In the collection of hydraulic valve components, flow control valves are any device manufactured for the purpose of modifying the rate of fluid flow in a portion of a hydraulic circuit. Flow variability occurs through the adjustment of the effective orifice size inside the valve, and this is achieved through various techniques based upon the type of valve and its manufacturer.
The most basic flow control is simply an orifice, which is a disc, plug or component with a simple hole drilled through the middle to limit the rate in which flow can pass. The flow rate is dictated by both the size of the hole and the pressure applied to push fluid through that hole. The difference in pressure between the upstream and downstream sides of the orifice are referred to as the pressure drop, and with consideration to orifices, the pressure drop is a predictable split between pressure and flow.
For applications requiring modification to actuator speed while the machine is running, a variable orifice is beneficial. Variability can be achieved in various ways, but a needle valve is the most common method. An inline body will have a through passage with a cone-shaped pocket machined into that flow path. A threaded needle installed into the valve will open and close via its threaded adjustment, opening and closing the gap between the cone and the needle, thereby varying flow through the valve.
A true flow control valve by most definitions is a needle valve with a reverse flow check valve. The check valve supplies free flow in one direction while restricting flow in the returning direction. Flow control valves installed into the work ports of a cylinder, for example, can be installed to restrict flow either going into or coming out of the cylinder port. When flow is restricted from entry we call it meter in control, and when flow is restricted from exit we call it meter out control.
A basic flow control valve such as a needle valve, while adjustable, is still susceptible to fluctuations in flow because of variable pressure drop. For example, as downstream load pressure rises, the difference between pressure on the inlet compared to the outlet of the needle valve is reduced. This reduced pressure drop results in reduced flow.
To limit load induced pressure fluctuations, flow control valves add a pressure compensator to the mix. A pressure compensator maintains the valve’s set pressure drop by comparing upstream and downstream pressure and then varies the orifice to maintain desire flow. For example, the increased downstream load pressure will act upon the pressure compensation mechanism to further open the orifice. The wider orifice will allow more flow to compensate for the reduced pressure drop. The effect is seamless and unnoticeable, simply providing the desired flow rate regardless of load pressure fluctuations.