How to Order Branch Fittings (Olets)
Branch fittings (commonly called “olets”) are self-reinforced branch connections that are welded directly onto the header (run) pipe. They eliminate the need for a reinforcing pad when the branch-to-header diameter ratio and design conditions permit.
Olets are manufactured per MSS SP-97 (dimensions and tolerances) and are designed in accordance with ASME B31.3 (process piping) or ASME B31.1 (power piping). The fitting is contour-machined to match the header pipe OD and is attached with a full-penetration or fillet weld.
Core Attributes
Every branch fitting line item must include all of the following attributes.
| # | Attribute | Values / Options | Notes |
|---|---|---|---|
| 1 | Fitting type | Weldolet (butt weld outlet), sockolet (socket weld outlet), threadolet (threaded outlet), elbolet (welded to elbows), latrolet (45° lateral branch), nipolet (integral nipple outlet), sweepolet (contoured low-stress branch), insert weldolet (for thinner headers) | Each type defines the outlet end connection and geometry |
| 2 | Header (run) size | NPS of the main pipe | Must match the actual OD and schedule of the run pipe; weldolets are contour-bored to fit a specific header OD |
| 3 | Branch (outlet) size | NPS of the branch connection | The branch is always smaller than or equal to the header. Typical ratios: branch/header up to 1:1 for weldolets, though most applications are 1:2 or smaller |
| 4 | Header schedule | Schedule or wall thickness of the run pipe | Critical: the weldolet saddle contour is machined to match this specific header OD. A mismatch means the fitting will not seat properly |
| 5 | Branch end details | For butt weld olets (weldolet, elbolet, latrolet, nipolet): branch schedule or WT. For SW/THD olets (sockolet, threadolet): pressure class (3000 or 6000) | Do not mix schedule and class designations; weldolets use schedule, sockolets/threadolets use class |
| 6 | Standard | MSS SP-97 (dimensions) | Design per ASME B31.3 or B31.1 as applicable |
| 7 | Material specification | See material table below | Must match or exceed the header pipe material in strength, corrosion resistance, and toughness |
Material Specifications
| Service | ASTM Standard | Common Grades | Matching Pipe Spec |
|---|---|---|---|
| Carbon steel | ASTM A105 | A105 | A106 Gr.B, API 5L Gr.B |
| Alloy steel | ASTM A182 | F11, F22, F5, F9, F91 | A335 P11/P22/P5/P9/P91 |
| Stainless steel | ASTM A182 | F304/L, F316/L, F321, F347 | A312 TP304/L, TP316/L |
| Duplex stainless steel | ASTM A182 | F51 (2205), F53 (2507) | A790 S31803, S32750 |
| Low-temperature CS | ASTM A350 | LF2 | A333 Gr.6 |
Optional and Special Attributes
| Attribute | When Required | Details |
|---|---|---|
| Reinforcement calculation | When required by design code | ASME B31.3 para. 304.3.3 or B31.1 may require area-replacement calculation. The olet manufacturer should confirm the fitting provides adequate reinforcement for the specified header/branch/schedule combination |
| NDT on attachment welds | Almost always for process piping | MT (magnetic particle) or PT (liquid penetrant) on the header-to-olet attachment weld per ASME B31.3 Table 341.3.2. Specify acceptance criteria (e.g., ASME Sec. V, Art. 7) |
| NACE MR0175 / ISO 15156 | Sour service | Hardness limits, heat treatment requirements, and material restrictions apply |
| PWHT | Per design code or project spec | Post-weld heat treatment of the attachment weld may be required depending on material, thickness, and service. Specify the PWHT procedure or reference the applicable code requirement |
| Certificates | Almost always required | EN 10204 Type 3.1 (MTR) minimum. Specify PMI if required |
Example Line Items
Correct
10 pcs - Weldolet, MSS SP-97, 8” x 2”, Header Sch 40, Branch Sch 80, ASTM A105, MTR EN 10204 3.1
This line item specifies: fitting type, standard, header and branch sizes, header and branch schedules, material, and certificate type.
Incorrect
10 - Weldolet 8” x 2”, CS
This line item is missing: the dimensional standard (MSS SP-97), header schedule (the saddle contour depends on it), branch schedule, the exact material specification (A105 vs A350 LF2), and certificate requirements. The supplier cannot manufacture the fitting without the header schedule.
Common Pitfalls
| Pitfall | Why It Matters |
|---|---|
| Omitting the header schedule | The olet saddle is contour-machined to match the OD of a specific pipe schedule. If the header schedule is not stated, the fitting may not seat flush against the pipe, causing weld defects and stress concentrations |
| Material mismatch with the header pipe | The olet material must match or exceed the header material. An A105 weldolet on an A335 P11 header creates a dissimilar metal joint that requires special welding procedures and may not meet design code requirements |
| Confusing branch schedule with class | Weldolets, elbolets, latrolets, and nipolets use schedule for the branch end (since it is a butt weld connection). Sockolets and threadolets use class (3000 or 6000) for the branch end. Specifying “Sockolet, Class Sch 80” is incorrect |
| Wrong header/branch size combination | Not all header/branch combinations are standard. Very large header-to-branch ratios (e.g., 24” x 1/2”) or near-equal sizes (e.g., 4” x 4”) may not be available as standard catalog items and may require custom manufacturing |
| Forgetting NDT requirements for attachment welds | The olet-to-header weld is a pressure-containing weld. ASME B31.3 requires NDE depending on service class and examination category. Failing to specify this in the PO means the weld may not be examined until field installation, when rework is far more costly |
| Not specifying the branch angle for latrolets | Latrolets are 45° lateral branches by default, but the angle must be confirmed. If a 90° branch is intended, a standard weldolet is the correct choice |
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