Slip on flanges are pipe flanges designed to slip over the pipe with the bore of the flange slightly larger than the outer diameter of the pipe. The pipe is then connected to the flange by two fillet welds, one conducted inside and outside the flange. Aligning the flange requires much less effort than the weld neck type. 
 
Slip on flanges are preferred over weld neck flanges due to lower cost. However, the cost of the two fillet welds offsets the advantages it has over the weld neck flange. They are mostly available with a raised face or flat face. These flanges are suitable for low pressure applications and has lower lifespan when compared to weld neck flange.