Abstract: An O-ring seal system for pipe flanges having a backing ring, an O-ring seal positioned within the backing ring, and a vacuum ring positioned within the O-ring seal. The backing ring and the vacuum ring form a rigid base against which mating flanges on pipes can be bolted together, thereby preventing bending of the mating flanges and permitting flange bolts to be torqued to their full tensile load limit without damage to the mating flanges. The O-ring seal system is constructed so that the O-ring seal is retained between the mating flanges without the need to position the O-ring seal in a groove in one of the mating flanges. Bending of the flanges is minimized since the flanges are clamped against the backing ring, minimizing stresses due to bending, thereby extending the service life of the flanges.