What does BS 3799 cover?

BS 3799:1974 is a British Standard specification for forged carbon and alloy steel screwed and socket-welding pipe fittings specifically for the petroleum industry . It covers fittings in sizes NPS 1/8 through NPS 4, with both screwed (BSPP and BSPT) and socket-weld end connections. Product types include elbows (90° and 45°), tees, equal and reducing tees, crosses, straight and reducing reducers, caps, unions, plugs, and bushes. BS 3799 is a dimensional and material specification — it defines geometry, tolerances, materials, testing, and marking requirements. Note: BS 3799 has not been formally revised since 1974 (incorporating Amendments Nos. 1 and 2). For new projects, the applicability of BS 3799 should be verified against current project specifications and applicable codes.

What is the difference between BSPP and BSPT threads in BS 3799?

BSPP (British Standard Pipe Parallel) : Parallel internal/external threads where the thread seal is made by a gasket, O-ring, or sealing compound in a recessed seat — the thread does not provide the pressure seal. BSPP fittings require a bonded seal washer or a flat-face union with a gasket. BSPT (British Standard Pipe Taper) : Tapered external threads (male) that seal by thread interference — the taper creates a pressure-tight metal-to-metal seal without a gasket. BSPT is used where a gasket-free joint is required. Both BSPP and BSPT are covered by BS 21 (ISO 7-1 equivalent) and are NOT interchangeable with NPT (National Pipe Taper) — NPT has a different thread angle (60° vs 55°) and taper ratio (1/16 vs 1/16), so NPT and BSPT threads will not seal together.

How does BS 3799 compare to ASME B16.11?

BS 3799 and ASME B16.11 both cover forged fittings with socket-weld and threaded connections, but they differ in: Thread standard : BS 3799 uses BSP/BSPT threads (BS 21, ISO 7-1); ASME B16.11 uses NPT threads (ASME B1.20.1). Pressure class : BS 3799 defines Table 1 (3000 psi) and Table 2 (6000 psi) ratings; ASME B16.11 defines Class 3000 and 6000 — pressure ratings are similar in magnitude but based on different standards. Dimensions : BS 3799 dimensions and socket bores differ from ASME B16.11 — the two are not interchangeable . Region : BS 3799 is used in the UK, Commonwealth countries, and petroleum projects following British/European codes; ASME B16.11 is used in the US, Canada, and projects following ASME/American codes. For international petroleum projects, verify which standard is specified by the project engineering specification.

What materials are specified in BS 3799?

BS 3799 covers forged carbon steel and alloy steel fittings. Common material grades referenced in BS 3799: Carbon steel : BS 1503 (forgings for pressure purposes — 261 to 430 MPa yield range); BS 1504 (castings); alternatively ASTM A105 is often accepted as equivalent for carbon steel forgings. Alloy steel : BS 1503 grades with Cr-Mo composition (1/2 Cr-Mo, 1-1/4 Cr-Mo, 2-1/4 Cr-Mo) for high-temperature petroleum service. The material specification in BS 3799 is project-specified — users should verify the specific material grades permitted by the applicable project specification or national standard. For modern projects, ASTM equivalents (A182 for alloy steel forgings) are frequently used alongside or in place of BS 1503.

Is BS 3799 still current for new projects?

BS 3799:1974 has not been formally revised since 1974 (with only Amendments Nos. 1 and 2 incorporated in the reprint). Its continued use for new petroleum projects is project-specific. Many modern international petroleum projects use ISO 4144 (Pipe fittings — Threaded carbon steel fittings — ISO series) or ASME B16.11 as the governing standard, with EN 10241 (Steel threaded pipe fittings) for European projects. However, BS 3799 remains a valid standard and is still referenced in some legacy and international petroleum project specifications (particularly in the Middle East, Southeast Asia, and Commonwealth countries). For any project, verify which standard is specified by the project engineering specification (PES) or company standard before specifying or sourcing fittings to BS 3799. When in doubt, use the standard specified on the purchase order or project datasheet.

BS 3799 Steel Pipe Fittings — Screwed & Socket-Welding for Petroleum Industry

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BS 3799Download

1. Scope

This standard covers forged carbon and alloy steel screwed and socket-welding pipe fittings for the petroleum industry, in sizes NPS 1/8 through NPS 4. The fittings are designed for use in petroleum, petrochemical, and associated process industries where British Standard pipe threads and dimensional conventions are applicable. Both screwed (BSP parallel and BSP taper) and socket-weld connections are covered. BS 3799 defines dimensional requirements, tolerances, materials, testing, and marking for the fittings.

Important Note on Status: BS 3799:1974 has not been revised since 1974 (incorporating Amendments Nos. 1 and 2). For new projects, verify applicability against the project engineering specification. Modern alternatives include ISO 4144 (ISO series threaded steel fittings), EN 10241 (European threaded steel fittings), or ASME B16.11 (American standard). When BS 3799 is specified, sourcing may be limited — some manufacturers produce BS 3799 fittings on request or as legacy stock.

Edition: BS 3799:1974 (incorporating Amendments Nos. 1 and 2) | Size Range: NPS 1/8–4 | Connection Types: Screwed (BSPP / BSPT) and Socket-Weld | Pressure Ratings: Table 1 (3000 psi) and Table 2 (6000 psi) | Materials: Forged carbon steel and alloy steel | Thread Standard: BS 21 (BSPP/BSPT) | Region: UK, Commonwealth, Middle East, international petroleum

2. Product Types Covered

Product Type	Connection	Sizes (NPS)	Notes
90° Elbow	SCREWED / SW	1/8 – 4	Equal elbow
45° Elbow	SCREWED / SW	1/8 – 4	Equal elbow
Tee (Equal)	SCREWED / SW	1/8 – 4	All three ends equal size
Tee (Reducing)	SCREWED / SW	1/8 – 4	Run and branch different sizes
Cross (Equal / Reducing)	SCREWED / SW	1/8 – 4	Four connections
Straight Reducer	SCREWED / SW	1/8 – 4	Concentric reducing
Eccentric Reducer	SCREWED / SW	1/8 – 4	Offset centreline for gas/vapor lines
Cap	SCREWED / SW	1/8 – 4	End closure
Union	SCREWED	1/8 – 4	Disconnectible screwed joint
Plug (Hex, Square, Round)	SCREWED	1/8 – 4	Male end closure fitting
Bushing	SCREWED	1/8 – 4	Reducing bushing (female to smaller male)

3. Pressure Ratings (BS 3799)

Table Reference	Pressure Rating	Approx. MPa	Typical Application	Equivalent ASME B16.11
BS 3799 Table 1	3000 psi	~20.7 MPa	General petroleum process	Class 3000
BS 3799 Table 2	6000 psi	~41.4 MPa	High-pressure petroleum, gas	Class 6000

Pressure ratings per BS 3799:1974. Actual pressure-temperature ratings are specified in the standard tables for each fitting type and size. BS 3799 does not use ASME B16.5 class designations — instead, it defines Table 1 and Table 2 pressure ratings. The pressure-temperature relationship for BS 3799 fittings must be verified from the standard tables or the project specification.

4. Thread Standards — BSPP vs BSPT vs NPT

Parameter	BSPP (BS 21 / ISO 228)	BSPT (BS 21 / ISO 7-1)	NPT (ASME B1.20.1)
Thread Angle	55°	55°	60°
Taper Ratio	Parallel (no taper)	1 in 16 taper	1 in 16 taper
Seal Mechanism	Gasket / O-ring / seal compound	Thread interference (metal-to-metal)	Thread interference (metal-to-metal)
Sealant Required	Yes (PTFE tape or compound)	Yes (PTFE tape or compound)	Yes (PTFE tape or compound)
Interchangeable with NPT?	NO — different thread angle	NO — different thread angle	Reference standard
Used In	UK, Commonwealth, petroleum	UK, Commonwealth, petroleum	USA, Canada, ASME projects

5. Material Specifications (BS 3799)

Material	BS Standard	Grade	Min Yield (approx. MPa)	Common Equivalent
Carbon Steel Forgings	BS 1503	261 to 430 range	260–430	ASTM A105
1/2Cr-1/2Mo Steel Forgings	BS 1503	621 (1/2Cr-1/2Mo)	~310	ASTM A182 F1
1Cr-1/2Mo Steel Forgings	BS 1503	622 (1Cr-1/2Mo)	~310	ASTM A182 F11/F12
1-1/4Cr-1/2Mo Steel Forgings	BS 1503	625 (1-1/4Cr-1/2Mo)	~310	ASTM A182 F11
2-1/4Cr-1Mo Steel Forgings	BS 1503	629 (2-1/4Cr-1Mo)	~310	ASTM A182 F22

BS 3799 material specifications per BS 1503 (forgings for pressure purposes). ASTM equivalents are commonly accepted in international petroleum projects where BS 1503 grades are not readily available. Always verify material specification requirements with the project specification before sourcing.

6. Tolerances, Testing, Finish and Marking

Requirement	Specification
Thread Tolerance	BSPP/BSPT threads per BS 21: gauge acceptance for parallel and taper threads
Dimensional Tolerances	Per BS 3799:1974 dimensional tables — centre-to-end, socket bore, overall dimensions within specified tolerances
Hydrostatic Test	Per BS 3799 — fittings hydrostatically tested at 1.5× rated pressure; no visible leakage; test pressure held for minimum 10 seconds
Surface Finish	Free from fins, laps, seams, tears; threads clean and properly formed; socket bores free from obstructions
Marking	Per BS 3799: manufacturer/trademark, material grade (BS 1503 or equivalent), pressure table number (Table 1 or Table 2), size (NPS), standard designation (BS 3799), heat/batch number. BS 3799 does not use MSS SP-25 — it has its own marking requirements per the standard.

7. Related Standards

Standard	Title
BS 21	Specification for pipe threads for tubes and fittings (BSPP and BSPT — parallel and taper threads)
ISO 7-1	Pipe threads where pressure-tight joints are made on the threads — Part 1: Threads having form (taper BSPT equivalent)
ISO 228-1	Pipe threads where pressure-tight joints are not made on the threads — Part 1: Threads (BSPP equivalent)
ASME B16.11	Forged Fittings, Socket-Welding and Threaded (American standard — NPT threads, Class 2000/3000/6000/9000)
BS 1503	Specification for forged steel fittings, valves and parts for pressure purposes (material specification for BS 3799 fittings)
ISO 4144	Pipe fittings — Threaded carbon steel fittings — ISO series (modern replacement for BS 3799 in ISO-adopted projects)
EN 10241	Steel threaded pipe fittings (European standard — BSP threads)

8. How to Order (Per BS 3799)

Parameter	Example	Notes
Standard	BS 3799	Always specify the standard designation
Product Type	90° Elbow, Tee, Reducer	As per product types above
Pressure Table	Table 1 (3000 psi)	Table 1 (3000 psi) or Table 2 (6000 psi)
Connection Type	Screwed (BSPT) or Socket-Weld	Specify BSPP or BSPT for threaded
Size (NPS)	NPS 2 (equal) or NPS 2 × 1 (reducing)	NPS 1/8 to NPS 4
Material / Grade	BS 1503 Grade 261 or ASTM A105	Verify against project specification
Testing / Certification	Hydrostatic Test Certificate, MTC, PMI	Specify if required by project QA plan

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