Check valves are mechanical valves that permit gases or liquids to flow in only one direction, preventing flow from reversing down the line. They are classified as one-way directional valves. Fluid flow in the desired direction opens the valve, while undesirable reverse flow forces the check valve to close. Most check valves contain a flat disc that is slightly larger in diameter than that of the port. Flow indicators on the side of the body indicate the direction of the flow for correct installation.
Spring check valves have a disc held in place by a spring, which means they and can operate in either a vertical or horizontal mounting position because they do not rely on flow pressure or direction to function. Spring check valves are available with many different types of seats, the most common being nylon, which tends to wear quickly, requiring replacement more often than more durable brass seats.
Swing check valves have a hinged disc which swings into position, and can operate in either horizontal or vertical (upward flow) positions. Movement through the line automatically swings the disc open to allow full flow, so swing check valves must be installed with normal flow opening the disc. Should reverse flow occur, the reversed pressure and the weight of the disc closes the disc back against the seat and back-flow is stopped immediately. This type of check valve offers minimal flow restriction, allowing full flow to be achieved through the valve.
Lift check valves work automatically with horizontal line pressure as the bonnet is positioned vertically above the valve, lifting up with forward flow. Like a globe valve, with its indirect line of flow, lift check valves are flow-restricting, and so are generally used as a companion to globe valves. Line pressure lifts the disc, and flow is allowed to occur. When flow reverses, the disc falls back onto its seat and immediately cuts off the flow, preventing back-flow from occurring.
