The basic principle.
A pressure-relief valve opens a route when the pressure acting on its mechanism overcomes the opposing force. It is commonly used to limit pressure by diverting flow to a lower-pressure return path.
It does not normally control cylinder speed in the way a metering valve does. Pressure behaviour depends on the valve design and flow through it; a nominal setting is not a perfectly constant pressure under every condition.
A load cannot move.
If pump flow enters a circuit that cannot accept it, pressure rises. A correctly designed relief path can divert flow rather than allow an uncontrolled pressure rise.
Follow the relationship.
- Pressure risesActs on the valve mechanism.
- Relief opensCreates a return path.
- Energy dissipatesFluid power becomes heat through the restriction.
Connect it to the lift.
Prolonged relief flow can heat the oil and waste power. A lift that fails to rise should not be “fixed” by increasing a relief setting: load, pump delivery, valve paths and mechanical conditions require investigation against the design.
A closer look
Direct-acting and pilot-operated relief valves have different response characteristics. Pressure-reducing valves perform a different function by controlling pressure in a downstream branch.
Check your understanding
Does a relief valve make unused hydraulic power disappear?
Reveal the explanation
No. Power dissipated through the relief path becomes heat, which the system must manage.