Oil contamination causes most hydraulic system failures. A proportional servo valve has precision micron-level matching parts. It needs very clean hydraulic oil. Tiny solid particles can jam the spool, wear throttle edges, increase zero drift and trigger system oscillation. In severe cases, the valve will fail completely. Welding, installation and storage leave welding slag, iron chips and oxide skin inside pipes. We must flush the pipe circuit before startup. Do not mount the proportional servo valve until oil cleanliness meets the required standard.
Flushing principle and flow requirement
What flow state can remove pipe wall contaminants? Turbulent flow. Turbulence strips dirt off pipe inner walls.
- Pipe flow speed ≥8 m/s
- Reynolds number: 2000～4000
- Oil temperature: 40～70℃. Lower oil viscosity helps form turbulent flow.
How to protect precision components during flushing?
- Isolate precision parts. Use high pressure hoses to short cylinder and motor ports. Remove proportional servo valves and install flushing plates instead. Disconnect accumulators from the flushing loop to block dirt entering precision parts.
- Use the system’s official hydraulic oil as flushing medium. Fill the tank to 30%~40% of rated volume. Use the main pump to build circulation.
- Raise pressure step by step. Pressure must overcome pipe and valve block pressure drop. Tap pipes section by section with wooden hammers to shake off attached dirt. Do not use steel hammers.
- Use duplex filters for on-line element replacement. Start with coarse filter elements to trap large debris. Switch to fine filter elements later. Add a portable filter cart to speed up oil cleaning.
How to avoid secondary contamination during flushing?
- Seal all shorted ports and pump ports with clean tape. Keep dust out.
- Monitor filter pressure difference. Replace filter elements immediately when pressure difference rises too high. Prevent element rupture and dirt backflow.
- Inspect the whole loop continuously. Fix leaks quickly. Track pressure and oil temperature.
How do we confirm flushing is finished?
Do not judge by flushing time only. Test oil cleanliness per the proportional servo valve datasheet (ISO 4406 standard). Two consecutive qualified oil samples pass the test. After acceptance, clean the tank, install new filter elements, mount proportional servo valves and start function tests.
