A swing valve is a butterfly-style check valve in which the disc, the movable part to block the flow, swings on a hinge or trunnion, either onto the seat to block reverse flow or off the seat to allow forward flow. The seat opening cross-section may be perpendicular to the centerline between the two ports or at an angle. Although swing check valves can come in various sizes, large check valves are often swing check valves.
A swing valve is normally recommended for use in systems employing gate valves because of the low pressure drop across the valve. Swing check valves are available in either Y-pattern or straight body design. In either style, the disk and hinge are suspended from the body by means of a hinge pin. Seating is either metal-to- metal or metal seat to composition disk. Composition disks are usually recommended for services where dirt or other particles may be present in the fluid, where noise is objectionable, or where positive shutoff is required.
Swing check valve shut off depends on gravity (weight of disc) and reverse flow. The pivot point of the swing check is outside the periphery of the disc and, the greater the head, the greater the possibility that the fluid will flow back through the valve before the disc can shut off. To effect complete shut off, the disc of a swing check valve must travel through a 90 angle arc to the swing check valve seat.