What Is MSS SP-97?
MSS SP-97 is the Manufacturers Standardization Society standard that covers integrally reinforced forged branch outlet fittings. These fittings; commonly called o-lets or branch fittings; create branch connections from a header pipe to a smaller outlet pipe without requiring a separate reinforcing pad.
What MSS SP-97 Covers
The standard defines dimensions, tolerances, testing, and marking requirements for forged branch fittings. It does not specify materials; material requirements come from ASTM specifications (A105, A182, A350).
| Aspect | Requirement |
|---|---|
| Standard title | Integrally Reinforced Forged Branch Outlet Fittings |
| Issuing body | Manufacturers Standardization Society (MSS) |
| Fitting types | Weldolet, sockolet, threadolet, latrolet, elbolet, nipolet, sweepolet |
| Header size range | NPS 2 to NPS 60 (depending on type) |
| Branch size range | NPS 1/8 to NPS 24 (depending on type) |
| Branch angles | 90° (standard), 45° (latrolet) |
| End connections | Butt weld, socket weld (Class 3000/6000), threaded (NPT) |
| Testing | Visual, magnetic particle (MT), liquid penetrant (PT) per manufacturer procedure |
| Marking | Manufacturer, size, material, heat/lot number |
How MSS SP-97 Relates to Other Standards
MSS SP-97 is not an ASME B16 standard. It sits outside the ASME B16 family but is referenced by ASME piping codes (B31.1, B31.3) as an acceptable standard for branch connections. The relationship to other fitting standards:
| Standard | Scope | Typical Size |
|---|---|---|
| ASME B16.9 | Buttweld fittings (elbows, tees, reducers, caps) | NPS 1/2 to NPS 48 |
| ASME B16.11 | Forged socket weld and threaded fittings | NPS 1/8 to NPS 4 |
| MSS SP-97 | Integrally reinforced branch fittings | Header NPS 2+, branch varies |
| MSS SP-75 | High-yield BW fittings for pipeline | NPS 1/2 to NPS 48 |
Integral Reinforcement
The defining feature of MSS SP-97 fittings is integral reinforcement. The fitting body itself provides the additional material needed to compensate for the hole cut in the header pipe. This eliminates the need for reinforcing pads (pad-on-pipe, or POP), which require additional welding, inspection, and potentially introduce crevice corrosion points between the pad and the header.
ASME B31.3 Paragraph 304.3.3 provides the branch reinforcement calculation. An MSS SP-97 fitting that meets the area replacement requirement needs no additional reinforcement.
Weldolet vs Tee: When MSS SP-97 Applies
The choice between a reducing tee (ASME B16.9) and a branch fitting (MSS SP-97) depends on the size ratio:
- Ratio > 0.5: Reducing tee preferred (standard BW fitting, lower stress intensification)
- Ratio 0.25-0.5: Either option works; branch fitting is often more economical
- Ratio < 0.25: Branch fitting (weldolet/sockolet/threadolet) is the practical choice
For dimensional data on all o-let types, see the branch fitting dimensions chart.
Leave a Comment
Have a question or feedback? Send us a message.