The calculated strength from a Slip On flange under internal pressure is of the order of two-thirds that of Welding Neck flanges, and their life under fatigue is about one-third that of the latter.
The slip on flange has a low hub and slips onto the pipe prior to welding. Welding is carried out on the inside and outside to provide sufficient strength and prevent leakage. Slip on flanges are all bored slightly larger than the OD of the matching pipe to allow for the welds to hold. They are many times preferred over weldnecks due to their lower cost, though net installation cost is often the same due to the additional welding.
Slip-On Flanges are probably the most common type of flanges in the industry and are ideal for lower pressure applications. You do not have to specify the pipe schedule when using slip-on flanges due to the fact that its inside diameter is determined by the outside diameter of the pipe. This outside diameter of the pipe does not change for the different schedules. This often makes these easier to stock for vendors. Fabricators also like the fact that these flanges are slightly easier to align than weld neck flanges.