What Is a Weldolet?

Quick Answer

A weldolet is a forged branch connection fitting that creates a 90-degree butt weld outlet from a header pipe to a smaller branch pipe. It provides integral reinforcement at the branch intersection, eliminating the need for a separate reinforcing pad. Weldolets are governed by MSS SP-97 and are preferred when the branch-to-header size ratio is below 0.5.

The name “weldolet” was originally a trade name by Bonney Forge, now used generically across the industry. The fitting contours to the header pipe OD on one end and provides a standard butt weld bevel on the branch end. This allows direct butt welding of the branch pipe without additional reinforcement.

Weldolet Specifications

Property	Detail
Standard	MSS SP-97 (branch connection fittings)
Branch angle	90° to header
Branch end	Butt weld bevel (per ASME B16.25)
Header end	Contoured to match header OD
Size range	Header: NPS 2 to NPS 36; Branch: NPS 1/2 to NPS 24
Materials	ASTM A105 (CS), A182 (SS/alloy), A350 LF2 (low-temp)
Reinforcement	Integral (no pad required per ASME B31.3)

When to Use a Weldolet

The decision between a reducing tee and a weldolet depends primarily on the branch-to-header diameter ratio:

Branch/Header Ratio	Preferred Fitting	Reason
> 0.5	Reducing tee	Standard BW connection, lower SIF
0.25 to 0.5	Weldolet	Cost-effective, integral reinforcement
< 0.25	Weldolet or half coupling	Branch too small for practical tee

A 2-inch branch on a 12-inch header (ratio = 0.17) would use a weldolet. A 10-inch branch on a 12-inch header (ratio = 0.83) would use a reducing tee.

Installation

1. Mark the branch location on the header pipe
2. Cut the branch hole to the required diameter (matching weldolet bore)
3. Fit the weldolet contoured base to the header, ensuring full contact
4. Weld the weldolet to the header with a full-penetration groove weld
5. Butt weld the branch pipe to the weldolet outlet

The weld between the weldolet and the header is typically inspected by radiographic or ultrasonic testing depending on the piping class and service.

Weldolet vs Other O-Lets

Weldolets are just one type of branch fitting (o-let). Other variants include sockolets (socket weld branch end), threadolets (threaded branch end), latrolets (45° lateral branch), and elbolets (branch from an elbow). The branch end connection type defines the o-let variant; the header-side connection is always a contoured weld.

For a dimensional reference of all o-let sizes, see the weldolet/sockolet/elbolet dimension charts.

Read the full guide to pipe fittings