HYDAC Filter Bypass: Which Oil Reaches the System Unfiltered?
When a filter bypass opens, oil can flow around the media directly to the downstream circuit. That portion of the flow does not receive filtration from the element.
Oil may still pass through the element at the same time. As element restriction increases, a larger share can take the bypass route. The machine may continue moving even though filtration has deteriorated.
A 6 bar bypass responds to differential pressure
Take a pressure filter with a bypass opening differential of 6 bar. An inlet pressure of 200 bar and outlet pressure of 198 bar produce a 2 bar differential, below the opening setting. With the inlet still at 200 bar and the outlet at 194 bar, the differential reaches 6 bar.
High system pressure alone does not necessarily open the bypass. The main relief valve limits system pressure; the filter bypass responds to the pressure difference across the filter.
Reaching the opening setting begins bypass operation. Opening changes with flow and resistance. A 6 bar opening setting does not mean that all oil instantly bypasses the media.
Alarm, bypass and collapse ratings
HYDAC clogging-indicator information includes a pressure-filter combination with a 5 bar indication and 6 bar bypass opening. These values describe this combination; other configurations can differ.
| Parameter | Purpose | Result |
|---|---|---|
| Alarm differential | Prompt filter inspection or replacement | Electrical or visual indicator operates |
| Bypass opening differential | Provide a path around the media | Part of the flow bypasses filtration |
| Collapse rating | Specify the element's structural differential-pressure capacity | Match the element to the circuit requirement |
In this combination, investigate the cause when differential pressure reaches 5 bar. At approximately 6 bar, bypass flow begins. Even if the element's collapse rating exceeds 6 bar, filtration protection has already changed; waiting until near the collapse rating is inappropriate.
Housing working pressure and element collapse pressure are also different. The housing contains system pressure, while the element withstands pressure difference across its media. A high-pressure housing can contain an element protected by a bypass. Housing pressure rating cannot substitute for element differential-pressure capacity.
A cold-start alarm clears as the oil warms
Cold, viscous oil creates more pressure drop through the media at the same flow. A high-flow movement immediately after startup can reach the alarm or bypass setting even with a clean element.
For example, an alarm appears on a winter startup and differential pressure falls substantially after warm-up during the same movement at similar flow. Check startup flow and filter capacity. If warm differential pressure remains high, check element loading, specification and passage restriction.
Compare similar movements and flows. A cold high-speed stroke and warm standby are not comparable: standby flow is already much lower.
| Observation | Checks |
|---|---|
| Cold alarm clears during the same warm movement | Viscosity, startup flow and filter capacity |
| Alarm persists when warm | Element loading, filtration area and correct element specification |
| Alarm occurs only during high-flow movement | Peak flow and filter operating limits |
| Unusually low differential after replacement, but oil cleanliness worsens | Element seating, seals and bypass closure |
An indicator with low-temperature suppression may withhold its alarm when cold. A mechanical bypass still responds to differential pressure. No cold alarm therefore does not prove that all oil is passing through the media.
For alarms after replacement, see why a new HYDAC filter element can still trigger a differential-pressure alarm.
Prolonged bypass affects downstream components
The bypass offers an alternative to a highly restricted filtration path, but particles in its flow continue downstream. If the filter supplies servo valves, precision proportional valves or other sensitive components, prolonged bypass weakens contamination control.
Replace a genuinely clogged element. If a new element still operates near bypass pressure at normal temperature and flow, check filter sizing, flow peaks and circuit restrictions.
Blocking the bypass, fitting a stiffer spring or raising the alarm setting changes the original protection arrangement. Silencing the alarm leaves the underlying high differential pressure unresolved.
Replacing the element may leave the old bypass in service
Some HYDAC return filters incorporate the bypass in the element; some pressure filters locate it in the head. Establish where the assembly is fitted.
A head-mounted bypass remains in service when the element is replaced. Debris that holds it open can prevent a new element from restoring filtration. Inspect the head's bypass assembly and seals in this situation.
A filter without bypass has no alternative route around the media. Differential pressure continues to rise as it clogs, so the element needs suitable collapse resistance. HYDAC's BN/BH information distinguishes elements for bypass-protected and non-bypass applications.
Match collapse rating to the existing circuit. An element that fits physically can still deform or fail if its differential-pressure capacity is inadequate. See filter-element deformation and damage.
Conclusion
Bypass flow avoids the media. Assess filter differential pressure together with temperature and flow. Replace a clogged element, correct insufficient flow capacity, and repair a bypass that does not close. Continued machine movement is not evidence that filtration is still adequate.