Stainless steel slip on flanges is preferred by some contractors over the weld-neck because of 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 sample for its 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 slip on 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.
The slip on 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. Slip on flanges are commonly lower in price than weld-neck flanges and to this effect is a popular choice for our customers. It is welded both inside and out to provide sufficient strength and prevent leakage. Slip on flanges are all bored slightly larger than the O.D. of the pipe. They are preferred over welding neck flanges but many users due to their lower initial cost, but final installation cost is probably not much less than that of the welding neck flange because of the additional welding involved.