Abstract: A gasket and seat system for sealing a joint is disclosed. The gasket comprises, for example, an inner portion with a wall facing an interior of the coupling and an outer portion, which extends axially forming radial end walls. The inner portion of the gasket is formed with an angled wall, making the inner portion expand inwards, which angled wall interacts with a matching angled wall of the seat, such that the joint has provisions for increasing the sealing ability in cases with increased pressure in the coupling. The gasket may further have the wall facing the interior being adapted to be flush with an interior surface when mounted in the seat and may also have provisions for heat expansion and elevated pressures, in order to remain flush with the coupling at elevated temperatures and in order to ensure sealing at elevated pressures.