ASME/ANSI BW FITTINGS
Butt weld fittings are the most common type of steel fittings used for pipelines above 2 inches of diameter. They are used to change the pathway of the pipeline (elbows), reduce / increase its flow size (reducers), branch (tees, cross) or close it (cap), and are connected to pipes by welding. Butt welding fittings are available in multiple shapes as elbows, tees, reducers, crosses, etc and in a wide range of diameters (from 1/2 to 48 inches), schedules (5/5s, 10/10s, 40/40s, 80/80s, XS/XH, XXS, 160, etc) and material grades (the most common are ASTM A234 WPB for carbon steel and ASTM A403 WP 304 to 316 for stainless steel wrought fittings). BW fittings are available in seamless and wekded execution (respectively < 24 / > 24 inch)
Carbon steel fittings are widely used for linepipes and piping systems in the oil & gas and petrochemical industries, whereas stainless steel buttweld fittings find extensive application in processes subject to corrosion and erosion (and are also common within the pharmaceutical and food sectors). The material of the pipe shall generally match the material of the connecting butt welded fittings.
For specific applications, steel fittings are either coated on the outer surface or lined internally.
The dimensions and the manufacturing tolerances of carbon steel fittings and alloy steel fittings are defined by the ASME B16.9 norm, while stainless steel and nickel alloy buttweld comply to the ASME B16.9 and MSS SP43 norms. The dimensions of butt weld fittings match the dimension of the pipes they have to be mechanically joined to (so for instance a 6" sch. 40 pipe will match exactly to a butt weld elbow 90 deg 6" sch. 40).
ANSI / ASME B16.9 - B16.28
"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
The MSS SP 43 sets the dimensions, tolerances, and markings for butt weld fittings for low pressure, corrosion resistant applications. It covers only fittings made 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 light weight 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 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).
BENEFITS OF WELDED FITTINGS
Welded pipe / fittings connections:
- are strong and leak-proof
- minimize the pressure drops and the turbolence along the pipeline
- have a long service life
The allowable pressure ratings for fittings designed in accordance with the standard ASME B 16.9–1993 may be calculated as for straight seamless pipe in accordance with the rules established in the applicable section of the ASME B 31 codes for pressure piping. Hydrostatic testing of wrought fittings is not required. All fittings shall be capable of withstanding, without leakage, a test pressure equal to that prescribed in the specification for the pipe with which the fitting’s marking identifies.
The most known manufacturers of butt welding fittings are: Allied Fttings, 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.