Back-Up Rings are flat, split, spiral or contoured washer-type fittings used to protect an O-Ring seal from extrusion at higher pressures and temperatures. A Back-up Ring works by blocking or reducing the extrusion gap, thus preventing the o-ring from extruding.
Back-up rings can be rubber or plastic rings that prevent the O-ring from entering the clearance gap. High pressures can force the O'ring's soft rubber material into the clearance gap, causing it to fail. Preventing premature failure by securing the gap with a rigid backing ring material will help keep the O-ring material in place.