1. Scope

MSS SP-43 provides requirements for wrought and fabricated butt-welding fittings intended for low pressure, corrosion resistant applications. Per the official MSS scope statement, this Standard Practice covers only fittings made for use with Schedule 5S or 10S pipe, for all Nominal Pipe Sizes (NPS) listed in ASME B36.19M, except that short pattern stub ends suitable for use with Schedule 40S are also shown.

ASME B16.9 points to SP-43 directly: B16.9 "does not cover low pressure corrosion resistant buttwelding fittings. See MSS SP-43." The two standards share centre-to-end geometry; SP-43 differs in that it is built on the stainless steel wall schedules of ASME B36.19M rather than the carbon steel schedules of ASME B36.10M.

2. Fitting Types Covered

SP-43 tabulates the following types. Note the absence of short radius elbows and of short radius returns — those are ASME B16.9 items, not SP-43 items.

3. Pressure Rating — What SP-43 Actually Says

MSS SP-43 is a dimensional and material standard for low pressure, corrosion resistant service. It does not publish a pressure-temperature table. The allowable working pressure of an SP-43 fitting is governed by the matching pipe and the applicable piping code — in practice ASME B31.3 for process work, or the owner's specification for utility and ventilation service.

Because SP-43 fittings are made to Schedule 5S or 10S wall, their pressure capability is well below that of a Schedule 40S or B16.9 standard-weight fitting of the same NPS. Do not apply a B16.9 rating to an SP-43 fitting on the basis of identical centre-to-end dimensions — the centre-to-end is the same, the wall is not.

Wall thickness values above are the ASME B36.19M nominal pipe walls that SP-43 fittings are built to match. Sch 40S is listed only for reference and for short pattern stub ends; SP-43 fittings are not made to Sch 40S wall in the elbow, tee, reducer, cap and return tables.

4. 90° Long Radius Elbow — Centre-to-End (A)

The centre-to-end dimension is identical to ASME B16.9 for the same NPS and is independent of schedule. There is no short radius column: SP-43 does not tabulate short radius elbows.

5. Tee — Centre-to-End (C = Run, M = Outlet)

6. Concentric Reducer — Overall Length (H)

7. Lap Joint Stub Ends — Long and Short Pattern

SP-43 tabulates lap joint stub ends in two patterns. The long pattern is the standard item and is made to match Schedule 5S and 10S pipe. The short pattern is the one place in SP-43 where Schedule 40S enters — it is intended for use with Sch 40S pipe. Both are used with a lap joint flange, which slips over the stub end and is free to rotate for bolt hole alignment.

8. Tolerances and End Preparation

Exact tolerance values are banded by NPS group in the standard and shall be taken from the published tables. The values published by SP-43 differ from ASME B16.9 mainly in the outside diameter at the bevel.

9. Material Grades

SP-43 is written around stainless steel. Other corrosion resistant alloys — nickel alloys to ASTM B366, titanium to ASTM B363 — are routinely supplied to SP-43 dimensions by agreement between manufacturer and purchaser, but they are not the subject of the standard's material tables. Confirm the material specification on the purchase order.

10. Related Standards

11. How to Order

12. FAQ — Frequently Asked Questions

Does MSS SP-43 cover Schedule 40S fittings?

No, with one exception. The SP-43 scope states that the standard covers only fittings made for use with Schedule 5S or 10S pipe, and that short pattern stub ends suitable for use with Schedule 40S are also shown. Elbows, tees, reducers, caps and returns are not made to Sch 40S wall under SP-43. If you need a Sch 40S stainless fitting, specify ASME B16.9.

Does SP-43 include short radius elbows?

No. SP-43 tabulates 90° and 45° long radius elbows and 180° long radius returns. Short radius elbows and short radius returns are ASME B16.9 items — B16.9 absorbed them from the withdrawn ASME B16.28. Specifying a "SP-43 short radius elbow" is a contradiction and will be queried by any competent manufacturer.

What is the size range of MSS SP-43?

NPS 1/2 through NPS 24 in the dimensional tables. The scope refers to all NPS listed in ASME B36.19M, and B36.19M itself runs NPS 1/8 to NPS 30; in practice the SP-43 tables cover 1/2 to 24. If you need NPS 26 to 30 stainless thin-wall fittings, treat them as specials.

Can an SP-43 fitting be substituted for a B16.9 fitting?

Not without an engineering check. The centre-to-end dimensions match, so the fitting will fit the line geometrically. The wall does not: an SP-43 Sch 10S fitting is considerably thinner than a B16.9 standard weight fitting, so its pressure capability and its corrosion allowance are both lower. Substitution is acceptable only where the design basis already assumed a Sch 5S or 10S wall.

What pressure rating should I put on the data sheet?

SP-43 publishes none. State the fitting as MSS SP-43 with the schedule and material, and let the pipe and the governing piping code set the rating. Where a rating is required for the line, calculate it per ASME B31.3 using the B36.19M wall you have specified.

Is SP-43 only for stainless steel?

SP-43 is written for corrosion resistant service and its material tables are built around austenitic and duplex stainless steel. ASME B16.9 refers to it as "MSS SP-43, Wrought Stainless Steel Butt-Welding Fittings". Nickel alloy and titanium fittings are supplied to SP-43 dimensions in the market, but that is a commercial practice, not something the standard's material section covers — always name the ASTM material specification on the order.