In Handbook of Valves and Actuators, 2007
126.96.36.199 Flat face (FF)
Flat face flanges are used for low pressure applications and with cast iron or non-metallic valves and fittings. The gasket seals across the full flange face width surrounding the bolting. No part of the flange is subjected to direct bending as a result of the bolting loads. The wide flange faces allow the use of soft gasket materials such as rubber and cork but care must still be taken to avoid over-torqueing.
Corrugated brass or copper gaskets with plenty of graphite compound are used on low pressure steam applications. It is possible to use a steel RF flange with a cast iron FF flange but skill and judgement are necessary. Alignment of the flanges is accomplished solely by the bolting. Maximum misalignment is equal to the bolt clearance. Flat face flanges are most often used with mirror, rough, smooth or stock finish.