Buttweld fittings are used to change the pathway of a pipeline (elbows), reduce/increase its bore size (reducers), branch (tees, cross) or blind it (butt weld cap). BW fittings are used for pipes above 2 inches in diameter, whereas socket weld and threaded fittings are used for smaller size pipes.
Buttweld fittings are very popular products in piping engineering and plant construction. They are available in multiple shapes (elbows, tees, reducers, crosses, caps), material grades (carbon, high-yield carbon, low-alloy, stainless, duplex, and nickel alloys), and sizes (2 to 24 inches in seamless execution, and welded for larger pipe sizes).
The key specifications for buttweld fittings are the ASME B16.9 (carbon and alloy fittings) and the MSS-SP 43 (that integrates ASME B16.9 for stainless steel, duplex, and nickel alloy BW fittings).
The material of the pipe and the buttweld fitting shall match (for example, an ASTM A106 Gr. B pipe shall be connected to an ASTM A234 WPB BW fitting, or an ASTM A312 T304 pipe has to be welded to an ASTM A403 WP304 fitting and so on). It is not possible to use pipes and buttweld fittings of different materials. Likewise, the size of the buttweld fitting and the size of the connected pipe shall match.
Buttweld fittings are manufactured starting from seamless or welded pipes, or in some cases, from steel plates. After construction, they can be tested with different approaches, as the hydrostatic test and the non-destructive tests – similarly to pipes.
BENEFITS OF BUTT WELD FITTINGS
Butt welding fittings have the following benefits
- allow strong, and leak-proof, piping connections
- minimize pressure drops and the turbulence inside the pipeline
- have a long service life
- are rather cheap to purchase and deploy
TYPES OF PIPE FITTINGS (BUTT WELD)
Each of this type of pipe fitting is reviewed in separate articles of the projectmaterials.com knowledge base.
BW FITTINGS MATERIALS
The most common material grades for buttweld fittings are ASTM A234 WPB for carbon steel/alloy steel, ASTM A403 WP304 and WP316 for stainless steel wrought fittings, and ASTM A815 for duplex and super duplex fittings.
Carbon steel fittings are common for pipelines and process piping applications in the oil and gas and power generation industry, stainless steel fittings are for higher temperature, pressure and corrosion applications (desalination, pharmaceutical and, food sectors).
In some cases, buttweld fittings can be externally coated with protective materials, internally lined (example PTFE may be inserted to enhance the resistance of the bare metal to corrosive fluids) or clad (the picture shows a clad tee: the tee is made of carbon steel, but the inner side is covered by a higher grade material, stainless steel 316 in this case).
SEAMLESS VS WELDED FITTINGS
Butt weld fittings are available in seamless and 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 BW fittings are manufactured by cutting, heating and shaping seamless pipes into desired forms. Welded buttweld fittings have one, two or more welds, depending on the dimension, the type of fitting and the manufacturing process adopted by the mill.
BUTT WELD FITTINGS MANUFACTURERS
- Bassi Luigi
- Close Bend
- Colonial Machine
- Erne Fittings
- Houston Pipe Benders
- IRC Spa
- Mills Iron Works
- OMR Raccordi (Officine Meccaniche Righi)
- Shanghai Gateway Fittings Co
- Steel Forgings
- Sungkwang Bend
- TK Bend
- Tube Forgings of America
BUTTWELD ENDS (ASME B16.25)
ASME B16.25 sets standards for the preparation of the ends of components that need to be welded together. As stated in the ASME website: “This Standard 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 tolerances). Coverage includes preparation for joints with the following: no backing rings; split or noncontinuous backing rings; solid or continuous backing rings; consumable insert rings; gas tungsten arc welding (GTAW) of the root pass” (source: ASME).
Cut square or slight chamfer, at manufacturer’s option for :
- t ≤ 0.19” carbon steel or ferritic alloy steels
- t ≤ 0.12” austenitic alloy steels
(1) The value of t mini is whichever of the following is applicable: a) the minimum ordered wall thickness of the pipe, b) 0.875 times the nominal wall thickness of pipe ordered to a pipe schedule wall thickness that has an under tolerance of – 12.5 %.
(2) The maximum thickness at the end of the component is: a) the greater of (t mini + 0.16”) or (1.15 t mini), when ordered on a minimum wall basis, b) the greater of (t mini + 0.16”) or (1.10 t nom.), when ordered on a nominal wall basis.
(3) Weld bevel is shown for illustration only.
(4) The weld reinforcement permitted by the applicable code may lie outside the maximum envelope.
(5) Where transitions using maximum slope do not intersect outside surface within the transition region, as shown by phantom outline, maximum slopes shown shall be used. Alternately, radii lying within the envelope may be used
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