RFSO Flanges, also known as “Raised Face Slip-On Flanges,” is a type of pipe flange on the edges of expansion joints in which the bolting circle face is slightly elevated. Moreover, as a slip-on flange, it can slide over the pipe to provide a better seal and protect against leakage. This is best used in low-to-moderate pressure applications, and is preferred in situations where welding access is easier and lower-cost fabrication is desired.