A typical check valve design is the

A typical check valve design is the swing check, which has a hinged discthat slams shut when flow reverses (Figure 2.10). Flow lifts the disc andkeeps it lifted until flow stops or reverses. The body of the check valve hasa cap for easy access to the flow-control element. Another design is thelift check, which has a disc that rests on the seat when flow is idle and liftswhen flow is active (Figure 2.11). Special guides keep the disc in place. Likethe swing check, it is designed to close when flow reverses. Lift checks areideal for systems in which flow rates fluctuate. The lift check is more durablethan the swing check. In the horizontal or vertical lift check design, a pistonor ball is lifted up and out of the seat by process flow. A third design is theball check design, which has a ball-shaped disc that rests on a beveled,round seat (Figure 2.12). The ball is down when flow is idle and up whenflow is active. Special guides keep the ball disc in place. Like the swingcheck, it is designed to close when flow reverses. Ball checks are idealfor systems in which flow rates fluctuate or the fluid contains some solids.The ball check is as durable as a lift check and more durable than a swingcheck. A fourth design is the stop check design, which has characteristicsof a lift check and a globe valve (Figure 2.13). In the closed position, thestop check disc is firmly seated. In the open position, the stem rises out ofthe body of the flow-control element and acts as a guide for the disc. In theopen position, the stop check functions like a lift check with one exception.The degree of lift can be controlled.