2/2 directional control valve
Two ports, two positions: opens or closes a single flow path (on/off).
ISO 1219-1
Valves
Directional control, pressure control and flow control valves that govern the system.
3/2 directional control valve
Three ports, two positions: connects the actuator alternately to pressure or to the reservoir.
ISO 1219-1
4/3 directional control valve (closed centre)
Four ports, three positions. The centre position blocks all ports (closed centre).
ISO 1219-1
Pressure relief valve
Normally closed. Opens to the reservoir when pressure reaches the set limit, and protects the system.
ISO 1219-1
Pressure reducing valve
Normally open. Throttles down to a lower, constant secondary pressure regardless of inlet pressure.
ISO 1219-1
Throttle valve (fixed)
Fixed restriction giving constant flow resistance, limiting flow rate.
ISO 1219-1
Adjustable throttle valve
Restriction with an adjustable opening. The diagonal arrow indicates that the resistance can be adjusted.
ISO 1219-1
Check valve (non-return valve)
Allows free flow in one direction and blocks the opposite. The ball is pressed against the seat under reverse flow.
ISO 1219-1
Pilot-operated check valve
Check valve that can be forced open for return flow via a pilot line (dashed).
ISO 1219-1
Test yourself on these in the symbol trainer.
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