The most common type of CHECK VALVES employ a ball valve element for seating within a valve body valve to shut off flow therethrough. A spring is used to urge the ball toward the seat, which is opposite to the direction of the incoming fluid flow. Thus, the flow of fluid therethrough is cut off when the pressure of the fluid flowing around the ball is insufficient to overcome the force generated by the spring. Such valves have a common handicap in that a shut off valve must be provided on the inlet of the check valves so that the Check Valves may be reopened after closing. Accordingly, it is necessary to insert a shut off valve upstream of the check valve which thereby makes the use of such check valves unnecessarily expensive.

Another difficulty with such prior art check valves is that a single seal is provided between the valve element and seat. Accordingly, such prior art valves fail to provide a primary (soft) seal to prevent fluid flow and a secondary (metal to metal) seal, which acts as a fire seal.

In accordance with the invention, a Check Valves is provided which uses a valve body having a bore with first and second sections, the cross sectional area of the first section being smaller than the cross sectional area of the second section; an inlet communicates with the first section of said bore for receiving flowing fluid into the valve body; a shoulder is disposed between the first section of the bore and an inlet for providing a valve seat; and an outlet communicates with the second section of the bore for discharging flowing fluid from the valve body. A valve element is mounted within the inlet of the valve body for engaging with the valve seat to shut off fluid flow through the valve body. The movable piston is mounted within the inlet of the valve body for engaging with the valve seat to shut off fluid flow through the valve body. The movable piston is mounted with the valve body for controlling movement of the valve element with the piston having a body portion slidably mounted in the second section of the bore, the body portion having a shoulder facing away from the valve element. The piston also has a male portion extending from the body portion and is slidably mounted in the first section of the bore, the male portion having a shoulder facing toward the valve element. A stem extends from the male portion into the inlet of the valve body, and the valve element is mounted with the stem. A passage is provided in the piston for placing the first section of the bore in communication with the second section so that an opening force is provided on the movable piston by the pressure from the fluid in the second section acting on the body portion shoulder. Also, a closing force is provided on the movable piston by the pressure of the incoming fluid acting on the valve element and the male portion shoulder. The valve element is thus moved into engagement with the valve seal when the pressure of the incoming fluid is sufficiently great to cause the closing force to override the opening force and thereby check the uncontrolled flow of fluid out of the valve body.

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