raw material: metal; plate or pipe.
- Welding: for fittings that are not simple elbows, returns, bends, reducers or caps.
- Inspection and tests: according to standards and client requirements.
- Coating (if required).
- Preparation of documentation.
What is a heat induction mandrel ?
- In this very efficient manufacturing method, a pipe is pushed over a die called “mandrel”, then expanded and bent to desired dimensions.
- In this process, heat is transferred to a pipe by applying an electric current (known as “induction”) around the steel pipe and gas flames under the neck (intrados) to minimize material thinning on this region and to make forming operation easier. Mandrels are used to make elbows from ½ inch to 72 inch diameters.
Destructive and non-destructive testing
Why and how often are pipe fittings tested ?
- Pipe fittings are tested to assure conformity with standards and performance in their applications.
- Standard, commodity fittings are tested by sampling during manufacturing process, with both destructive and non-destructive approaches.
- Custom-made fittings are regularly tested during manufacturing process with non-destructive approaches, as required for the application or as specified by end-users.
What is the difference between a destructive and a non-destructive test ?
- After a destructive test, fittings can no longer be used in the field, because they were bent, pulled or abused until breakage point. For this reason, destructive tests can only be performed on a small sample of fittings; otherwise, all fittings would be destroyed during tests.
- A non-destructive test measures performance for regular operating conditions, or by means that do not destroy the fitting.
What are most frequent destructive tests for fittings?
- Bending: fittings are bent until deformation appears on their side, face or root.
- Charpy impact: named after Georges Charpy who developed it, this test measures toughness of a fitting by determining the amount of energy absorbed during its fracture from impacts.
- Corrosion: fittings are exposed to corrosive fluids or materials until corrosion appears.
- Metallographic: this test studies physical structure and components of metal samples, typically by microscopy. Samples are taken from actual fittings.
- Tension: fittings are stretched until deformation appears; measures are taken for strength, yield, elongation.
What are most frequent non-destructive tests for fittings?
- Dye or liquid penetrant: low-cost test to detect surface hairline cracks or porosity in fittings, by applying a special liquid to its surface.
- Ferrite content: ferrite content in Duplex, Super Duplex and stainless steel has to be just right to achieve yield strength, fracture toughness and corrosion resistance. Ferrite content is measured in weld seams by magnetic induction.
- Hydrostatic: a fitting is filled with a liquid, often colored water, and pressure is applied; measurements are taken to see if there is pressure loss due to any defects.
- Magnetic particles: uses a magnetic field to detect surface or slightly subsurface discontinuity or weakness in a fitting.
- Macrography: makes photographs, sometimes in 3 dimensions, of items visible to naked eye.
- Micrography: uses microscopes to make photographs of metal and obtain information about its properties.
- Pneumatic pressure: a fitting is filled with pressurized air; measurements are taken to see if there is pressure loss due to any defects.
- Positive material identification: analysis of a metal alloy by identifying percentage for each of its elements. Uses specialty equipment, including X-ray fluorescence or optical emission spectrometry.
- Radiography: uses X-rays or gamma rays to detect any weakness in fittings.