Hydraulic flow control valves are used to control the speed of actuators when directional switching valves are used. Most hydraulic machines will contain several actuators, most of which need to move at different speeds. Flow control valves are used to control the speed of movement and allow changes to be made during setup and/or operation.
Pressure compensated flow control valves are used to give constant flow/speed on equipment where the load changes.
In the simplest form a flow control valve is just an orifice. The fluid flow will increase until the pressure drop (p.d) across the orifice is the same as the supply pressure, less the load pressure. Supply pressure in must equal load pressure out. The p.d. load across the orifice is the brake that controls the actuator speed.
Adjustable flow control valves contain an adjustable orifice. The better valves have sharp edge orifice designs that provide more consistent flow across the working temperature range.
With meter in flow control we throttle the flow before the actuator. With meter out flow control we throttle the flow after the actuator.
Pressure compensated flow control valves contain an orifice whose size changes as the pressure drop across it changes e.g. a spool element. This means the flow through the valves stays the constant under most operating conditions and the orifice size varies instead.
See flow control valves in our symbols sections for more details.
In mobile equipment, smooth control of actuator speed is generally very important so tends to be by proportional control valves. Cost is also very important so a simple orifice or crude adjustable orifice are often ideal.
A common format for flow control in industrial applications are the double flow control and bypass check slices that sit below CETOP directional control valves. Some high end industrial flow control valves can give very stable flow across the pressure and temperature ranges, if required.
As a general rule don't use an orifice below 0.8mm.
Another good rule is 'if in doubt, meter out'.
The manufacturer datasheet should provide a p.d. against flow curve.
Beware of temperature effects on performance. Some valves have better temperature compensated designs.
Consider backflow and bypass check options.
The risk of meter in is that the actuator load tries to move the cylinder quicker than the flow controller should allow creating negative pressure and a dangerous uncontrolled load as a result. With meter out the load is braked by the orifice so cannot run away. The risk now is that the bore pressure creates an intensified annulus pressure that is higher than the cylinder or valves are designed to take.
Always check and beware that shutting a flow control valve will not over pressurise another part of the circuit.
Check cold start up is not an issue.
Design calcs TBC