1. Role of the Relief Valve
The relief valve performs two duties: it limits the maximum system pressure to protect the pump and piping, and in fixed-displacement systems it passes surplus flow back to tank. A solenoid-controlled version can also unload the pump at low pressure or switch between two pressure levels, which reduces heat during holding phases.
2. Pressure Setting Sequence
- Back the adjusting knob fully out so the valve is open at low pressure.
- Jog the pump and confirm there is no abnormal noise or leakage before raising pressure.
- Increase pressure gradually to the working value and watch the gauge to avoid a pressure spike.
- Lock the knob after setting, then re-check once oil temperature has stabilised.
3. Commissioning the Unloading Function
Unloading is achieved by opening or closing the pilot oil path. Verify coil voltage and wiring, then confirm that system pressure falls to near zero when unloading is commanded. If pressure does not drop, check for a blocked pilot orifice, a wrongly connected unloading port or a spool stuck in the closed position. If pressure rises slowly, check whether the remote port is open or the pilot stage is leaking.
4. Main Sources of Noise
Relief noise comes from high-velocity flow and bubble collapse. Typical causes are: actual flow far below the rated flow of the valve, producing intermittent relief; return lines that are too small, or shared with high-flow circuits, creating back pressure and resonance; air entrainment causing cavitation at the seat; and a valve size mismatched to pump flow. Check flow matching first, then confirm the return line discharges below oil level and that suction lines are tight.
5. Routine Maintenance
Keep oil cleanliness within the specified class, replace filter elements on schedule and inspect pilot stage damping orifices. Compare gauge readings with the set value every quarter. Systems that relieve for long periods should use an accumulator or an unloading circuit to cut heat generation and noise.
6. Graded Response to Pressure Faults
If pressure cannot be built at all, check whether the pilot valve is held open, whether the remote port is open and whether the pump is worn. If pressure builds but fluctuates, suspect air in the oil, a partly blocked pilot orifice or system flow below the minimum stable flow of the valve. If pressure exceeds the set value and keeps rising, check for a stuck spool or a restricted return line.
Work from the outside in: confirm the gauge and tapping are reliable and the return line is clear before opening the pilot stage. After any strip-down, reset and record the pressure so later readings remain comparable.
7. Common Unloading Circuit Arrangements
Fixed-displacement systems usually unload through the solenoid relief valve: during holding, the coil shifts the pilot path to tank so the pump circulates at low pressure; when pressure is needed, the coil resets and the pilot stage re-establishes pressure. Variable-displacement systems rely on the pump compensator, with the relief valve acting only as a pressure limit.
- Run the unloading line separately to tank to avoid back pressure from other returns.
- Add an accumulator where unloading is frequent, to smooth pressure and reduce coil cycling.
- Check the duty rating before holding a coil energised continuously, since coils heat up.
References
- GB/T 3766-2015 Hydraulic fluid power — General rules and safety requirements for systems and their components
- GB/T 7935-2005 Hydraulic fluid power — General technical requirements for hydraulic components
- GB/T 14039-2002 Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles
- GB/T 17446-2012 Fluid power systems and components — Vocabulary
