ASME B16.11

ASME B16.11 covers forged socket-welding and threaded pipe fittings in sizes NPS 1/8 to NPS 4 and pressure classes 2000#, 3000#, 6000# and 9000#. It specifies dimensions, tolerances, pressure-temperature ratings and marking for elbows (45° and 90°), tees, crosses, couplings, half-couplings, reducing couplings, caps, bushings and plugs. It does not cover flanges (ASME B16.5), buttweld fittings (ASME B16.9), or pipe nipples.

Class 3000# socket weld fittings are suitable for Sch 80 pipe and are the most common choice for standard process piping in NPS ½–2. Class 6000# corresponds to Sch 160 / XXS pipe and is required when higher pressure, higher temperature, or smaller bore pipe is involved. In practice, many projects specify 6000# as a conservative standard for all SW fittings to avoid class mixing on site. Verify against ASME B31.3 Table 302.3.1 pressure-temperature tables for your material and temperature. A105 at 100°F can sustain significantly higher pressure than at 900°F.

Socket weld fittings have a recessed socket bore — the pipe slides in and is fillet-welded around the outside. They provide better structural integrity and are preferred for hazardous and high-pressure services. Threaded fittings screw onto NPT or BSPT threaded pipe — no welding is needed, making them quick to install and removable. ASME B31.3 restricts threaded fittings for fluid services involving flammable, toxic or highly volatile materials, so socket weld is preferred wherever leak-tightness is critical.

ASME B31.3 requires a 1/16" (approximately 1.6mm) gap between the pipe end and the bottom of the socket before welding. This gap allows for thermal expansion of the pipe during welding and prevents weld shrinkage from stressing the pipe. Failing to maintain this gap (or zero-gapping) is a common fabrication error that leads to stress cracking in service — particularly in alloy steel (P91, P22) at high temperatures. The gap must be visible in the weld procedure and verified by the QC inspector before welding.