How to judge whether a HYDAC filter element needs replacement according to differential pressure, and what is the standard limit differential pressure?

I. Differential Pressure Standards: Initial, Alarm, Limit and Bypass

Initial differential pressure (new filter element): ≤ 0.2 MPa for return line filters, ≤ 0.3 MPa for pressure line filters. A much higher value generally indicates undersized selection or abnormal oil viscosity.

Alarm differential pressure (replacement recommended): 0.35 MPa (3.5 bar) for return line filters and 0.5 MPa (5 bar) for pressure line filters. Replace the element immediately once the indicator alarms; this is the most commonly adopted standard on site.

Limit differential pressure (mandatory replacement): 0.5 MPa for return line filters and 0.6 MPa for pressure line filters. The filter media is prone to deformation near this value, leading to a sharp rise in contamination risks.

Bypass valve opening pressure: 0.6–0.8 MPa for return line filters and 1.0 MPa for pressure line filters. Once the bypass valve opens, filtration fails. Long-term operation under this condition is strictly prohibited.

II. On-site Differential Pressure Judgment Methods

Differential pressure under stable operation: Normally less than 0.3 MPa. A gradual rise means normal clogging; keep relevant records.

Replace on alarm (strict rule): Replace the filter element right away when the differential pressure reaches 0.35 MPa (return line) or 0.5 MPa (pressure line), regardless of service time.

Check for sudden pressure rise: A pressure increase of over 0.1 MPa within one hour usually results from sudden system contamination, excessively low oil temperature or reversed installation. Shut down the equipment for inspection.

Be alert to abnormally low pressure: A sustained reading below 0.1 MPa may be caused by damaged filter elements, stuck bypass valves or reversed installation, which allows unfiltered oil to flow back directly.

Viscosity correction: The rated differential pressure is calibrated at 32 mm²/s. At low temperatures with high viscosity (e.g., No.46 hydraulic oil below 20℃), the differential pressure will show a false high value. Re-measure after the oil temperature rises.