Slip on pipe flanges actually slip over the pipe. These pipe flanges are typically machined with an inside diameter slightly larger than the outside diameter of the pipe. 

Slip on pipe flanges are secured to the pipe with a fillet weld at the top and the bottom of the slip on pipe flanges. Slip-on flanges are most frequently used at lower pressure - Class 150 (PN 20) or Class 300 (PN 50) primary service pressure ratings.

Many pipe designers are reluctant to use slip-ons for higher pressures, since (1) the joint between the flange and pipe is not as strong as in the welding neck type; and (2) the junction of the flange and pipe is more susceptible to corrosion.