Lap-joint Flanges with Stub-end
A lap joint flange is a two component assembly comprising of a stub end and a backing lap-joint ring flange. The stub end face acts as a raised face of the flange as its outside diameter corresponds to the dimensions of the raised face portion of a flange. Only the stub end comes in contact with the process fluid. Hence, lap-joint flanges with stub ends are used as a cheaper alternative to slip-on flanges in piping specifications for corrosion resistant materials such as stainless steel or titanium. The cost of corrosion resistant stub end with carbon steel backing lap-joint ring flange is less than a complete slip-on flange in corrosion resistant material. The difference becomes significant for larger dimension and rating flanges.
Because the backing ring is loose fit on the stub end, it can be rotated after the welding the stub end on the pipe to align with the matching flange. This advantage reduces the installation effort during the fabrication process as no prior alignment is necessary. This feature is also useful for large diameter flanges, at locations where rotational adjustment is required and in services that require frequent dismantling for inspection and cleaning.
Stub ends are available as Type-A, Type-B and Type-C. Type-A stub end has flare radius on the outside. This radius matches the radius on the inside of the standard lap-joint flange. Type-B stud end does not have a flare radius. It is designed to be used with a standard slip-on flange as the backing ring joint flange. Note that a slip-on flange cannot be used as a backing ring flange for Type-A stub ends as the flange will not fit on the stub end due to radius on the outside. Type-C stub end is a fabricated form pipe. Figure 328.5.5 of ASME B31.3 shows design of five typical fabricated laps. Type-C lap-joint stub-end welding fittings shall not be used under severe cyclic conditions as specified in section 306.1.4 of ASME B31.3.