For valves with more ports, the spool is designed with more pistons or lands on a common shaft. The sliding spool is the most commonly used type of valving element used in directional control valves. Check valves are used in fluid systems to permit flow in one direction and to prevent flow in the other direction. They are classified as one-way directional control valves. The check valve may be installed independently in a line to allow flow in one direction only, or it may be used as an integral part of globe, sequence, counterbalance, and pressure-reducing valves.
Check valves are available in various designs.
They are opened by the force of fluid in motion flowing in one direction, and are closed by fluid attempting to flow in the opposite direction. The force of gravity or the action of a spring aids in closing the valve. Swing check valve. Figure 6-24 shows a swing check valve. In the open position, the flow of fluid forces the hinged disk up and allows free flow through the valve.