Flat flange vessels are large-volume reaction vessels consisting of a lower part and a lid. These two parts are held together by a clamping ring. The red O-ring made of silicone is seamlessly coated with FEP and serves as an optimal seal for flat flange vessels with an annular groove. It prevents jamming of the ground surfaces between the base and the lid and is convincing due to its good elasticity in combination with outstanding chemical resistance.