Check Valves

Inline spring check valves are common, easy to understand, and simple to design. Spring loaded in-line check valve in open and closed positions. Arrows indicate flow direction. When fluid enters the valve input port, there should be sufficient pressure (force) to overcome the opening pressure and spring force. The pressure pushes against the disc, opening the orifice and allowing fluid to flow through the valve. When the input pressure is no longer high enough or there is sufficient back pressure, the back pressure and spring push the valve disc toward the orifice and seal the valve closed. The short stroke of the spring and disc allows for fast closing response times. This valve design also prevents pressure surges in the line, thereby preventing water hammering.

Spring-loaded Y-check valves operate similarly to in-line spring-loaded check valves. The difference is that the spring and movable disc are positioned at an angle. This forms a "y" shape, hence the valve's name. It works in the same way as an inline valve, but because the movable components are angled, the valve can be inspected and serviced while it is connected to the system. These valves are larger and take up more space within the system.

Ball check valves use a free-floating or spring-loaded ball that sits on a sealing seat to close an orifice. The sealing seat is usually conical in shape to guide the ball into the valve seat and form a reliable seal to prevent reverse flow. When the fluid pressure on the inlet side exceeds the cracking pressure, the ball moves away from its seat, allowing flow. When the inlet pressure does not exceed the opening pressure, or there is back pressure, the ball will close with the help of back pressure or spring, effectively closing the orifice. True union ball check valves allow for easy ball removal and replacement, eliminating the need to purchase a new valve.

Diaphragm check valves consist of a rubber diaphragm that flexes open when inlet pressure increases. A normally open diaphragm check valve with a "minimum" inlet pressure that allows the flow of media. As the inlet pressure increases, the diaphragm will flex open, allowing more flow. If back pressure occurs (or a normally closed diaphragm check valve), the diaphragm will be forced against the opening and sealed against any backflow. Diaphragm check valves are ideal for low-pressure or vacuum applications.

Lift check valves consist of a guide disc that raises (lifts) the valve seat to allow media flow. It requires opening pressure to overcome gravity and/or spring resistance. Guides hold the disc in a vertical line for reseating with proper alignment and sealing.

Most commonly, lift check valves force the media to rotate 90 degrees. If there is no spring to assist closing, the installation direction must be considered to ensure that the valve disc swings closed under the action of gravity.

Swing check valves are known as "swashplate" or "flapper" check valves. They have a disc on their hinge (or trunnion) that opens under inlet pressure. When the inlet pressure decreases or backflow occurs, the valve disc will swing closed. If there is no spring to assist closing, the installation direction must be considered to ensure that the valve disc swings closed under the action of gravity. Double disc or double door check valves have a center disc that is divided into two semi-circular doors that are hinged on a central pivot point to operate independently.

Globe check valves are typically spring-loaded Y-check valves or lift check valves with manual override. This allows the valve to function as a normal check valve and prevent backflow. An external mechanism can be used to maintain the valve in an open or closed position. Therefore, a stop check valve can function as two in one: a flow regulating valve and a backflow prevention valve. They are commonly used in power plants, boiler cycles, steam generators, turbine cooling, and safety systems.

The terms butterfly check valve and wafer check valve are often used interchangeably. They consist of butterfly or wafer-shaped discs and springs on a hinge. When the inlet pressure overcomes the opening pressure, both sides open, as shown in Figure 9. When the inlet pressure decreases or backflow occurs, the spring (or back pressure) on the hinge will close the valve disc, effectively sealing it. This valve type allows direct flow of media with minimal resistance. Wafer swing check valves are stylishly designed to fit into tight flange spaces.

A duckbill valve allows fluid to flow through a hose that has a naturally flat shape at the end. This flat shape resembles a duckbill, hence the valve's name. The flow of water opens the flat end of the duckbill, allowing media to pass through. When pressure is removed from the inlet side, the duckbill end returns to its flattened state, cutting off flow.