Check valves have two ports and restrict the flow of fluids and other
loose materials through a pipe or passageway to one direction. Flow
enters through one port and exits through the other, closing when flow
reverses or stops. Two common types of check valves are ball cone valves
and swing check valves. A ball cone valve utilizes a spring-loaded ball
seated against a cone-shaped interior surface to stop the flow of fluid
in a forward direction, and a swing check valve has a movable disc on a
hinge to block reverse flow. Check valves are rated by their cracking
pressure (the minimum upstream pressure required for the valve to
operate), and they are used in various water, gas, air, and steam lines.