Double Piston Seat Ball Valve. A trunnion mounted valve with two seats such that when the closed valve is pressurised from one side, the downstream seal is designed to seal if the upstream seat should fail. This feature may be provided on both sides or on one side only. Note: In the latter case the valve becomes directional and provides double isolation in one direction only, but the pressure relieving capability of the unmodified seat is preserved. Note: Upstream and downstream in this context is not to be confused with flow direction and that the double isolation direction required is often opposite to the flow direction (e.g. in the case of a pump outlet isolation valve).