What is the difference between MSS SP-43 and ASME B16.9 for stainless steel fittings?

MSS SP-43 is a simplified version of ASME B16.9 written specifically for stainless steel and nickel alloy butt-weld fittings. Key differences: (1) SP-43 uses Schedule 10S and Schedule 40S dimensions per ASME B36.19M — the standard wall schedules for stainless steel pipe. B16.9 uses Schedule STD/XS dimensions based on ASME B36.10 carbon steel pipe, which differ slightly for the same NPS. (2) SP-43 provides only two schedule options (10S and 40S), simplifying procurement and dimensional consistency with ASTM A312/A403 pipe. (3) SP-43 covers stainless steel AND nickel alloy fittings; B16.9 covers all materials. (4) The centre-to-end dimensions are identical between SP-43 and B16.9 for the same NPS and radius type; the difference lies only in the wall thickness and end preparation dimensions that match the respective pipe schedule.

What is Schedule 10S vs Schedule 40S in stainless steel piping?

Schedule 10S and Schedule 40S are the two primary stainless steel pipe wall schedules defined in ASME B36.19M. Schedule 10S is a lighter wall: NPS 6 has 3.40 mm wall vs 7.11 mm for Schedule 40S — approximately 52% thinner. Schedule 10S fittings are used in low-pressure utility, instrument air, and general corrosion-resistant service. Schedule 40S is the standard weight (equivalent to carbon steel Schedule 40 wall) for most process piping. Both SP-43 schedules share the same centre-to-end dimensions; only the wall thickness and end preparation dimensions differ.

What stainless steel and nickel alloy grades are covered by MSS SP-43?

MSS SP-43 covers fittings in stainless steel and nickel alloy materials. Austenitic stainless: ASTM A403 TP304/TP304L (general corrosion), TP316/TP316L (chemical/marine), TP321/TP347 (elevated temperature). Duplex stainless: ASTM A815 WP2205 (PREN ~35, sour service per NACE MR0175/ISO 15156, chloride SCC resistance), WP2507 super-duplex (PREN ~43, seawater and subsea). Nickel alloys per ASTM B366: Inconel 625 (N06625), Hastelloy C-276 (N10276), Monel 400 (N04400), Incoloy 825 (N08825), and other grades listed in the applicable ASTM specifications. The material grade must be compatible with the matching ASTM A312 pipe grade and the service conditions.

Are MSS SP-43 fittings pressure-rated, and how do I determine the pressure rating?

MSS SP-43 fittings do not carry an independent pressure-temperature rating. The fitting rating is governed by the matching pipe and the applicable piping code (ASME B31.3 for process piping, B31.4/B31.8 for pipelines). For Schedule 40S fittings in B31.3 service at 100°F, the maximum allowable working pressure is determined by the Barlow formula using the Schedule 40S pipe wall and the B31.3 MAWP for the material class (e.g., TP316L at approximately 1,400 psi for NPS 6 Sch. 40S at 100°F). SP-43 Schedule 10S fittings have a lower pressure rating than Schedule 40S because of the thinner wall. Always verify the pressure rating with the project piping engineer and applicable code case.

When should MSS SP-43 be specified instead of ASME B16.9?

Specify MSS SP-43 when: (1) the piping system uses ASME B36.19M stainless steel pipe (Schedule 10S or 40S) and dimensional compatibility with the pipe schedule is critical; (2) the project specification mandates SP-43 (common in offshore oil and gas, chemical processing, and FPSO vessels); (3) a simplified two-schedule procurement is preferred over the multiple schedule options in B16.9; (4) nickel alloy fittings are required — SP-43 explicitly covers ASTM B366 nickel alloys. Specify ASME B16.9 when: the governing piping code (B31.3) requires B16.9 fitting dimensions, when intermediate or extra-strong wall options beyond 10S/40S are required, or when Schedule 80S/160S fittings are needed.

MSS SP43 | HT PIPE

Your position:Home > standard > Manufacturing Standard

PRODUCT

Special Products

BW Pipe Fittings

Forged Pipe Fittings

Steel Pipe Tube

Flanges

Fasteners

Steel Sheet Plate

Round Bar

Welding Wire Electrodes

Prefabrication Pipe Spools

PRODUCT MATERIAL

Special Alloys

Duplex Stainless Steel

HOT PRODUCTS

STANDARDS

MSS SP43Download

1. Scope

This standard provides requirements for factory-made wrought and fabricated butt-welding fittings intended for low-pressure, corrosion-resistant applications. MSS SP-43 is a simplified dimensional specification derived from ASME B16.9, adapted specifically for stainless steel and nickel alloy piping using Schedule 10S and Schedule 40S dimensions per ASME B36.19M.

Edition: MSS SP-43-2026 (supersedes MSS SP-43-2019) | Size Range: NPS 1/2 – 24 (all sizes in ASME B36.19M) | Schedules: 10S and 40S (5S available on request) | Materials: ASTM A403 (austenitic stainless), A815 (duplex/ferritic stainless), B366 (nickel alloy) | Application: Low-pressure corrosion-resistant piping per ASME B31.3, B31.4, B31.8

2. Fitting Types

MSS SP-43 covers the same fitting types as ASME B16.9, adapted for stainless steel service and thin-wall schedules:

Fitting Type	Radius Options	Available Sizes (NPS)
90° Elbow	Long Radius (1.5D), Short Radius (1.0D)	1/2 – 24
45° Elbow	Long Radius (1.5D)	1/2 – 24
Equal Tee	—	1/2 – 24
Reducing Tee	—	3/4 – 24
Concentric Reducer	—	3/4 – 24
Eccentric Reducer	—	3/4 – 24
Buttweld Cap	—	1/2 – 24
180° Return Bend	Long Radius (1.5D) only	1/2 – 12
Stub End (Type A and B)	Lap Joint (A) / Slip-On (B)	1/2 – 24

3. Pressure Rating Guidance

MSS SP-43 fittings carry no independent pressure-temperature rating. The fitting pressure-temperature rating is governed by the matching pipe and the applicable piping code. The table below provides indicative Schedule 40S wall thicknesses and the corresponding maximum allowable working pressure at 38°C (100°F) for austenitic stainless steel per ASME B31.3:

NPS	Schedule	Wall (mm)	B31.3 MAWP at 38°C TP316L (psi)	B31.3 MAWP at 38°C TP304L (psi)
2	10S	2.77	~1,390	~1,210
2	40S	3.91	~1,970	~1,715
4	10S	3.05	~720	~625
4	40S	6.02	~1,445	~1,255
6	10S	3.40	~505	~440
6	40S	7.11	~1,070	~930
8	10S	3.76	~430	~375
8	40S	8.18	~950	~825
10	10S	4.19	~385	~335
10	40S	9.27	~870	~755
12	10S	4.57	~350	~305
12	40S	10.31	~810	~705

Note: Pressures are indicative B31.3 MAWP at 38°C (100°F) based on ASME B36.19M wall thicknesses and ASTM A403 TP316L/TP304L allowable stresses. Always calculate actual MAWP using the governing code and service conditions. Schedule 10S fittings have approximately 40-55% lower pressure rating than Schedule 40S for the same NPS.

4. 90° LR Elbow — Centre-to-End (A) for Schedule 10S and 40S

NPS	DN	LR (1.5D) A — 10S & 40S (mm)	SR (1.0D) A — 10S & 40S (mm)
1/2	15	38	—
3/4	20	57	—
1	25	76	51
1-1/4	32	95	64
1-1/2	40	114	76
2	50	152	102
2-1/2	65	190	127
3	80	229	152
3-1/2	90	267	178
4	100	305	203
5	125	381	254
6	150	457	305
8	200	610	406
10	250	762	508
12	300	914	610
14	350	1067	711
16	400	1219	813
18	450	1372	914
20	500	1524	1016
24	600	1829	1219

Note: LR and SR centre-to-end dimensions are identical for Schedule 10S and Schedule 40S. The wall thickness difference affects only the fitting body wall and end preparation dimensions — not the centre-to-end dimension.

5. Equal Tee — Centre-to-End Dimensions (C = Run, M = Branch)

NPS	DN	C = M (mm) — 10S & 40S
1	25	102
2	50	152
3	80	229
4	100	305
6	150	457
8	200	610
10	250	762
12	300	914
16	400	1219
20	500	1524
24	600	1829

6. Concentric Reducer — Overall Length (L)

Larger NPS	Smaller NPS	Overall Length L (mm)
2	1	152
4	2	305
6	4	457
8	6	610
10	6	610
12	8	610
16	12	610
20	16	610
24	18	610

7. Stub End Dimensions — Type A (Lap Joint) and Type B (Slip-On)

Stub ends per MSS SP-43 are used with lap joint flanges (Type A) and slip-on flanges (Type B). The lap dimension (G) and overall length (F) are specified per ASME B16.9 for the matching NPS and schedule:

NPS	OD (mm)	Schedule 10S		Schedule 40S
NPS	OD (mm)	T (mm)	F (mm)	T (mm)	F (mm)
1/2	21.3	2.11	50.8	2.77	50.8
3/4	26.7	2.11	50.8	2.87	50.8
1	33.4	2.77	50.8	3.38	50.8
1-1/2	48.3	2.77	50.8	3.68	50.8
2	60.3	2.77	63.5	3.91	63.5
3	88.9	3.05	63.5	5.49	63.5
4	114.3	3.05	76.2	6.02	76.2
6	168.3	3.40	88.9	7.11	88.9
8	219.1	3.76	101.6	8.18	101.6
10	273.1	4.19	127.0	9.27	127.0
12	323.9	4.57	152.4	10.31	152.4
16	406.4	4.78	152.4	7.92	152.4
20	508.0	5.54	152.4	9.53	152.4
24	609.6	6.35	152.4	9.53	152.4

OD = outside diameter at bevel. T = lap (hub) thickness. F = overall length from socket end to bevel face. Type A (lap joint flanges) has larger fillet radius; Type B (slip-on flanges) has smaller fillet radius. Wall thickness must be not less than 87.5% of nominal.

8. End Preparation (Beveling)

Parameter	Requirement
Bevel angle	30 degrees +0 / -5 from pipe axis
Tolerance land	1.6 mm +0.8 / -0.8 mm (1/16 in +1/32 / -1/32 in)
End preparation standard	ASME B16.25

9. Tolerances (Summary)

Parameter	Tolerance
Centre-to-end (NPS up to 12)	+1.5 / -1.5 mm
Centre-to-end (NPS greater than 12)	+3.0 / -3.0 mm
Outside diameter at bevel	+1% / -1% of OD (min 0.8 mm)
Wall thickness at ends	Not less than minimum; not more than 1.14 times minimum
Overall length (reducers, caps)	+3.0 / -1.5 mm

10. Material Grades — Stainless Steel and Nickel Alloy

Grade	ASTM Spec	UNS	Category	Typical Application
TP304	A403	S30400	Austenitic SS	General corrosion, food, dairy
TP304L	A403	S30403	Austenitic SS	Carbon steel overlay welds (L = low carbon)
TP316	A403	S31600	Austenitic SS	Chemical, marine, chloride < 200 ppm
TP316L	A403	S31603	Austenitic SS	Marine, pharmaceutical, chloride up to 1,000 ppm
TP321	A403	S32100	Austenitic SS	Elevated temperature up to 870 degrees C, nitric acid
TP347	A403	S34700	Austenitic SS	Elevated temperature up to 650 degrees C, petrochemical
WP2205	A815	S31803/S32205	Duplex SS	Sour service (NACE MR0175/ISO 15156), chloride SCC, offshore
WP2507	A815	S32750	Super-duplex SS	Seawater, subsea, high-chloride environments
Inconel 625 (N06625)	B366	N06625	Nickel alloy	High-temp, highly corrosive, seawater, nuclear
Hastelloy C-276 (N10276)	B366	N10276	Nickel alloy	Strong acids, chlorides, sour gas, wet HCl
Monel 400 (N04400)	B366	N04400	Nickel alloy	Hydrofluoric acid, seawater, caustic, food processing
Incoloy 825 (N08825)	B366	N08825	Nickel alloy	Sulfuric acid, phosphoric acid, seawater, nuclear

11. Related Standards

Standard	Title
ASME B16.9	Factory-Made Wrought Buttwelding Fittings (all materials, all schedules)
ASME B36.19M	Stainless Steel Pipe (Schedule 5S, 10S, 40S dimensions)
ASTM A312	Seamless and Welded Austenitic Stainless Steel Pipe
ASTM A403	Wrought Austenitic Stainless Steel Piping Fittings (WP/CR/WP-WX classes)
ASTM A815	Wrought Ferritic/Duplex Stainless Steel Fittings
ASTM B366	Factory-Made Wrought Nickel and Nickel Alloy Fitting
ASME B16.25	Buttwelding Ends
ASME B31.3	Process Piping
MSS SP-25	Standard Marking System for Valves, Fittings, Flanges and Unions

12. How to Order (Per MSS SP-43-2026)

Order Element	Specification	Example
Standard	MSS SP-43-2026	SP-43
Fitting type	Elbow/Tee/Reducer/Cap/Stub End/Return	Elbow
Radius	LR (1.5D) / SR (1.0D) for elbows	LR
Size	NPS	6
Schedule	10S or 40S	40S
Material grade	ASTM grade (TP316L, WP2205, Inconel 625, etc.)	TP316L
Stub end type (if applicable)	Type A (lap joint) / Type B (slip-on)	Type A
Ends	Beveled (standard per ASME B16.25)	Beveled
Surface finish	Pickled and passivated (standard); electropolished on request	Pickled + Passivated
Quantity	Number of pieces	50
Material test report	EN 10204 3.1 MTC if required	3.1 MTC
Third-party inspection	EN 10204 3.1 MTC if required	3.1 MTC

13. FAQ — Frequently Asked Questions

Question	Answer
Why are Schedule 10S and 40S the only options in MSS SP-43?	Stainless steel pipe per ASME B36.19M is dimensioned differently from carbon steel pipe per ASME B36.10. B36.19M defines two primary wall schedules: Schedule 10S (thin wall, for low-pressure and utility service) and Schedule 40S (standard weight). Intermediate schedules (20S, 30S, 60S, 80S, 100S, etc.) exist but are less commonly used in standard process piping. SP-43 provides the two most common schedules to simplify procurement, dimensional standardization, and inventory management for stainless fittings. Schedule 5S is available on request for ultra-thin-wall applications.
What is chloride stress corrosion cracking (CSCC) risk for SP-43 stainless fittings?	Austenitic stainless steels (TP304/TP316) are susceptible to chloride stress corrosion cracking (CSCC) when exposed to chlorides above approximately 20 ppm at temperatures above 50-60 degrees C. In cooling water or geothermal service with chlorides, consider upgrading to: TP321/TP347 (better but not immune to CSCC above 100 degrees C); 2205 duplex stainless (PREN approximately 35, good chloride SCC resistance up to approximately 250 degrees C); 2507 super-duplex (PREN approximately 43, for seawater service). Always consult the project materials engineer for chloride and temperature limits.
Can 2205 duplex stainless fittings per SP-43 be welded to 316L austenitic pipe?	Yes, but careful filler metal selection is required. The recommended filler is ER2594 (AWS A5.9 E2594) or ER2209 (AWS A5.9 E2209) for matching duplex composition. Using 316L filler (ER316L) creates a weld metal with a mixed microstructure and lower ferrite content, reducing corrosion resistance and mechanical properties. For sour service per NACE MR0175/ISO 15156, the PNo. classification and specific heat treatment condition of the fitting must be documented and verified.
Are SP-43 fittings pressure-tested individually?	Like ASME B16.9, MSS SP-43 does not require individual hydrostatic testing of each fitting. The completed stainless steel piping system is hydrostatically tested per the governing ASME B31.3 (process) or B31.4/B31.8 (pipeline) code. For critical offshore and subsea applications, some operators require factory hydrostatic testing of fittings at 1.5 times design pressure, documented in the manufacturer's test certificates. For sour service fittings, additional PMI (positive material identification) and hardness testing may be required per NACE MR0175/ISO 15156.
What surface finish is standard for SP-43 stainless fittings?	Standard surface finish for SP-43 fittings is pickled and passivated. The pickling process removes surface oxides from heat treatment and forming operations; passivation (nitric acid or citric acid treatment) dissolves free iron from the surface and forms a passive chromium oxide layer that restores corrosion resistance. For high-purity pharmaceutical or food service, electropolished (EP) fittings may be specified, providing a smoother, more chemically inert surface. Surface finish requirements should be stated on the purchase order.

SEND MESSAGES CHAT NOW

> USEFUL LINKS: Special Products; BW Pipe Fittings; Forged Pipe Fittings; Steel Pipe Tube; Flanges; Fasteners; Steel Sheet Plate; Round Bar; Welding Wire Electrodes; Prefabrication Pipe Spools

> APPLIATION: AEROSPACE; CHEMICAL PROCESSING; MEDICAL; OIL and GAS; POWER GENERATION; THERMAL PROCESSING

> TOOLS: Size Chart; National timetable

> FOLLOW US: facebook; linkedin

Email:info@htpipe.com
Tel:+86-371-60953359
Fax:+86-371-60953359
Cell Phone:+86-19339900201
Wechat/Whatapp:86-19339900201