Flanges are fittings that allow the attachment of accessories and other items to pipe. They can also increase the strength of pipe. Flanges are a disc-like fitting that create a tight seal by pressing two surfaces together with bolts, clamps, edges, or a compressive force. Most flanges, however, use bolts to join the two surfaces together. PVC flanges are available with slip or threaded ends. Because of the strength required of flanges.