Lap Joint Flanges are primarily installed with lap joint stubs, the combined initial cost of which is approximately 1/3 higher than that of comparable weld neck flanges. Their pressure holding ability is little, if any, better than that of a slip-on flange and their fatigue life is only 1/10 that of welding neck flanges. The primary use of lap joint flanges is in carbon or low alloy steel piping systems in services necessitating frequent dismantling for inspection and cleaning and where the ability to swivel flanges and to align bolt holes simplifies the erection of large diameter or unusually stiff piping. Their use at points where severe bending stress occurs should be avoided.

Threaded Flanges made of steel should be confined to special applications. Their chief merit lies in the fact that they can be assembled without welding. They are often used in extremely high-pressure service applications, particularly at or near atmospheric temperature, where alloy steel is essential for strength and where the necessary post-weld heat treatment is impractical. Threaded flanges are unsuited for conditions involving temperature or bending stresses of any magnitude, particularly under cyclic conditions, where leakage through the threads may occur in relatively few cycles of expansion or contraction or stress. Seal welding is sometimes accomplished to overcome this weakness, but cannot be considered as entirely satisfactory.