As far as directional valves go, the check valve is the simplest in both concept and function. They are designed to allow flow in one direction while blocking flow in the opposing direction — that’s it. Some valves get more complicated, such as the pilot-operated check valve, but the one most commonly used as a supplemental component to a fluid power system is the inline check valve.
Inline check valve, image courtesy of Parker Hannifin
An inline component is any useful object located in a circuit and coupled to adjacent fluid conduits, in the line of the fluid connection, as it were. Inline components are convenient and versatile, often added to enhance an existing circuit or as an upgrade to an existing machine. The inline check valve is no different and has many uses.
Many functions of a fluid power circuit require a one-way flow path, and the inline check valve provides the best solution. For hydraulic applications, a check valve will consist of a steel body with a spring-loaded ball or poppet fixed in place with a retainer. Pneumatic applications have more freedom of construction because of their lower pressure rating, so the materials can be brass, aluminum or even some type of plastic or composite.
The most common tactic for an inline check valve is to prevent the backwards transmission of pressure, rather than flow, and it’s pressure that creates flow anyway. An example of this tactic is the check valve typically installed after a pump in a system which employs an accumulator. The check valve prevents stored energy from making its way back into the pump, an especially important task when there is stored energy in the accumulator and the pump is not even running. Energy stored in an accumulator can be dangerous or damaging and must be controlled for hydraulic systems to be safe and reliable.
In complex load-sensing applications, the pump or compensator used to read the load sense signal from downstream of all control valves needs only to sense the highest pressure of all functioning actuators. The load sense check valve network opens a parallel path of hydraulic fluid from each actuator to the primary compensator, and the highest pressure channel overcomes its check valve and subsequently closes every other check valve in the network. Although not an individually plumbed component true to the “inline” nomenclature, they are installed in valve banks or manifolds and exist only to open or close an inline flow path.
Inline check valves can be added to existing hydraulic circuits to add functionality. For example, an inline needle valve may have been added to a compression-loaded cylinder application, but the machine operator felt the cycle time could be improved. A needle valve reduces flow in both directions, and this application really only required metering of flow under retraction, so the addition of an inline check valve plumbed around either side of the needle valve creates a true flow control. Fluid will now bypass the needle valve as it travels through the check valve unimpeded, providing full flow when extending. The check valve blocks free flow in retraction, forcing fluid across the needle valve and controls the downward motion of the cylinder.
Filed Under: Valves & Manifolds
