1. Scope

This standard covers 90° and 45° integrally reinforced forged branch outlet fittings (commonly known as thredolets, sockolets, sockweldolets, weldolets, and elbowlets) with socket welding, threaded, and buttwelding branch connections. The fittings are forged — the reinforcement pad at the branch is an integral part of the forging, not a separate weld-on pad. MSS SP-97 defines essential dimensions, tolerances, finish, testing, marking, material, and minimum strength requirements. The pressure-temperature ratings are based on ASME B16.5 Class 3000 and 6000, or the governing piping code for buttweld connections.

Edition: MSS SP-97-2025 (revised March 2025 — supersedes SP-97-2018) | Branch Sizes: NPS 1/4–12 | Branch Connection Types: Threaded (NPT), Socket-Weld (SW), Buttweld (BW) | Pressure Classes: CL3000 and CL6000 | Materials: Carbon steel, alloy steel, stainless steel, nickel alloys | Outlet Angles: 90° and 45°

2. Product Types and Configurations

3. Material Specifications

4. Weldolet — Common Run × Branch Combinations (Reference)

Standard run × branch combinations per MSS SP-97-2025. Consult manufacturer for full dimensional tables (NPS 3/4 through NPS 12 branch). Thredolets and sockolets available for NPS 1/4–2 branch on run sizes NPS 1–12.

5. Integral Reinforcement — Design Principles

The integral reinforcement pad of an MSS SP-97 outlet fitting provides the reinforcement required by ASME B31.3 paragraph 304.3.3 (or equivalent paragraphs in B31.4, B31.8). The reinforcement area calculation must account for the:

• Metal area removed (A₁): Cross-sectional area of pipe wall cut away by the branch opening in the run pipe wall
• Reinforcement area available in the run pipe (d): Depth of reinforcement zone extending into the run pipe (typically 2.5 × wall thickness or to the run pipe OD, whichever is less)
• Reinforcement area available in the branch pipe (t_b): Branch wall thickness in the reinforcement zone, up to t_b (branch nominal wall)
• Integral pad area (A₂ + A₃): The integral reinforcement pad provides metal area A₂ and A₃ in the reinforcement zone
• Weld metal area (A₄): Required fillet weld areas connecting the branch outlet to the run pipe

The total available reinforcement area must equal or exceed the metal area removed (A₁). The integral pad on an SP-97 fitting is calculated to provide the required A₂ + A₃ automatically — the designer must still verify the weld area A₄ and the branch pipe wall adequacy per the applicable code.

6. Tolerances, Testing, Finish and Marking

7. Related Standards

8. How to Order (Per MSS SP-97)