The non-return valves or check valves are used to block the reverse flow of oil or gas in a fluid power circuit. Additionally this valve may be used for pressure control or directional control. The check valve consists of a valve body with inlet and outlet ports and an internal movable member biased by the spring. The movable member can be a flapper or a plunger but most often in valves of hydraulic systems, it is a ball or poppet. They are built to provide free flow in one direction and a check in the other direction and prevent flow-back in hydraulic systems.
Check valves are of several styles: swinging disc, ball, plunger, and
poppet. Check valves of the pilot-operated type are also popular. The
connection at the bottom is connected to a remotely operated pilot
valve, As in the ordinary check valve, free flow is permitted in one
direction when the line pressure lifts the plunger thus opening the
valve. The flow is checked in the opposite direction until pilot
pressure is applied to the blind end of the plunger through the pilot
connection. This causes the plunger to unseat the valve spool and oil is
allowed to flow through the valve.
When system pressure at the check valve inlet port is high enough to
overcome the biasing spring force on the poppet, the poppet is pushed
off its seat allowing oil to flow through the valve. This is known as
the check valve's free flow direction. When the fluid attempts reverse
direction of flow, the spring pushes the poppet back on the seat to
block flow through the valve. Check valves can be direct acting or pilot
operated. A pilot operated version is used where the no-flow
characteristic of the valve is desired only for a portion of the system
cycle. A pilot operated check valve also allows free flow from its inlet
port to its outlet port just as in an in-line check valve.
The design and construction of a check valve makes it one of the
simplest fluid power components. Though simple, its use can make the
system sophisticated. By application of check valves one can also design
many sophisticated circuit diagrams where a constant flow can be
maintained in both directions by using them in a bridge.