API 6A (6B/6BX) Weldneck Flange

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. 

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 should specify the O.D., I.D., and strength of tubular material used for attachment to any of these special weld necks.