“Flange Gaskets: Full-Face or Ring” explains the difference between the two types. Choosing which to use is largely a matter of clamping force needed and flange design.
A gasket prevents leaks by taking up imperfections between mating surfaces. This is achieved by firmly compressing the material so it conforms to both faces. This compression also helps the gasket resist the internal pressure within the pipe. The higher the pressure the greater the compression needed, (and it helps to keep the gasket as thin as possible.)
Ring-type flanges seal over a smaller area than full-face designs and apply higher compressive loads for a given bolt tightness. This needs to be considered when selecting a gasket material. Elastomeric gaskets can be crushed if over-tightened.
Because they seal over a larger area, full face flanges can’t achieve the same compressive load without tightening the bolts more than would be done for a ring face flange. Thus full face flanges, and also full face flange gaskets, are reserved for lower pressure and less critical applications. Conversely, most process plant piping systems use ring face flanges and gaskets.
