Valves and Flow Control
Relief, directional, flow control, pressure reducing and check valves, orifice flow and proportional and servo valves.
Three jobs: pressure, direction, flow
Every valve does one of three things: limit or set pressure, direct the flow to the right place, or control how much flow passes. Pressure sets the force, direction sets the movement and flow sets the speed.
Pressure control valves
| Valve | What it does | Typical use |
|---|---|---|
| Relief valve | opens when the pressure reaches its setting and dumps flow to the tank | system protection: every circuit with a positive-displacement pump needs one |
| Pressure-reducing valve | holds a lower pressure in a branch | clamps and secondary circuits |
| Sequence valve | opens at a set pressure to start the next movement | clamp then drill |
| Counterbalance valve | holds a load and lets it down under control | vertical cylinders, winches |
| Unloading valve | dumps the pump flow at low pressure when a set pressure is reached or a signal comes | accumulator circuits, idle times |
| Brake valve | controls a motor that is overrun by its load | travel and slew drives |
A relief valve is not a flow limiter. It starts to crack open at its setting, and needs a higher pressure to pass the full pump flow (the override): a valve set at 160 bar may need 175 bar (9 percent more) to pass its rated flow, which must be accounted for when setting the pressure under a load. Set the relief about 10 percent above the working pressure, never as a regular working control (it wastes all the power, see Fluids, Filtration and Reservoirs).
Example of a reducing valve: a clamp cylinder of 50 mm bore fed through a valve set to 60 bar presses with 11.8 kN, however high the main pressure rises.
Directional valves
A directional valve connects ports in different combinations. The code “4/3” means four ports (P pressure, T tank, A and B to the actuator) and three positions. The center position decides what happens when nothing is commanded:
| Center type | What happens at neutral | Use |
|---|---|---|
| Closed center | all ports blocked: pump flow must go over the relief or an unloader | with an accumulator or a pressure-compensated pump; holds a load |
| Open center | P connected to T: pump unloaded, actuator floats | simple circuits, low idle losses |
| Tandem center | P to T, A and B blocked: pump unloaded and the load held | the most common: hold and unload |
| Float center | A and B to T, P blocked: actuator free | motors that should coast |
Actuation is by solenoid (direct or pilot operated), hydraulic pilot, lever or cam; spool valves are fast but leak slightly (a spool is never perfectly sealed), poppet valves are leak-tight. Valves are sized to the ISO 4401 patterns (NG6, NG10 and so on), where the NG number is the nominal size in mm and the maker’s data gives the flow limit at a pressure drop.
Flow control valves
A needle valve or orifice restricts the flow, so it sets the speed (v = Q ÷ A) of the actuator. The flow through an orifice depends on the pressure drop, so a plain throttle gives a speed that changes with the load. A pressure-compensated flow control holds the pressure drop across the orifice constant with an internal compensator, so the flow stays set whatever the load.
Worked example: a throttle in front of the press cylinder
- A 1.5 mm orifice (1.77 mm²) in the cap-side line with 100 bar across it (pump 160 bar, load 60 bar): Q = Cd A √(2 Δp ÷ ρ) = 0.62 × 1.77 mm² × √(2 × 100 × 10⁵ ÷ 870) = 10.0 L/min, so the cylinder moves at 0.033 m/s.
- Where the load rises to 120 bar (Δp = 40 bar) the flow falls to 6.3 L/min: the speed drops by 37 percent, unless the valve is pressure compensated.
- The throttled oil carries the pressure drop as heat: 100 bar × 10.0 L/min ÷ 600 = 1.66 kW into the tank.
- Meter-in (throttle before the actuator) works well for resistive loads; meter-out (after it) controls running-away loads (see Circuits and Accumulators).
Orifice flow and valve pressure drop
Q = Cd A √(2 Δp ÷ ρ) Δp ∝ Q²
Worked example: orifice and valve losses
- To pass 30.2 L/min with a 5 bar drop, the orifice area is A = Q ÷ (Cd √(2 Δp ÷ ρ)) = 23.9 mm², a diameter of 5.5 mm.
- A directional valve rated at 5 bar for 30 L/min has 20 bar drop at 60 L/min (twice the flow, four times the drop). Choose a valve with the flow limit above the real flow, or the drop becomes a heat load.
- At every pressure drop the energy appears as heat in the oil: Δp × Q.
Proportional and servo valves
A proportional valve moves its spool in proportion to a current, giving smooth control of the flow, pressure or both and the possibility of ramps and closed-loop control with feedback. A servo valve is faster and more accurate (response in a few milliseconds, with a torque motor and a null-bias leakage), and needs very clean oil (ISO 4406 16/14/11). Both are driven by amplifiers and controlled by a PLC or a motion controller (see Motion Control and Actuators).
Cartridge and manifold valves
Cartridge (screw-in or slip-in) valves are mounted in a machined manifold block. They are compact, leak-tight, can handle large flows (to 1 000 L/min), and let a designer build an entire circuit in one block. They suit production machines and mobile equipment. Subplate and sandwich (modular) valves stack on a standard pattern for easy service.
Related
- Previous: Cylinders and Actuators. Next: Circuits and Accumulators.
- Fittings, Valves and Minor Losses for the loss coefficients of water valves.
- Hydraulics and Pneumatics: the whole guide.
FAQ
Why does my relief valve screech?
It chatters when it is set too close to the working pressure or is undersized for the flow. Set it about 10 percent above the working pressure and size it for the full pump flow.
Do I need a check valve?
To hold pressure in one direction, to stop reverse flow through a pump or to bypass a throttle on the return stroke. A pilot-operated check holds a load without drift.
What does NG6 mean?
Nominal size 6 mm: the ISO 4401 interface for a small valve (about 30 L/min). NG10 is about 80 L/min.
Why does the speed vary when the load changes?
The throttle’s flow depends on the pressure across it. A pressure-compensated flow control (or a proportional valve with closed-loop feedback) cures it.
References
- Esposito, A., Fluid Power with Applications, Pearson (valves); Merritt, H. E., Hydraulic Control Systems, Wiley.
- ISO 4401 (valve mounting surfaces), ISO 1219 (symbols) and ISO 6263 (compensated flow controls).
- Bosch Rexroth, Parker Hannifin and Eaton valve catalogs.
Last reviewed: October 2026. Orifice coefficients and valve data are typical.