1. Scope

BS 3799 is the British Standard "Specification for steel pipe fittings, screwed and socket-welding for the petroleum industry". It covers forged carbon and alloy steel fittings with screwed and socket-welding ends. Thread forms are to API Std 5B or ANSI B2.1 — that is, NPT taper threads. Bore diameters of socket-welding fittings are compatible with BS 3600 and BS 1600, and the standard substantially agrees with ANSI B16.11, ASTM A234 and ASTM A403.

Status: BS 3799:1974 is current (published 30 August 1974, BSI committee ISE/110). It has not been revised or withdrawn. It remains widely specified on UK, Commonwealth and Middle East petroleum work.

2. Product Types Covered

The standard lists the following types. Note that BS 3799 does not use the word "reducer" for a reducing fitting in the way ASME B16.11 does — reducing is achieved with bushings (hexagonal head) and swage nipples.

Solid plugs and bushings do not carry a specific rating; they are intended for use with screwed fittings of any designation.

3. Designations and Pressure / Temperature Ratings

Fittings to BS 3799 are designated 3000 lb and 6000 lb. These designations represent the maximum cold non-shock working pressure in pounds force per square inch. Pressure/temperature ratings are listed in Table 1 of the standard.

Bore dimensions tabulated for 3000 lb socket-welding fittings correspond to Schedule 40 pipe. The 3000 lb and 6000 lb designations line up with the Class 3000 and Class 6000 of ASME B16.11, which is why the two standards are frequently cross-referenced on the same project — but the dimensions are not identical and the two should not be mixed on one joint.

4. Thread Form — NPT, Not BSP

Clause 2.3 of BS 3799 requires screwed fittings to be taper threaded in accordance with API Std 5B or ANSI B2.1 (NPT). This is the single most commonly mis-stated fact about the standard. British Standard pipe threads to BS 21 / ISO 7-1 are not what BS 3799 specifies.

NPT and BSPT are not interchangeable. The flank angle differs and the pitch differs for most sizes, so a BSPT male in an NPT female will bind or leak. If a project genuinely needs BSPT ends, that is a deviation from BS 3799 and must be stated explicitly on the purchase order.

5. Manufacture, Materials and Certification

Design note: screwed elbows, tees and crosses are required to be designed with wide reinforcing bands of heavy section at the crotch, giving additional wall thickness over the full length of the tapped ends. That extra wall is what makes a 3000 lb screwed elbow heavier than a comparable socket-weld one.

6. Related and Alternative Standards

7. How to Order

8. FAQ — Frequently Asked Questions

Is BS 3799 still current?

Yes. BS 3799:1974 was published on 30 August 1974 and remains listed as current by BSI under committee ISE/110. It has not been revised or withdrawn. On new designs, check that the project specification still accepts it — some operators have moved to ISO 4144 or EN 10241 for threaded work — but it is not obsolete.

Are BS 3799 fittings BSP threaded?

No. Clause 2.3 requires taper threads to API Std 5B or ANSI B2.1, which is NPT. BSPT or BSPP ends are a deviation from the standard and must be declared on the order. NPT and BSPT are not interchangeable — different flank angle, different pitch for most sizes.

What sizes does BS 3799 cover?

Screwed fittings run from 1/8 in (6 mm) up to and including 4 in (100 mm). Socket-welding fittings run from 1/8 in (6 mm) up to and including 3 in (80 mm) — the socket-welding range stops an inch short of the screwed range.

What do 3000 lb and 6000 lb mean?

They are the maximum cold non-shock working pressure of the fitting in pounds force per square inch. Pressure/temperature ratings above ambient are given in Table 1 of the standard. 3000 lb screwed and socket-welding fittings are normally used with Schedule 80 pipe; 6000 lb with double extra strong (screwed) or Schedule 160 (socket-welding).

Is BS 3799 the same as ASME B16.11?

No, though BS 3799 substantially agrees with it. The ratings align — 3000 lb / 6000 lb against Class 3000 / Class 6000 — but the dimensional tables are separate and the thread and bore bases differ. Do not mix BS 3799 and B16.11 components in the same joint without checking the fit.

Does BS 3799 cover 2000 lb fittings?

No. The standard designates fittings as 3000 lb and 6000 lb only. A 2000 lb class is common in ASME B16.11 practice but has no counterpart in BS 3799.