About Weld Neck Flanges

API Spec 6A, 6B weld neck flanges, do not have full bores, they have restricted bores that match nominal pipe sizes. As an example, 4-1/16″ 5M, 6B flanges have a maximum bore of 3.47″ approximating the I.D. of 4 inch schedule 160 line pipe, 4-1/2″ O.D.  API Spec 6A specifies 45K yield strength material for 6B flanges with standard (restricted bore) weld necks. The smaller bore lowers the flange stress and allows the use of more easily welded material. An illustration of a conventional flow line composed of 4-1/16″ 5M weld neck flanges, welded to compatible line pipe, as described, and joined by bolting these flanges together with an R-39 ring gasket, appears below. 

Standard API Spec 6A, 6B weld neck flanges present some interesting fluid dynamic problems when connected to full bore valves or flow lines. When bolted downstream onto flanges of full bore equipment, these weld neck flanges present a restrictive shoulder in the flow bore.  Rapidly moving fluids will experience turbulence upon contacting this shoulder, premature failure of equipment may result. 

To prevent erosion problems weld neck flanges may have a special entrance opening bevel, or they may have full bores and special weld necks which may or may not be compatible with commercial pipe sizes. API 6A does not specify full bore 6B weld neck flanges, so manufacturers must utilize 60K yield strength material for these as they would for full bore integral flanges.  See illustrations below.

All API Spec 6A, 6BX weld neck flanges have full bores. Manufacturers making 6BX weld neck flanges rated for 10,000 psi working pressure shall use 60K minimum yield strength material, and for 15,000 psi and higher working pressure flanges they shall use 75K minimum yield strength material. Users of full bore weld neck flanges should¹ specify the O.D., I.D., and strength of tubular material used for attachment to any of these special weld necks. 

In the past API Spec 6A specified concentric reducer transition pieces that allowed joining 6BX Flanges to standard pipe, usually resulting in derating the working pressure to that of the pipe chosen. API no longer specifies these transition pieces.

Regardless of published specifications, to satisfy their field service needs, many users specify special weld necks that match standard pipe sizes on 6BX flanges. Such users do so at high risk unless they include engineering and welding considerations in their decision process. Many 6BX flanges with special weld necks that match standard pipe sizes cannot properly bear the API monogram.

WOODCO USA provides a Flange Slide Rule, which includes links to Weld Neck Dimensions on most of the popular API flanges. All of the weld necks shown meet API requirements but some 6B flanges illustrated have bores smaller than the maximum allowed by API Spec 6A.

WOODCO USA recommends that pipe of 2″ nominal size (2-3/8″ O.D.) or larger, used for pressure service of 960 psi or more, have a minimum wall thickness of .200 inches. This wall thickness can resist some external damage and internal erosion. All bores of 2″ nominal size and larger weld neck flanges shown on this web site match pipe with a wall thickness equal to or greater than .200 inches.

API Spec 6A does not specify compatible pipe. 6B flange weld necks shown on this web site match commonly available pipe which WOODCO USA identifies. Joining these flange weld necks to the identified pipe should ¹ allow full rated working pressure as indicated for the flange.

API Spec 6A does not specify test pressure for field welded, weld neck flange to pipe, assemblies.

6BX weld neck flanges must meet all material and design requirements for Integral flanges so welded assemblies using them, joined to equal strength tubing of the indicated size, may experience a test pressure equal to that specified for the same pressure rated flanged equipment with integral flanges.

For flanges which have Weld Neck Dimensions other than those shown on this web site, determine matching pipe size by referencing Carbon Steel Pipe Dimensions and Weights on this web site. WOODCO USA suggests no grade or strength for pipe so selected and suggests no maximum service pressure or test pressure for flanges joined to such selected pipe.

Always use Qualified Welding Procedures supported by a Performance Qualification Record and performed by Qualified Welders when joining weld neck flanges to pipe or tubing.  

1	Words in bold italic indicate they have subjective meaning and persons using this information must use experience to improve the reliability of their judgement when the meaning of these words can have impact on performance.