Anti-syphon check valves are placed in return lines to stop fluid draining out of the reservoir when valves are removed for maintenance.
Check valves can be used as simple, low-pressure relief valves although they don't usually have a good pressure rise rate characteristic. Typical applications for this are as filter bypass valves.
PO check valves are commonly used for load holding e.g. locking a cylinder in position so that it does not move once the directional control valve or power supply has been switched off. See our understanding load control course or hydraulic circuit simulation to learn more.
In-line check valves can be used to provide positive back pressure on a system.
When arrange in numbers, PO check valves can also be used as directional valves when very low leakage is required. Also, when very high flows are required they may make a cheap alternative to a large directional valve. See our logic valve section which covers more specialist check valves (logic valve) applications.
In some situation return line check valves can stop the flow from one actuator accidentally moving or affecting another actuator.
Check valves allow fluid to flow in one direction only.
When pressure is higher on the ball side it pushes the ball against the spring and opens a path for the fluid to pass.
When pressure is higher on the spring side the ball closes against the seat and flow stops.
The spring force determines the pressure difference at which the valves starts to open.
Leak-free sealing of flow in one direction.
A shuttle valve feeds the highest pressure from either of two supply lines, back into the T line. Shuttle valves may be used within a daisy chain of valves to feed the highest of many lines back into one.
Shuttle valves are commonly used to feedback load sensing signals back to the supply pump controller. On larger installations such as ships, a boost valve may be required to ensure the correct level of pressure signal is returned to the pump e.g. compensating for losses in the pilot return lines.
Check valves come in a wide range of formats and installation styles. These include line mounted, cartridge, platen mounted, sandwich plate and flange inserts.
Check valves can have a range of spring setting. Check valves without springs provide less resistance and therefore pressure loss. These are often fitted vertically to gravity close or they may not work.
Pilot-to-open (often called pilot operated) check valves allow flow in both directions (backwards through the valve) when a pressure is applied to the pilot line.
Pilot-to-close check valves will not open in either direction when pressure is applied to the pilot.
Other types of check valves include mechanically operated or vented.
See check valves in our symbols sections for more details.
Although the function of check valves looks very simple. There are a wide range of different quality, performance and usage considerations.
In its simplest form, a check valve may just be a ball bearing against a round hole for a seat. The design is very cheap and robust but may leak later in life as the ball and seat wear.
More expensive versions use machined poppets, hardened seats and spool bodies to ensure accurate location. Be careful to select the size and quality for your application taking into account the fluid contamination level expected and required life and leak free performance.
A common topic for lively discussions is whether any poppet valve, including check valves, can be leak free. It's certainly better to always expect a few drops per minute which usually means that your cylinder will drop after a few hours or overnight.
Of course this is not always the case and if you have trapped in pressure you can guarantee that the valve will not leak a drop and a high daytime temperature change runs the risk of increasing the trapped in pressure to above the limits of your system.
Make sure you look at the manufacture's datasheet for cracking pressure, reseat pressure and pressure rise rate against flow.