Understand where and why check valves are used. The different types available, how they work and their specification or usage issues.
Check valves are used for a wide range of different functions. The type of check valve will vary with different equipment and flow rates. The name can also change as they may be known as anti-siphon, non-return, bypass, load holding, PO check, one-way plus a few others.
Their functions include:
Check valves only allow flow in one direction to provide a range of sequencing or safety functions.
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 with springs can be used as simple, low-pressure relief valves although they don't have a good pressure rise rate characteristic. Typical applications may be a filter bypass valves. They can also be used to maintain slight back-pressure on systems if required for control purposes.
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 supply pressure has been switched off. Try our 'understanding load control' course or circuit simulation to learn more.
Shuttle valves are common in load sensing, pilot line feeds or pressure switching circuits.
When arranged in numbers, PO check valves can be used as directional valves when very low leakage is required. Also when very high flows are required they may make an alternative to a large directional valve. In these applications, they are normally called logic valves. More details on this are covered in our logic valve training modules.
Preventing back flow or anti-siphon.
Low-pressure relief or filter bypass.
Holding loads with minimal leakage.
Pilot pressure feeds.
Check valves only allow fluid to flow in one direction.
When pressure is higher on the ball side it pushes the ball or poppet against the spring and opens a path for the fluid to pass.
When pressure is higher on the spring side the ball or poppet closes against the seat and flow stops.
The spring force determines the pressure difference at which the valves start to open.
There should be virtually leak-free sealing of flow in the other direction.
Pilot operated check valves (PO checks) have the facility whereby an external pressure can be used to open the valve to allow flow in either direction.
Shuttle valves have two seats and three connecting lines. They always feed the highest pressure from either of the feed lines back into the sense line.
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 as cartridge inserts or line-mounted valves. They can, however, be very difficult to spot as they can be hidden inside manifolds or look just like line mounted fittings.
Check valves can generally be purchased with a range of spring setting.
Check valves without springs provide less resistance and therefore pressure loss.
Pilot-to-open (often called pilot operated) check valves allow flow in both directions (backward 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.
Check valves come in a wide range of formats and installation styles. These include line mounted, cartridge, CETOP mounted, sandwich plate and flange inserts.
See check valves in our symbols sections for more details.
Check valves without springs should be fitted vertically such that gravity will close the valve, otherwise, they may not work.
In days gone by there was an old trick whereby, if a valve was not sealing, people would tap the ball or poppet with a hammer, to bed onto its seat. However, valves are made better these days and if it's leaking you've probably got some dirt on it. Hitting with a hammer is likely to wreck the seat with a dirt indentation. Remove the poppet, clean and replace. Check there are no wear marks around the seat contact area, particularly un-concentric ones. Remember the poppets will rotate around the seat.
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.
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 seat wears.
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, required life, and leak free performance. We say this because a ball and seat are very simple and robust. But as soon as you start adding spool elements you have additional spool clearances that may be prone to contamination. Also maintaining the spool bore to seat concentricity is particularly difficult so low leakage performance could be sensitive to quality issues, on less expensive valves.
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 the flow.