The Shuttle Valve brings together products from two pipelines into one pipeline (W valve), or distributes a product from one pipeline into two pipelines (X valve). As always there is a seal in the valve disk, one-piece, which separates the pipelines from each other (top - middle or bottom - middle). If there is a faulty seal it can lead to a mixing of the product contained in the two pipelines (like with the Single Seat Shut-Off Valve).