Check Valves

Check valves are designed to pass flow in one direction with minimum resistance and to prevent reverse or backflow with minimal leakage. The principal types of check valves used are the tee-pattern lift check, the swing check, the tilting-disc check, the wye-pattern lift check, and the ball check, illustrated in Figs. A10.13 to A10.17, respectively.

Construction of a Check Valve

A basic check valve consists of a valve body, bonnet or cover, and a disc which is attached to a hinge and swings away from the valve seat to allow fluid to flow in the forward direction, as in a swing- or tilting-disc check valve, and returns to valve seat when upstream flow is stopped. Thus, reverse flow is prevented. In foldingdisc check valves, the disc consists of two halves attached in the middle. The two halves fold backward when upstream flow is initiated. Activated by a spring, the two halves quickly close the flow path when upstream flow ceases. In the case of lift-check valves, the disc is in the form of a piston which is moved out of the flow path by upstream flow and returns to the valve seat by gravity to stop back flow. Ball-check valves have a disc in the form of a ball.

Check valves are available in sizes (Courtesy BTR Inc./Edward Valve) from NPS ¹⁄₄ (DN 6) through NPS 72 (DN 1800). Other sizes may be made available to meet specific size requirements. Depending upon the design requirements of a piping system, a check valve may have butt welding, socket welding, threaded, or flanged ends.

Advantages of Check Valves

They are self-actuated and require no external means to actuate the valve either to open or close. They are fast acting.

Disadvantages of Check Valves

The following are some of the disadvantages that are attributed to check valves:

1. Since all moving parts are enclosed, it is difficult to determine whether the valve is open or closed. Furthermore, the condition of internal parts cannot be assessed.

2. Each type of check valve has limitations on its installation configurations.

3. Valve disc can stick in open position.