PIPE FITTINGS (BUTT WELD)
Pipe fittings are used to change the layout of a pipeline (elbows to change the direction, reducers to increase/decrease the bore size, tees to branch the pipeline, caps to blind it). In this article, we give an overview of butt weld fittings, their benefits and mention the main ASME/ANSI specifications related to this type of fittings, which find wide application in the petrochemical, power generation, and construction industries.
WHAT ARE BUTT WELD FITTINGS?
Buttweld fittings are the most common type of steel fittings used for pipelines above 2 inches in diameter. Pipe fittings change the pathway of the pipeline (elbows with 45, 90 and 180 degrees angle curve), reduce/increase its flow conveyance capacity (concentric and eccentric reducers), branch (tees, cross) or blind it (butt weld cap). Butt weld fittings are connected to pipes by welding.
Example of a butt weld fitting to pipe connection.
BENEFITS OF BUTT WELD FITTINGS
Butt welding fittings have the following main benefits:
- allow a strong and leak-proof pipe to fitting connection
- minimize the pressure drops and the turbulence along the pipeline
- have a long service life
DIMENSIONS AND MATERIALS
Butt welding fittings are available in multiple shapes as elbows, tees and crosses, reducers and caps, stub ends, etc (see image below) and in a wide range of dimensions (combinations of outside diameter and wall thickness, i.e. schedule) and material grades (the most common are ASTM A234 WPB for carbon steel/alloy steel, ASTM A403 WP304 and WP316 for stainless steel wrought fittings, ASTM A815 for duplex and super duplex fittings).
The material of the pipe shall generally match the material of the connecting butt welded fittings (hence an ASTM A106 Gr. B pipe shall be connected to an ASTM A234 WPB fitting, or an ASTM A312 T304 pipe to an ASTM A403 WP304 fitting and so on). Also, the dimension a butt weld fitting shall match the dimension of the pipe it has to be connected to (so, for instance, a 6″ schedule 40 pipe matches a 6″ schedule 40 butt weld elbow).
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 applications where corrosion and erosion are a key issue (desalination, pharmaceutical and, food sectors).
For specific applications, steel fittings are either coated on the outer surface or internally lined or clad (see the example of an overlay clad tee below).
TOLERANCE OF BUTT WELD FITTINGS (ASME B16.9)
|All Fittings||90 Deg. & 45 Deg.
Elbows and Tees
& Lap Joint
A ,C ,B
|1/2 to 2-1/2||+0.06||Not less than 87.5%
of nominal thickness
|+/- 0.06||+/- 0.06||+/- 0.12|
|3 to 3-1/2||+/- 0.06||+/- 0.06||+/- 0.06||+/- 0.06||+/- 0.12|
|4||+/- 0.06||+/- 0.06||+/- 0.06||+/- 0.06||+/- 0.12|
|5 to 8||+/- 0.06||+/- 0.06||+/- 0.06||+/- 0.06||+/- 0.25|
|10 to 18||+0.16||+/- 0.12||+/- 0.09||+/- 0.09||+/- 0.25|
|20 to 24||+0.25||+/- 0.19||+/- 0.09||+/- 0.09||+/- 0.25|
|26 to 30||+0.25||+/- 0.19||+/- 0.12||+/- 0.19||+/- 0.38|
|32 to 48||+0.25||+/- 0.19||+/- 0.19||+/- 0.19||+/- 0.38|
SEAMLESS VS WELDED FITTINGS
BW fittings are available in seamless and welded execution (seamless for bore sizes below 24 inches, welded for sizes above 24 inches). They are manufactured out of seamless pipes (<24 inches), welded pipes (>24 inches) or steel plates (large diameter fittings, like metered ones).
KEY SPECIFICATIONS FOR PIPE FITTINGS
ASME B16.9, ASME B16.28 and, MSS SP43 are the key specifications covering the dimensions and the manufacturing tolerances of butt weld fittings. ASME B16.25 covers the preparation of butt weld connections between pipes and fittings. ASME B16.49 covers the marking details.
ASME B16.9 AND B16.28 (CARBON/ALLOY PIPE FITTINGS)
“This Standard covers overall dimensions, tolerances, ratings, testing, and markings for wrought carbon and alloy steel factory-made buttwelding fittings of NPS 1/2 through 48. It covers fittings of any producible wall thickness. This standard does not cover low pressure corrosion resistant buttwelding fittings. See MSS SP-43, Wrought Stainless Steel Butt-Welding Fittings. Short radius elbows and returns, which were previously included in ASME B16.28-1994, are included in this standard. B16.9 is to be used in conjunction with equipment described in other volumes of the ASME B16 series of standards as well as with other ASME standards, such as the Boiler and Pressure Vessel Code and the B31 Piping Codes”. (source: ASME)
MSS SP 43 (STAINLESS STEEL/NICKEL ALLOY PIPE FITTINGS)
The MSS SP 43 sets the dimensions, tolerances, and markings for butt weld fittings for low pressure, corrosion resistant applications. It covers only fittings for use with Schedule 5S or 10S pipe, for all NPS sizes listed in ASME B36.19M, except that for short pattern stub ends suitable for use with Schedule 40S are also shown. Essentially, MSS SP 43 is the reference standard for lightweight corrosion resistant butt weld fittings (stainless steel, duplex).
ASME B16.25 (BUTT WELD ENDS)
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
The ASME B16.49 Norm covers the manufacturing, marking, design, material, testing, and inspection requirements for factory-made bends of carbon steel materials having controlled chemistry and mechanical properties, produced by the induction bending process, with or without tangents. This standard covers induction bends for transportation and distribution piping applications (e.g., ASME B31.4, B31.8, and B31.11).
The most known manufacturers of butt welding fittings are Allied Fittings, Erne Fittings, Interfit, TK Bend, SK Bend, Benkan. A large number of butt weld fittings suppliers has also emerged in low-cost countries, mainly China, India, and Cambodia.