Flushing a New Hydraulic System
A new hydraulic system is never clean inside: scale, chips, hose debris and assembly dirt sit in pipes, tank and hoses. Start it up straight away and that dirt travels through pump and valves, risking early failures. This guide shows step by step how to flush a system to the required oil cleanliness before its first run.
View hydraulics guidesWhy does a new system need flushing at all?
Manufacturing, transport and assembly leave particles in the system: weld scale and rust from pipes, chips from threads, debris from hoses and dust from open ports. Just a few grams of hard particles can permanently damage piston pumps and proportional or servo valves with their tight control clearances. A new system therefore often starts dirtier than a run-in one. The goal of flushing is to carry out this residual dirt before it reaches the sensitive components.
- Protect the pump, proportional and servo valves from wear and sticking.
- Reach the oil cleanliness required by the component makers before the first load.
- Avoid early failures, leaks and costly downtime during the run-in phase.
- Documented cleanliness as evidence for acceptance and warranty.
How do I prepare the system for flushing?
Before flushing, the sensitive actuators are bypassed so dirt is not driven through cylinders and valves. Replace servo valves with flushing plates and connect cylinder ports with flushing bridges to form a loop. The flushing medium is usually the later operating oil of the same viscosity, so no mixing problems arise.
How do I flush right - flow, temperature and turbulence?
Turbulent flow is decisive: only it lifts particles off the pipe wall and carries them to the filter. Aim for a Reynolds number above 4000, usually reached at 2 to 3 times the operating flow. Warm oil of 40 to 60 °C lowers viscosity and raises turbulence further. Thermal cycling and tapping on pipe bends free stubborn scale.
Choose a flow that keeps the Reynolds number above 4000, often 2-3x operating flow.
More hydraulics basics- Reverse the flushing direction several times to clear dead legs and blind holes.
- Check the flushing filter regularly and change it as the differential pressure rises.
- Reconnect the actuators only after the target class is stably reached.
When is the system clean enough for release?
Release is not by gut feeling but by measured particle counts to ISO 4406. A particle counter or a lab sample determines the oil cleanliness during operation. The target class must be stable over several measurements and at least as clean as the spec of the most sensitive component. Only then are the servo valves fitted and the system put into service.
Frequently asked questions
Can I just start a new hydraulic system on operating oil?
Better not. New pipes, tanks and hoses contain scale, chips and debris. Without flushing this dirt travels through pump and valves and causes wear and early failures. Flush to the required ISO 4406 class before the sensitive actuators are loaded.
Which oil do I use for flushing?
Usually the later operating oil at the same viscosity, so no mixing problems arise. More important than a special flushing oil are high, turbulent flow, warm temperature and a fine flushing filter in the return line.
How long do I need to flush?
Until the measured oil cleanliness to ISO 4406 stably reaches the target class of the most sensitive component over several measurements. Depending on system size and contamination that can take hours to several days. The particle count, not the clock, decides.
Commission your hydraulic system clean
Our hydraulics specialists help with the flushing concept, target cleanliness and filter selection for your system.
Defined cleanliness
Target classes to ISO 4406 instead of gut feeling.
Component protection
Pump and valves stay bypassed while flushing.
Backed by measurement
Release only after a stable particle count.
Expert advice
Hydraulics specialists support commissioning.