Weldolet & O-Let Branch Fittings

Quick Reference

Branch fittings (O’lets) create 45° or 90° reinforced branch connections per MSS-SP 97. Types: Weldolet (buttweld), Sockolet (socket weld, Class 3000/6000/9000), Threadolet (NPT threaded), Latrolet (45° lateral), Elbolet (on elbows), Nipolet (with integral nipple), Sweepolet (smooth flow). Materials: ASTM A105, A182, A350.

Branch Fittings (O’Lets)

Branch fittings, or O’lets, are forged fittings that create 45- or 90-degree reinforced branch connections from a header pipe to a smaller outlet pipe. They replace buttweld tees, reinforcing pads, and saddles, with fewer welds, less space, and lower installed cost.

Branch Fittings (Olets)Outlet fittings are forged products with high strength and clear cost advantages over conventional pipe branching techniques. They can be used as an alternative to buttweld tees, reinforcing pads, and saddles.

Classification of branch fitting types (olets)

O’lets come in size-on-size or reducing configurations, with buttweld, socket weld, or threaded end connections. Common materials:

Material	Specification
Carbon steel (high temp)	ASTM A105
Carbon steel (low temp)	ASTM A350 LF2, LF3
Alloy steel	ASTM A182 F5, F9, F11, F22
Stainless steel	ASTM A182 F304, F316, duplex, super duplex
Nickel alloys	Inconel, Hastelloy, Monel, Incoloy
Exotic	Titanium, Cupronickel

Installation

Installation follows the same basic sequence regardless of O’let type:

1. Mark and prepare the header pipe surface at the branch location. Clean the area back to bright metal.
2. Cut the hole in the header. The bore should match the O’let’s ID (not the branch pipe OD). Bevel the hole edge for full weld penetration.
3. Fit the O’let snugly against the header contour and align the outlet in the branch pipe direction.
4. Weld the run-pipe side: for Weldolets, this is a full-penetration buttweld around the base. Sockolets and Threadolets also require a weld to the header.
5. Connect the branch pipe: butt weld (Weldolet), insert and fillet weld (Sockolet), or thread on with sealant (Threadolet).
6. Inspect: visual examination of all welds, followed by NDE per project spec (typically RT or UT on the header-side weld) and hydrostatic pressure testing.

Pro Tip

When cutting the hole in the header pipe for a Weldolet, always use a beveled cutter or grind a bevel on the hole edge - this allows full weld penetration at the root and prevents incomplete fusion defects. The hole diameter should match the Weldolet’s ID, not the branch pipe OD.

Advantages over Conventional Fittings

Compared to buttweld tees, reinforcing pads, and saddles, O’lets win on several fronts:

Factor	O’let	Buttweld Tee / Pad
Number of welds	2 (header + branch)	3 or more
Installation time	Faster; weld directly to header	Requires cut-out, fit-up, alignment
Space required	Compact; fits congested pipe racks	Tee needs full-bore branch length
Flow restriction	None (funnel-shaped bore)	Tee can restrict flow at small branch ratios
Material use	Less forging weight, especially on large headers	Full tee body uses more material
Field modifications	Weld on without cutting pipe	Must cut out pipe section to insert tee
Reinforcement	Integral; no separate pad needed	Pad must be calculated, fabricated, welded

In practice, I have seen O’lets cut branch installation time nearly in half on congested offshore modules where every millimeter of clearance matters. The real payoff comes on large headers (20” and above) with small branches: a reducing tee in those sizes is expensive and heavy, while a Weldolet weighs a fraction and installs in an afternoon.

Common Mistake

Using Weldolets for branch-to-run ratios greater than 0.5 (branch OD is more than half of header OD). When the branch is that large relative to the header, use a reducing tee instead - the Weldolet’s reinforcement is insufficient for the stress concentration at that size ratio, and your stress analysis will likely flag it.

Key Specifications

Standard	Scope
MSS SP-97	The primary standard for O’lets. Covers types, dimensions, tolerances, material grades, and marking for Weldolets, Sockolets, Threadolets, Elbolets, Nipolets, and others
ASME B16.11	Dimensions and pressure-temperature ratings for socket weld and threaded forged fittings (Sockolets, Threadolets)
ASME B16.25	Buttweld end preparation (Weldolets, Sweepolets)
ASME B31.3	Process piping code; governs design, fabrication, examination, and reinforcement calculations for branch connections
ASTM A105	Carbon steel forgings (high temperature)
ASTM A182	Alloy and stainless steel forgings (F304, F316, F11, F22, etc.)
ASTM A350	Carbon/low-alloy forgings for low-temperature service (LF2, LF3)

Buttweld O’lets (Weldolets, Sweepolets) follow the schedule/wall thickness of the branch pipe. Socket weld and threaded O’lets (Sockolets, Threadolets) are rated by class (3000#, 6000#, or 9000#) matching ASME B16.11 pressure-temperature tables.

Branch Fittings Types

Weldolet

The Weldolet is the most widely used branch fitting. It creates a 90-degree buttweld branch connection from a header pipe to a smaller branch line. Both the header-side and branch-side connections are full-penetration butt welds, which is why Weldolets belong to the buttweld fittings family.

The contoured body sits flush against the header pipe curvature and provides integral reinforcement, so no separate reinforcing pad is needed. The branch-side bevel matches the branch pipe schedule, which can equal or exceed the header schedule. Weldolets are available in a wide range of dimensions.

Of all O’let types, the Weldolet handles the broadest size and pressure range. On any project I have worked on, if the branch-to-header ratio is below 0.5 and the line is NPS 2 or larger, the Weldolet is the default choice. It gives you a fully radiographable weld, unrestricted bore, and the strongest joint of all O’let variants.

How to install a Weldolet: Bonney Forge

Sockolet

A Sockolet works like a Weldolet but uses a socket weld connection on the branch side instead of a butt weld. The branch pipe inserts into the socket and is secured with a fillet weld per ASME B16.11. The header side is still welded to the run pipe.

Sockolets are available in size-on-size or reducing configurations, in classes 3000#, 6000#, and 9000#. They work well for small-bore, high-pressure branch connections (typically NPS 2 and below) where the compact socket joint is easier to fabricate than a buttweld in tight spaces.

Sockolet vs. Weldolet

Criterion	Sockolet	Weldolet
Branch-side joint	Socket weld (fillet weld)	Butt weld (full penetration)
Classification	Forged fitting per ASME B16.11	Buttweld fitting
Ratings	Class 3000#, 6000#, 9000#	Matches branch pipe schedule
Typical branch size	Up to NPS 2	NPS 2 and above (no practical upper limit)
NDE	Fillet weld, limited to MT/PT	Butt weld, fully radiographable (RT/UT)
Fatigue resistance	Lower (fillet weld root is a stress riser)	Higher (full-penetration weld)
Best use	Small instrument taps, drains, vents, tight spaces	Process branches, high-cycle thermal services

Field Note

For branch connections in high-cycle services (like steam systems with frequent thermal cycling), Weldolets are generally preferred over Sockolets - the full-penetration buttweld better handles fatigue stress than the fillet weld of a socket connection. Reserve Sockolets for small instrument taps and drains where accessibility for welding is limited.

Thredolet

A Thredolet (also spelled Threadolet) creates a 90-degree branch with female NPT threads per ASME B1.20.1. The body is welded to the header pipe; the branch pipe simply screws in. Available in classes 3000# and 6000#.

Threadolets are the go-to fitting for low-pressure instrument taps, pressure gauge connections, drain points, and any branch where you need a removable connection without hot work on the branch side. They also see use where welding is restricted, such as live plant tie-ins or areas classified as hazardous where hot-work permits are difficult to obtain.

Latrolet

A Latrolet creates a 45-degree lateral branch from a header pipe, the only standard O’let that does so. Available with butt weld, socket weld, or threaded ends (classes 3000# or 6000# for SW/threaded).

Latrolets are commonly used for drain connections, instrument taps, and auxiliary tie-ins where a 45-degree take-off improves flow dynamics or gravity drainage. They are also useful during retrofits when a 90-degree branch would interfere with adjacent piping or steel.

Elbolet

An Elbolet is the O’let you reach for when you need a branch connection directly on the curved surface of a pipe elbow. The body is contoured to match the elbow’s curvature (long-radius or short-radius), so a standard Weldolet or Threadolet, shaped for straight pipe, cannot substitute.

Available with butt weld, socket weld, or threaded ends (classes 3000# or 6000# for SW/threaded). Typical uses include instrument taps, vent lines, and sampling ports on elbows where there is no straight run nearby.

Nipolet

A Nipolet combines an O’let body with an integral nipple extension. The header side welds to the run pipe; the branch end has external (male) threads or a plain end, so instrumentation, valves, or small-bore piping can connect directly without a separate nipple piece.

This makes the Nipolet a one-piece solution for instrument taps, thermowell connections, drains, and vents. It saves a threaded joint (and a potential leak point) compared to a Threadolet plus a separate nipple.

Sweepolet

A Sweepolet has a gradually curved, swept profile that provides the smoothest flow transition of any branch fitting. Where a standard Weldolet creates an abrupt 90-degree turn into the branch, the Sweepolet’s radius guides flow with less turbulence, lower pressure drop, and reduced erosion at the branch entry.

Both ends are butt-welded (header and branch), and the header-side weld is fully accessible for radiographic (RT) or ultrasonic (UT) non-destructive testing.

Sweepolets are specified on critical process lines, high-velocity services, erosive slurries, and high-temperature steam systems, anywhere that flow-induced vibration or erosion is a concern. They cost more than a standard Weldolet but pay for themselves in reduced maintenance on aggressive services.

Coupolet

Coupolet Branch FittingA Coupolet is a half-coupling style branch fitting with a contoured base that welds to the header pipe, providing a full-penetration connection with integral reinforcement. The branch side accepts a threaded or socket weld connection, similar to a half coupling, hence the name.

Coupolets are typically used where the branch is less than half the header size. They follow MSS SP-97 and are available in the same materials and classes as other O’lets. Common applications include instrument connections (pressure gauges, thermowells), small drain and vent branches, and auxiliary system tie-ins on process piping in oil and gas, petrochemical, and power generation plants.

How to Order Branch Fittings

Suppliers of Weldolets, Thredolets, Sockolets, and other branch fittings need the following information to identify the needed product:

• Header run pipe size (which is generally expressed as a range of possible sizes, for example, 36-22)
• Branch pipe size (the NPS of the connecting pipe to the run pipe, for example, 6 inches)
• Branch pipe schedule (which can be STD, XS, 160, XXS) for buttweld types of connections (rating shall be provided for socket weld and NPT threaded Olets)
• Style of branch fitting (Weldolet, Thredolet, Nipolet, Sockolet, etc)
• Material Grade (example A105, A350 LF2, A182 F304)

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EXAMPLE: 36-22 | 6 INCH Sch. STD | Weldolet | ASTM A105***

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Conventional Reinforced Branch Connections

While O-lets provide integral reinforcement, traditional branch connections often use reinforcing pads (also called pad-type reinforcement or saddle reinforcement) to strengthen the branch-to-header junction. Understanding both methods helps engineers select the most appropriate solution for each application.

Reinforcing Pads (Pad-Type Reinforcement)

A reinforcing pad is a circular or oval plate welded around a branch connection to compensate for the material removed when cutting the hole in the header pipe. The pad restores the structural integrity lost due to the opening and distributes stress over a larger area.

Feature	Description
Shape	Circular or elliptical plate with center hole matching branch OD
Material	Same as header pipe (or compatible material with equivalent strength)
Thickness	Typically equal to or greater than header pipe wall thickness
Vent hole	Small drilled hole (typically 1/4” or 6mm) for leak detection and venting during welding
Welds	Fillet welds at outer edge and around branch pipe

Set-On vs. Set-In Connections

Branch connections are classified by how the branch pipe meets the header:

Type	Description	Application
Set-on	Branch pipe sits on top of header; hole equals branch ID	Most common; simpler fabrication
Set-in (set-through)	Branch pipe penetrates through header; hole equals branch OD	Better flow characteristics; requires more precise fit-up

Both set-on and set-in connections may require reinforcement depending on the pressure-area calculation per ASME B31.3 or other applicable codes.

O-Lets vs. Reinforcing Pads: Selection Criteria

Factor	O-Lets (Integrally Reinforced)	Reinforcing Pads
Branch/header ratio	Best for ratios below 0.5	Any ratio; required calculation per code
Cost (small quantities)	Higher unit cost	Lower material cost, higher labor
Cost (large quantities)	Economical at scale	Labor-intensive
Inspection	Simple; fewer welds	More welds to inspect
Fabrication time	Faster installation	Requires pad cutting, fit-up, multiple weld passes
Code compliance	Pre-engineered per MSS SP-97	Requires reinforcement calculation per ASME B31
Availability	Standard sizes readily available	Custom pads easily fabricated on-site
Large branches	Limited by size ratio	Preferred for branch/header ratios > 0.5

When to Use Reinforcing Pads

Reinforcing pads remain the preferred choice in several scenarios:

1. Large branch-to-header ratios (above 0.5) where O-lets provide insufficient reinforcement
2. Field modifications where standard O-let sizes are not available
3. Cost-sensitive projects with sufficient fabrication capacity
4. Non-standard angles (other than 45° or 90°) where O-lets are not manufactured
5. Existing systems where pad reinforcement can be added without removing branch pipes

Reinforcement Calculation

Per ASME B31.3 (and similar codes), the required reinforcement area must equal or exceed the area removed from the header pipe. The calculation considers:

• Header pipe diameter and wall thickness
• Branch pipe diameter and wall thickness
• Design pressure and allowable stress
• Weld strength reduction factors
• Credit for excess wall thickness in header and branch

When the inherent reinforcement (from excess wall thickness) is insufficient, external reinforcement via pads or integrally reinforced fittings (O-lets) is required.