Wrought Buttweld Fittings Selection Guide in Industry

  1. Scope
  2. References
  3. Definitions
  4. General
    1. Pressure Temperature Rating and Design Basis
    2. Inline Dimension
    3. Taper Boring
    4. Match Piping
    5. Tolerances
    6. Lateral Fittings
    7. Different Designs
  5. Requirements for Specific Fittings
    1. Fabricated Fittings
    2. Seamless and Welded
    3. Fittings for Services Requiring Stress Relief
    4. Elbows, Reducing Elbows, and Return Bends
    5. Tees and Reducing Tees
    6. Reducers
    7. Stub Ends
  6. Materials
  7. Services
  8. Marking
  9. Testing and Inspection

TABLE I – Services and Materials

Wrought Buttweld Fittings Selection Guide in Industry

Wrought Buttweld Fittings Selection Guide in Industry

Wrought Buttweld Fittings Selection Guide in Industry

1. Scope

1.1 This article covers pressure ratings, dimensions, tolerances, testing, marking and materials of wrought buttweld fittings for piping and pipelines. These fittings include but are not limited to elbows, reducers, tees, crosses, U-bends, lap joint stub ends, and swage connections, in sizes ranging from NPS 1/2 to 48.

1.2 This article supplements the requirements of ASME B16.9, B16.28, B31.1, B31.3, B31.4, B31.8 and MSS SP-75.

1.3   Cast and  forged flanged fittings, forged threaded and socket weld fittings, tube fittings, and pipe bends used in lieu of fittings, are excluded from the scope of this standard.

2.  References

Reference is made in this article to the following documents.

American Petroleum Institute (API)

5L Specification for Line Pipe

941 Steel for Hydrogen service at elevated temperatures and pressures in petroleum Refineries and petrochemical plants.

American Society for Testing and Materials (ASTM)

A 234 Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Moderate and Elevated Temperatures

A 403 Specification for Wrought Austenitic Stainless Steel Piping Fittings

A 420 Specification for Piping Fittings of Wrought Carbon Steel and Alloy Steel for Low- Temperature Service

A 860 Specification for Wrought High – Strength Low Alloy Steel Butt-Welding Fittings

B 366 Specification for Factory-Made Wrought Nickel and Nickel Alloy Fittings

American Society of Mechanical Engineers (ASME) B16.9 Factory-Made Wrought Butt Welding Fittings B16.25 Butt Welding Ends

B16.28 Wrought Steel Butt Welding Short Radius Elbows and Returns

B31.1 Power Piping

B31.3 Process Piping

B31.4 Pipe Line Transportation system for liquid hydro carbons and other liquids

B31.8 Gas Transmission and distribution Piping systems Section VIII, Div 1 Boiler and Pressure Vessel Code

Manufacturers Standardization Society (MSS)

SP-75 Specification for High Test Wrought, Butt-Welding Fittings

National Association of Corrosion Engineers (NACE)

MR0175/ISO 15156 Petroleum and natural gas Industries-Materials for use in H2S Containing environments in oil and production-Part 1, Part 2 & Part 3

3. Definitions

For the purpose of understanding this standard, the following definition apply.

Crotch:  The area covering a 90° section of the circumference of the elbow along its inner radius.

4. General

4.1      Pressure Temperature Rating and Design Basis

4.1.1    The allowable pressure-temperature rating of fittings shall equal or exceed that of the mating pipe of the same or equivalent material, size, and nominal thickness, as specified in ASME B16.9.

4.1.2    The allowable pressure rating shall be  determined by calculation as  for straight   seamless   pipe   of   equivalent   material,   in   accordance   with ASME B31.3,  paragraph  302.2.2.  The  pipe  NPS   and   wall  thickness (schedule number) and material shall be identified on the fitting in lieu of pressure rating markings.

4.1.3    The design of fittings shall be established by:

a. Burst test strength in accordance with ASME B16.9, Section 9

b. Mathematical analysis in accordance with ASME Section VIII

4.1.4    When requested by SABIC, records of mathematical analysis or burst test data shall be provided.

4.2      Inline Dimensions

Dimensions shall relate to a fixed position for the welding ends with reference to the centerline of the fitting.

4.2.1    Pipe fitting dimensions shall conform to ASME B16.9.

4.2.2    Short radius elbows shall conform to ASME B16.28.

4.2.3    Fittings for high pressure gas and oil transmission, and distribution systems (pipeline fittings), shall conform to MSS SP-75.

4.3      Taper Boring

4.3.1    For fittings and pipe of the same grade, the nominal pipe thickness of the fitting shall be the same as the pipe to which it is welded.

4.3.2    Transition from welding bevel to outside surface of the fitting, and from the root face to the inside surface of the fitting, shall be in accordance with ASME B16.9, Figure 1.

4.3.3    Fittings which have average wall thickness 15 percent or more in excess of the nominal wall thickness specified, shall not be preferred, as they add undue   stiffness   to   the   system,   resulting   in   an   increase   in   stress intensification. Such fittings may be acceptable with taper boring, subject to prior SABIC approval.

4.3.4    When proposing taper bored fittings, the bidder shall provide the end-to-end or center-to-end dimensions, or both, if tapering will result in non-standard dimensions.

4.4      Match Piping

4.4.1    For fittings and pipe of different grades, when the specified minimum yield strength (SMYS) for the fitting material is different from that specified for the pipe to be matched, the nominal wall thickness of the fitting shall not be less than the nominal wall thickness of the matching pipe multiplied by the ratio of the specified minimum yield strength of the matching pipe to that of the fitting material.

4.4.2    The wall thickness ratio shall not exceed 1.5.

4.4.3    When the wall thickness of the fitting at the welding end exceeds the wall thickness  of  the  matching  pipe,  the  welded  joint  design  shall  be  in accordance with ASME B31.4, Figure 434.8.6(a)-2.

4.5      Tolerances

Tolerances shall be in accordance with ASME B16.9, Table 2. The wall thickness of fittings designed using lower tolerances, rolled out of a plate for example, shall not be less than the nominal pipe thickness. That is, no negative tolerance shall be permitted. These fittings shall be identified in the purchase description.

4.6      Lateral Fittings

Lateral connections shall be made by an integrally reinforced outlet fitting.

4.7      Different Designs

Non-ASME B16.9 designs will be acceptable subject to SABIC approval if similar fittings pass the bursting test specified in ASME B16.9.

5       Requirements for Specific Fittings

5.1      Fabricated Fittings

5.1.1    Fittings fabricated out of more than two pieces (two halves), and other fittings employing circumferential welds, shall conform to ASME B31.3, paragraph 304.7.2, and shall require prior SABIC approval. Wrought fittings shall not be substituted by fittings machined from a block of metal.

5.2      Seamless and Welded

5.2.1    Fittings for welded pipe may be welded. Seamless fittings can be supplied in place of welded fittings (made out of plates) but with maximum negative tolerance of 0.3 mm of wall thickness.

5.2.2    Fittings for seamless pipe shall be seamless. Welded fittings shall not be acceptable in place of seamless fittings.

5.2.3    When a fitting has to be seamless, any material used to make the fitting shall also be seamless.

5.3      Fittings for Services Requiring Stress Relief

5.3.1    Stress relieving for service requirements, for example caustic and H2S, shall be performed in a furnace, covering the entire spool.

5.3.2    Field welds shall be wrapped and stress relieved. This process does not stress relieve a fabrication weld in the fitting. So SABIC will specify seamless or stress relieved fittings for use in these cases.

5.4      Elbows, Reducing Elbows, and Return Bends

5.4.1    To mitigate stress distribution in bends, the wall thickness at the crotch area of elbows and U-bends shall be greater than the nominal thickness of the body of the fittings.

5.4.2    When elbows or U-bends are fabricated from two or more sections of pipe, the circumferential weld seams of the pipe shall be located outside the crotch area of the bend.

5.4.3    Long radius elbows (1.5 D) shall be preferred. The use of short radius elbows shall  be  minimized,  as  short  radius  elbows  have  a  lower  pressure temperature rating and higher modulus of rigidity.

5.4.4    Miter elbows shall not be used without prior SABIC approval. Miter welding causes turbulence due to abrupt change in direction of flow, resulting in increased stresses, friction loss, and erosion. This leads to premature failure and costly shutdowns.

5.5      Tees and Reducing Tees

5.5.1    The wall thickness at the crotch area of wrought tees shall be no less  than 150% of the nominal thickness of the body of the fittings.

5.5.2    When tees are fabricated from plate, the weld seams shall be parallel to the axes of the run and the branch.

5.5.3    Pulled Outlets. The branch outlet of tees shall be extruded or forged to the run. Welded on branch connections and fittings made by a welded stub-in shall not be used. Pulled outlets on a piece of pipe, which will later be buttwelded into piping, may be used. Pulled outlets (sweep-o-lets) shall also be acceptable on a spool piece.

5.6      Reducers

5.6.1    Reducer Shape:  Concentric and  eccentric reducers  shall  have  smoothly curved wrought transitions and parallel ends (ASME shape) unless otherwise specified in purchase order. ASME shape reducers not listed in ASME B16.9 or MSS SP-75 shall be designed in accordance with ASME B31.3.

5.6.2    Reducer Sizes: Reducer sizes shall be limited to those listed in ASME B16.9.

5.6.3    Sizes and Walls: Each reducer has two sizes, and two walls. Reducers may have the same schedule on both ends, but not the same wall. In case of taper boring on reducers, the reducer shall be provided with a wall thickness at least the wall thickness of the large end. When selecting reducers for a material class, all the calculated pipewalls shall be available.

5.6.4    Size Convention: The sizes of a reducer should be on the large size. The large size usually defines the body wall. Reducers are bulk materials, therefore odd sizes shall be avoided, as they will probably be uneconomical, and not of timely delivery.

5.6.5    Conical Reducers: Conical reducers have no tangents, which causes stress concentration at the weld. Conical reducers shall have a stress check in accordance  with  ASME B31.3,  Section 304.61(b)  or  ASME  Section VIII, Div 1. The included angle shall not exceed 12°.

5.6.6    Swage: Swages or swage nipples are made from pipe, but are wrought. Wrought swages shall conform to this subsection.

5.7      Stub Ends

5.7.1    Stub ends may be either ASME, or MSS, as in the purchase order.

5.7.2    The thickness of the raised face portion shall equal the nominal pipe wall thickness.

5.7.3    The outside diameter of the lap shall be in accordance with ASME B16.9.

5.7.4    Taper boring shall be minimized to avoid stress risers.

5.7.5    Lapped joint stub ends should not be used in high pressure and temperature services & cryogenic service application.

5.7.6   The gasket contact surfaces of stub ends shall be flat with face finish specified in the table-I of SES P26-G01

6       Material

6.1      Materials shall conform to the PIP specifications listed in SES P02-S01, P02-S14, P03-S01 and P04-S01 as applicable.

6.2      Fittings for NACE Service

6.2.1    Plant Service: When an in-plant service has water and H2S in concentrations that conform to the levels specified in NACE MR0175/ISO15156, that service shall be considered NACE service. The purchase order for all fittings in those services shall contain the word ‘NACE’.

6.2.2    Fittings of High Strength Material for Pipeline Service: High strength material are fittings conforming to API 5L, pipe Grades X42 to X65.

6.3      Fittings shall have a purchase description as per  SES P31-G01. Additional service and material limitations, for example face to face dimension, operation requirements, service classification, type of fittings, end connection, material grade, testing requirements, nominal size of fitting, wall thickness as defined by schedule, weight, or actual decimal wall, shall be specified on the purchase/requisition order.

6.4      Post weld heat treatment (PWHT) shall be specified in purchase order when required by the applicable code. See ASME B31.3, Table 331.1.1.

6.5      Galvanized fittings shall be coated with zinc by hot dip process confirming to ASTM A 153.

7. Services

7.1   Material requirements are specified in the relevant SES/PIP.

7.2   Table I lists materials for typical services, based on general relationships between service, temperature, and size. These are derived from the relevant PIP/SES, but also include additional materials and service requirements for generation of future material classes.

8.  Marking

8.1   Fittings shall be marked in accordance with respective fittings standards such as ASME B16.9, paragraph 4, ASME 16.28, paragraph 4 and MSS SP-75 paragraph 17. Fittings shall be  protected against mechanical damage during shipping. Visually fittings shall have a smooth surface and shall be free of burrs and sharp edges. Mating surfaces shall fit-up without gaps.

9       Testing and Inspection

9.1      Weld surfaces shall have a smooth contour. Fittings with defects having a depth in excess of 6 1/2 % of specified nominal wall shall be rejected.

9.2      Fitting dimensions shall conform to ASME B16.9, B16.25, MSS SP-75, or approved vendor’s drawings.

TABLE I – Services and Materials

TABLE I - Services and Materials

TABLE I – Services and Materials

Wrought Buttweld Fittings Selection Guide in Industry

Notes

1. Use of Welded fittings instead of Seamless is permissible in case of Carbon Steel for sizes 18” & above and in case of Stainless Steel for sizes 16” & above. This relaxation is acceptable for general process piping classes of 150 & 300 rating only. The weld shall have straight seam & 100% Radiographed.

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