Weldolet vs Sockolet vs Threadolet

Weldolets, sockolets, and threadolets are the three most common branch connection fittings (o-lets) per MSS SP-97. All three create a 90-degree branch from a header pipe. The difference lies in how the branch pipe connects to the fitting outlet: butt weld, socket weld, or threaded.

Comparison at a Glance

Feature	Weldolet	Sockolet	Threadolet
Branch end	Butt weld bevel	Socket (recess)	NPT or BSP threads
Branch pipe connection	Full-penetration butt weld	Fillet weld	Threaded (no welding)
Branch size range	NPS 1/2 to NPS 24	NPS 1/8 to NPS 4	NPS 1/8 to NPS 4
Pressure classes	Matches pipe schedule	3000, 6000	2000, 3000, 6000
Typical application	Large branch lines	Small-bore process connections	Instrument taps, drains, vents
NDE on branch weld	RT or UT possible	PT or MT on fillet weld	None (no weld)
Leak integrity	Highest	High	Moderate
Disassembly	Requires cutting	Requires cutting	Unscrew

When to Use Each

Weldolet: Use for branch sizes NPS 2 and above, or wherever the piping specification requires butt weld connections. The full-penetration weld allows radiographic inspection and provides the highest joint integrity. Weldolets are standard on process piping headers in oil and gas plants.

Sockolet: Use for small-bore branch connections (NPS 2 and below) where a welded, leak-proof joint is needed. The socket design simplifies alignment during fabrication. Sockolets in Class 3000 match SCH 80 pipe; Class 6000 matches SCH 160. Common for instrument connections, chemical injection points, and small-bore drain and vent lines.

Threadolet: Use for instrument connections, sampling points, and temporary installations where disassembly without cutting is required. Most piping specifications limit threadolets to NPS 1 or NPS 1.5 maximum and restrict them from hydrocarbon service above certain pressures.

Size Selection

The o-let size is specified as “header size x branch size.” For example, a 12 x 2 weldolet fits a 12-inch header pipe and provides a 2-inch butt weld branch outlet. The header side is always contoured to match the header pipe OD.

Header NPS	Typical Weldolet Branch	Typical Sockolet Branch	Typical Threadolet Branch
4	1 to 2	1/2 to 2	1/4 to 1.5
8	1 to 4	1/2 to 2	1/4 to 1.5
12	1 to 6	1/2 to 2	1/4 to 1.5
16	2 to 8	1/2 to 2	1/4 to 1.5
24	2 to 14	1/2 to 2	1/4 to 1.5

For complete dimensional tables, see the o-let sizes chart.

Materials

All three types share the same forged material specifications: ASTM A105 (carbon steel), A182 (stainless and alloy steel), and A350 LF2/LF3 (low-temperature carbon steel). The material must match the header pipe grade per project piping material specifications.

Read the full guide to pipe fittings