1. Scope and Class Designations

ASME B16.11 covers ratings, dimensions, tolerances, marking and material requirements for forged socket-welding and threaded fittings in sizes NPS 1/8 through NPS 4 (DN 6 through DN 100). It applies to elbows, tees, crosses, couplings, half couplings, caps, plugs, bushings and bosses.

Class 9000 socket-weld fittings and Class 6000 threaded fittings are made only in the smaller sizes: Class 9000 stops at NPS 2 and Class 6000 socket-weld at NPS 2. Sizes above those are supplied in Class 3000.

Sizes are shown in this page as NPS to match the standard. The corresponding DN values are DN 6, 8, 10, 15, 20, 25, 32, 40, 50, 65, 80 and 100 for NPS 1/8 through 4.

2. Dimensional Letter Designations

Figure 1 — Socket-weld tee shown in section. The same letters are used for elbows, crosses and couplings. A, B and J are the dimensions that govern correct fit-up.

3. Socket Bore (B) and Socket Wall Thickness (C)

The socket bore B is given as a minimum and maximum. Filling the column with the pipe outside diameter of one schedule (for example 21.34 mm for NPS 1/2) is a common error: the socket must be slightly larger than the pipe so that the pipe can enter, and the tolerance band is what the standard controls. Socket wall thickness C is stated as an average, with a lower minimum permitted in localised areas.

4. Bore Diameter (D) by Pressure Class

The bore narrows as the class rises, because the wall is thicker for the same outside envelope. A Class 9000 NPS 2 fitting has a bore of about 37–39 mm, against 52–53 mm for the Class 3000 fitting of the same size.

5. Socket Depth (J) and Centre to Bottom of Socket (A)

Socket depth J does not depend on the pressure class: it is fixed by size. The centre-to-bottom dimension A does depend on the class, because a thicker fitting body moves the socket bottom further from the centre. Values below are for 90° elbows, tees and crosses. A for 45° elbows is a separate, shorter value.

6. Couplings and Half Couplings — Laying Lengths (E, F)

E and F are laying lengths as defined in the ASME B16.11 sketch; they are not the overall length of the fitting. E is the distance the pipe ends stand apart inside a full coupling, and F the corresponding distance for a half coupling. Where a full coupling is used, the raw length of stock to allow is roughly E + 2J.

7. Product Types — What B16.11 Does and Does Not Cover

8. Material Specifications

The forging material is called out separately from B16.11. Common combinations:

9. Socket-Weld Preparation and the 1.6 mm Expansion Gap

Figure 2 — Socket-weld assembly. The pipe is withdrawn about 1.6 mm (1/16 in) from the socket bottom before welding, so that differential thermal expansion cannot push the pipe into the socket bottom and crack the weld.

ASME B16.11 defines the fitting. It does not define how the joint is welded. The welding requirements come from the applicable piping code, normally ASME B31.1 or B31.3, which call for:

• A fillet weld at the socket mouth, with a minimum leg size of 1.09 × the pipe wall thickness.
• A gap of about 1.6 mm (1/16 in) between the pipe end and the socket bottom before welding, so that thermal expansion of the pipe cannot load the weld. The gap appears in B31.1 / B31.3 Fig. 328.5.2C, not in B16.11.
• A plain end on the pipe Socket-weld fittings take an un-beveled (square-cut) pipe end. A beveled end must be cut back flush before insertion.
• Pre-cleaning both surfaces free of oil, scale and rust before welding.

10. Threaded Ends

Threaded B16.11 fittings use NPT taper threads in the sizes covered. Class 2000 is the standard threaded class, with Class 3000 and 6000 available in the smaller sizes. Threaded joints are used where welding is impractical or where the line must be broken for maintenance; they are not accepted in most hydrocarbon specifications for sizes above NPS 2. Threads must be inspected with a plug or ring gauge, and a thread sealant compatible with the process is required at assembly.

11. Marking and Traceability

12. Related Standards

13. How to Order

14. FAQ — Frequently Asked Questions

What sizes does ASME B16.11 cover?

NPS 1/8 through NPS 4 (DN 6 through DN 100). Socket-weld fittings are made in Class 3000, 6000 and 9000; threaded fittings in Class 2000, 3000 and 6000. Class 9000 stops at NPS 2 and Class 6000 socket-weld also stops at NPS 2.

Is a B16.11 Class 3000 fitting the same as an ASME B16.5 Class 300 flange?

No. The class numbers come from two different systems. B16.11 rates a forged fitting by its body wall; B16.5 rates a flange by a pressure–temperature table. There is no Class 3000 in B16.5 at all, and a Class 3000 fitting and a Class 300 flange have no relationship.

What should the socket bore B value be for NPS 1/2?

Between 21.8 mm and 22.2 mm. The pipe outside diameter for NPS 1/2 is 21.3 mm, so B is deliberately larger than the pipe. Filling the socket bore column with 21.34 mm (the pipe OD) is a common catalogue error.

Do I need a 1.6 mm gap when socket welding?

Yes, as good practice and as required by ASME B31.1 and B31.3 for hot service. Withdraw the pipe about 1/16 in from the socket bottom before welding so that expansion of the pipe cannot drive it into the socket bottom and crack the fillet weld. The requirement is in the piping code, not in B16.11.

Is a socket-weld cap the same as a plug?

No. A cap fits over the pipe and has no drive head. A plug is threaded (or socket-weld) and has a hex or square head so that it can be tightened with a wrench. Only the plug has a drive head.

Does ASME B16.11 include straight reducers?

No. There is no straight forged reducer in B16.11. Where a reduction is needed with small-bore connections, use a swage nipple to MSS SP-95 or a butt-weld reducer to ASME B16.9.

Are unions covered by ASME B16.11?

No. Socket-welding and threaded steel pipe unions are covered by MSS SP-83. B16.11 covers couplings, which are permanently installed fittings, not unions.

What weld size is required on a socket joint?

A fillet weld with a minimum leg of 1.09 × the nominal pipe wall thickness, applied at the socket mouth. The weld size, examination and any preheat requirements are set by the applicable B31 code and the project specification.