Slip-On Flanges are preferred by some contractors over the Weld-neck Flange as slip-on flanges have the lower initial cost. However, this may be offset by the added cost of the two fillet welds required for proper installation. The strength of the slip-on flange is enough for rating, but its life under fatigue conditions is considered to be only one-third that of the weld-neck flange.

The slip-on flange may be attached to the end of a piece of pipe or to one or more ends of a pipe fitting. The slip-on flange is positioned so the inserted end of the pipe or fitting is set back or short of the flange face by the thickness of the pipe wall plus 1/8 of an inch. This allows for a fillet weld inside the SO flange equal to the thickness of the pipe wall without doing any damage to the flange face. The back or outside of the flange is also welded with a fillet weld.

A variation of the Slip-On flange also exists. This is the Slip-On Reducing Flange. This is simply a larger Slip-On flange blank that, instead of the Center (pipe) hole being cut out (or drilled out) for 14″pipe it is cut out for a 6″(or some other size) pipe. The SO Reducing flange is basically used for reducing the line size where space limitations will not allow the length of a weld neck flange and reducer combination. The use of the Slip-On Reducing Flange should only be used where the flow direction is from the smaller size into the larger size.