Pressure control valve symbols
Learn about hydraulic relief and reducing valves
Hydraulic pressure relief valve symbols
The top symbol shows a simple, direct operated pressure relief valve.
Note how the arrow is shown in it's deactivated position e.g. with the spring force higher than the pressure in the pilot. The pilot pressure also comes straight off the supply line upstream of the valve showing that as the pressure before the valve increases it pushes the arrow against the spring and relieves the pressure.
The lower symbol show a pilot operated pressure relief valve. This valve operates in the same way as the top, direct operated valve except that it will have at least two stages of valve within it. Learn more about relief valves here.
Electrically operated pressure relief valves
These symbols show similar valves to the ones above. Both work in a the same way and will relieve the pressure when it gets to high but these also have electrical load or unload facility. Neither shows enough detail to say whether the solenoid needs to be energised to make the valve raise pressure or de-energised to raise pressure.
The bottom symbol shows two stage pilot operation with an external Y drain. This may be important for holding a constant or stable pressure because without it the balanced spring chamber pressure would be fed from the valve tank return line which could be subject to pressure peaks or oscillations.
Hydraulic pressure reducing valves
Pressure reducing valve symbols look very similar to those for pressure relief valves. This is because the valves work in similar ways except that the pressure sense line feeds from down stream of the reducing valve instead of upstream as in the relief valve. This means that the reducing valve controls the downstream pressure rather than the upstream pressure. As such the reducing valve cannot generate a downstream pressure that is not there but it can limit its value. Not how the arrow is shown in the middle positions indicating that it is normally open but closes the flow off when the pressure feed forces the arrow against the adjustable spring.
The top valve shows an external Y drain line but only has 2 ports.
The lower symbol has a third tank line port and arrow heads pointing in both directions. In this valve if the pressure downstream rises above the reducing valve's setting then the valve will vent this pressure down the third line to provide a constant pressure in the sensed port.