CHECK VALVES

The closing member, the movable part to block the flow, is a spherical ball. 

In some ball check valves, the ball is spring-loaded to help keep it shut.

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For those designs without a spring, reverse flow is required to move the ball toward the seat and create a seal.

The interior surface of the main seats of ball check valves are more or less conically-tapered to guide the ball into the seat and form a positive seal when stopping reverse flow.

Uses a flexing rubber diaphragm positioned to create a normally-closed valve.

Pressure on the upstream side must be greater than the pressure on the downstream side by a certain amount (pressure differential) for the check valve to open allowing flow

Once positive pressure stops, the diaphragm automatically flexes back to its original closed position.

A check valve with override control to stop flow regardless of flow direction or pressure. 

In addition to closing in response to back-flow or insufficient forward pressure (normal check-valve behavior), it can also be deliberately shut by an external mechanism

Thereby preventing any flow regardless of forward pressure.

A check valve in which the disc, sometimes called a lift, can be lifted up off its seat by higher pressure of inlet or upstream fluid to allow flow to the outlet or downstream side 

A guide keeps motion of the disc on a vertical line, so the valve can later reseat properly

When the pressure is no longer higher, gravity or higher downstream pressure will cause the disc to lower onto its seat, shutting the valve to stop reverse flow

A 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 center line 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.